Modeling volcano growth on the Island of Hawaii: Deep-water perspectives

Transcription

1 Modeling volcano growth on the Island of Hawaii: Deep-water perspectives Peter W. Lipman and Andrew T. Calvert U.S. Geological Survey, Menlo Park, California 925, USA ABSTRACT Recent ocean-bottom geophysical surveys, dredging, and dives, which complement surface data and scientific drilling at the Island of Hawaii, document that evolutionary stages during volcano growth are more diverse than previously described. Based on combining available composition, isotopic age, and geologically constrained volume data for each of the component volcanoes, this overview provides the first integrated models for overall growth of any Hawaiian island. In contrast to prior morphologic models for volcano evolution (preshield, shield, postshield), growth increasingly can be tracked by age and volume (magma supply), defining waxing alkalic, sustained tholeiitic, and waning alkalic stages. Data and estimates for individual volcanoes are used to model changing magma supply during successive compositional stages, to place limits on volcano life spans, and to interpret composite assembly of the island. Volcano volumes vary by an order of magnitude; peak magma supply also varies sizably among edifices but is challenging to quantify because of uncertainty about volcano life spans. Three alternative models are compared: (1) near-constant volcano propagation, (2) near-equal volcano durations, (3) high peak-tholeiite magma supply. These models define inconsistencies with prior geodynamic models, indicate that composite growth at Hawaii peaked ca. 8 ka, and demonstrate a lower current rate. Recent age determinations for and Kohala define a volcano propagation rate of 8.6 cm/yr that yields plausible inception ages for other volcanoes of the Kea trend. In contrast, a similar propagation rate for the less-constrained Loa trend would require inception of Loihi Seamount in the future and ages that become implausibly large for the older volcanoes. An alternative rate of 1.6 cm/yr for Loa-trend volcanoes is reasonably consistent with ages and volcano spacing, but younger Loa volcanoes are offset from the Kea trend in age-distance plots. Variable magma flux at the Island of Hawaii, and longer-term growth of the Hawaiian chain as discrete islands rather than a continuous ridge, may record pulsed magma flow in the hotspot/plume source. INTRODUCTION This overview, inspired by the 1th anniversary of the U.S. Geological Survey (USGS) Hawaii Volcano Observatory (HVO) in 212, focuses on results of underwater studies of Hawaiian volcanoes that provide new perspectives on the growth of intraplate volcanoes. Recent studies have been especially productive for the Island of Hawaii (Fig. 1), where sonar surveys, dives, and dredging by the University of Hawaii, Monterey Bay Aquarium Research Institute, National Oceanic and Atmospheric Administration (NOAA), and USGS, and collaborations with the Japan Agency for Marine- Earth Science and Technology (JAMSTEC) have complemented on-land scientific drilling and abundant data from HVO. These results, in combination with a vast body of older data, provide new insights about volcano growth on Hawaii. More prior studies than can be acknowledged have evaluated growth of Hawaii in relation to longer-term evolution of the Hawaiian Ridge. Notable were early recognition of the southeastward younging of volcanoes and distinction of the parallel Kea and Loa volcanic trends (Dana, 1849; Jackson et al., 1972), and of course the insights about ocean-island volcanism that emerged in the 196s from the plate-tectonic paradigm. Among recent critical observations and interpretations are: quantifying propagation rates along the Hawaiian-Emperor Ridge by isotopic dating (McDougall and Swanson, 1972; Jackson et al., 1972; Clague and Dalrymple, 1987), volume estimates from submarine bathymetry (Bargar and Jackson, 1974; Robinson and Eakins, 26), recognition of an early-alkalic ( preshield ) stage at Loihi Seamount (Moore et al., 1982; Garcia et al., 1995a), insights into volcano growth based on ages of submerged slope breaks and coral reefs (Moore and Campbell, 1987; Ludwig et al., 1991), and geodynamic models for growth rates and compositional evolution in response to plate motion over a hotspot (Moore and Clague, 1992; DePaolo and Stolper, 1996; Ribe and Christensen, 1999; DePaolo et al., 21). Until recently, compositions and ages bearing on growth of Hawaiian volcanoes have largely come from subaerial sampling of late eruptive stages, and estimates of inception and early evolution have been heavily model dependent, using volcano spacing and plate motion to infer propagation rates and duration of edifice growth. The present interpretive synthesis, by combining recent chemical and Ar/ Ar isotopic-age data (mainly from underwater and drill-hole samples), revised edifice volumes, limitations from volcano structures and eruptive processes, and geodynamic constraints, attempts to interpret the growth histories of individual volcanoes and the composite growth of the entire island. While focused on construction of Hawaii Island, some data from older islands are referenced briefly where helpful to constrain volcano-growth models. In part, this analysis is a sequel to the impressive synthesis by Moore and Clague (1992), while benefiting from compilations for the geologic map of Hawaii Island (Wolfe and Morris, 1996a, 1996b) and the state map of Hawaii (Sherrod et al., 27). Particularly useful samples, compositional data, and imaging of deep structure have come from the Hawaii Scientific Drilling Project (HSDP; Stolper et al., 1996, 29) and ~1 submersible dives and bathymetric surveys during JAMSTEC cruises in (Takahashi et al., 22; Robinson et al., 23; Coombs et al., 26a). Reliable age determinations for old Hawaiian lavas remain sparse, but recent application of Ar/ Ar methods to young basalts, especially Geosphere; October 213; v. 9; no. 5; p ; doi:1.113/ges935.1; 15 figures; 11 tables. Received 2 April 213 Revision received 6 July 213 Accepted 29 July 213 Published online 14 August For permission to copy, contact editing@geosociety.org 213 Geological Society of America

