The Devils Postpile and its Unusual Surroundings. Shannon B. Carpenter A May 3, 2001

Transcription

1 The Devils Postpile and its Unusual Surroundings Shannon B. Carpenter A May 3, 2001 Abstract: The Devils Postpile is located in the eastern Sierra Nevada region just a few miles southwest of the Mammoth Lakes. Even though the cause of this highly volcanic area is still under debate, the source could be due to transtension due to the extension of the Basin and Range of the Sierra Nevada. About 100,000 years ago, a fissure was formed just south of the Mammoth Mountain from which poured basaltic lava. This lava then pooled and cooled in a distinctive way to form posts by columnar jointing.

2 The Devils Postpile and its Unusual Surroundings Introduction-The Devils Postpile National Monument is located in the Sierra Nevada region east of Yosemite National Park and only a few miles southwest of the Mammoth Lakes (Fig. 1). This geologic feature is considered to be one of the finest examples of basaltic columnar jointing in the world. It is contained in only an 800 acre piece of land, making it one of the smallest national monuments (Fig. 2). This site was designated as a National Monument in 1911, only after threatened to be destroyed for mining motives (Kiver and Harris, 1999, p.239). Regional Geology-The Sierra Nevada has been an area of abundant tectonic activity generating scattered areas of volcanism from the Cenozoic to the most recent eruptions in the last 10,000 to 20,000 years (Hill and Bailey, 1985). The cause of this tectonic activity is still under conflict but the most recent theory relates the San Andreas Extension Theory in combination with the strike-slip movement of the San Andreas Fault (Kiver and Harris, 1999, p.242). The movement of the San Andreas Fault is not as simple as it seems because there are bends in the fault, which are affected by the northerly movement of the Pacific Plate. In areas where the fault is parallel to the plate boundary of the North American Plate and the Pacific Plate, the movement will be right-lateral strike-slip. However, in areas where the fault movement is not parallel (i.e. the bends), there will be areas of either extension or compression (Harden, 1998, p ). This movement, along with the various strike-slip movement of the normal fault strands along the eastern Sierra Nevada (Fig. 3), cause transtensional movement (divergent strike-slip). Uplift and basin extension are all characteristics of this transtensional movement, which in turn is the possible cause for the expansion of the Sierra Nevada region (Kearey and Vine, 1996, p ). Hill and Bailey suggest that this extension causes the lithosphere to rapidly pull apart in this region and is "drawing up basaltic magmas to fill up the void", hence producing the high levels of volcanic activity (Hill and Bailey, 1985) in the Sierra Nevada region. The topography of the neighboring Basin and Range Providence is quite intricate with numerous areas of sequences involving depression and elevated features. The extensional simple shear model developed

3 by Wernicke in 1985 is capable of explaining these dramatic elevation changes. This model illustrates that continued tension and fault movement in a region will cause the thinning of the hanging wall and the basin. This deformation of the basin and hanging wall then will form a normal fault duplex, which is an area having a number of rotated faults and thin fault blocks (Fig. 4). As a consequence, extensional allochthons of the Basin and Range form next to the thinning crust causing an uplift of the Sierra Nevada block (Fig. 5) (Kearey and Vine, 1996, p ). Wernicke also makes certain to note that this simple shear theory does not always predict the presence of hot mantle in nearby areas of the rift. However, his studies do show that the Basin and Range Providence is depicted as having low P n velocities ( km/s) and high topography which are characteristic of these regions (Wernicke, 1985). Over the years, the result of this uplift has caused the formation of the rift valley into the eastern side of the Sierra Nevada Batholith which, in turn, is causing the high levels of activity near the Long Valley region (Cousens, 1996). The Devils Postpile Geology-The Devils Postpile is located within the Middle Fork basin which shares its borders with the Ritter Range, with peaks ranging in elevation from 10,000 and 13,000 feet, and the more gentle Sierra Crest. This setting is partly forest and partly alpine which gives visitors a very complex and beautiful spectacle to look at and hike through (Huber and Rinehart, 1965). The oldest volcanic rocks in the Devils Postpile are of Quaternary age and most likely originated from a basalt flow that came from the western Buttresses. There is also the Reds Meadow Tuff which is just east of the monument and is about the same age as the huge eruption of the Long Valley Caldera about 760,000 years ago. Scientists estimate that during this time about 600 km 3 of magma was erupted, mainly as hot pyroclastic flows, generating a partially evacuated the magma reservoir which caused the ground to collapse into it forming what we now know to be the Long Valley Caldera. This massive eruption was followed by hundreds of smaller eruptions over the next few hundred thousand years concentrated in the central and western parts of the caldera. At this same time, the central section of the caldera had new magma rising into the shallow reservoir, which caused an upward rise of the caldera floor forming the "resurgent dome (Cousens, 1996). These recent activities most likely have something to do with the numerous eruptions and lava

