Long Valley Caldera. *5& Field Trip September 18-20

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1 Long Valley Caldera *5& Field Trip September Mammoth Pacific binary generating plants at Casa Diablo in Long Valley Caldera. View is from the top of a peak within the caldera s resurgent dome looking southwest toward the southern topographic margin of the caldera. The caldera moat in the background is the focus of recent seismicity during the last two decades of caldera unrest. Paleozoic metamorphic rocks in the Sierra Nevada range to the south are the source area for a landslide block that slid into the caldera on a gassy cushion of ash late in the caldera s collapse. Photo by R. Sullivan 1

2 LONG VALLEY CALDERA GEOTHERMAL AND MAGMATIC SYSTEMS Gene A. Suemnicht EGS Inc., Santa Rosa, CA INTRODUCTION Long Valley Caldera in eastern California has been explored for geothermal resources since the 1960s. Early shallow exploration wells (<300m) were located around Casa Diablo near the most prominent hot springs and fumaroles on the southwest flank of the Resurgent Dome (Figure 1). Later deep (±2000m) wells explored the southeastern caldera moat and evaluated federal lease offerings in and around the caldera s Resurgent Dome. Data from these wells revealed that the principal geothermal reservoir in Long Valley is not located directly beneath the Casa Diablo Hot Springs and is not currently related to the Resurgent Dome. Instead, the hydrothermal system appeared to be more complex with shallow production at Casa Diablo supplied by outflow from a more extensive upwelling geothermal source deep beneath the western caldera moat. Geothermal development at Casa Diablo occurred in stages under various operating companies. Ormat Nevada Inc. currently owns the project, operating it under the Mammoth Pacific Limited Partnership (MPLP). The facility generates 40 MWe (gross) from three power plants utilizing 6 production wells and 5 injection wells. Until 2005, production of ~630 L/sec (~10,000 gal/min) of moderate temperature (170 C) fluids for the geothermal plants was limited to ~0.7 km 2 (~165 ac) around Casa Diablo from shallow wells (<200m) completed in permeable Early Rhyolite eruptive units on the southwestern edge of the Resurgent Dome. MPLP drilled deeper (~450m) production wells in 2005 in the southwest caldera moat around Shady Rest in a development area that Ormat refers to as Basalt Canyon. The deeper west moat wells produce 185 C fluids from the upper part of the Bishop Tuff. In 2006, an average of 126 L/sec (2000 gal/min) of higher temperature brine from the Basalt Canyon wells began to be piped 2.9 km to sustain production at the Casa Diablo plants allowing several older shallow wells to be shut in. Ormat plans to expand production from Basalt Canyon, install new generation facilities and increase generation to approximately 60 MWe (gross). GEOLOGIC SETTING Long Valley Caldera is a 17 X 32 km (10 X 20 mi) depression created by the eruption of an estimated 600 km 3 of Bishop Tuff 760,000 years ago (Bailey and others, 1976). The caldera is the largest feature in the Mono-Long Valley volcanic field that includes Pleistocene-Recent eruptive centers of Mammoth Mountain and the Mono Inyo volcanic chain. Volcanism associated with Long Valley began ~ 4 Ma ago with widespread 1

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4 eruptions of intermediate and basaltic lavas accompanying the onset of large-scale transtensional faulting that formed the eastern front of the Sierra Nevada and the Owens Valley graben (Figure 1). Discontinuous erosional remnants of precaldera extrusive rocks are scattered over a 4000 km 2 area around the caldera suggesting an extensive mantle source region (Bailey and others, 1989). Rhyolitic eruptions began ~ 2 Ma ago from multiple vents of the Glass Mountain eruptive complex along the northeast rim of the present-day caldera (Metz and Mahood, 1985) (Figure 1). These more silicic eruptions suggest the initiation of magma accumulation and differentiation in the shallow crust (Bailey and others, 1989; Hildreth, 2004). The caldera-forming eruption partially evacuated the underlying magma chamber and the floor of the caldera subsided up to 2 km in segments separated along pre-existing inherited basement faults (Suemnicht and Varga, 1988). Approximately km 3 of the Bishop Tuff filled the caldera depression. Post-collapse eruptions have continued to fill the caldera over the last 600,000 years (Bailey and others. 1976; Bailey 2004; Hildreth 2004). A series of rhyolite flows and tuffs (Early Rhyolite) mark the onset of resurgence in the west-central part of the caldera approximately 600,000 years ago (Figure 1). Coarsely porphyritic Moat Rhyolites erupted later around the Resurgent Dome beginning in the north approximately 500,000 years ago progressing to the south around 300,000 years ago and approximately 100,000 years ago in the west (Bailey and others, 1976; 1989). The western Moat Rhyolites erupted during a period of more voluminous basaltic and andesitic flows that began approximately 200,000 years ago in the western caldera extending beyond the caldera margins to the south and west. These more mafic eruptions include basalts in the southwestern caldera moat (Casa Diablo flow of Bailey and others, 1976), eruptive events such as the 82,000 year-old Devil s Postpile basaltic andesite and more recently, the 8,000 year-old Red Cones vents south of the caldera. A series of rhyodacitic eruptions also occurred in the western caldera moat 110,000 50,000 years ago (Mahood and others, 2010); the most prominent of these is the Mammoth Mountain Dome Complex on the southwestern topographic rim of the caldera (Bailey 2004). Subsequent mapping and age dating (Hildreth, 2004; Hildreth and others, 2014)) suggested that the mixed maficrhyodacitic volcanism represented a separate magmatic system outside the caldera s ringfracture system. The most recent eruptions in the area occurred along the Mono-Inyo volcanic chain extending from the western caldera moat northward to Mono Lake (Figure 2). Eruptions along the chain began approximately 40,000 years ago and have continued to historic times. Bursik & Sieh (1989) identified 20 small eruptions (erupted volumes <0.1 km3) within the chain over the past 5000 years occurring at intervals ranging from 250 to 700 years. The most recent dome-forming eruptive events began at the north end of the Mono Craters about 600 years ago and culminated with phreatic eruptions in the south at the Inyo Craters and the north flank of Mammoth Mountain (Bursik & Sieh 1989; Mahood and others, 2010). The magma source for these eruptions is an 8 10-km-long dike that trends north out of the caldera. In 1987, the DOE funded several core holes that penetrated the dike at Obsidian Dome north of the caldera and at the Inyo Craters in the caldera s west moat. Petrologic studies of the core indicate that the eruptions may have tapped magmas of three different compositions venting at several places along strike (Eichelberger, 2003). A shallow intrusion beneath Mono Lake approximately 200 years 3

5 ago lifted lake-bottom sediments to form Pahoa Island erupting a small volume of andesitic lava from vents on the island s north side (Bursik & Sieh, 1989). The progression of eruptions over the past 2 Ma from Glass Mountain on the eastern caldera margin to Mammoth Mountain on the west and the Mono Inyo volcanic chain to the north suggests that the magmatic system that erupted to form Long Valley Caldera has declined with time and has been supplanted by complex mixtures of magma erupting contemporaneously from the active Mammoth Mountain Inyo Domes magmatic system at times triggered by the intrusion of more mafic magma into the shallow crust (Hildreth, 2004; Mahood and others, 2010; Hildreth and others, 2014). Figure 2. Locations ages and types of eruptive events in the Mono-Inyo volcanic zone (from USGS) 4

6 Tectonics Long Valley Caldera lies within an east-west embayment or offset in the northwest trending Sierran escarpment (Mayo, 1934). The east-dipping normal faults that form the escarpment mark the western edge of crustal extension in the Basin and Range Province. The caldera is located at the northern end of the Owens Valley graben, part of the Eastern California Shear Zone, a region of transtensional deformation along the western edge of the Basin and Range that extends to the north along the Walker Lane in western Nevada (Figure 2) (Hill, 2006). Transtensional deformation and active magmatism within the Eastern California Shear Zone and the Walker Lane reflect the combined influence of dextral slip along the San Andreas Fault transform boundary between the Pacific Plate and North American Plates and the westward progression of crustal extension across the Basin and Range Province. Right-lateral slip distributed across this transtensional zone accounts for 15-25% of the relative motion between the Pacific and the North American plates (Dixon and others. 2000, Faulds, 2004). Active magmatic processes in the region are generally attributed to upwelling of the underlying asthenosphere as the crust stretches, thins and fractures in response to transtensional extension across the Eastern California Shear Zone - Walker Lane corridor (Hill, 2006). The tectonic interaction between the Eastern California Shear Zone and the Walker Lane system localizes volcanism at transtensional pull-apart sections in the Mono-Long Valley region. Long Valley Caldera is located at the western end of the Mina Deflection, a broad zone of northeast-striking left-lateral faults that form a right jog in the regional right-lateral fault system of the Owens Valley providing a link to the Walker Lane to the northeast (Figure 3). Crustal extension in the Mono Basin is the result of left lateral shear across the Mina Deflection and the extension increases southward into Long Valley Caldera. Extensional strain is complexly resolved within the caldera because of the faulting and fracturing that defined the pre-existing Sierran range front embayment. The western and central parts of the caldera are superimposed over several pre-existing fault systems that defined the pre-caldera eastern Sierra range front. The floor of the caldera, rather than being a simple planar feature, is segmented into a number of discrete blocks with varying offsets that accommodated 1-2 km of subsidence as the caldera s floor foundered. The basement structure of Long Valley, buried under nearly 400 km 3 of Bishop Tuff and later volcanic fill, has been interpreted from detailed mapping, regional geophysical surveys and drilling data (Bailey, 1976; Suemnicht and Varga, 1988; Bailey, 2000). The Discovery Fault Zone in the western caldera is a prominent surface feature that is a direct extension of the Mina Deflection and an expression of the original range front embayment before the caldera collapsed. The pre-existing tectonic framework controlled the configuration of the caldera floor and, through faults and fault intersections, controlled the location of postcollapse eruptive centers. The inherited deep basement structures of the western caldera provide the high fracture density and deep permeability for the source of the present geothermal system in Long Valley. 5

7 Figure.3 Shaded relief map of east-central California and western Nevada, showing the location of Long Valley Caldera (LVC) and the Mono Inyo volcanic chain (MIVC) outlined by red ovals. Increasing elevation is indicated by colors ranging from dark green (at approximately sea-level) to light grey (>2400 m with maximum elevations reaching c m). The transition from yellow-green to brown marks the 1700 m contour. Heavy black lines indicate major Quaternary faults. The red line in the Owens Valley indicates surface rupture from the M 7.6 Owens Valley earthquake of White arrows indicate the sense of displacement across the Eastern California Shear Zone (ECSZ), the Mina Deflection (MD), and the Walker Lane (WL). The white outline in the inset shows the map location with respect to the San Andreas Fault (SAF) in coastal California (CA) and the Basin and Range Province in western Nevada (NV) with opposing white arrows indicating the sense of extension. (After Hill, 2005) 6

8 Caldera Unrest Moderate to strong historical earthquakes occur regularly in the eastern Sierra Nevada and the Owens Valley south of Long Valley Caldera (Ellsworth, 1990). The northern end of the rupture zone of the M ~ 7.6 Owens Valley earthquake of 1872 extended to within 60 km of Long Valley Caldera (Figure 4) and earthquakes M>5 occurred outside the caldera before 1970 (Cramer & Toppozada 1980; Ellsworth 1990). Long Valley Caldera began a period of unrest in the late 1970s that included earthquake swarms, approximately 80 cm (2.6 ft.) of inflation within the resurgent dome, changes in the outflow from hot springs and fumaroles and increased CO 2 emissions around the flanks of Mammoth Mountain (Figure 1). The gas emissions on Mammoth Mountain have been accompanied by rising 3 He/ 4 He ratios interpreted as potential indicators of magma moving to shallower crustal levels. The largest magnitude earthquakes occurred within the Sierran block south of the caldera while caldera activity was marked by earthquake swarms, long-period (LP) and very-long period (VLP) volcanic earthquakes. An intense earthquake sequence that included four M>6 earthquakes within and around the caldera on May 25, 1980 occurred within days of the May 18, 1980 eruption of Mount St Helens and in that context, raised strong concerns about the eruptive potential of a large active magma chamber beneath the caldera. Volcanic hazard concepts related to the continuing unrest within the caldera evolved rapidly as research progressed on the Mono-Long Valley magmatic system (Hill, 2006). Based on Long Valley data and a better understanding of restless calderas worldwide, large silicic calderas can go through sustained periods of episodic unrest, separated by years to decades of relative quiescence, all without producing an eruption (Newhall & Dzurisin 1988; Newhall, 2003). Caldera unrest can also be more intense and may extend beyond the comparatively short restless periods associated with central vent volcanoes. Volcanic earthquakes, increased magmatic gases and changes in geothermal manifestations have all occurred in Long Valley Caldera without an eruption. Long Valley remains on an active volcanic hazard alert status although the US Geological Survey states that earthquake activity within and adjacent to the caldera has remained at a comparatively low level since The caldera is probably not underlain by a laterally extensive, upper-crustal magma body capable of feeding a major eruption (Eichelberger, 2003), but none of the current data exclude the possibility of smaller (<1 km3) eruptions from smaller magma sources or a series of phreatic explosions similar to the historical eruptions that occurred along the Inyo-Mono dike (Miller, 1985). The period of unrest most likely was associated with the addition of ~0.3 km 3 of magma and hydrous fluids at a depth of 6-7 km beneath the resurgent dome. Seismic tomography studies might resolve a 1 2 km diameter magma body but not the smaller melt volumes noted above (Hill and Prejean, 2005). The dacitic magma chamber beneath Mammoth Mountain has probably crystallized because the last eruption occurred 52,000 years ago (Hildreth, 2004). Hill & Prejean (2005) ascribed the 1989 earthquake swarm beneath Mammoth Mountain that included mid-crustal long-period earthquakes and 7