2 Volcano growth on Hawaii 21 N K HA A Hana Ridge M-G KP M-C KO B Kiholo R N Kona Slump HU MK B Laupahoehoe Slump H Hilo Ridge A Figure 7 Puna Ridge ML KL MSB 19 N LO Ka Lae R 1 km Figure 1. Map of the Island of Hawaii and adjacent sea floor, showing locations of volcanoes (Kea trend in black; Loa trend in blue): HA Haleakala; HU Hualalai; K Kahoolawe; KL ; KO Kohala; LO Loihi; M-G Mahukona (summit location of Garcia et al. [199]); M-C Mahukona (summit location of Clague and Moore [1991]); MK Mauna Kea; ML Mauna Loa. Historical eruptions are in red. Dashed line is the inferred buried east rift of Kohala that continues into Hilo Ridge. H Hilo (site of HSDP hole); KP Kohala platform; MSB mid-slope bench offshore of. Modified from Robinson et al. (26). Cross sections A A and B B (short dotted lines) are depicted in Figure 6. Arrow at the base of Hilo Ridge is the site of dive K215 (Fig. 7). underwater and subsurface samples, has improved controls on volcano growth (Table 1). Alkalic lavas have yielded stratigraphically coherent results for Mauna Kea and (Sharp and Renne, 25; Calvert and Lanphere, 26), but dating of low-k tholeiites continues to be problematic because of low radiogenicargon yields. In such tholeiites, K resides mainly in glassy selvages next to groundmass minerals, leaving samples vulnerable to argon loss and/or argon-recoil problems in the reactor. Some tholeiite samples fail to yield meaningful ages without apparent petrographic or chemical reasons. In the most detailed Ar/ Ar study TABLE 1. RECENT Ar/ Ar GEOCHRONOLOGIC RESULTS, ISLAND OF HAWAII Rock type Volcano (number of ages) References Alkalic and transitional basalt (13) Transitional basalt (2) Calvert and Lanphere (26) Hanyu et al. (21) Mauna Loa Tholeiite (1) Tholeiite (1) Tholeiite (14) Mauna Kea Alkalic and tholeiitic basalt (9) Alkalic and tholeiitic basalt (9) Sharp et al. (1996) Sharp and Renne (25) Jicha et al. (212) Sharp et al. (1996) Sharp and Renne (25) Kohala Transitional-alkalic basalt (2) Lipman and Calvert (211) Mahukona Transitional and tholeiitic basalt (2) Transitional and tholeiitic basalt (3) Note: Ages and analytical data are listed in Appendix A. Clague and Calvert (29) Garcia et al. (212) Geosphere, October

3 Lipman and Calvert Total volcano volume, 1 3 km Mahukona Robinson & Eakins (26) This review Kohala Hualalai to date of Hawaiian tholeiites, on the 1.5 km scarp along the submarine southwest rift zone of Mauna Loa, only 14 of 45 analyzed samples yielded successful ages (Jicha et al., 212). Less precise K-Ar determinations remain the main data for subaerial lavas on Kohala and Mauna Kea, and attempts to improve resolution by unspiked K-Ar methods for and Loihi basalts (Guillou et al., 1997a, 1997b) yielded some results that are internally contradictory or inconsistent with Ar/ Ar dates (Calvert and Lanphere, 26). Appendix A lists the published Ar/ Ar and some recent K-Ar age determinations used for estimating volcano growth rates for the Island of Hawaii and underwater slopes. Volumes of individual volcanoes are also difficult to estimate, with probable uncertainties of 1% 2%, but constrained by the composite island construct (213, km 3 ; Robinson and Eakins, 26). Deep subsidence along the Hawaiian Ridge, defined by seismic profiling (Hill and Zucca, 1987), requires volumes nearly twice early estimates that assumed growth on flat ocean floor (Bargar and Jackson, 1974). The prior estimates used vertical contacts between volcanoes; here, edifice volumes are adjusted for sloping and interfingering boundaries (Table 2; Fig. 2; and discussion below). Effects of old seamounts and other irregularities of the ocean floor are neglected, as in prior estimates, but are unlikely to increase significantly uncertainties about total volume of the island construct. Positive gravity anomalies at volcano summits and proximal rift zones (Kinoshita et al., 1963; Kauahikaua et al., 2), interpreted as recording dense intrusions and olivine cumulates, also help locate volcano boundaries where concealed by younger deposits. Each edifice is divided into subsections for which volumes can be calculated from simplified models, as done previously for (Lipman et al., 26). Further constraints come from eruption and lava-accumulation rates, especially for younger volcanoes. For volcanoes that onlap older edifices, an additional adjustment is made for the volume of deep intrusions and olivine cumulates. Details of volume calculations are tabulated in Appendix B. The age, composition, structural, and volume data for individual volcanoes can then be used to model changing magma supply during sequential compositional stages, to place limits on the duration of volcano growth, and to evaluate the composite assembly of the island (Table 3). These data show that the growth stages of Hawaiian volcanoes are more diverse than previously documented, define inconsistencies at various scales with geodynamic models, and indicate that composite volcanic growth at Hawaii peaked ca. 8 ka. MORPHOLOGIC AND COMPOSITIONAL GROWTH STAGES Building on pioneering insights by Stearns (1946), Hawaiian volcanoes have commonly been discussed in terms of preshield, shield, postshield, and rejuvenated stages (e.g., Clague and Dalrymple, 1987; Peterson and Moore, 1987; Clague and Sherrod, in press). These TABLE 2. ESTIMATED VOLCANO VOLUMES, ISLAND OF HAWAII Total volume (1 3 km 3 ) Basis for changed volume Volcano Published* Revised Alternate (see text for details) Loihi Built on Punaluu slump Overlies south flank of Mauna Loa Mauna Loa Includes sub- flank; underlain by south Hualalai Mauna Kea Above large east rift (Hilo Ridge) of Kohala Hualalai Projected south, beneath Mauna Loa Kohala Includes Hilo Ridge; underlies Mauna Kea Mahukona Unlikely to extend NE, beneath Kohala (Garcia et al., 212) Total Island volume held constant Note: Blue italics Loa-trend volcanoes; others are Kea-trend volcanoes. *Robinson and Eakins (26). Using larger Mahukona (Clague and Moore, 1991; Clague and Calvert, 29) and decreased Kohala and Hualalai. Mauna Kea Mauna Loa Loihi Figure 2. Interpreted volumes of volcanoes, Island of Hawaii: published (Robinson and Eakins, 26) and proposed revised values (assumption of small Mahukona; Garcia et al., 212). terms have been used, somewhat ambiguously, concurrently to reference both morphology and composition. As chemical and age data become more abundant, especially for underwater samples that record early growth, tracking volcanic evolution primarily by composition and time seems increasingly desirable. Much more is known about later stages than earlier ones, although reliable age and eruption-rate data remain sparse. Modeling of growth commonly has assumed uniformly changing compositions and magma supply as the Pacific plate moves across a hotspot source, but recent data document considerable variation in stage durations, transitions between stages, periods of quiescence, and interactions between concurrently active edifices. This paper distinguishes three main compositional stages: waxing early alkalic, sustained main tholeiite, and waning late alkalic, versus morphologic evolution of the edifice (submarine, subaerial shield, late submergence). No volcano on Hawaii Island contains highly alkalic late volcanism considered characteristic of the rejuvenated stage, and the age significance of this stage at older Hawaiian volcanoes currently seems uncertain. Rocks assigned to this stage at volcanoes like East Maui (Haleakala) form an age continuum with prior waning-alkalic eruptions, while similar late-erupted rocks are separated from waning-alkalic lavas by long intervals at West Maui or are absent at volcanoes such as Lanai (Sherrod et al., 27; Clague and Sherrod, in press). All Hawaiian volcanoes are broadly shield shaped in morphology regardless of composition, with sustained slopes rarely >15 both underwater and on land, except along fault and 135 Geosphere, October 213

