Volcano growth and evolution of the island of Hawaii

JAMES G. MOORE U.S. Geological Survey, Menlo Park, California 94025 DAVID A. CLAGUE U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii 96718

ABSTRACT INTRODUCTION directed toward these submerged reefs and asso- ciated volcanic products from the region west of The seven volcanoes comprising the island The island of Hawaii and its submarine pedes- north Hawaii (Fig. 2). In this paper, we examine of Hawaii and its submarine base are, in tal are composed of at least seven independent compositional data on subaqueous lavas to iden- order of growth, Mahukona, Kohala, Mauna volcanoes (Fig. 1), each with a distinct growth tify the volcanic centers that produced the lavas, Kea, Hualalai, Mauna Loa, Kilauea, and history. These volcanoes were constructed and to trace the chemical evolution of these cen- Loihi. The first four have completed their above the Hawaiian mantle hot spot and then ters. This information is then combined with shield-building stage, and the timing of this carried northwest on the drifting Pacific litho- newly acquired 234U-238U ages (Ludwig and event can be determined from the depth of the spheric plate. Five of the volcanoes are presently others, 1991) for the drowned reefs in this re- slope break associated with the end of shield above sea level. The two submarine volcanoes, gion. The relation between lava erupted from building, calibrated using the ages and depths Loihi and Mahukona, and the submarine flanks the various volcanic centers and growth of the of a series of dated submerged coral reefs off of the other volcanoes have been investigated by dated reefs provides chronologic information on northwest Hawaii. The composition of lavas marine surveys. the growth and development of volcanoes which collected adjacent to these reefs helps to de- The sequence of the end of shield building coalesced to form the present island of Hawaii. fine the eruptive history of the various vol- or of "extinction of volcanoes" in the Hawaiian canic centers. The island of Hawaii has chain is generally from northwest to southeast. FORMATION OF SLOPE BREAKS grown at an average rate of about 0.02 This concept, first discussed by J. D. Dana in his AND DROWNED REEFS 2 km /yr for the past 600 k.y. and presently is report on the 1840 U.S. Exploring Expedition close to its maximum size. Mahukona com- (see Dana, 1890, p. 259), was based on the During the shield-building stage, when the pleted shield building about 465 ka; Kohala, greater dissection of volcanoes to the northwest volcanoes are most active, the shoreline is ex- 245 ka; Mauna Kea, 130 ka; and Hualalai, and the presence of younger-appearing volcanic tended rapidly by flowing lava, and the off- 130 ka. On each volcano, the transition from features and active volcanism to the southeast. shore slope is continuously mantled by sub- eruption of tholeiitic to alkalic lava occurs Stearns (1946) made use of such features and aqueously chilled and fragmented volcanic prod- near the end of shield building. The larger developed models of the growth of the islands ucts derived from the entry of fresh lava into volcanic systems (which stood more than 4 from Oahu to Maui. Wilson (1963) first pro- the sea. The difference in character between the km above their shoreline at the end of shield posed that this decrease in age to the southeast subaqueously chilled material deposited below building) change in composition before the resulted from northwest movement of the sea the shoreline, and the subaerial lava above, pro- end of shield building, and the smaller volca- floor over sources of lava fixed in the astheno- duces a marked steepening of the volcano flank noes change near or after the end of shield sphere, and McDougall (1964) quantified the below sea level. This coastal-slope break at the building, or never make the change to erup- age progression by K-Ar dating of lavas from subaerial-subaqueous transition is well shown tion of alkalic lava. the volcanoes. by the Hawaiian volcanoes currently in the ac- The rate of southeastern (south 40° east) Further constraints on the timing of volcano tive shield stage. Kilauea and Mauna Loa have progression of the end of shield building (and growth at the island of Hawaii are based on the average subaerial slopes of about 4° up to 1,000 hence the postulated movement rate of the ages of a series of drowned coral reefs that drape m elevation. Below sea level, this slope steepens Pacific plate over the Hawaiian hotspot) in the submarine flanks of five of the volcanoes. abruptly to an average of 13° at 500-m depth the interval from Haleakala to Hualalai is The first extensive drowned reef discovered on (Mark and Moore, 1987). When activity wanes about 13 cm/yr. Based on this rate and an the submerged flanks of the island of Hawaii at the end of shield building, and lava no longer average spacing of volcanoes on each loci line was found in 1983 off Kealakekua Bay (Moore flows across the shoreline in copious amounts, of 40-60 km, the volcanoes require about 600 and Fornari, 1984). Since then, six reefs off this primary slope break is maintained, and con- thousand years to grow from the ocean floor northwest Hawaii in a stairstep configuration, tinued isostatic subsidence carries it below sea (generally from a point on the southeastern the oldest being deepest, have been sampled and level. Hence, the age of drowning of this sub- submarine flank of the next older volcano of dated (Szabo and Moore, 1986; Moore and merged slope break provides a basis for dating the same loci line) to the time of the end of Campbell, 1987; Ludwig and others, 1991). the cessation of shield building. shield building. They arrive at the ocean sur- During 1987 and 1988, two dredge pro- Coral reefs are generally preserved on ma- face about midway through this period. grams and one submersible diving program were rine volcanoes at this latitude only after each

Additional material for this article (3 tables in appendix) may be secured free of charge by requesting Supplementary Data 9234 from the GSA Documents Secretary.

Geological Society of America Bulletin, v. 104, p. 1471-1484, 9 figs., 2 tables, November 1992.