9 Figure 4. Shaded relief map illustrating the relation of Long Valley Caldera (LVC) to major tectonic elements and M>5 earthquakes in eastern California (the red rectangle in inset shows the map area). Heavy black and red lines are faults with Holocene slip and historic (post-1870) slip, respectively. The surface trace of the M=7.8 Owens Valley earthquake of 1872, labeled Orange circles are post-1977 earthquakes (small for M>5; large with decadal year included for M>6). Yellow circles are M>6 earthquakes (not including aftershocks to the 1872 main shock with decadal year indicated. WC is Wheeler Crest (from Hill, 2005). 8

10 Increased CO 2 venting to a mid-crustal plexus of basaltic magma (that) remains capable of feeding future mafic eruptions. This magma plexus presumably fed eruptions of the mafic field surrounding Mammoth Mountain, including the 8,000 year-old Red Cones vents, and it is the likely heat source for the 700 year-old phreatic explosion vents on the northeast flank of Mammoth Mountain. Some long period earthquakes have occurred west of the Mono Domes (Pitt & Hill 1994) but the area has remained comparatively quiet during the unrest in Long Valley. Based on a geologic history of 20 eruptions over the last 5000 years and the eruption at Pahoa Island approximately 200 years ago (Bailey 2004), the young silicic domes of the Mono Inyo volcanic chain still have the potential to produce significant eruptive events. GEOTHERMAL SYSTEM The geothermal system in Long Valley has varied through time (Bailey and others, 1976, Sorey and others, 1978; 1991). Different mineral assemblages in and around the Resurgent Dome and differing age dates indicate that mineralization occurred in two separate phases (Sorey and others, 1991). The caldera supported an intense hydrothermal system from 300,000 to 130,000 years ago producing widespread hydrothermal alteration in and around the Resurgent Dome. The current hydrothermal system has probably been active for only the last 40,000 years, but prominent surface manifestations occur in many of the older system s established outflow zones at comparatively low elevations in the south central portion of the caldera around the Resurgent Dome (Figure 5). As much as 75% of the current hydrothermal outflow occurs in the thermal springs at Hot Creek on the southeastern edge of the Resurgent Dome and geochemical estimates of source reservoir temperatures range from 200 C 280 C (Sorey and others, 1978; 1991; Mariner and Wiley, 1976). Figure. 5. Generalized distribution of surface manifestations in Long Valley Caldera 9

11 Hydrothermal manifestations are notably absent in the western caldera moat (Bailey and others, 1976); however, detailed mapping (Suemnicht and Varga, 1988) and remote sensing studies of the western caldera (Martini, 2002) identify many high-temperature alteration mineral assemblages <100,000 years old that are related to vigorous hydrothermal outflow along penetrative faults in the western caldera. The alteration mineralogy shows that significant surface manifestations occurred at higher elevations in the western caldera in the early phases of the current hydrothermal system and the current pattern of outflow to the southeast toward Casa Diablo may have resulted from active fracturing and faulting opening older hydrothermal flow zones allowing outflow along permeable zones at lower elevations. GEOTHERMAL EXPLORATION Early exploration drilling in Long Valley focused on the southern and central part of the caldera. Production wells drilled in the 60 s evaluated shallow production around Casa Diablo s fumaroles and hot springs. The first deep well (66-29) was drilled in 1976 to evaluate the resource potential in the southeast moat (Figure 6). Numerous shallow gradient holes evaluated the heat flow associated with the Resurgent Dome in the 1970s and geothermal lease sale opportunities in the late 70s and early 80s prompted shallow and intermediate drilling to assess lease blocks within the Resurgent Dome. Clay Pit-1 and Mammoth-1 drilled in 1979 were the first deep wells drilled in the Resurgent Dome of Long Valley and the first deep tests to penetrate the entire section of the caldera fill (Figure 6). Mammoth-1 drilled through 390m of Early Rhyolite, 863m of Bishop Tuff and 230m of metasedimentary rocks that correlate with the Mt. Morrison roof pendant to the south, bottoming at 1605m. Mammoth-1 was also the first well within the caldera to encounter a block of chaotically mixed metapelite and granite at 466m in the upper section of Bishop Tuff that was interpreted as a landslide block based on cuttings alone (Suemnicht, 1987). Drilling to evaluate federal lease offerings during the 1980s and scientific drilling to evaluate various eruptive processes expanded the understanding of the western part of the caldera. Unocal s deep well IDFU penetrated the caldera fill, Tertiary volcanic rocks and metamorphic rocks to a depth of 2000m near the Inyo Craters (Figure 6). The well encountered temperatures of 218 C at 1100m, the highest yet measured in Long Valley, but proved unproductive because of a limited thickness of reservoir rocks and the incursion of cold water beneath the production zone (Suemnicht, 1987). Later scientific drilling by Sandia National Labs west of (Figure 6) established that 1000 m of vertical offset on the caldera s western ring fracture system occurs within a kilometer distance between the two wells along the western structural margin of the caldera (Eichelberger and others, 1988). Additional scientific drilling in Long Valley included a core hole at Shady Rest (Figure 6) (Wollenberg and others, 1989), and an ultra-deep (3 km) Long Valley Exploratory Well (LVEW, Figure 6) intended to test the presence of magma near the center of the Resurgent Dome (Finger and others, 1995). 10

12 Figure 6. Caldera outline and locations of temperature gradient holes (T designation), intermediate and deep wells drilled in Long Valley. Wells in orange drilled into intracaldera Bishop Tuff and wells in red have penetrated the intracaldera fill and are completed in the Paleozoic metamorphic rocks of the caldera basement. (Suemnicht and others, 2006) The results of deep drilling on the Resurgent Dome indicate that present-day thermal conditions are controlled in places by vertical flow of relatively cold water in steeply dipping faults that formerly provided channels for high-temperature fluid upflow (Sorey and others, 2000). In the central caldera, current temperatures and gradients are relatively low and show little evidence of possible magmatic temperatures at depths of 5-7 km postulated on the basis of recent deformation, seismic interpretations and shearwave attenuation of teleseismic waves (Rundle and others, 1986; Rundle and Hill, 1988). Drilling results establish that magmatic activity beneath the central part of the caldera has waned over the past ~100,000 years while similar activity in the western caldera has increased. 11

13 RECENT DRILLING Mammoth Pacific LP has continued to assess the productive potential of the caldera s western moat. Shallow test wells confirmed the general temperature distribution within the shallow hydrothermal system and expanded the understanding of the intra-caldera geology. A similar section of metapelite/granite was encountered at the top of the Bishop Tuff in both the corehole drilled to 353m less than a kilometer south of Casa Diablo (Bailey, pers. com.) in 1992 and the BC corehole drilled to 600m approximately 2 km west of the production facilities in 2002 (Figure 7). Figure 7. Distribution of the landslide block (blue) of Paleozoic metasedimentary rocks from the southern rim of Long Valley Caldera (Suemnicht and others, 2006) The intracaldera landside block is relatively coherent and covers approximately 3 km 2 of the southern moat between Mammoth-1 on the north, and BC on the west. The landslide block is absent in surrounding wells that penetrate into the Bishop Tuff on the Resurgent Dome and at Shady Rest, approximately 1 km northwest of BC The landslide lies at or near the top of the Bishop Tuff. Lithologic interpretations of cuttings from Mammoth-1 indicate that the well penetrated an upper ash-rich section of highly altered Bishop Tuff before passing through a 43m thick section of landslide breccia and then back into highly altered ash-rich Bishop Tuff. Core from and BC indicate the landslide block is a highly indurated relatively undisturbed meta-breccia or conglomerate with a dense grey matrix of rock fragments and larger rounded or 12

14 subrounded clasts of granite, hornfels, metapelite and metaquartzite typical of lithologies that crop out in the southern topographic wall of the caldera (Figure 8). Figure 8. Structural cross-section of the southern caldera margin showing the landslide block encountered in exploration corehole and the Mammoth-1 deep test well at Casa Diablo (Suemnicht and others, 2006). The landslide block does not contain pumice or phenocrysts from the Bishop Tuff. The southern wall lithologies and indurated metamorphic matrix preclude the possibility that the block might be glacial till. Based on the excellent core samples from and BC 12-31, the Bishop Tuff below the landslide block is lithic rich with an intensely altered glassy matrix permeated with gas void spaces. The landslide block lies entirely at the top of the Bishop Tuff in both and BC and the underlying remarkably undisturbed soft ash probably represents the upper ash cloud at the top of the Bishop Tuff that acted as a cushion beneath the landslide block (Figure 7). Intense clay alteration has obscured many of the primary features of the ash but much of the porous tuff matrix appears to be a gassy froth that retains doubly terminated quartz crystals diagnostic of the Bishop Tuff. SHALLOW HYDROTHERMAL SYSTEM Landslide blocks are common features that have been mapped in several eroded collapsed calderas (Lipman, 2003). In Long Valley, a block of metasedimentary rock from the southern caldera rim slid into the southern Long Valley moat on a gassy cushion of 13

15 Bishop ash late in the eruption and collapse sequence. The temperature distribution within the caldera indicates that the present-day outflow of the deeper hydrothermal system occurs along penetrative NW-SE faults related to the Resurgent Dome and E-W ring fracture faults that control the southern structural margin of the caldera. Upflow at these fracture intersections occurs at comparatively low elevations at the base of the Rhyolite Plateau northwest of Shady Rest. Within the corresponding shallow hydrothermal system, the landslide block controls the flow of hydrothermal fluids southeast around the southern part of the Resurgent Dome (Figure 9). Intense alteration of the over/underlying ash froth, overlying clay-rich lacustrine sediments and the impermeable nature of the landslide block isolates the shallow hydrothermal system from deeper flow and maintains moderate-temperature outflow in shallower Early Rhyolite or lacustrine units east of Shady Rest. Figure. 9. Lithology and temperatures ( o C) from wells in the southern moat of Long Valley caldera. Wells extend from the projected outflow zone of he current geothermal system at Shady Rest to existing production area at Casa Diablo represented by the deep Mammoth-1 well (Suemnicht and others, 2006). Early conceptual models used the available temperature data to suggest that hotter outflow was separated from colder recharge water because of density (Blackwell, 1985). While density separation might prevail for a time, it would be a transient condition. Shallow hydrothermal circulation could only be sustained where hot water is physically separated from the underlying permeable Bishop Tuff. All of the drilling results within the caldera show that the strong head of cold recharging waters from the caldera rim has a 14

16 significant effect on the periphery of the hydrothermal system. Sharp temperature reversals of nearly 100 C are commonly found on the structural caldera margins where high temperature upflow is affected by cold recharge penetrating into the deeper fractured Bishop Tuff (Suemnicht, 1987). The physical separation between the Bishop Tuff and the overlying impermeable landslide block in the southern part of the caldera allows the shallow moderate temperature hydrothermal system to survive. Once thermal outflow has made it into the shallow permeable Early Rhyolites east of Shady Rest, it is effectively separated from underlying rocks and the pervasive influence of cold recharge waters around the rim of the caldera (Figure 10). Production wells at Casa Diablo are completed in an upper 170 C zone in Early Rhyolites while injection wells are completed in the deeper (~700m) permeable Bishop Tuff below the landslide block. The impermeable landslide block, lacustrine clays or intensely altered clay-rich upper part of the Bishop Tuff control shallow hydrothermal circulation in the southern caldera allowing sustained shallow production at Casa Diablo by isolating warm shallow outflow from deep cold recharge that might quench the system. ACKNOWLEDGEMENTS Ormat Nevada Inc. graciously allowed publication of their proprietary information and John Bernardy of MPLP arranged our tour of the Casa Diablo production facilities. Thanks to Dave Hill, Bill Evans and Jim Howle of the USGS and Mack Kennedy of LBNL for providing updates on caldera monitoring and recent geochemical data. This field guide is dedicated to Roy Bailey of the USGS whose pioneering work in Long Valley provided vital insight into the workings of caldera eruptive systems worldwide. 15