4 Volcano growth on Hawaii TABLE 3. SUMMARY OF ESTIMATED INCEPTION AGES AND DURATIONS OF MAIN-THOLEIITE STAGE, BASED ON ALTERNATIVE MODELS FOR VOLCANO GROWTH Near-constant propagation Near-constant lifespan Variable life (high-tholeiite) Notes Tholeiite-stage duration (k.y.) Inception (ka) Tholeiite-stage duration (k.y.) Inception (ka) Tholeiite-stage duration (k.y.) Inception (ka) Volcano Island of Hawaii Loihi Early-alkalic stage similar to? Only dated duration of early-alkalic stage Mauna Loa Loa-trend propagation rate Mauna Kea k.y. late-alkalic stage Hualalai End of main-tholeiite stage, ~12 ka Kohala Only dated duration of tholeiite-stage Mahukona End of main-tholeiite stage, ~45 ka? Maui Nui Haleakala End of main-tholeiite stage, ~1 ka Kahoolawe End of main-tholeiite stage, ~12 ka Note: Complete age-volume models are presented in text sections for individual volcanoes. Italics indicate Loa-trend volcanoes; others are Kea-trend volcanoes. Bold indicates best-constrained ages. landslide scarps. Slopes are steeper underwater (Mark and Moore, 1987), in part because much shoreline-generated hyaloclastite breccia accumulates at angle of repose, in part because of steep slope-failure scarps at heads of submarine landslides. The most pronounced change in topographic profile, from steeper submarine slopes to more gentle subaerial deposition as a seamount becomes an island (Mark and Moore, 1987), typically occurs during eruption of relatively uniform tholeiite that constitutes >9% of volcano volume. Although subaerial slopes are generally low during the tholeiitic stage (commonly <5 ), much variation is present as a function of eruptive style. Sustained tube-fed pahoehoe sheet flows that grow by inflation have dips of only a few degrees (Hon et al., 1994), while slopes reach 1 or steeper where built by small tholeiite eruptions as on upper slopes of Mauna Loa (above ~3 m; Mark and Moore, 1987, their figure 3.2). Independently of morphology and whether on land or underwater, edifice growth can be tracked by composition and magma supply. Tholeiitic lavas typically define coherent majorelement arrays varying mainly in olivine content, as well documented for and Mauna Loa (Wright, 1971; Clague et al., 1995; Sisson et al., 22). Early- and late-alkalic lavas are more varia ble. Compositions that plot between the dominant tholeiite array and the alkali-basalt boundary of MacDonald and Katsura (1964) have commonly been designated transitional basalt (Wolfe and Morris, 1996a, 1996b; Sisson et al., 22; Coombs et al., 26b), a usage continued here (see Fig. 4). Such transitional basalts, which are abundant during shifts between compositional stages at some vol canoes, have also been described as low-silica tholeiite, especially in HSDP studies (e.g., Stolper et al., 24; Rhodes et al., 212). Even more subtle compositional variations among tholeiites at individual volcanoes, over varied time scales, are documented by trace-element and isotopic studies (Frey and Rhodes, 1993; Kurz et al., 1995; Pietruszka and Garcia, 1999; Marske et al., 27; Weis et al., 211). The change from sustained-tholeiite ( shield ) to waning-alkalic eruptions ( postshield ) typically coincides with declining eruption rates, accompanied by submergence of the shoreline as volcano loading outpaces lava accumulation (Moore and Clague, 1992). Late-alkalic lavas also form steeper slopes than during the sustained tholeiite stage (up to 2 ; Mark and Moore, 1987), a change probably resulting from smaller and briefer eruptions of alkalic basalt and more silicic lavas (hawaiite, mugearite) that form thicker and more viscous flows. Both compositional and morphologic changes have been widely referenced as the shield-postshield transition. However, capacity of a volcano to sustain subaerial growth is a function of volcano size in relation to magma supply. At a large volcano such as Mauna Loa, subsidence can outpace coastal lava accumulation late during the tholeiitic stage (Lipman, 1995; Lipman and Moore, 1996). In contrast, Mauna Kea continued subaerial growth well after the change to late alkalic volcanism (ca. 33 ka; Sharp and Renne, 25), with submergence beginning to outpace growth only at ca. 13 ka (Moore and Clague, 1992). As a result of such competing processes, the dueling balance between growth and sub sidence at the shoreline can terminate at different stages of compositional evolution. Accordingly, the record of slope-break (shoreline) submergence (Moore and Campbell, 1987; Moore and Clague, 1992) provides critical evidence for declining eruption rates, but does not necessarily coincide with the shift from main-tholeiite to late-alkalic stage. Additional factors modulating volcano growth include changes in eruption sites and duration: distal segments of rift zones can shut down as volcano size increases (e.g., Mauna Loa southwest rift zone [Moore et al., 199b], Hilo Ridge of Kohala [Lipman and Calvert, 211]), eruptions become focused higher on the edifice, and shorter-lived eruptions with high proportions of a a to pahoehoe tend to generate steeper slopes higher on volcanoes. Sparse age-volume results suggest modest asymmetry in volcano growth, with rapid early increase in magma supply, followed by a more protracted waning stage. As detailed later, the only documented duration for early-alkalic stage volcanism () is fairly brief (~15 k.y.), but volume and average magma supply are relatively large at (~25 km 3,.17 km 3 /yr) and Loihi (~1 km 3 in ~125 k.y.,.8 km 3 /yr), in comparison to late-stage alkalic volcanism at Mauna Kea (~8 km 3 in 33 k.y.,.25 km 3 /yr), Kohala (~3 km 3 in ~23 k.y.,.13 km 3 /yr), Haleakala (3 km 3 in 95 k.y.:.3 km 3 /yr), and none at Lanai (Sherrod et al., 27). Shifts between compositional stages are probably all gradational to varying degree (Clague and Sherrod, in press). The shift from waxing-alkalic to tholeiitic stage is in progress at Loihi Seamount, where compositional types interfinger on upper slopes (Moore et al., 1982; Garcia et al., 1995a). Thick lava sequences also inter finger during prolonged transitions from sustained-tholeiite to waning-alkalic stage (described as late-shield ; Sherrod et al., 27) at Mauna Kea (Wolfe et al., 1997; Rhodes and Vollinger, 24) and Kohala (Lanphere and Frey, 1987). Geosphere, October