4 flows which have been constructing the present-day Postpile and its region. One such instance of this was that after the collapse of the caldera, new magma erupted from vents along the rim of the western caldera and along the edge of the Sierra Nevada. This produced the youngest volcanic rocks in the area, which are of Pleistocene to Holocene age consisting of mainly rhyodacite lava (Kiver and Harris, 1999, p.242). Formation of the Posts-Less then 100,000 years ago, basaltic lava ran down the sides of the Middle Fork Valley and pooled as a lake in the valley of the San Joaquin River about 400 feet deep. Scientists have estimated it was around this time based upon radiometric age determinations and by the correlation on rocks of various locations within the monument. The characteristic columnar jointing of these posts were formed when both the top (by air) and the bottom (by the coolness of the underlying granite) were cooled, shrinking the lava and cracking it into hexagonal patterns (Geology of U.S. Parklands, 244). If all conditions are perfect and the ground beneath the flow is horizontal, then these posts will form perpendicular to the ground. However, there may have been some irregularities in the surface such as the layers of a porous lava could have insulated the material underneath which might have produced many of the columns in the Postpile which seem to be tilted or bent (Hartesveldt, 1955). The columns have an average diameter of 2 feet and some are 60 feet long. In a study, it was found that not all of the columns in the Devils Postpile have six sides due to imperfect environmental conditions; by a sample of 200 posts: 2% had 4 sides, 37% had 5 sides, 55% had 6 sides, and 5% had 7 sides (Huber, 8). Pleistocene Glaciers-The last major glaciation were the Wisconsin glaciers which flowed down the valley of the San Joaquin River and eroded most of the lava flows from the area. These glaciers cut a vertical slice of the Devils Postpile exposing the interior and sides of the columns (Fig. 6). If one looks closely at these exposed ends of the columns on top of the Postpile, some glacial polish and striations still remain even after years of weathering and erosion (Huber, 9-10). Destruction of Columns-Over the years, the Postpile has disintegrated considerably resulting in the accumulation of broken posts laying along side the remaining standing ones. One reason as to why these posts have fallen is due to the accumulation of water in the cracks between the columns. Because the columns

5 are only a mere 12 inches apart, this water most likely expanded while freezing causing the posts to be pried apart and causing them to fall. Another cause of some of these broken posts is due to the May 1980 series of earthquakes in the Eastern Sierra region. Four of these earthquakes had a Richter Magnitude of 6.0 or higher within a 58-hour period. During this period, many of the columns along the outside edge of the monument tumbled and others near the center of the Postpile were only cracked (Huber, 1985, p.20). Conclusions- Even though there has not been a great deal of data collection to confirm the exact cause of the high levels of tectonic activity in the Sierra Nevada region, there are numerous theories which attempt to explain the cause of this very unique area. Recently, scientists are all in agreement that the most probable cause is due to some sort of extension produced by the change in motion between the Pacific and North American Plates. More data testing in this region, especially concerning the mantle under the Basin and Range Providence and the Sierra Nevada block as a whole, is needed in order to determine the specifics behind the theory and determine what for sure formed the scenic Devils Postpile.

6 Fig. 2. Devils Postpile (Source: Fig. 1. Red diamond shows the location of Devils Postpile. (Source: s/map_html/15041gd.htm) Fig.3. Distribution of Seismic Activity in the Western States. Solid arrows give relative plate movement and open arrows indicate stress direction. (Source:

7 Fig. 4. Model of an extensional simple shear system. Note that the Sierra Nevada Uplift is occurring on the right side of the diagram. (Source: Global Tectonics) Fig. 5. Central California shaded relief map showing state topography and landforms. (Source: Fig.6. (Source: Devils Postpile Story)

8 References Cited Cousens, B.L., Magmatic evolution of Quaternary mafic magmas at Long Valley Caldera and the Devils Postpile, California: Journal of Geophysical Research, v.101, p.27, , 689. Harden, D.R., California Geology: Prentice Hall, Inc., Upper Saddle River, NJ, vii pp. Hartesveldt, R.T., The Devils Postpile: Natural History, v.64, p Hill, D.P., and Bailey, R.A., Active Tectonic and Magmatic Processes Beneath Long Valley Caldera Eastern California: Journal of Geophysical Research, v.90, p.11,111-11,120. Huber, N.K., Devils Postpile Story: Sequoia National History Association, Three Rivers, CA, ii+29pp. Huber, N.K., and Rinehart C.D., The Devils Postpile National Monument: Mineral Information Service, v.18, p Kearey, P., and Vine, F.J., Global Tectonics: Blackwell Science, Malden, MA, v pp. Kiver, E.P., and Harris, D.V., Geology of U.S. Parklands: John Wiley & Sons, Inc., New York, NY, ix pp. Wernicke, B., Uniform-sense normal simple shear of the continental lithosphere: Canadian Journal of Earth Sciences, v.22, p