17 REFERENCES Bailey, R.A., Dalrymple, G.B. and Lanphere, M.A., 1976, Volcanism, structure and geochronology of the Long Valley caldera, Mono County, California. Journal of Geophysical Res., Vol. 81, No. 5, pp Bailey, R.A., Dalrymple, G.B. and Lanphere, M.A., 1989, Quaternary Volcanism, of the Long Valley Caldera and Mono-Inyo Craters Eastern, Field Trip Guidebook. 28 th International Geological Congress, 36 pp. Bailey, R.A., 1989, Geologic map of Long Valley caldera, Mono-Inyo Craters Volcanic Chain, and Vicinity, Eastern California. US Geol. Survey Map I Bailey, R.A., 2004, Eruptive History and Chemical Evolution of the Precaldera and Postcaldera Basalt-Dacite Sequences, Long Valley, California: Implications for Magma Sources, Current Seismic Unrest and Future Volcanism. US Geol. Survey Prof. Paper pp Benioff, H. and Gutenberg, B., 1939, the Mammoth Earthquake Fault and Related Features in Mono County, California. Seismological Soc. of America, Bull. 29/ Blackwell, D.D., A transient model of the geothermal system of Long Valley caldera, California. Journal of Geophysical Res., Vol. 90, No. B13, pp. 11,229-11,242. Brophy, P., 2008, Geothermal reservoir evaluation. GRC Annual Meeting workshop, Bursik, M., 2009, A general model for tectonic control of magmatism: Examples from Long Valley Caldera (USA) and El Chichón (México). Geofísica Internacional 48 (1), (2009) Bursik, M., 2003, Overview: tectonic-magmatic interactions at Mono-Inyo Craters. in: Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Processes in Long Valley Caldera-Mono Craters. Bursik, M. and K. Sieh,1989. Range Front Faulting and Volcanism in the Mono Basin, Eastern California, J. Geophys. Res., 94, 15,587-15,609. Cathles, L.M., Erendi, A.H.J. and Barrie, T., 1997, How long can a hydrothermal system be sustained by a single intrusive event? Econ. Geo. V 92 n 7-8 pp Chesterman, C.W., Chapman, R.H. and Gray, C.H. Jr.,1986, Geology and ore deposits of the Bodie Mining District, Mono County, California. Calif. Div. of Mines and Geology (Calif. Geologic Survey) Bull

18 Cramer, C. H. & Toppozada, T. R A seismological study of the May, 1980, and earlier earthquake activity near Mammoth Lakes, California. In: Sherburne, R. W. (ed.) Mammoth Lakes, California earthquakes of May California Division of Mines and Geology Special Reports, 150, Dixon, T. H., Miller, M., Farina, F., Wang, H. & Johnson, D Present-day motion of the Sierra Nevada block and some tectonic implications for the Basin and Range province, North America. Tectonics, 19, Eichelberger, J.C., Lysine, P.C., Miller C.D., and Younker, L.W., 1985, Research drilling at Inyo Domes, California: 1984 results. EOS Trans. AGU. Vol. 66, No. 17, pp Eichelberger, J.C., Vogel, T.A., Younker, L.W., Miller C.D., Heiken, G.H., and Wohletz, K.H., 1988, Structure and stratigraphy beneath a young phreatic vent: South Inyo Crater, Long Valley caldera, California. Journal of Geophysical Res., Vol. 93, No. B11, pp. 13,208-13,220. Eichelberger, J.C., 2003, Where have all the magmas gone (long time passing)? in: Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Processes in Long Valley Caldera-Mono Craters, NSF Ellsworth, W. L Earthquake history, In: Wallace, R. E. (ed.) The San Andreas Fault System, California. US Geological Survey, Washington DC, Farrar, C.D., Evans, W.C., Venesky, D.Y., Hurwitz, S. and Oliver, L.K., 2007, Boiling Water at Hot Creek The Dangerous and Dynamic Thermal Springs at California s Long Valley Caldera. US Geol. Survey Fact Sheet ( Finger, J.T. and Jacobsen, R.D., 1999, Phase III drilling operations at the Long Valley Exploratory Well (LVF 51-20). Sandia National Laboratories SAND pp Goehring, L., and Morris, S.W., 2008, The scaling of columnar joints in basalt, J. Geophys. Res., 113, B10203, doi: /2007jb Hildreth, 1979, The Bishop Tuff: Evidence for the origin of compositional zoning in silicic magma chambers. Geol. Soc. Am. Special Paper 180, pp Hildreth, W. and Mahood, G.A., 1986, Ring-fracture eruption of the Bishop Tuff. Geol. Soc. Am. Bulletin v.97, pp

19 Hildreth, W Volcanological perspectives on Long Valley, Mammoth Mountain, and Mono Craters: several contiguous but discrete systems. Journal of Volcanology and Geothermal Research, v.136, pp Hildreth, W., Fierstein, J., Champion, D. and Calvert, A., 2014, Mammoth Mountain and its mafic periphery A late Quaternary volcanic field in eastern California. Geosphere (gsapubs.org). Hill, D.P., 2006, Unrest in Long Valley Caldera, ; in Troise, C., De Natale, G. & Kilburn, C. R. J. (eds) Mechanisms of Activity and Unrest at Large Calderas. Geological Society, London, Special Publications, 269, Hill, D. P. & Prejean, S Volcanic unrest beneath Mammoth Mountain, California. Journal of Volcanology and Geothermal Research, 146, pp Hollister, V. F. and Silberman, M. L., 1995, Geology of epithermal silver-gold bulkmining targets, Bodie district, Mono County, California. NATURAL RESOURCES RESEARCH Volume 4, Number 2 (1995), , Huber, N. K. and W. W. Eckhardt The story of Devils Postpile. Three Rivers, CA: Sequoia Natural History Association. Izett, G.A., 1982, The Bishop ash bed and some older compositionally similar ash beds in California, Nevada and Utah. U.S. Geol. Survey Open File Report Jaques, H.L., 1939, Mono Craters tunnel construction problems. Am. Water Works Assn. (Pacific Section Mtg), pp John, D.A., dubray, E.A., Blakely, R.J., Fleck, R.J., Vikre, P.G., Box, S.E., and Moring, B.C., 2011, Miocene magmatism in the Bodie Hills volcanic field, California and Nevada: A long-lived eruptive center in the southern segment of the ancestral Cascades arc. Geosphere; February 2012; v. 8; no. 1; p Lachenbruch, A.H., Sass, J.H., Munroe, R.J., and Moses, T.H., Jr., 1976, Geothermal setting and simple heat conduction models for the Long Valley Caldera: Journal of Geophysical Research, v. 81, no. 5, p Lajoie, K.R., 1968, Late Quaternary stratigraphy and geologic history of Mono Basin, eastern California. PhD dissertation, Univ. Calif. Berkeley, 271 pp. Lipman, P., 2003, Internal structure of exhumed calderas; in: Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Processes in Long Valley Caldera-Mono Craters, NSF 18

20 Lipshie, S.R., 2001, Geologic Guidebook to the Long Valley-MonoCraters Region of Eastern California (2 nd Ed.). South Coast Geological Society, 270p. Long, K. R., DeYoung, J.H. and Ludington, S.D., 1996, Database of significant deposits of gold, silver, copper, lead and zinc in the United States. USGS Open File Report A. Mahood, G.A., Ring J.H., Manganelli, S. and McWilliams M.O., 2010, New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California. GSA Bulletin; March 2010; v. 122; no. 3-4; p ; DOI: /B Mariner and Wiley, 1976, Geochemistry of thermal waters in Long Valley, Mono County, California. Journal of Geophysical Res., Vol. 81, No.5, pp Marsh, T., 1995, Duration of well-dated hydrothermal systems. pangea.stanford.edu/research/odex/tim/table24.htm Martini, B. A., 2002, New insights into the structural, hydrothermal and biological systems of Long Valley caldera using hyperspectral imaging. PhD. Thesis, Univ. of Calif. Santa Cruz, 291 p. Mastin, L.G. and Pollard, D.D., 1988, Surface deformation and shallow dike-intrusion processes at Inyo Craters, Long Valley caldera, California. J. Geophys. Res. v.93, pp13, Mayo, E.B., 1934, Geology and mineral resources of Laurel and Convict basins, southeastern Mono County, California. Calif. J. Mines and Geology v.30 (1) pp McConnell, V.S., Shearer, C.K., Eichelberger, J.C., Keskinen, M.J., Layer, P.W. and Papike, J.J., 1995, Rhyolite intrusions in the intracaldera Bishop Tuff, Long Valley caldera, California. Journal of Volcanology and Geothermal Research, v.67 pp Metz, J.M., and Mahood, G.A., 1985, Precursors to the Bishop Tuff eruption: Glass Mountain, Long Valley, California. J. Sedimentary Petrology, v. 45, pp Millar, C.I., King, J.C., Westfall, R.D., Alden, H.A. and Delany, D.L., 2006, Late Holocene forest dynamics, volcanism and climate change at Whitewing Mountain and San Joaquin Ridge, Mono County, Sierra Nevada, CA, USA. Quaternary Research 66 (2006) pp Miller, C.D., 1985, Holocene eruptions at the Inyo Volcanic chain, California: implications for possible eruptions in Long Valley. Geology, v. 13, pp

21 National Park Service (NPS), 2009, Devils Postpile National Monument Geologic Resources Inventory Report. NPS/NPRC/GRD/NRR 2009/160 Newhall, C. G. & Dzurisin, D Historical unrest at large calderas of the world, Volumes 1 and 2. US Geological Survey Bulletin, Newhall, 2003, A global perspective on restless calderas in: Understanding a Large Silicic Volcanic System: An Interdisciplinary Workshop on Volcanic Processes in Long Valley Caldera-Mono Craters Pitt, A. M. & Hill, D. P Long-period earthquakes in the Long Valley Caldera region, eastern California. Geophysical Research Letters, 21, Putnam, W.C., 1950, Moraine and shoreline relationships at Mono Lake. Geol. Soc. Am. Bulletin v. 61, pp Rinehart, C.D., and Ross, D.C., 1964, Geology and mineral deposits of the Mount Morrison Quadrangle, Sierra Nevada, California: U.S. Geological Survey Professional Paper 385, 106 p. Roeloffs, E., Sneed, M., Galloway, D.L., Sorey, M.L., Farrar, C.D., Howle, J.F., and Hughes, J., 2001, Water-level changes induced by local and distant earthquakes at Long Valley caldera, California, Journal of Volcanology and Geothermal Research, v. 127, issues 3-4, Roth, R, 2010, Homicide rates in the Nineteenth-Century west: Tables. Ohio State University Criminal Justice Research Center. Rundle, J.B., Carrigan, C.R., Hardee, H.C. and Luth, W.C., 1986, Assessment of Long Valley as a Site for Drilling to the Magmatic Environment. Sandia National Labs (DOE) Rept. no Rundle, J.B. and Hill, D.P., 1988, Geophysics of a Restless Caldera. Ann. Rev. Earth and Planetary Sci : Russell, I.C., 1889, Quaternary History of the Mono Valley, California. U.S. Geol. Survey, Eighth Annual Report, pp Sarna-Wojcicki, A.M., Morrison, S.D., Meyer, C.E., and Hillhouse, J.W. 1987, Correlation of Upper Cenozoic tephra layers between sediments of the western United States and the East Pacific Ocean, and comparison with biostratigraphic and magnetostratigraphic age data. Geol. Soc. Am Bulletin v. 98 pp Sieh, K., and Bursik, M., 1986, Most recent eruption of the Mono Craters, eastern central California: J. Geophys. Res., v. 91, no. B11, pp. 12,539 12,