5 Lipman and Calvert Where stage transitions involve prolonged interfingering, dating of the change is inherently approximate. Perhaps the shift from earlyalkalic to main-tholeiite stage should be defined by the initial appearance of abundant tholeiite (as currently at Loihi) a time of increasing magma supply, when the continued eruption of alkalic basalt becomes volumetrically overwhelmed by tholeiitic lavas. For the change from main-tholeiite to late-alkalic stage, the transition could similarly be defined at the initial appearance of abundant transitional and alkalic lava. For the growth models in this overview, however, uncertainties about transition ages are rarely significant at the precision of available age control. GROWTH AND MAGMA-SUPPLY MODELS Modeling growth of Hawaiian volcanoes is complicated by many interacting processes. Factors favoring augmented growth in edifice size and height include increasing lava-accumulation and magma-supply rates during the progression from early-alkalic to main-tholeiite stage, along with intrusion-driven inflation and expansion. At large tholeiite-stage edifices, volcano height can be negatively impacted by loaddriven subsidence, summit deflation, caldera collapse, flank spreading, and catastrophic slope failures. All Hawaiian volcanoes likely increase in height rapidly during early submarine growth because of initially small size. Subaerial volcanoes rise more slowly, even when eruption rates are higher, as the volcano area becomes large and subsidence modulates growth by lava accumulation. Most previous volcano-growth models for Hawaii have portrayed age-volume relations as variants of a flattened bell curve, in which magma-supply and lava-accumulation rates increase during early growth, peak during the tholeiitic stage, and diminish during late alkalic volcanism. A perceptive early model for Mauna Kea (Wise, 1982; Fig. 3A) has been proposed with only modest differences for other volcanoes (Clague, 1987; Garcia et al., 1995a; Lipman, 1995). Duration of eruptive stages has also been evaluated by geodynamic models involving steady-state plate motion over a fixed hotspot, as recorded by volcano spacing (Fig. 3C 3D; Moore and Clague, 1992; DePaolo and Stolper, 1996; DePaolo et al., 21), but eruptive behavior in Hawaii appears to be non steady state over a wide range of scales. Magma supply has increased markedly during the last few million years (Bargar and Jackson, 1974; Clague and Dalrymple, 1987), volcano spacing along the young end of the Hawaiian Ridge varies by at least a factor of two ( 8 km), major fluctuations in magma-generation and eruptive processes are recorded by the gaps between islands and seamounts, volcano volumes vary by an order of magnitude (Table 2), propagation rates are inconsistent for some adjacent volcanoes, historical magma-supply and eruption rates have varied at individual volcanoes, and life spans of volcanoes also may vary substantially as discussed later. Simple time-volume models, such as depicted in Figs. 3A and 3B, likely are generalizations of magma-supply fluctuations with fractal geometry on time scales from decades or less to that for growth of individual volcanoes, entire islands, and the ocean-channel gaps that separate them. Determining long-term magma supply is especially challenging (Wright and Klein, 213; Poland et al., in press). Short-term shallow magma supply at volcanoes like has been estimated by combining historical observations, rates measured during eruptions, and intrusion volumes determined from geodetic data (Swanson, 1972; Dzurisin et al., 1984; Dvorak and Dzurisin, 1993; Cayol et al., 2; Wright and Klein, 213). Changes in magma supply also have been inferred from lava-accumulation rates at dated stratigraphic sections (e.g., Lipman, 1995; Sharp et al., 1996; Quane et al., 2), but accumulation rates inevitably vary greatly with distance from vents and relation to local topography. Late growth histories of the older Hawaiian volcanoes that are extinct or nearly so are constrained by data from subaerial lavas, but reconstructions of early evolution depend heavily on deep-water sampling. Even with recent drill-hole and submarine sampling, no Hawaiian volcano exposes a complete record of all growth stages. Accordingly, to evaluate magma supply during assembly of Hawaii, eruption rates that have been determined for a stage at one volcano are used to approximate volume-age evolution at others. For each volcano, one or more growth models are developed for 1 k.y. intervals (25 k.y. for and Loihi), constrained by available composition, age, and volume data, and also by analogies with growth stages at other volcanoes, to permit inter-volcano comparisons and to interpret overall growth of Hawaii (Table 3). As these growth models are variably subjective and dependent on data availability, uncertainties are accordingly large. One major uncertainty involves volcanoes of differing size and volume: do smaller volcanoes have briefer life spans than large ones, or are they characterized by lower eruption rates, especially during the main-tholeiite stage? Nevertheless, available age, compositional, and volume data provide a framework to infer overall growth histories and make comparisons with geodynamic models. The time-volume distributions can be adjusted to varying degrees without violating available data, but application of consistent assumptions to the entire suite of volcanoes potentially provides insights about their diverse histories and the composite assembly of Hawaii. Existing data are inadequate to evaluate whether the main-tholeiite stage is characterized by sustained near-constant magma supply (Fig. 3A; Wise, 1982), by a bell-curve peak (Fig. 3B; Holcomb et al., 2), or by major variability from volcano to volcano. For the historical time frame at and Mauna Loa, eruption and magma-supply rates have fluctuated on intervals of decades to centuries, perhaps antithetically, with periods of intense eruptions alternating with sustained intervals of reduced activity (Stearns and Macdonald, 1946; Lipman, 198a; Klein, 1982; Swanson et al., 211; Wright and Klein, 213; Gonnermann et al., 212; Poland et al., in press). Similar or longer-wavelength fluctuations are likely to have characterized earlier activity, but data to evaluate long-term trends are sparse. Because the volcanoes of Hawaii differ substantially in volume (by an order of magnitude or more; Table 2), either the duration of volcano growth or peak magma supply must vary greatly. Several alternatives are explored for growth of less-constrained volcanoes: (1) near-steady-state progression of volcano inception, in accord with plate-motion models; (2) semi-equal durations (~11 k.y.) but varied peak-eruption rates; and (3) shorter durations at smaller volcanoes that maximize peak-eruption rate during the tholeiite stage (Table 3). In addition, recent ages suggest that volcano progression has been asynchronous between the Kea and Loa trends (~N35 W on Hawaii). Measured and modeled propagation rates and growth stages, discussed in later sections, suggest that volcanoes grew earlier along the Loa trend than for similar positions along the Kea trend, at least for the more recent volcanoes. Accordingly, growth along each trend is summarized separately, in general order from younger volcanoes to less-documented older ones. These discussions of available age, composition, and volume data, which are the framework for proposed growth models of individual edifices, provide the basis for evaluating overall island growth and resulting implications for geodynamic models of the Hawaiian hotspot/ plume. Readers mainly interested in general interpretations and conclusions may prefer to go directly to the sections Assembly of the Island of Hawaii and Discussion Geosphere, October 213