22 Smith, G.H., 1943, The History of the Comstock Lode. University of Nevada Press. Sorey, M.L., Lewis, R.E. and Ohlmstead, F.H., 1978, The hydrothermal system of Long Valley caldera, California. US Geological Survey Prof. Paper 1044-A, 60p. Sorey, M. L, G. A. Suemnicht, N. J. Sturchio, and G. A. Nordquist, 1991, New Evidence on the Hydrothermal System in Long Valley caldera, California, from Wells, Fluid Sampling, Electrical Geophysics, and Age Determinations of Hot Spring Deposits, Journal of Volcanology and Geothermal Research, 48 (1991), pp Sorey, M.L., Evans, W.C., Kennedy, B.M., Farrar, C.D., Hainsworth, L.J., and Hausback, B., 1998, Carbon dioxide and helium emissions from a reservoir of magmatic gas beneath Mammoth Mountain, California, Journal of Geophysical Research, v. 103, no. B7, 15,303-15,323. Sorey, M.L. and McConnell, V.S., 2000, Scientific drilling in Long Valley caldera-an update. Calif. Geology v. 53 pp Sorey, M. L., 2005, Hydrology of the geothermal system in Long Valley caldera, California, Unpublished report for the Long Valley Hydrologic Advisory Committee, Mammoth Lakes, California. Sorey, M.L., and Sullivan, R., 2006, Quantitative analyses of warm spring waters at the Hot Creek Fish Hatchery, Mammoth Lakes, California, Transactions of the Geothermal Resources Council, v. 30, Stein, H.J. and Cathles, L.M. (eds.) (1997) Preface: The Timing and Duration of Hydrothermal Events: Economic Geology, Special Issue, v. 92, no. 7/8. Suemnicht, G. A., 1987, "Results of Deep Drilling in the Western Moat of Long Valley, CA," EOS, Vol. 68, No. 40, pp Suemnicht, G. A. and R. J. Varga, 1988, Basement Structure and Implications for Hydrothermal Circulation Patterns in the Western Moat of Long Valley caldera, California, Journal of Geophysical Res., Vol. 93, No. Bll, pp Suemnicht, G. A., Sorey M.L. Moore, J. N. and Sullivan R., 2006, The Shallow Hydrothermal System of Long Valley Caldera, California, Geothermal Resources Council Trans., Vol. 30, pp Wilkerson, G., Milliken, M., Saint-Amand, P., Saint-Amand, D., 2007, Roadside geology and mining history, Owens Valley and Mono Basin. U.S. Bur. of Land Mgmt. Buena Vista Museum of Natural History 21

23 Wilson, C.J.N. and Hildreth, W., 1997, The Bishop Tuff: new insights from eruptive stratigraphy. J. Geol. v. 105 pp Wollenberg, H., Eichelberger, J., Elders, W., Goff, F., and Younker, L., 1989, DOE thermal regimes drilling program through EOS Transactions of the Amer. Geophys. Union v. 70, n. 28 pp Wood, S.H., 1977, Chronology of late Pleistocene and Holocene volcanics, Long Valley and Mono Basin geothermal areas, eastern California. US Geological Survey Open File Report p. 22

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25 Long Valley Caldera Field Trip Log September 18-20, 2015 Prepared by: Gene Suemnicht Bastien Poux Duncan Foley Waterfall Towers Suite A Bennett Valley Road Santa Rosa, CA

26 Tentative schedule, subject to change (as field trips always are). Topographic and geologic reference maps for each day show stops and include a geologic correlations table for ease of reference. Dinner Hosted by GRC Breakfast on own at Juniper Lodge Breakfast on own at Juniper Lodge 2

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31 DAY 1: Long Valley Caldera and Mammoth Mountain The trip leaves from the Peppermill Hotel in Reno, Nevada (see reference maps for Day 1). We drive south on Highway 395 passing the Steamboat Springs geothermal field and hot spring areas around Bridgeport before driving through the Mono Lake basin and entering Long Valley Caldera at Deadman Summit. Continue south on Highway 395 to the Mammoth Lakes/Devils Postpile exit at Highway 203. At the bottom of the exit ramp, turn right (west) toward Mammoth Lakes. Continue uphill and west through the Town of Mammoth Lakes. Turn right (north) on Minaret Summit Rd. Drive to the Mammoth Mountain Main Lodge and park at the base of the gondola facilities. Ride the gondola to the top of the mountain. Walk out of the gondola building uphill to the northeast to the Inyo National Forest summit sign: Mammoth Mountain. Elevation 11, 060 feet. Incremental distance (to the nearest mi/km) 166 mi. / 267 km Stop 1 Caldera Overview, Mammoth Mountain Summit The view from the summit of Mammoth Mountain encompasses Long Valley Caldera, the northern Owens Valley and the Sierra crest. The topographic margin of the caldera extends from the Palisades below San Joaquin Ridge on the west, Bald Mountain on the north, the Glass Mountain eruptive complex on the northeast, Lake Crowley to the east and Laurel Mountain and the Sierra front to the south. The White Mountains, on the eastern skyline beyond the Glass Mountain complex represent the eastern side of the Owens Valley rift with elevations of 4373m (14347ft). Long Valley Caldera formed when the roof of the caldera magma chamber ruptured 760,000 years ago erupting 600 km 3 of Bishop Tuff. The roof of the partially emptied magma chamber collapsed to form the 17 x 32 km (10x20 mi) depression of Long Valley Caldera. Drilling confirms that the structural margin of the caldera and the inherited faults that controlled the caldera collapse can be as much as 2 km (1.2 mi) inboard of the current topographic rim of the caldera. As pressures recovered within the Bishop Tuff magma chamber after the cataclysmic eruption, the roof began rising and lifting the overlying caldera fill. The low hills in the west central part of the caldera are the composite Early Rhyolite eruptive centers of the Resurgent Dome that began erupting at ~600,000 years ago. The caldera moat is the space between the caldera s topographic rim and the Resurgent Dome. Smaller Moat Rhyolite eruptions began in the north around 500,000 years ago, southeast near the airport approximately 300,000 years ago and in the western moat around 100,000 years forming a rhyolite plateau above the Town of Mammoth Lakes. Most eruptive events over the last 160,000 years have occurred in the western caldera varying from mafic to silicic compositions suggesting a complex mixture of contemporaneous magma sources (Hildreth, 2004; Mahood and others, 2010). The Mammoth Mountain Dome Complex on the western caldera rim is a composite of a dozen dacite/rhyolite centers that erupted between 100,000 and 50,000 years ago (Hildreth an others, 2014). Mammoth Mountain and Lincoln Peak were erupted between 86,700-58,000 years ago. The summit dome where we stand is dated at 61,400 years. 8

32 Detailed age dating indicates that the main bulk of the mountain was formed by dacite eruptions in less than 30,000 years (Mahood and others, 2010; Hildreth and others, 2014). The Mono-Inyo volcanic chain extends from the Inyo Domes in the northwestern caldera moat to the Mono Craters and Mono Lake to the north. Eruptions began in the Mono Craters approximately 40,000 yrs. ago. Based on tree ring data, The Deadman, Glass Creek and Obsidian domes all erupted in the late summer in 1350 AD (Millar and others, 2006); an exceptionally well constrained pre-historic eruption age. The Mono-Inyo chain is the most prominent surface evidence of current magmatic activity in Long Valley. Phreatic explosions related to the Mono-Inyo dike intrusion excavated phreatic explosion pits on the north face of Mammoth Mountain yrs. ago (Sorey and others, 1998). Many of these features have been modified by construction of the mountain s recreational facilities. Only a tephra hill remains of what used to be an explosion crater that formed the base of Discovery Chair (formerly Chair 19) west of the lodge. Stratified eruption deposits are found in drainages eroded between ski runs. Areas of dead trees began to appear where high levels of CO 2 soil gas were noted following a six-month period of seismic swarms beneath the mountain in 1989 (Sorey and others, 1998)(Figure 1). Elevated soil gas flux and tree-kill areas occur along faults or fault intersections around all but the eastern flank of Mammoth Mountain, most notably along the northern flank near Chair 12 and along the southern flank at Horseshoe Lake just outside the southwestern margin of the caldera. The cold CO 2 discharge does not contain 14 C but is accompanied by elevated levels of magmatic helium ( 3 He/ 4 He ratios ~ 5 times the atmospheric ratio [R/Ra]). Groundwater with the same inorganic carbon and helium isotopic compositions flows off the flanks of the mountain. The source of these inorganic carbon and helium emissions appears to be a longlived gas reservoir formed above a region of magmatic Figure 1. Tree kill areas around Mammoth Mountain and Horseshoe Lake to the south (USGS). intrusion, sealed at the top by hydrothermally altered volcanic and metasedimentary rocks that are periodically penetrated by intrusions of basaltic magma (Sorey and others, 1998). 9

33 Ride down the gondola and return to the vehicles. Exit the ski area parking lot and head east toward Mammoth Lakes on Minaret Summit Road. Continue south at the intersection of Minaret Summit Road and Highway 203 through the Sierra Star golf course to the intersection of Minaret Road and Meridian Boulevard. Turn right (west) on Meridian Boulevard. Cross Majestic Pines Drive and park at the entrance of Juniper Ridge resort. Incremental Distance 6.1 mi / 9.8 km Cumulative distance mi. / km - END OF THE FIRST DAY Photo opportunities: Sunset - 7:00 pm, Last light - 7:20 pm NOTES: 10

34 12

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37 DAY 2: Central Caldera Photo opportunities: 1st light - 6:15 am, Sunrise - 6:40 am Exit the parking lot and head east on Meridian Boulevard (see reference maps for Day 2). Turn left (north) onto Minaret Summit Road. Continue through the intersection onto Highway 203 heading uphill towards the Main Lodge at Mammoth Mountain. Turn right (north) at the small side entrance road to the Earthquake Fault parking lot. Stop 1 Earthquake Fault Incremental Distance 3.4 mi/ 5.5 km Figure 2. Earthquake Fault, view south. The Earthquake Fault is the best-known dilational ground crack in the western part of Long Valley Caldera (Figure 2). A more appropriate name for the fracture would be Earthquake Fissure because there is little evidence that the rocks on either side of the fracture have moved vertically or laterally relative to one another. Irregularities in the joint faces across the fissure nearly match perfectly with some suggestion of right-lateral pull-apart separation. The fissure is up to 3m (10 ft) wide and 18m (60 ft) deep and cuts through ± 7,500 year old dacites of Earthquake Dome a satelite eruption in the north part of the Mammoth Mountain complex (Hildreth and others, 2014). Snow from the winter months commonly lasts all year round in the deep bottom portions of the fissure and local Indians stored their food in the fissure during warmer summer months. Earthquake Fault is one of a series of western caldera fissures and faults (see stop #7) aligned along the Inyo-Mono Craters volcanic chain that, in a regional sense, are part of the east-west stretching that is gradually widening the entire Basin and Range. Early studies (Benioff and Guttenberg, 1939; Rinehart and Ross, 1964; Bailey and others, 1976) concluded that these features are tectonic in origin and suggested that the cracks and faults might represent a southern extension of the Harley Springs Fault from the north. Mastin and Pollard (1988) concluded that the faults and fissures in the western part of Long Valley are related to crustal dilation above the southern portion of 8 10-km-long dike that strikes north out of the caldera supplying dome-forming eruptive events in the north Mono Craters about 600 years ago (Bursik and Sieh1989) and here along the south end of the Inyo Domes (Sorey and others, 1998). Both interpretations are compatible because the Inyo-Mono dike is probably controlled by preexisting faults that allowed magma into the shallower crust similar to previous intrusive events in the western caldera while the cracks and fractures are related to shallow uplift and extensional stresses immediately above a rising Mono-Inyo dike. 15

38 The age of Earthquake Fault is unkown or at least poorly constrained. Tree rings indicate that the trees growing in the crack are ~160 years old. The crack lacks pumice fill so the fissure formed after the eruption of the Inyo Craters 600 years ago. Indian legends recount a massive earthquake in the area ~ 200 years ago causing the Earthquake Fault rift. This legendary event roughly corresponds to a shallow intrusion beneath Mono Lake ~200 years ago that pushed up lake-bottom sediments to form Pahoa Island and produced a small volume of andesitic lavas from vents on the island s north side (Lajoie, 1968; Bursik and Sieh1989) (Day 3, Stop #3). Benioff and Guttenberg (1939) placed stainless steel pins in opposite walls of the fissure and they or CalTech colleagues periodically measured the distance between the pins with a steel rod to determine strain over time. Through the 1940s, the fissure widened by approximately a millimeter but after a 20-year break, 1967 measurements found the crack had closed enough that the steel caliper bar would no longer fit between the pins. No firm evidence was ever presented to substantiate rumors that Earthquake Fault was reactivated during the 1980s Mammoth Lakes earthquakes. Soil settling noted at the bottom of the rift was probably related to consolidation of loose material during the series of M6 earthquakes in May 1980 and M5 events in January A number of collapse or settling pits up to 5 m (16 ft) in diameter also formed along this and other ground cracks in the western caldera during the Mammoth Lakes earthquake sequence. The US Forest Service restricted access to the fissure during the seismic unrest of the 1980s and removed damaged ladders that had previously allowed tourist access to deeper levels of the rift. Exit the Earthquake Fault parking lot and turn left (east) on Minaret Summit Road toward the Town of Mammoth Lakes. BEWARE OF TRAFFIC! Turn left (east) at the stoplight intersection of Minaret Summit Road and Highway 203. Turn left (north) on Shady Rest Campground road. Drive past the camping area and bear right through the parking lot. Turn right (east) onto a dirt road at a gap in the parking lot logs. Continue to the BC well pad on the left and park on the western side of the pad. Incremental Distance 3.8 mi/ 6.3 km Cumulative distance 7.2 mi. / 11.6 km Stop 2 Production Wells, Shady Rest Figure 3. Drilling BC exploration corehole in Basalt Canyon (photo by R. Sullivan) The USGS and DOE drilled core hole RDO- 8 to 732 m (2401 ft.) near the Shady Rest campground in the southern caldera moat in 1984 (Wollenberg and others, 1989). The lower portion of the well was lost because of wellbore instability but measured temperatures at 400m (1312 ft.) were 203 o C (397 F) and appeared to persist at depth based on extrapolated survey temperatures. This scientific effort and exploration wells drilled in the western caldera added to 16

39 understanding the Long Valley geothermal system. Drilling established that the principal geothermal reservoir in Long Valley was not located directly beneath Casa Diablo and did not appear to be related to the Resurgent Dome. Instead, the hydrothermal system appeared to be more complex with shallow production at Casa Diablo supplied by upflow from a western geothermal source and outflow at or near Shady Rest. Mammoth Pacific LP has continued to evaluate the geothermal potential of the southern caldera moat, drilling intermediate-depth exploration wells and in 1992, BC in 2002 (Figure 3) and production wells 57-25, 66-25, and east and north of the Shady Rest (RDO-8) core hole. The production wells are nearly 500 m (1640 ft.) deep and produce from the upper fractured portion of the Bishop Tuff. The current production of ~2000gpm is piped to Casa Diablo and commingled with production from earlier wells. Continue east on Sawmill Flat Road to Highway 203 (BEWARE OF TRAFFIC). Continue east on Highway 203 crossing beneath the Highway 395 overpass. Continue east toward Casa Diablo. Turn left (north) on Casa Diablo Road and enter the power plant headquarters at the control gate at the top of the hill. Access to the geothermal development is by permission from Ormat. Incremental Distance 5.0 mi/ 8.0 km Cumulative distance 12.2 mi. / 19.6 km Stop 3 Mammoth Pacific Production Facilities, Casa Diablo Casa Diablo Hot Springs is an active thermal area on the southwestern side of the caldera s Resurgent Dome (Figure 4). Most of the prominent hot springs within the caldera including Hot Creek (Stop 4) occur in the southern caldera moat or are localized along faults around the southern edge of the Resurgent Dome. The springs at Casa Diablo were an early gathering place for local Piute Indians, settlers and prospectors. During the late 1800s, the site was a way station on the Bodie- Mammoth City stage line and Figure 4. Generalized distribution of surface manifestations in Long Valley Caldera. 17

40 later an automobile rest stop on Old Highway 395. The old route survives as a paved roadway dividing the east and west portions of the geothermal field (Figure 5). Historical records and photographs recount the erratic discharge of the thermal features that varied from relatively quiet outflow to sporadic geysering. Figure 5. Wells and temperature gradient holes in and around the Casa Diablo development area (modified from Calif. Div. of Oil, Gas and Geothermal Resources). Active and relict fumaroles, mudpots and hot springs at Casa Diablo are localized along a major northwest striking normal fault system that forms a graben within the Resurgent Dome. Hydrothermal alteration marks the trace of a fault that cuts 600,000 year-old Early Rhyolite of the Resurgent Dome on the northeastern side of the field. Younger 179,500-92,400 year-old postcaldera mafic lavas flood the southwestern caldera moat and lap against the Resurgent Dome. Active fumaroles on the western side of the geothermal field are aligned along a fault scarp that uplifts and exposes these moat units. Casa Diablo was the location of several seismic swarms that occurred during the initial phases of caldera unrest and Resurgent Dome uplift from 1980 to Some of these events were marked by ground cracking, changes in spring discharge, vigorous reactivation of a previously dormant fumarole immediately east of the old highway and new or reactivated hot springs around the edge of the Resurgent Dome. 18

41 Magma Power Co drilled the first geothermal exploration wells in Long Valley at Casa Diablo between 1959 and A series of nine wells were drilled to total depths ranging from 125m to 324m ( ft) adjacent to active fumaroles west of Old Highway 395. The wells encountered reservoir temperatures of ~170 C (338 F) and Magma planned to construct a 15 Mw generation facility in 1962; however, flash/binary production technologies were still in the early stages of development and the project was shelved. Initial geothermal development in 1984 included four production wells and 2 injection wells supplying the MP-1 pilot 10 Mw (gross) binary production facility within the original Magma well field. Successful generation led to additional development drilling and in 1990 the installation of two 15 Mw binary MP-II and PLES-1 generating plants. The project currently uses 6 production wells and 5 injection wells in the Casa Diablo area plus production from 2 wells in the western caldera (referred to as Basalt Canyon; Stop 2) to produce ~757 L/sec (~12,000 gal/min) of moderate temperature (170ºC) fluids to generate a combined 40 MWe (gross) of electricity. The project is managed and operated by Mammoth Pacific LP, a partnership between Ormat Nevada Inc. as operator and Constellation Power supplying base load electricity under contract to Southern California Edison (SCE). Ormat has modernized the older MP-1 plant and plans additional production and injection in the southwest moat to increase production to ~1135 L/sec (~18,000 gal/min) and 60MWe (gross). Mammoth-1, drilled by Unocal east of Old Highway 395 in 1979, was the first well at Casa Diablo to evaluate potential deep production from the intracaldera Bishop Tuff and the caldera basement. Mammoth-1 drilled through 390m (1280ft) of Early Rhyolite, 863m (2831ft) of Bishop Tuff and 230m (755ft) of metasedimentary rocks that correlate with the Convict Lake roof pendant to the south, bottoming at 1605m (5265 ft.). Mammoth-1 was also the first well within the caldera to encounter a landslide block of chaotically mixed metapelite and granite at 466m in the upper section of Bishop Tuff (Suemnicht, 1987). The well is currently used as a backup injector. The results from Mammoth-1 and other deep tests indicate that production at Casa Diablo is limited to shallow (<200m; 650 ft.) permeable Early Rhyolite flows and that the source of the current system is not directly beneath the Resurgent Dome (Suemnicht, 1987). Mineralization ages (Sorey and others, 1991) indicate that the Bishop Tuff magma chamber supported a vigorous geothermal system from 300,000 to 130,000 years ago producing widespread hydrothermal alteration in and around the Resurgent Dome. Drilling results establish that magmatic activity beneath the central part of the caldera has waned over the past 100,000 years while activity in the western caldera has increased. Shallow wells at Casa Diablo produce 170 o C (338 F) outflow that is supplied by upflow from an active geothermal system to the west. Injection wells return the produced fluid to deeper (750m; 2460 ft.) permeable zones in the underlying Bishop Tuff. The shallow Early Rhyolite reservoir is stratigraphically separated from the underlying Bishop Tuff by an impermeable landslide block that controls the vertical distribution of shallow hydrothermal circulation in the southern caldera allowing sustained production and injection at Casa Diablo by isolating warm shallow outflow from deeper cold natural recharge and injection fluids that might quench the system. Refer to the geothermal discussion in the general geology section of the trip guide for additional information. 19

42 Exit the geothermal facility and turn left on Casa Diablo Road. Turn right (west) on Highway 203. Cross under the freeway as if returning to Mammoth Lakes. Bear left (southeast) onto Highway 395 and drive approximately 1.5 miles (2.4 km) to the intersection with Hot Creek Fish Hatchery Road. Turn left (north) on Hot Creek Fish Hatchery Road and continue to the intersection with Hot Creek Rd. Continue east on Hot Creek Rd. and park at the public access area at the top of Hot Creek gorge. Incremental Distance 7.5 mi/ 12.1 km Cumulative distance 19.7 mi. / 31.7 km Stop 4 Surface Manifestations, Hot Creek The most prominent hot springs and fumaroles in Long Valley are located at Hot Creek, Little Hot Creek, and the Alkali Lakes around the southern and eastern edge of the Resurgent Dome (Figure 4). The hot springs and fumaroles in Hot Creek Gorge discharge ~250L/sec (3968 gpm) of near-boiling water representing 70-75% of the geothermal system s outflow. The creek cuts through a 288,000 year-old rhyolite flow that originated from a Moat Figure 6. Hot Creek Gorge view west (USGS photo). Rhyolite dome 4 km to the south (Bailey and others, 1976; Bailey and others, 1989). The lower portion of the flow erupted into Pleistocene Long Valley Lake burying existing lake sediments and pervasively hydrating the rhyolite. Brecciated and perlitically altered flow units are exposed along the paved trail that leads down to the Hot Creek. Bailey and others (1976) described the Hot Creek flow as sparsely phorphyritic, sanidine-augite bearing rhyolite; a composition slightly different from other hornblende-biotite rhyolites of other moat eruptive sequences. He interpreted the mineralogic difference as evidence that new mafic magma injections rejuvenated the Long Valley magma chamber, inflating the Resurgent Dome and initiating basaltic eruptions in the western caldera moat. Age dates on the hot spring mineralization indicate that part of the hydrothermal alteration common in Hot Creek gorge resulted from a vigorous geothermal outflow from 300, ,000 years ago (Sorey and others, 1991). Intense hydrothermal alteration and boiling (93 C; 199 F) springs at Hot Creek are localized along two north-striking faults that cut the rhyolite flow and form a small graben that contains the Hot Creek swimming hole. Smaller or cooler springs are localized along the contact between the flow and underlying lake sediments. Numerous earthquakes that occurred during caldera unrest that began in 1980 have often affected the flow of the springs. Additional boiling springs developed or were reinvigorated in May 2006 expanding beyond the protective fencing on the north side of the gorge. Changes in spring discharge were accompanied by temperature increases in nearby 20

43 monitoring wells and pressure increases in adjacent cold-water aquifers in response to above normal precipitation in the preceding winter (Farrar and others, 2007). The swimming area remains closed as a precautionary measure. Exit the Hot Creek parking lot and head northeast on Hot Creek Road. Turn left (north) on Owens River Road and immediately left (southwest) onto Little Antelope Valley Road. The road parallels Little Hot Creek for approximately 3 miles. Pull of to the right (north) side of the road at the Hundley Clay Pit. Incremental Distance 9.6 mi/15.5 km Cumulative distance 29.3 mi. / 47.2 km Stop 5 Clay Pit The Hundley Clay Pit is a large area of intense hydrothermal alteration in the Little Antelope Valley in the northern part of the Resurgent Dome. Alteration is controlled by a series of north-striking normal faults that are northern splays of the Quaternary/Recent Hilton Creek fault system that Bailey and others (1976) considered the principal postcollapse structural element in the central caldera. Hilton Creek fault splays also control hydrothermal outflow at Hot Creek (Stop 4) to the south and east. Similar hydrothermal alteration at Casa Diablo (Stop 3) occurs along northwest fault splays that define a medial graben in the west-central part of the Resurgent Dome roughly along the route of Highway 395. Clay Pit alteration is predominantly kaolinite with lesser amounts of alunite and opal veins in Pleistocene tuffaceous lakebeds or older 600,000-year-old Early Rhyolite units. The pit faces expose relict primary bedding in the lakebeds and relict flow banding in altered Early Rhyolite units. The deposit is capped by an opaline claystone that ranges in thickness from 3 in. (8 cm) to 20 ft. (6 m). Alteration occurred before deposition of the youngest lakebeds suggesting that Clay Pit is related to the vigorous older hydrothermal system developed within the central caldera 300, ,000 years ago. Older or reactivated faults still control circulation in the current hydrothermal system that has been active for the last 40,000 years. The Little Hot Creek springs occur along fault splays that define the limits of alteration approximately 1 mile (1.6 km) east of Clay Pit. The Hundley mine has operated intermittently since Standard Industrial Minerals, the current owner, trucks kaolinite from the Hundley pit to the company mill north of Bishop. Kaolinite uses include paint filler, plastic, rubber, paper processing, Portland cement, ceramics, insecticides, pharmaceuticals and stucco (Wilkerson and others, 2007; Lipshie, 2001). Prior to the federal Geothermal Steam Act that defined geothermal as a mineral resource, Magma Power Company maintained claims on this and other minor kaolinite prospects during the 1970 s to retain grandfather mineral/geothermal rights and potential leasing rights for federal geothermal lease sales in the 1980 s. Clay Pit-1, on the southern side of the Hundley mine, drilled by Unocal in 1979, was another deep test to 1847m (6060 ft.) that evaluated potential production from the most prominent hydrothermally altered area within the caldera. Clay Pit-1 drilled through 335m (1280ft) of Early Rhyolite, 1085m (3559 ft.) of Bishop Tuff and 230m (755ft) bottoming in 128m (420 ft.) rhyodacite intrusive. Pervasive alteration of the rhyodacite 21