6 Volcano growth on Hawaii B A D C Figure 3. Some prior age-volume and volcano-propagation models for growth of Hawaiian volcanoes. (A) Volume-time framework for the evolution of Mauna Kea volcano (Wise, 1982); the inferred rapid inception, sustained tholeiite stage, and prolonged late-alkalic stage are consistent with much of the more recent data summarized in this review. (B) Diagrammatic growth models for Hawaiian volcanoes (Holcomb et al., 2, their figure 5B), inferring constant volcano volumes and propagation rates. (C) Estimated ages for stages in the life histories of volcanoes on or adjacent to the Island of Hawaii (Moore and Clague, 1992, their figure 8), inferring growth at constant propagation rates based largely on the end of shield building as determined from submerged slope breaks and the compositional change from tholeiite (shield) to late-alkalic (postshield) stage. (D) Map of Hawaii showing volcano locations as a function of time (DePaolo et al., 21, their figure 1B), assuming a Pacific plate velocity of 9 cm/yr (numbered circles indicate volcano positions for which isotopic data are available), superimposed on the melt-supply model of DePaolo and Stolper (1996). HSDP Hawaii Scientific Drilling Project. Abbreviations: H Hualalai; HA Haleakala; KI ; KO Kohala; L Loihi; MK Mauna Kea; ML Mauna Loa. See cited papers for details about construction and interpretation of these published figures. KEA-TREND VOLCANOES Because age and volume data are more robust for the Kea trend, these volcanoes are discussed first, starting with where composition, age, and eruptive evolution are constrained by study of its young subaerial deposits, abundant seismic and other geophysical data on three-dimensional structure, several multi-kilometer-deep drill holes, and especially the submersible dives and samples obtained during the Japan-USA research supported by JAMSTEC during (Takahashi et al., 22; Coombs et al., 26a). Subaerial and underwater slopes of display strikingly different records of growth. The on-land surface is mantled by tholeiite lava varying mainly in olivine content (Wright, 1971), mostly erupted <1.5 ka (Holcomb, 1987; Neal and Lockwood, 23). Interlayered thin tephra deposits record prolonged intervals of volumetrically minor explosive activity, during which lava eruptions were sparse (Fiske et al., 29; Swanson et al., 212). Drill holes along s subaerial east rift zone have penetrated similarly uniform tholeiites to depths as great as 17 m below present sea level (Quane et al., 2). Offshore of, all sampled pillow lavas along Puna Ridge, the submarine continuation Geosphere, October

7 Lipman and Calvert of the east rift zone, are similar tholeiite (Clague et al., 1995; Johnson et al., 22). In contrast, no outcrops of tholeiite have been found along the submarine south flank downslope from the summit. Below a prominent mid-slope bench at ~3 mbsl (meters below sea level) (MSB, Fig. 1), bedded volcaniclastic rocks interpreted as debris-flow deposits from ancestral (Lipman et al., 22) contain clasts of diverse submarine-erupted (high sulfur) alkali basalt, including nephelinite and tephriphonolite (Sisson et al., 22), that are more compositionally diverse than known elsewhere on Hawaiian volcanoes except during the late rejuvenated stage. These have been interpreted as recording initial growth of, broadly comparable to the current Loihi Seamount but including lessevolved alkalic compositions. Breccia-matrix and turbidite sands interbedded with the debrisflow breccias contain glass grains of submarine-erupted alkali basalt, mixed with degassed tholeiitic grains generated by shoreline entry of subaerially erupted lavas. This submarine volcaniclastic sequence thins westward against breccias of Loa-type tholeiite interpreted as the underlying flank of Mauna Loa. Above the midslope bench, to the shallowest exposures at 18 mbsl, scattered bedrock ribs expose only weakly alkalic to transitional pillow basalts (Fig. 4). The change to subaerial-type tholeiitic lavas must lie concealed in shallower water, beneath the angle-of-repose mantle of shoreline-derived hyaloclastite. Age and Volume Prior geometric analysis of s volume, including contrasts between subaerial and submarine lava compositions, suggested a volume of ~1, km 3 for the edifice, with about one-quarter emplaced during the waxing-alkalic stage (Lipman et al., 26). This volume for the alkalic part of the edifice, substantially larger than that of Loihi Seamount at present, may have been modestly overestimated, because subaerially erupted shoreline-derived tholeiitic sand forms matrix between alkalic clasts in some deep debris-flow deposits. The prior estimate of total volume also neglected deep parts of associated summit and rift-zone intrusions emplaced within the underlying Mauna Loa flank, here roughly approximated as an additional 75 1 km 3 (Appendix A, Table A1); further interpretation assumes a total volume of ~11, km 3. Multiple Ar/ Ar incremental-heating ages on early-alkalic and transitional basalts from s submarine south flank provide especially tight constraints on ancestral growth of this volcano, as well as a possible template for early evolution of other Hawaiian volcanoes Age, years before present (log scale) 5, 25, 1, 5, 25, 1, ?? Lower Hilina Upper Hilina Lower Puna Upper Puna Historical Tholeiitic Transitional Above bench - east Pillow rib - west (Calvert and Lanphere, 26). Inception of no earlier than ca ka is inferred from high-precision plateau ages of 234 ± 9 and 238 ± 1 ka on phlogopite from nephelinites that record low magma supply generated by small degrees of source melting at initial stages of volcano growth. Ages on weakly alkalic pillow basalt above the mid-slope bench range down to 135 ka. Two ages from a thick breccia section of transitional lava are as young as 65 ± 28 ka (Hanyu et al., 21), suggesting that main-stage tholeiites only became dominant at ca. 1 ka or? Weakly alkalic Two lower flows, Hilina Pali (Chen et al., 1996) Kulanaokuaiki Tephra (Fiske et al., 29) Three flows (of 437 analyses) (Wolfe & Morris, 1996b) Clasts below bench Alkalinity Strongly alkalic Figure 4. Summary of alkalinity versus ages of lavas, illustrating intermittent eruption of volumetrically minor transitional basalt during the sustained-tholeiite stage since >5 ka. Calculated alkalinity [(Na 2 O + K 2 O).37*(SiO 2 )] is defined as the weight percent difference in Na 2 O + K 2 O between the sample and the alkali/tholeiitic basalt line of MacDonald and Katsura (1964). Left column is the subaerial stratigraphic sequence of (Wolfe and Morris, 1996a); more alkalic lavas in the upper right of the diagram are from the submarine south flank (italic labels). The strongly alkalic samples are clasts in debris-flow deposits from below the mid-slope bench (MSB, Fig. 1); weakly alkalic basalts are from prominent rib outcrops above the western side of the bench; and submarine transitional basalts form continuous exposures above the eastern mid-slope bench. even later (Fig. 4). These young ages for initial eruptions and shift to the tholeiite stage contrast with prior inference of earlier volcano inception (6 7 ka) based on plate-motion models (DePaolo and Stolper, 1996). Geothermal drill holes along s east rift have penetrated tholeiite sections 17 m or more thick (Quane et al., 2), documenting proximal emplacement of this basalt type at depths nearly to that of the shallowest alkalic pillows on the offshore slope, helping to bracket the shift between compositional stages and 1354 Geosphere, October 213

8 Volcano growth on Hawaii suggesting that the change may have been fairly abrupt. Drill-hole samples have ages as old as 351 ± 12 ka by the unspiked K-Ar method (Guillou et al., 1997b), but their reliability has been questioned because of inconsistency with Ar/ Ar dates from submarine alkalic rocks and potential for excess Ar and K loss to disturb ages in low-k samples (Calvert and Lanphere, 26). Rare transitional flows and tephra with atypically high TiO 2 and alkalis at low SiO 2 (3 analyses of 437 tabulated for ; Wolfe and Morris, 1996b), erupted in the last few thousand years at, support inference that the change to main-tholeiite stage is complex (Fig. 4) and may still be incomplete, consistent with the relatively modest current volume estimated for the growing volcano. Examples include the A.D. 6 1 Kulanaokuaiki tephra (Dzurisin et al., 1995; Fiske et al., 29), an associated lava flow (Lipman et al., 26, their table 5), older flows of transitional basalt in Hilina fault scarps (Chen et al., 1996), and a young alkalic flow at the base of the distal Puna Ridge (Clague et al., 1995; Johnson et al., 22). Magma-Supply and Growth Models A prior effort to model growth of (Lipman et al., 26), based on compositions and ages of submarine samples collected during JAMSTEC research, a revised edifice volume, and published estimates of late-2th-century magma supply (~.1 km 3 /yr; Swanson, 1972; Dvorak and Dzurisin, 1993), became the starting point for this summary that refines the result and applies similar methods to the older volcanoes. Diverse observations now suggest that magma supply at has varied sizably in geologically recent time. Data from the continuing east rift eruption (since 1983) have documented varied eruptions rates, up to.2 km 3 /yr (Wolfe, 1988; Wright and Klein, 213; Poland et al., 212). Interpretation of geodetic data suggests that the total magma supply has been close to.18 km 3 /yr since 1961, including intermittent dike intrusions along rift zones during this interval (Cayol et al., 2). Evaluation of the longer-term historical record of eruptions, in conjunction with analysis of seismic data on magma-accumulation sites, also suggests supply rates to ~.18 km 3 /yr since ca. 196, increasing from 19th- and earlier 2th-century rates of only.1.8 km 3 /yr (Pietruszka and Garcia, 1999; Wright and Klein, 213). Examination of prehistorical eruptive deposits has begun to document even more complex variability in eruptive rates at, with multihundred-year periods of lava eruption at high rates alternating with similarly long intervals Figure 5. Age and magma-supply growth models for, at 25 k.y. intervals. (A) Linear scale for magma supply. (B) Semi-log scale that better illustrates variations during low magma supply. Data are from Table 4, which also lists interval volumes. Magma supply, km3/yr (linear) Magma supply, km3/yr (semi-log) dominated by explosive eruptions of only modest volume (Swanson et al., 211, 212). Age-volume relations for the overall growth of, as modeled here (Table 4; Fig. 5), show that magma-supply rates as high as.2 km 3 /yr must be recent, intermittent, or both. Such a rate, if representative during the ~1 k.y. duration of main-stage tholeiite eruptions, would have yielded a volcano volume almost twice that estimated from geometric modeling. Early growth during the waxing-alkalic stage at is modeled to fit the estimated volume for this interval (~ km 3, during ka; Lipman et al., 26), so even a present-day rate of.2 km 3 /yr, constrained by a total volume of ~11, km 3, requires rapidly increasing average magma supply since 1 ka, with a convex-upward slope that projects toward higher future rates (Fig. 5B). Such a supply rate also seems improbably high to be applicable for the sustained-tholeiite stages at older volcanoes. As presently modeled, a volcano as large as Mauna Loa could maintain a rate of.2 km 3 /yr for at most 1 k.y., even if its sustained-tholeiite stage were relatively brief (~7 k.y.; see section on Mauna Loa, especially Fig. 12). Even a lower present magma supply, reaching.1 km 3 /yr at after 1 k.y. in the sustained-tholeiite stage, produces a growth rate as high or higher than modeled for a modestly asymmetrical TABLE 4. ALTERNATIVE KILAUEA GROWTH MODELS, AT 25 K.Y. INTERVALS A..1 km 3 /yr current rate B..2 km 3 /yr current rate Age (ka) Event Magma supply (km 3 /yr) Volume (1 3 km 3 ) Magma supply (km 3 /yr) Volume (1 3 km 3 ) 275 Inception Waxing alkalic Waxing alkalic Waxing alkalic Alkalic-transitional Transitional Transitional Transitional-tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Total: 11. Total: 11. Note: Italics indicate the interval of compositional transition; colors indicate times of sustained compositional uniformity km3/yr current rate.1 km3/yr current rate 2.2 km3/yr current rate.1 km3/yr current rate Age, ka Geosphere, October

9 Lipman and Calvert magma-time plot for other volcanoes (see sections on Kohala, Mauna Kea, and Hualalai, especially Figs. 9, 1, and 13; also Wise, 1982; Frey et al., 199; Garcia et al., 1995a; Lipman, 1995). More rapid onset of tholeiitic magma supply to rates as high as.2 km 3 /yr would require a strongly asymmetrical growth-time curve, relatively brief period of peak magma supply, and prolonged decline in supply rates. As an additional factor, the high supply rates estimated from geodetic and seismic data for rift extension during the past 5 years (Cayol et al., 2; Wright and Klein, 213) omit any component of passive flank motion and slumping driven by gravitational spreading (Fiske and Jackson, 1972; Borgia et al., 2; Morgan et al., 23; Byrne et al., 213). The model of Cayol et al. (2) infers average dike-induced rift opening of cm/yr, which seems unsustainable for the prolonged duration of the tholeiitic stage at. Such a dike-intrusion rate, if active since inception of tholeiite eruptions at ca. 1 ka, would have produced a zone km wide of 1% dikes along s proximal east rift. In contrast, the intense dike swarm forming >% (to 7%) of rock along the northwest rift at the deeply eroded Koolau volcano on Oahu, which appears geometrically analogous to, is ~1 km wide adjacent to its caldera and decreases to ~5 km width 15 km down rift (Walker, 1986, 1987). Because of these complexities and uncertainties, the preferred model for long-term magma supply at is that in Fig. 5A, gradually reaching a multi-thousand-year average of.1 km 3 /yr since