44 precluded age dating; however, major oxide chemistry closely matches moat rhyodacites that erupted 500,000 years ago less than a mile from the Clay Pit mine. Clay Pit-1 was the first well within the caldera to encounter younger intrusions into or beneath the Bishop Tuff that could be a heat source for a hydrothermal system. The bottomhole temperature of 148 C (298 F) was less than shallow production wells at Casa Diablo and a conductive geothermal gradient and low permeability confirmed a lack of convective circulation (Suemnicht, 1987). The results from Clay Pit-1 and Mammoth-1 confirm that the Resurgent Dome is not the source of the current active hydrothermal system in Long Valley. Continue west on Antelope Valley Rd approximately 2.5 miles (4 km). Turn right (north) at the intersection of USFS Rd. 3S13 Stay to the right and continue approximately.8 mi (1.3 km) to the Long Valley Exploratory Well site. Turn left (west) onto the wellsites and park on the south side of the wellpad. Incremental Distance 5.0 mi/ 8.1 km Cumulative distance 34.3 mi. / 55.2 km Stop 6 LVEW DOE well The Long Valley Exploration Well was initially drilled to 2.3 km (7545 ft.) in 1991 and then deepened to its current total depth of 3 km (9842 ft.) in The well was drilled in the approximate center of 80 cm (2.6 ft.) of uplift within the Resurgent Dome to evaluate the potential for extracting energy from magmatic or nearmagmatic conditions in Long Figure 7. Long Valley Exploration Well in (USGS photo) Valley. Measurable uplift closely related to seismicity clusters were interpreted as evidence for potential intrusion of new magma that initiated a period of unrest in the caldera. The DOE originally funded the drilling but the well is currently part of the International Continental Scientific Drilling Program ( The well penetrated 662m (2171 ft.) of Early Rhyolite, 1178m (3865 ft.) of Bishop Tuff and entered metamorphic roof pendant rocks at 1891m (6204 ft.). The metamorphic rocks in the lower portion of the well correlate with the Mt. Morrison metamorphic roof pendant exposed in the southern caldera rim and at Convict Lake to the south (McConnell and others, 1995). Although permeable fluid zones were encountered in the crystalline metamorphic rocks at depths of 2-3 km, the maximum temperatures in the well are only 104 C (219 F) (Figure 8) 22

45 Figure 8. Measured (solid) and extrapolated (dashed) LVEW temperature profile (Sorey and McConnell, 2000). Generalized lithologies: post-caldera Rhyolites (PCR), Bishop Tuff (BT), Mesozoic metavolcanic rocks (Mzmv), and Paleozoic metasediments (Pzms). Fracture zone projected ductile conditions included. Significant data from the well were summarized in a special issue of the Journal of Volcanology and Geothermal Research (v. 127, no. 3-4, 2003). The well remains an observation point for studying caldera unrest. Water level response to rock strain is significantly more sensitive in this well than in shallower wells (Roeloffs and others, 2001). The LVEW well is one of the best sites for monitoring rock strain related to seismicity and potential intrusion. A downhole seismometer was installed in 2003 and remains the deepest seismic monitoring point within the caldera (Peter Malin, pers. comm.) Return to Antelope Valley Rd. and turn right (southwest). Return back past the Casa Diablo production facilities. Turn right (north) on Highway 395 to Mammoth Scenic Loop. Turn left (west) and continue on Mammoth Scenic Loop to Inyo Craters Road Turn right (northwest) on Inyo Craters Road and follow the road north to the Inyo Craters parking lot. The trail to the craters is on the northwest side of the parking lot and wanders approximately 0.3 mi /500m northwest through the forest to the base of the craters. Incremental Distance 14.0 mi/ 22.5 km Cumulative distance 48.3 mi. / 77.8 km 23

46 Stop 7 Inyo Craters Phreatic explosions created the Inyo Craters approximately 600 years ago (Bursik and Sieh1989) (Figure 9). The magma source for these eruptions is an 8 10km (5-6 mi)-long dike that trends north out of the caldera. Phreatic craters were created where the dike encountered groundwater. Other craters extend from the north flank of Mammoth Mountain to the Mono Basin. Dome-forming eruptive events occurred where magma made it to the surface at the north end of the Mono Craters (Day 3, Stop 2) and south within Long Valley Caldera at the Inyo Domes immediately north of the craters. Figure 10. Conceptual diagram of the Inyo dike (USGS adaptation after J. Fink) Figure 9. Inyo Craters, view north to Deadman Dome (USGS photo) The Inyo Craters are aligned north south along the presumed strike of the Inyo dike (Figure 10). Two southern craters are approximately 200m (650 ft.) across and 60m (200 ft.) deep and contain small cold (11 C; 52 F) lakes that show no evidence of thermal influence. The color of the water in each lake is the result of differing types of algae or organic matter. A phreatic eruption also excavated a small northern crater at the summit of Deer Mountain, a 115,000 year-old moat rhyolite dome similar in age and composition to the rhyolite plateau domes north of Mammoth Lakes. The light colored phreatic explosion debris at the summit (Figure 9) is fragmented hornblende rhyolite of the dome and the darker debris around the two lower craters is predominantly andesite derived from underlying flows exposed in the walls of the southern crater (Bailey and others, 1989). The stratigraphy exposed in the craters includes andesite flows overlain by a thin (1m) layer of pumiceous Inyo Craters rhyolite tephra overlain by a succession of poorly sorted phreatic explosion debris. Based on detailed stratigraphic studies, Mastin and Pollard (1988) determined that the craters erupted from north to south with lightcolored explosion deposits from Deer Mountain summit overlain by dark debris from the middle crater in turn overlain by darker debris from fragmented andesites underlying the 24

47 southern crater. Phreatic debris from the southern crater extends as far as 0.6 mi/1km to the northeast and includes granite and metamorphic fragments probably from underlying till or basement rocks. The Inyo Craters lie within a small.6 km wide graben extending 2.5 km (1.5 mi) north and south (Figure 15) defined by numerous faults and fissures with as much as 20 m of displacement. Similar fissure and fracture zones are common in the western moat of Long Valley Caldera and control similar phreatic eruptive centers. Mastin and Pollard (1988) attributed these fissure zones to uplift and extension above a rising Mono-Inyo dike although the intrusion probably followed some existing inherited structures similar to previous intrusive events in the western caldera over the last 200,000 years (Suemnicht and Varga, 1988). Phreatic eruptions were widespread along the Mono- Inyo dike and numerous equally impressive but less accessible Figure 11. Generalized map of faults and ground cracks around the Inyo Craters (After Mastin and Pollard, 1988) craters occur to the north and west of the Inyo Craters that do not conform to a uniform north-south orientation. A series of DOE -funded core holes penetrated and sampled the Mono-Inyo dike inside and outside the caldera. The RDO-4 (Inyo-4) corehole was drilled from the top of the fault scarp on the western side of the south Inyo Crater at an angle of 68, and penetrated 320m (1050 ft.) of post caldera andesites and basalts, 50m of gravel, 360m of Early Rhyolite tuffs and flows, 63m of glacial till bottoming in 30m of Sierran metamorphic rocks (Eichelberger and others, 1988) (Figure 12). The highest temperature in the hole was 81 C (178 F at 500m (1640 ft.). The hole encountered limited amounts of juvenile high-silica rhyolite and vent breccias composed of Early Rhyolite and andesite directly under the southern crater. Petrologic studies of the core indicate that the eruptions may have tapped magmas of three different compositions venting at several places along strike (Eichelberger, 2003). 25

48 Return to Inyo Craters Road. Turn left (northeast) at the first Forest Service road on the left. Park on the northwest side of the wellpad. Incremental Distance 0.6 mi/ 1 km Cumulative distance 48.9 mi. / 78.7 km Stop 8 Inyo Domes Unocal drilled well approximately 1 km (0.6 mi.) east of the Inyo Craters to test the geothermal potential of the western caldera moat. While is the hottest well drilled in Long Valley (218 C at 1100m)(424 F at 3608 ft.), it is unproductive because a strong incursion of cold water penetrates beneath a limited in thickness of Bishop Tuff reservoir rocks (Suemnicht, 1987). The Inyo-4 corehole and establish the location of the western structural margin of Long Valley Caldera. Between the two wells, the caldera s ring fracture system offsets the metamorphic rocks of the caldera floor by approximately 1km in less than 1km lateral distance (Figure 13). As with many calderas, the structural margin of the ring fracture system can be considerably different from the topographic margin or rim of the caldera that we see today. Figure 12. Inyo-4 core hole (from Eichelberger and others, 1988) Figure 13. Stratigraphic correlations between the Inyo-4 corehole and IDFU adjacent to the western ring fracture system of Long Valley (From Eichelberger and others, 1988) 26

49 Return on Inyo Craters Road to Mammoth Scenic Loop. Turn right (southwest) and continue on Mammoth Scenic Loop to the intersection with Minaret Summit Road. Turn left (south) and continue on Minaret Summit Road to the intersection of Highway 203. Cross Highway 203 and continue through the Sierra Star golf course to the intersection of Minaret Road and Meridian Boulevard. Turn right (west) on Meridian Boulevard. Cross Majestic Pines Drive and park at the entrance of Juniper Ridge resort. Incremental Distance 6.5 mi/ 10.5 km Cumulative distance 54.8 mi. / 88.2 km - END OF THE SECOND DAY Photo opportunities: Sunset - 7:00 pm, Last light - 7:20 pm NOTES: 27

50 28

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53 DAY 3: Mono Basin The final day of the trip leaves from Juniper Lodge and all road mileage is based on distances from that point (see reference maps for Day 3). Once we leave Bodie, we will be driving north on Highway 395 back to Reno retracing our route from the first day. Exit the Juniper Lodge parking lot and head west on Highway 203 through the Town of Mammoth Lakes. Turn right (north) on Minaret Summit Rd. Continue west to the intersection with the Mammoth Scenic Loop. Turn right (north) to cross a plateau of 100,000 year-old Moat Rhyolites and continue northeast through the Dry Creek basin. BEWARE OF TRAFFIC and cross the highway at the intersection with Highway 395. Turn left (northwest) toward June Lakes and Mono Lake. Exit the caldera at Deadman Summit. Continue approximately 1.5 mi. /2.4 km from the summit and pull over to the eastern side of the road at a prominent steep road cut. BEWARE OF TRAFFIC! Incremental Distance 17.2 mi/ 27.7 km Stop 1 - Bishop Tuff, Northern Lobe The Bishop Tuff eruption produced approximately 600 km 3 of rhyolite ash evacuating the upper part of the Long Valley magma chamber causing the chamber roof to subside approximately 1-2 km creating Long Valley Caldera. Ashfall from the eruption is recognized east as far as Kansas and Nebraska (Izett, 1982) and in deep-sea cores from the eastern Pacific (Sarna-Wojcicki and others, 1987). Collapse of the eruption column produced as many as seven ash flow lobes that blanketed 1500 km 2 surrounding the caldera (Hildreth, 1979). The ash flow completely covered precaldera topography and is thickest near the margin of the caldera thinning at the northern and southern distal edges. The ash flow sheet at this stop is the moderately welded upper part of the northern Mono Basin lobe of the eruption (Hildreth, 1979). Post-caldera deposits of glacial outwash, Lake Russell/Mono Lake sediments and Mono Craters tephra cover at least 75% of the original expanse of northern Bishop Tuff. The remnant outcrops are referred to as the Aeolian Buttes in the Mono Basin. The mineral assembly of quartz+sanidine+ biotite is typical of the Bishop Tuff. Hildreth (1979) noted that the mineral compositions within the Tuff changed as the eruption progressively tapped deeper and hotter levels of the magma chamber. Calculated Fe-Ti oxide eruption temperatures of o C ( F) establish the Mono Basin lobe of the Bishop Tuff as the highest temperature phase of the eruption. The pumice fragments within the Tuff are still relatively coherent at this location with some evidence of collapse or welding in the upper part of the ash flow sheet. Some xenoliths can be found in northern and eastern lobes of the Tuff but they are much more prevalent in southern ash flow lobes. Hildreth and Mahood (1986) interpreted lithic fragment rock types and prevalence as indicating that the Bishop Tuff eruption progressed from the south central margin near Lake Crowley counterclockwise to the north and west culminating on the northern caldera margin. Detailed stratigraphic studies and eruption modeling suggest that the Bishop Tuff eruption probably took a week or less (Wilson and Hildreth, 1997). 32

54 Most volcanic activity over the last ~200,000 years has occurred in the western moat of Long Valley and over the last ~50,000 years, north of the caldera into the Mono Basin. Obsidian Dome across Highway 395 to the west is one of a series of eruptions that occurred 600 yrs. ago along the Mono-Inyo volcanic chain. Pumice outfall from eruptions over the last 2,000 years mantle the outcrops at this location and much of the area including the interior of the Mono Basin. Carefully merge back onto Highway 395 and continue north past June Lake Junction. Turn right (east) on Highway 120. Drive approximately 3 mi. / 5 km. and turn left (north) on a dirt road poorly marked as the access to Panum Crater. Continue north approximately.5 mi. /.8 km. to the parking lot at the southeastern edge of Panum Crater. Incremental Distance 12.5 mi / 20.1 km Cumulative distance 29.7 mi. / 47.8 km Stop 2 Panum Crater Panum Crater is the farthest northern vent along the Mono-Inyo dike that erupted between AD (Bursik and Sieh1989). The vent is a textbook example of a rhyolite plug-dome volcano and this stop serves as the prime illustration of eruptive progressions common to Mono-Inyo dike events (Figures 14 and 15). The Panum tephra ring is approximately 0.8 km in diameter Figure 14. Panum Crater view SW(from USGS) and is composed of pumice, ash, lapilli, obsidian fragments and minor amounts of granitic or metamorphic fragments derived from underlying glacial deposits. Detailed studies by Sieh & Bursik (1986) indicate that phreatic explosions cleared the throat of the Panum eruptive center, progressed to pyroclastic flow and surge deposits, constructed the outer tephra ring and eventually extruded a high-silica rhyolite dome as magma made it to the surface. Stratigraphy of the surrounding pyroclastic deposits indicates two periods of explosive activity from the same eruptive center. The northern part of an early rhyolite dome apparently exploded or collapsed producing a block avalanche deposit to the northwest into Mono Lake (Wood, 1977; Sieh & Bursik, 1986). Later pyroclastic ashfalls healed the breach of the tephra ring and a composite dome was built by further intrusions. Panum rhyolite has a silica content of 75% and the rocks in the dome interior grade from highly pumiceous to dense obsidian. The flow textures within the dome illustrate the complex composite nature of dome formation. Breadcrust textures are common where viscous rhyolite surfaces cooled and degassed. Eruptive styles along the Mono-Inyo dike (Figure 10) are apparently controlled by magma type, the depth of penetration into the shallow crust, variations in country rock 33

55 and groundwater. Portions of the dike that intruded the local water table may have fragmented producing phreatic explosion craters like the Inyo Craters and the phreatic craters on the north face of Mammoth Mountain without extruding lavas at the surface. Early phreatic eruptions at Panum Crater cleared the path for later magma to make it to the surface and eventually build the rhyolite dome in the center of the tephra ring. The eruptive progression continued at other places along the dike producing high-silica rhyolite flows that overrode tephra deposits producing the Inyo Domes about 600 years ago (Miller 1985). Figure 15. Generalized map and cross sections of Panum Crater (after Bursik and Sieh, 1989) Return to Highway 120 and turn right (west). At the intersection with Highway 395, turn right (north) toward the town of Lee Vining. Approximately.25 mi (400m) north of town, turn right on the access road to the US Forest Service National Scenic Area Visitors Center. Incremental Distance 12.7 mi/ 20.4 km Cumulative distance 42.4 mi / 68.3 km Stop 3 Mono Craters Visitor Center The Mono Craters National Scenic Area is operated by the Inyo National Forest and provides an excellent overview of Mono Lake and the Mono-Inyo volcanic chain. The Mono Craters are the result of multiple volcanic eruptions that began ~40,000 years ago. At least 20 small volume eruptions have occurred along the chain over the past 5000 years at intervals of 250 to 700 years (Bursik & Sieh, 1989). The progression of eruptions over the past 2 Ma from Glass Mountain on the eastern margin of Long Valley Caldera to Mammoth Mountain on the west and the Mono Inyo volcanic chain in the north suggests that the caldera s magmatic system has declined with time and has been supplanted by mixed composition eruptions from the active Mammoth Mountain Complex Inyo Domes magmatic system (Hildreth, 2004; Hilreth and others, 2014). The progression of eruptions in the area did not end 600 years ago with Panum Crater or with the extrusion of the Inyo Domes. A shallow intrusion beneath Mono Lake ~250 years ago pushed up lake-bottom sediments to form Pahoa Island and produced a small volume of andesitic lavas from vents on the island s north side (Lajoie, 1968; Bursik and Sieh1989). Mixed magma compositions are consistent with regional crustal extension in the Eastern California Shear Zone and the Mina Deflection connection to the Walker Lane to the northeast. Bursik (2003) suggested that dikes beneath the domes intrude a pull-apart structure to accommodate strain otherwise resolved by purely extensional faulting. 34

56 The eastern Sierra is also a key location for understanding regional glacial events over the last 2-3 Ma (Lipshie, 2001). Prominent lateral and terminal moraines along the Sierran range front west of the visitor s center are the result of the latest Tahoe (75,000 60,000 years) and Tioga (25,000 12,000 years) glacial advances and retreats during the late Pleistocene. The Casa Diablo till sandwiched between 162,000 year-old trachyandesite and 125,000 year-old basalt in the southern caldera moat in Long Valley (Hildreth and others, 2014) provides well-dated evidence of a Mid-Late Pleistocene glacial event that may also correlate with Mono Basin glaciation of similar age. The Sherwin till underlies the Bishop Tuff at Sherwin Creek and marks a glacial retreat approximately 800,000 years ago. Tills on McGee Mountain south of Long Valley are evidence of an earlier 2.5 Ma glaciation (Table 1). Table 1. Correlation of glacial sequences in the eastern Sierra (modified from Lipshie, 2001) North American Continental Glacial Sequences Sierra Glacial Sequences Ages (years before present) Post Wisconsin Matthes Recess Peak Hilgard ,500 Late Wisconsin Tioga 12,000-25,000 Early Wisconsin Tenaya ~ 45,000 Illinoian Tahoe 60,000 75,000 Mono Basin Casa Diablo ~130,000 ~150,000(?) Kansan Sherwin ~800,000 Nebraskan McGee Ma Mono Lake is the saline remnant of Lake Russell, an extensive freshwater lake that formed in the Mono Basin during Pleistocene glacial retreats. The glacial lake is named for Israel Russell who mapped and studied the region in He interpreted the moraines along the range front as evidence of at least two major glacial events in the Sierra, mapped the glacial lake shorelines and correlated the glacial advances and retreats with high water levels in Lake Lahontan and Lake Bonneville elsewhere in the Great Basin (Russell, 1889). The shoreline terraces of Lake Russell extend to an elevation of 2188m (7180 ft.) (Putnam, 1950) or roughly 244m (800 ft.) above the present 1944m (6379ft) level of Mono Lake and are evident along the shore of the lake west of the visitor s center. The scarp along the western shoreline marks the Lee Vining Fault part of the Sierran range front fault system. As we drive north along the lakeshore, note the uplifted shoreline terraces and deformed young lakebeds along the fault scarp. The predominant water source for Mono Lake is snowmelt in the creeks that flow into the Mono Basin from the Sierran range front. The Los Angeles Department of Water and Power (LADWP) acquired water rights throughout the basin and in 1941 began exporting water from Walker, Parker, Lee Vining and Rush creeks through an 18.5km (11.5mi) tunnel under the Mono Craters to the Owens River in Long Valley, and eventually to the Owens Valley aqueduct that extends 359 km (223mi) from Lake Crowley in Long Valley to the northern part of the Los Angeles basin. Other minor streams that drain into the closed basin contribute relatively small volumes of water. Without continued freshwater 35

57 supply, evaporation eventually exceeded inflow resulting in declining lake levels and rising salinities. The total dissolved solid content (TDS) of Mono Lake water in 1941 was 50,000mg/L and at the lake s lowest level in 1982 the salinity was 99,000mg/L. The declining water levels eventually formed a peninsula on the north side of Negit Island exposing migratory bird nesting areas to predators. The expropriation of water was eventually restricted by court decisions and in 1994 the California State Water Resources Control Board ordered LADWP to release enough water to maintain the elevation of the lake at 1948m (6391 ft.). The current lake level is ~4m below that elevation but Negit is barely an island and salinities are anticipated to stabilize around 70,000 mg/l. The young siliceous Mono Craters eruptive complex is an obvious geothermal exploration target enhanced by the occurrence of several hot springs within the Mono Basin (Figure 16). Records from the LADWP Mono Craters tunneling project document large volumes of 36 C (97 F) water and CO 2 (Jaques,1939). Some recent publications cite boiling hot water where the tunnel crossed beneath the central axis of the crater chain; a claim unsubstantiated in LADWP records that cite vertical shafts sunk through sands and silts which, due to the water pressures behind or under them, as they were approached, would cause them to boil up and fill the shaft for many feet, (emphasis mine) (Jaques, 1939). The tunnel passes through 884 m (2,900 ft.) of glassy or fractured rhyolite, large crevasses (fault zones) and heavy flows of CO 2. The maximum water flow from all the tunnel headings was 1262 L/s (20,000 gpm) under substantial head that would certainly account for water boiling into the tunnel shafts. Wildcat exploration wells on the north and south shores of Mono Lake did not encounter encouraging temperatures to depths of 744m (2440 ft.)(figure 23). A Unocal temperature gradient hole at the southeast end of the volcanic chain encountered 28 C (82 F) at 893m (2929 ft.). The Mono Craters are certainly a spectacular example of comparatively recent silicic volcanism but the available drilling data indicate that neither the Inyo section of the dike (Stop 7 on Day 2) or the longer-lived Mono eruptive complex provide regionally extensive heat for a substantial hydrothermal system in the shallow crust. Return to Highway 395. Turn right (north) heading toward Reno. Leave the Mono Basin at Conway Summit and continue north to the intersection with State Route 270. Turn quickly right (east) and continue approximately 9 miles (14.5 km) to Bodie State Park. Incremental Distance 32.0 mi/ 51.5 km Cumulative distance 74.4 mi. / km 36

58 Figure 16 Generalized topographic map of the Mono Basin showing warm springs and geothermal exploration wells (from CDOGGR well files) 37

59 Stop 4 Bodie The gold and silver mining town of Bodie is one of the best preserved ghost towns in the old west and its designation as a National Historic Landmark and a California State Park has kept it from being carted away piece by piece over time (Figures 17 and 18). Most of the information about the town and the geology of the mineral deposit are taken from Figure 17. Bodie, CA. Chesterman and others, 1986; Hollister and Silberman, 1995; John and others, The main purposes of this stop are to review some of the extensive and well-documented hydrothermal processes responsible for the mineralization at Bodie and to allow time for exploration of this well-preserved wild-west mining town. Epithermal mineral deposits like Bodie provide excellent conceptual models and analogs for modern geothermal systems particularly in intermediate-composition volcanic environments that host more than 50% of the world s known and developed geothermal systems (Brophy, 2008). The Middle-Late Miocene Bodie Hills volcanic field is a large ~700 km 2 (270 mi 2 ) longlived (9my) but episodic andesite-dacite eruptive center that is part of the southern ancestral Cascades arc (Figure 19, John and others, 2011). Peak periods of eruptive activity include trachyandesite stratovolcanoes between my ago and trachyandesite-dacite dome fields between ~ my ago. Tertiary pyroclastic rocks, flows and intrusive plugs (Silver Hill series, Murphy Spring Tuff Breccia, Potato Hill Formation) around Bodie are equivalents of a regional suite of calc-alkaline basalt, andesite, dacite, and rhyolite that erupted in the eastern Sierra during the Miocene. Diffuse regional low-grade hydrothermal alteration over 30 km 2 (11 mi 2 ) is related to several earlier volcanic centers but not necessarily to economic mineralization. The precious metal mineralization at Bodie is closely related to the structurally controlled emplacement of late stage my old silicic (dacite-rhyolite) plugs late in the Silver Hill Volcanic Series (Chesterman and others, 1986; John and others, 2011). The largest plugs of Bodie Bluff and Standard Hill are the host rock for mineralized quartz veins that account for more than 90 percent of the total production of gold and silver. Equivalent plugs at Red Cloud Mine, Queen Bee Hill, and Sugarloaf to the south are smaller but have fewer quartz veins. The major NNE striking Moyle Footwall and the Standard Vein faults form a graben between Bodie Bluff and Standard Hill and were the principal focus of fluid circulation, intense alteration and mineralization. Later vein deposition occurred 38