inception of its main-tholeiite stage at ca. 1 ka or younger. Kohala Kohala is discussed before Mauna Kea because its overall growth history is better constrained by dating, providing a possible template for modeling early evolution of other Hawaiian volcanoes. Recent underwater studies provide unique information on early edifice growth at Kohala, long recognized as the oldest subaerial volcano on Hawaii, and show that this volcano is larger than previously thought (Table 2). Prior to the JAMSTEC-supported dives during , virtually all published compositional and age data for Kohala had been obtained on land, where mixed tholeiitic to weakly alkalic basalts (Pololu Volcanics) are capped by waning alkalic-stage lavas of the Hawi Vol canics (Stearns and Macdonald, 1946; Lanphere and Frey, 1987). However, many of the analyzed subaerial samples, especially tholeiites of the Pololu Volcanics, have been affected by varia ble to extreme alkali leaching and exchange (Lipman et al., 199). Potassium-argon ages that have sizable uncertainties suggest that exposed Pololu rocks range from greater than 45 to ca. 3 ka, and that the overlying waning-alkalic lavas were erupted from ca. 28 to 12 ka, possibly as recently as 6 ka (McDougall and Swanson, 1972; Sherrod et al., 27). Largely or entirely tholeiitic lower parts of the Pololu Volcanics become more compositionally diverse upward (Lanphere and Frey, 1987), recording a broad late-shield transition, here estimated at ca. 35 ka (28 ka in Moore and Clague [1992]). A prominent slope break at 1 11 mbsl, continuously traceable for at least 6 km around the north submarine flank of the volcano, records submergence associated with waning of the sustained-tholeiitic stage at ca. ka (Moore and Clague, 1992; Smith et al., 22) and shows that Kohala was once much higher than its present summit elevation of 1678 m. Two large submarine slope failures of Kohala s north flank, the Laupahoehoe and Pololu slumps, occurred late during the sustained-tholeiite eruptions, then were onlapped by younger lavas from Mauna Kea (Smith et al., 22). Only a few tholeiite lavas have been sampled from underwater slopes, but abundant turbidite sandstones from the submarine north flank (37 samples from 3 dives) have uniform tholeiitic glass compositions without intermixed transitional or alkalic compositions, providing an indirect record of a long-lived tholeiite stage at Kohala (Lipman and Calvert, 211; M.L. Coombs, 21, written commun.). The southeast-trending subaerial rift zone of Kohala is interpreted as continuing beneath Mauna Kea to reappear as the submarine Hilo Ridge (Figs. 1 and 6), as initially proposed by Holcomb et al. (2) based on correlation of submarine slope breaks. This interpretation, in contrast to the more common depiction of Hilo Ridge as a rift zone of Mauna Kea (Fiske and Jackson, 1972; Moore and Clague, 1992; Wolfe et al., 1997), is supported by a residual-gravity anomaly along the Hilo Ridge that pro jects more directly toward Kohala than toward Mauna Kea (Kauahikaua et al., 2) and by evidence for early inception of the ridge. Transitional to weakly alkalic pillow lavas that are overlain by tholeiitic picrite at the toe of Hilo Ridge (Fig. 7), interpreted to mark the change from waxingalkalic to tholeiitic volcanism, have yielded Ar/ Ar plateau ages of ca. 115 ± 35 ka (Lipman and Calvert, 211). The ridge also has an overall reverse magnetic direction, requiring the bulk of rift growth before 76 ka (Naka et al., 22, p. 46), much earlier than any dated tholeiitic lavas on subaerial Kohala (ca. ka). By analogy with the k.y. span interpreted for the waxing-alkalic stage at, inception of early-alkalic volcanism at Kohala is estimated at 13 ka (Table 5). These ages require the growth of Hilo Ridge before plausible inception of Mauna Kea related to semi-steady propagation along the Kea trend. For Hilo Ridge to be part of Mauna Kea would require rapid propagation from Kohala to Mauna Kea (at least 2 cm/yr, even if Kohala began as early as 1.5 Ma), then slowing greatly from Mauna Kea to (~5 cm/yr). The reinterpreted Kohala east rift zone, as inferred from the summit to the distal toe of Hilo Ridge, is the longest among volcanoes on Hawaii Island (135 km). In comparison, s east rift zone, including its submarine extension along Puna Ridge, is 115 km long. The only longer Hawaiian rift zone would be the east rift zone of Haleakala and its submarine continuation along Hana Ridge, with an overall length of 15 km. Hana Ridge offers an instructive geometric analog for evaluating size and geometry of the inferred rift-zone connection from subaerial Kohala to Hilo Ridge. This comparison can be illustrated (Fig. 8) by transposing major morphologic features of eastern Haleakala onto Kohala (present-day shoreline, submerged slope break at ~2 mbsl that marks decline in tholeiitic-stage eruptions, and approximate base of Hana Ridge adjusted on its north side for large-scale slumping). The distance from the present-day summits of the two volcanoes to submerged slope breaks along their ridge crests is similar (8 9 km), even though Hana Ridge continues 15 km farther underwater than the distal Hilo Ridge. The north-flank slope break is convex northward for both volcanoes, despite the presence of large submarine flank failures (Laupahoehoe slump for Kohala, Hana slump for Haleakala). The northward convexity along the northeast coast and submerged slope break of Hawaii results from younger infilling of lavas from Mauna Kea and also probably from late tholeiitic-stage Kohala (Smith et al., 22, their figure 4). The geometrically similar convexity on the north flank of Hana Ridge probably records continued lava accumulation along this originally subaerial segment after slope failure generated the Hana slump (Eakins and Robinson, 26). In contrast, the embayed south-flank slope break on Hana Ridge, which is asymmetrically close to the ridge crest, suggests that this side of the ridge was modified by late slope failures, and the resulting deposits are now concealed beneath younger rocks from the Island of Hawaii. Curvature of Hana Ridge appears somewhat less than that projected for the Kohala rift zone, but even so, the south flank of the transposed Hana Ridge would pass beneath Hilo and project beneath the summit of Mauna Kea Geosphere, October 213

10 Volcano growth on Hawaii A 4 S.L NW Paleo S.L. ( 1,1 m) Water Oceanic crust V.E. = 2.8 Curved longitudinal profile A-A, along crest of Kohala and its rift zones Summit Mauna Kea, N slope NW rift zone Kohala (subaerial) Hilo Ridge Kohala (submarine) Mahukona? 5 1 KM SE Section B-B Elevation, km 4 A A S.L No V.E. 5 1 KM B Km 5 MK summit H L Sea level 5 Shield tholeiite KOHALA-SA MK-SM KOHALA-SM Section A-A Figure 6. Cross sections illustrating interpreted long east rift of Kohala, onlapped by Mauna Kea (V.E. vertical exaggeration). Locations of sections are shown on Figure 1. (A) Arcuate longitudinal profile A A, along the crest of Kohala and its rift zones. Basal surface of Mauna Kea is constrained by the interpreted contact at the slope break at 11 meters below sea level along Hilo Ridge (Holcomb et al., 2). S.L. sea level. (B) Radial profile B B, from the summit of Mauna Kea to the northeast base of the island; compare Wolfe et al. (1997, their figure 3). H Hamakua Volcanics; L Laupahoehoe Volcanics; MK Mauna Kea; SA subaerial; SM submarine. - m slope break -1,1 m slope break V.E. = 5 Laupahoehoe slump 3 Km 1 Sea level B B L No V.E. Geosphere, October

11 Lipman and Calvert Figure 7. Bathymetric map of Japan Agency for Marine-Earth Science and Technology dive site K215, along pillow lavas on the south flank of distal Hilo Ridge, showing locations of dated transitional-alkalic basalt and overlying >6 m of tholeiitic picrite, and interpreted geologic relations (Lipman and Calvert, 211). Non-shaded areas are inferred to be largely exposed pillow basalt. Water depth contour interval is 1 m. Location shown on Figure 1. Age and Volume Dates from subaerial samples of the waningalkalic stage document a broad transition from tholeiitic eruptions at ca ka and probable termination of Kohala volcanism at ca. 12 ka (Sherrod et al., 27). Interpretation of Hilo Ridge as the distal east rift of Kohala (Holcomb et al., 2), in conjunction with ages of transitional-composition basalt at its toe (ca. 115 ka; Lipman and Calvert, 211), allows the first estimated duration for the sustainedtholeiite stage (~8 85 k.y.) of a Hawaiian volcano. This duration is substantially longer than that inferred from prior plate-motion models (~5 k.y. [Moore and Clague, 1992]; 6 k.y. [DePaolo and Stolper, 1996]). These results also imply that the 135-kmlong east rift zone developed to near-total length early during growth of Kohala and imply a volume (Table 2) substantially larger than the prior estimate of 36, km 3 (Robinson and Eakins, 26). Any geometrically simple topographic profile connecting Hilo Ridge to Kohala requires the rift zone to have been subaerial at shallow depth beneath the north flank of Mauna Kea, limiting Mauna Kea to a much smaller volcano perched on the south slope of the large Kohala rift zone (Fig. 6). Kohala would have begun largely or entirely on ocean floor as an elongate northwest-southeast edifice without significant interference from pre-existing volcanoes. Rapid early growth of Hilo Ridge (Lipman and Calvert, 211) suggests that at least distal parts of Kohala reached near-present size prior to major growth of Mauna Kea. Late interfingering of lavas from these volcanoes may have been relatively minor, as Kohala eruptions increasingly became focused closer to its present summit. A more modest addition to the total volume of Kohala results from reduced estimates for Mahukona, as discussed for that volcano. If the volume of Mahukona is ~6 km 3 (Garcia et al., 212), or if this construct were the distal Age (ka) TABLE 5. ALTERNATIVE KOHALA GROWTH MODELS, AT 1 K.Y. INTERVALS, CONSTRAINED BY ESTIMATED TOTAL VOLUME OF KM 3 Event A. Sustained-tholeiite magma supply B. High peak-tholeiite magma supply Magma supply (km 3 /yr).1 Volume (1 3 km 3 ) Cumulative (1 3 km 3 ) Magma supply (km 3 /yr).1 Volume (1 3 km 3 ) Cumulative (1 3 km 3 ) 13 Inception (alkalic) 12 Transition to tholeiite, ~ 115 ka Begin tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Sustained tholeiite Transitional volcanism, after ~ 3 ka Hilo Ridge submerged, after 13 ka Termination at 12 ka Note: Bold indicates best-constrained events and ages, and total cumulative volumes. Shading indicates duration of sustained-tholeiite stage Geosphere, October 213

12 Volcano growth on Hawaii 19 N 21 N K M-G HA Kiholo R M-C N Kona Slump 1 km Subaerial Hana Ridge HU KO ML MK Submarine Laupahoehoe Slump KL LO H Hilo Ridge Puna Ridge Figure 8. Diagram comparing the geometry of Hilo Ridge with Hana Ridge of Haleakala. Solid line is the constructional base of Hana Ridge; dotted line is the slope break marking the original submarine-subaerial shoreline at the end of shield growth; dashed line is the crest of the rift zone. South flank of Hana Ridge is embayed by landslide scars and partly covered by deposits of the Laupahoehoe Slump. Geometry is similar when these features of Hana Ridge are juxtaposed onto Kohala, Hilo Ridge, and the rift crest beneath Mauna Kea. Abbreviations as in Figure 1. west rift zone of Kohala onlapped by a Hualalai rift, then half or more of its previously estimated volume (15,5 km 3 ; Robinson and Eakins, 26) becomes part of Kohala. Based on a simple geometric model of elongate ellipsoidal prisms for Hilo Ridge and northwest rifts of Kohala, while retaining the smaller Mahukona estimate from Garcia et al. (212), a revised volume for Kohala is ~64, km 3 (Table 2; Appendix B, Table B2), nearly as large as previously estimated for Haleakala (69,8 km 3 ; Robinson and Eakins, 26). Of this increased Kohala volume, 75 km 3 was previously included with Mahukona and 2, km 3 with Mauna Kea (Hilo Ridge and landward continuation). Growth Model By these volume interpretations, Kohala is among the largest Hawaiian volcanoes. Its lifespan (~12 k.y.) and duration of main-tholeiitic stage (~8 85 k.y.) are relatively well constrained by the recent ages from the Hilo Ridge (Lipman and Calvert, 211), in conjunction with prior K-Ar dating of its late-alkalic stage and analogy with duration of the early-alkalic stage at. The Kohala ages provide the only measured duration for the main-tholeiite stage at a Hawaiian volcano, and simple geometric modeling of its growth as dominated by a mildly asymmetric trend of sustained tholeiite eruptions (Fig. 9A) yields peak magma-supply rates of ~.1 km 3 /yr that are similar to estimates of long-term historical rates at (Swanson, 1972; Dzurisin et al., 1984; Wright and Klein, 213). A rate this high can characterize only a fraction of Kohala s tholeiite stage; otherwise, its total volume would be even larger than the 64, km 3 estimated here (Table 2). An alternative growth model, in which magma supply becomes comparable to the.2 km 3 /yr peak rate during recent activity, produces a high-amplitude short-wavelength growth curve (Fig. 9B) that could persist for only a brief interval without exceeding the total volume of this volcano. Even Mauna Loa, with its greater total volume, yields a compressed growth curve at such magma production rates. Such a short-duration high magma supply would also be inconsistent with a large-diameter hotspot source (1 15 km), as commonly inferred for the Hawaiian chain from diverse geochemical and geophysical evidence (e.g., Ribe and Christensen, 1999; DePaolo et al., 21). No estimates have been published for volumes of the late-alkalic lavas at Kohala, but no more than a few hundred cubic kilometers seems likely, judging by the widely exposed transitional and tholeiitic flows low on subaerial slopes and absence of alkalic clasts or sand grains in landslide and turbidite deposits sampled on the north submarine flank during the JAMSTEC dives. For a volume of ~3 km 3, erupted between 35 and 12 ka, average late-alkalic magma supply Geosphere, October