60 on other pre-mineralization faults. Post-mineralization ENE and WNW faults (Mono and Tioga) offset many ore veins (Chesterman and others, 1986). Hydrothermal alteration is widespread in the district. For reference the general economic geology terms for low to high-grade alteration are: Propylitic chlorite, epidote, various clay minerals, albite, pyrite, and minor quartz (usually characterized as greenish in color) Argillic - montmorillonite, illite, sericite, quartz, and pyrite (progression to micaceous mineralization) Potassic adularia, quartz, sericite, (progressive silicification) These assemblages are analogous to the sequence of hydrothermal alteration minerals that are commonly logged at progressively greater depths and higher temperatures in geothermal wells and are characteristic of an alteration cap developed around many geothermal systems. The margins of the Bodie district are characterized by initial propylitic (greenish) alteration followed by later by more pervasive light-colored argillic and potassic alteration sequences. Potassically altered rocks are lighter colored, and tend to resemble the original rock in color and texture. Potassic alteration is accompanied by extensive and pervasive development of irregular quartz-adularia veins with or without calcite. Silicic alteration generally occurred last at Bodie and the silica-altered rocks are light colored, hard, and form a cap on argillicly-altered rocks. The paragensis or progression of alteration and mineralization is very important in mineral exploration and no less significant for identifying and evaluating a potential geothermal system. Age dating and the progression of mineralization at Bodie provide important constraints on the viability of active geothermal systems. Focused intense alteration occurs within the larger area of regional lower grade argillic alteration. Recent 40 Ar/ 39 Ar dating of adularia in the gold-bearing quartz veins and in surrounding hydrothermally altered rocks indicates that alteration and mineralization occurred over a period of ~1.5 my and were directly related to younger siliceous dacite intrusives at Bodie Bluff and Standard Hill (John and others, 2011). Epithermal mineralization accompanied repeated normal faulting resulting in vein development along the fault planes. Individual mineralized structures commonly formed along new fracture planes during separate repeated fault movements with resulting broad zones of veinlets in the walls of the major veined faults. For more than a century geologists have recognized that epithermal precious metal deposits are actually fossil hot spring systems. Bodie and many other well-studied epithermal mineral deposits are important analogs for exploring and evaluating current igneous-related geothermal systems. Vigorous hydrothermal systems are related to longlived, repeatedly intruded, tectonically active settings and share common characteristics: Heat sources Young shallow intrusions are fundamentally important for conventional igneousrelated geothermal systems. Highly siliceous systems are priority exploration targets because they are part of a larger differentiating intrusive system at depth 39

61 and siliceous intrusions are emplaced at relatively shallow levels in the crust resulting in high heat flow and fluid circulation at economically accessible depths. The epithermal system at Bodie is an example of a long-term series of regional andesitic systems culminating with the emplacement of more siliceous daciterhyolite intrusions at Bodie Bluff-and Standard Hill that resulted in a vigorous focused hydrothermal system. Shallow, easily accessible geothermal systems significantly enhance the economics of geothermal development and epithermal systems related to shallow silicic intrusions like Bodie generally occur within ~ 1km of the surface. Timing and duration Hydrothermal systems are ephemeral and short-lived. The pervasive permeability that is critical for a productive geothermal system is also an efficient means of convecting heat away from a crustal heat source. Simple conductive heat flow modeling for the Long Valley Caldera suggests that a 2 my history of eruptions from an upper crustal magma chamber could only be sustained by repeated magma replenishment (Lachenbruch, and others 1976). Numerical modeling suggests that an intrusion emplaced in a crustal setting with a minimal amount of convective heat loss would remain a viable heat source for only 800, 000 yrs. (Cathles and others, 1997). More precise 40 Ar/ 39 Ar dates for well studied mineral systems suggest a narrow range of ages for single intrusion systems (Marsh, 1995; attached) and some short-lived systems are younger than the lowest 10,000 yr. limit of current 40 Ar/ 39 Ar dating techniques of (Stein and Cathles, 1997). Still, geologic evidence indicates many geothermal systems are the current or youngest phase of much longer-lived hydrothermal systems. The current Long Valley geothermal system succeeds an older system that peaked ~300,000 yrs. ago. The Steamboat geothermal system has been active over a span of nearly 3 my. The Geysers geothermal system has apparently been active for more that 1.5 my. The implication is that repeated replenishment from multiple intrusive events is important for the development of ore deposits and active geothermal systems. Structural control Permeability is a critical element for any viable geothermal system. Based on detailed studies, epithermal mineral deposits and current active hydrothermal systems are directly controlled by faults, fault intersections, fault complexities (transfer and accommodation zones) and repeated fault movement creating permeability to allow circulation of large volumes of fluid in otherwise impermeable crystalline rocks. The tectonic setting and therefore the style of faulting is also a critical element. Transtensional and dilatant settings provide the vertical permeability that is often crucial in allowing vertical convection of geothermal fluids. Bodie is a prime example of how pre-existing or repeated complex faulting focused the emplacement of shallow crustal heat sources and generated permeable pathways for hydrothermal circulation that resulted in vein development along fault planes. 40

62 Geothermal exploration is the search for heat. Drilling is the only way of directly sensing heat but it is generally a much more advanced, intrusive and expensive part of an exploration program. Geology, geochemistry and geophysics are the core geothermal exploration methods before any holes are drilled but they evaluate the effects of heat. Interpreting geological, geochemical or geophysical data depends on drawing comparative analogies with known settings to develop a conceptual model of a potential resource. Unfortunately, most geothermal systems are only partially explored or poorly understood and none are exposed in three dimensions like a mine. The best exploration tool is a well-constrained conceptual model. The best quantified model constraints come from detailed evaluations of epithermal systems because ore bodies provide a chance to examine what a geothermal system looked like before, during and after it evolved. The Bodie area remains a potential Walker Lane exploration target. The lode mines operated productively between 1877 and 1942 but the lower-grade disseminated goldsilver part of the deposit has always attracted attention. The main geothermal attraction is the renewed post 5 my volcanism and rapid extension. The nearest (very) young eruptive enter is Aurora Crater, a 250,000 yr. old basaltic cinder cone on the southwest side of the town of Aurora. Gradient drilling partially compiled from mineral exploration holes suggest heat flow above Basin and Range background of F/100 (31-51 C/km). Precious Metal Mineralization Gold and silver production at Bodie came from the pervasive mineralization of the several systems of quartz veins in and around the intrusive dacite plugs of Bodie Bluff-Standard Hill. Including Silver Hill area farther south, gold and silver were recovered from quartz veins and irregular silicified zones in dacite flows and tuff breccia. Ore minerals include native Au, pyrargyrite (Ag 3 SbS 3 ), tetrahedrite (Cu,Fe,Zn, Ag) 12 Sb 4 S 13 stephanite (Ag 5 SbS 4 ), and Figure 18. Standard Mill ~1932 pyrite. The ratio of Au to Ag by weight in the northern part of the district was about 1:12, and in the southern part about 1:40. (Chesterman and others, 1986; Hollister and Silberman, 1994). The total value of production from Bodie through 1942, including the boom years , was $34m (Chesterman and others, 1986) or roughly one year of peak Comstock production valued at $36m (Smith, 1943). The total documented production of M oz. Au and 7.28 M oz. Ag combined would yield roughly $1.7B at 2015 prices ($1097/oz. Au and $15/oz. Ag). Fantastic high-grade production early in the district s history yielded incredible financial rewards. The Syndicate mill crushed 1,000 tons of bonanza ore from the Bodie Mine in 1878, yielding $601,103 in six weeks (Smith, 1943). An average 100-pound bar of the district s bullion contained about 73 pounds Ag and 27 41

63 pounds of Au. The term Bodie gold assumes a certain silver content. Bodie s bullion was very pale in color because of the high silver content that also decreased it s value since silver is less valuable than gold. Early Nevada placer miners at Washoe noted their gold flakes and nuggets were lighter in color than the California gold they were accustomed to panning. Dan De Quille (William Wright) in The Big Bonanza, his 1876 account of the Comstock Lode, explained that: The gold dug in the placer-mines of California is worth from $16 to $19 per ounce, whereas the gold taken from the croppings of the Comstock was worth no more than $11 or $12 per ounce. Smith (1943) recognized the effect noting: All of the gold in the (Bodie) district was heavily alloyed with silver, and was rarely worth more than $12 per ounce. In some of the shallow placer diggings, at the south end of the ridge, the gold was worth only from $3 to $8 per ounce. Silver/gold alloy was also the principal source of silver at Bodie but disseminated silver mineralization was a secondary source. White quartz veins were variably discolored by sulfides that 19 th century miners referred to as sulphurets ; an old mining term for Au/Ag bearing sulfide ore. Sulphurets at Bodie included argentite (Ag 2 S; known in the Comstock as the blasted blue stuff ), chalcopyrite (CuFeS 2 ), galena (PbS), pyrargyrite (Ag 3 SbS 3 ; a crimson mineral called ruby silver ), pyrite (FeS 2 ; fool s gold), stephanite (Ag 5 SbS 4 ) and tetrahedrite (Cu,Fe,Zn,Ag) 12 Sb 4 S 13 ). Sulphurets required specialized treatment beyond the milling and ore processing methods of the time. Microscopic particles of free gold often represented the primary value of sulfide ores. Scarcer forms of silver were also found at Bodie. Horn silver (AgCl) was especially valued because it could be easily processed and recovered by ordinary milling. Rare metallic or native Ag occurred deep underground beyond near-surface oxidation. Open-pit mining had been considered at Bodie whenever precious metal prices were high. Drilling by Galactic Resources during the 1980 s found economically viable veinlet swarms around the lode ore bodies. The USGS (Long and others, 1996) estimated the disseminated Bodie resource at 24.6 million tons averaging 0.07oz Au/ton and 0.42oz Ag/ton. Galactic shelved the project in the late 1980 s when precious metal prices declined and their openpit mining proposal provoked bitter controversy. Cleanup liabilities at Summitville, Colorado eventually forced Galactic into bankruptcy and the Desert Protection Act of 1994 withdrew 5,000 acres of existing Bodie claims appraised at $65 million. California State Parks acquired the acreage in 1997 for $6 million. (Wilkerson and others, 2007). History Bodie was one of the roughest, toughest, rip-roaringest, rootin-tootinest mining camps in the west. The town s name is supposedly a misspelling of the last name of William (a.k.a. Waterman or Wakeman) S. Bodey who discovered placer gold in1859 near what is now called Bodie Bluff. Bodey died in a blizzard the following November while making a supply trip to Monoville and never saw the development of the town that supposedly bears his name. The discovery of gold and silver at Bodie coincided with other discoveries at nearby Aurora, Nevada and the Comstock Lode in Virginia City, Nevada. While those towns boomed, interest in Bodie remained lackluster. By 1868 only two companies had built stamp mills at Bodie, and both had failed. A collapse at the Bunker Hill mine in 1875 exposed a rich quartz lode vein on Bodie Bluff that quickly attracted 42

64 speculators and investors who eventually purchased the claim and organized the Standard Company. By 1880 the town had an estimated population of 10,000 people bustling with robbers, miners, storekeepers, gunfighters and prostitutes. Town records list 65 saloons stretching for a mile along Main Street that included or were part of gambling halls, opium dens and houses of ill repute. Rosa May, a local madam, is credited with giving life-saving care to many during a serious epidemic at the height of the town s boom but, like many soiled doves of the day, she was not regarded as a respectable woman and is buried outside the town cemetery fence. Legend and fiction often masqueraded as fact in the 19 th century Wild West and at least some of Bodie s notoriety was based on exaggeration. Embellishing the potential value of a mineral deposit was an integral part of soothing speculators nerves and assuring continued investment. A notorious town legend held that a man was killed every day of the week in Bodie. That might be an exaggeration from a particularly violent week but, based on court records for murder convictions, the homicide rate was at least 30 times the current US average (Roth, 2010). An unattributed quote in one local newspaper commented that, Bodie is becoming a summer resort no one killed here last week. Presumably, the undertaker never had a slow day. Mark Twain tried his hand at mining in nearby Aurora and got his first newspaper job writing for the Esmeralda Star. He chronicled life in the Esmeralda Mining District for the Virginia City Territorial Enterprise and in his book Roughing It. Twain wrote that he never had occasion to kill anybody with the Colt Navy revolver he carried, but he had " worn the thing in deference to popular sentiment, and in order that I might not, by its absence, be offensively conspicuous, and a subject of remark." One story (sometimes wrongly attributed to Twain) is about a little girl whose family was moving to Bodie and wrote in her diary "Goodbye God, I'm going to Bodie." In one of the finest of early spin jobs, the ever-enterprising local newspaper editor Lying Jim Thompson denied the slight claiming that the girl was misquoted or the sentence mis-punctuated and that she had actually said Good! By God I m going to Bodie. Clean out your pockets before leaving. Bodie is a historic park and removing any artifact is strictly prohibited. More importantly, among the town s many ghost stories and legends of haunting the most infamous is the Bodie Curse. Anyone who removes something from Bodie is cursed with bad luck and misfortune until the item or items are returned. Every year park rangers receive objects mailed back to Bodie with letters (preserved in the Museum) apologizing to the town and imploring its guardian spirits to forgive their offense. If you have carried anything off with you, this is the first of your bad luck. We can t risk riding with you and you ll have to find our own way back. Return west on Highway 270 to Highway 395.Turn right (north) heading toward Reno. BEWARE OF TRAFFIC! Continue north through Bridgeport, the county seat of Mono County. Continue north of Highway 395 to the Peppermill Resort in Reno. Incremental Distance 131 mi/ 211 km Cumulative distance mi. / km 43

65 Figure 19. Generalized geologic map of the Bodie Hills (after John and others, 2011) 44