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Figure 1. Bathymetry of southeastern part of Ha- waiian Ridge; contour inter- val is 1,000 m. Heavy lines indicate the two lineaments, Kea and Loa, of volcanic centers (solid circles) which include Waianae (W), Koo- lau (KU), West Molokai (WM), East Molokai (EM), Lanai (L), Kahoolawe (K), West Maui (WI), Haleakala (HA), Kohala (KO), Mahu- kona (M), Hualalai (H), Mauna Kea (MK), Mauna Loa (ML), Kilauea (KI), Loihi (Lo), and hypothetical volcano Keikikea (Ke). Ho- honu seamount (Ho) is also shown. Areas of Figures 2 (north) and 6 (south) are boxed.

volcano has completed its shield-building cial ice on the continents. At the onset of a from volcano loading. The deepest (and old- stage. During the shield-building stage, the glacial period, the fall in sea level was com- est) terrace represents the first reef to form rapid advancement of the shoreline, and the monly about 1-3 mm/yr as water was re- following cessation of shield-building activity. mantling of the offshore slope by volcanic moved from the ocean. This rate of fall was Six coral reefs on the gentle subaqueous products inhibit reef growth. Waning of erup- about equal to the rate of isostatic subsidence slope west of north Hawaii (Fig. 2) have pro- tive activity at the end of shield-building of the active part of the Hawaiian Ridge duced topographic benches which range from causes more passive shoreline conditions that (Moore, 1987), producing a stable shoreline a few kilometers to about 10 km in width, and favor the growth and preservation of coral that favored the growth of large coral plat- are bounded seaward by a scarp or reef face as reefs. Hence, the reefs occur only shallower forms. At the end of a glacial period, the com- much as 200 m high (Fig. 3). Because of re- than the primary slope break related to the bined effects of subsidence and eustatic sea- gional tilting, the depths of the reefs are end of shield building. level rise caused by melting of the icecaps somewhat variable along their length; reef Hawaiian volcanoes undergo rapid subsid- resulted in submergence that exceeded the depths reported in Table 1 are average depths ence during their growth because of the load upward growth rate of coral and the reef of the top of the reef face rounded to the near- they impose on the lithosphere. During the drowned. Hence, the stair-step flights of coral est 5 m. The six reefs have been sampled by Pleistocene, this subsidence occurred simul- reef terraces are produced when sea-level submersible and dredging and dated by the taneously with eustatic fluctuations in sea changes caused by a sequence of glacial peri- 234y/238(j method. Determined ages range level caused by addition and removal of gla- ods modulate the steady subsidence resulting from 18 ± 5 ka for the shallowest reef (reef 1,

1472 Geological Society of America Bulletin, November 1992

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/104/11/1471/3381391/i0016-7606-104-11-1471.pdf by guest on 02 October 2021 Figure 2. Bathymetry in region west of north Hawaii (see Fig. 1 for location) from NOAA multibeam survey for area south of 20°N lat. and from Campbell (1987) for area to north; contour interval is 200 m. Shaded reef faces are designated by circled numbers, boundaries between lavas from named volcanoes, by heavy dashed lines; submersible bottom tracks, by beaded lines; dredge sites, by solid circles; northwest rift zone of Hualalai volcano, by dashed shaded line; upper boundary of North Kona slump, by dotted line; and Alika phase 2 debris avalanche, by pattern and light dashed line.

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E W (1989) reported only a single glass analysis. Be- o cause of bathymetric relations, however, it is un- likely that we have sampled any Kohala lavas Figure 3. Profiles of reef draping the terraces offshore. terraces west of north Ha- The two deepest reefs are draped with lavas

-500 waii from Campbell (1987). from Mahukona Volcano (Clague and Moore, E Profiles are along parallels of 1991a). All of these samples are tholeiitic basalt latitude 2 minutes (3.7 km) (D20, D25, P5-65, and P5-75 along reef 6; and .e a. apart: 20°00', dotted line; D21 and P5-73 along reef 5; Fig. 2) and are o a 20°02\ dashed line; 20°04', geochemically similar to the tholeiitic lavas re- -1000 solid line. Arrows indicate covered along the rift axis deeper than the main depth of numbered reefs slope-break. We conclude that the summit of as shown in Figure 2 and Mahukona is located east of reef 5 (-1,145 m), Table 1. but west of reef 4 (-925 m), which is draped by lavas that are geochemically more similar to -1500 0 10 20 30 40 50 lavas of the Hamakua Volcanics of Mauna Kea Distance, km volcano. Mauna Loa lavas occur along the shoreline for about 15 km between Mauna Kea lavas to TABLE 1. DEPTH AND AGE FOR REEFS ers, 1989), and Hualalai (2 whole-rock analyses, the north and Hualalai lavas to the south. Off- Moore and others, 1982; 3 glass analyses; Garcia shore from these Mauna Loa lavas, the -150-m Reef no. Depth (m) Model age (ka) U age (ka) and others, 1989; 5 whole-rock analyses, Wright reef (18 ka) is still easily discerned in the bathym-

1 -150 18 17, 19 and Clague, 1989). New glass and whole-rock etry, indicating that few flows crossed the 2 -430 130 133 analyses of lavas dredged from the submarine reef. We have not sampled flows in this area, but 3 -695 245 225, 226, 276 1 4 -925 340 314, 287 rift zones of Mauna Loa are given in Table A , we interpret the bathymetry to indicate that 5 -1,145 430 406,360, 475 6 -1,335 463 of Mauna Kea in Table B, of Hualalai in Table Mauna Loa flows have been entering the sea via C, and of Mahukona in Clague and Moore the saddle between Mauna Kea and Hualalai (1991). Analyses of submersible-collected sam- Volcanoes only since about 18 ka. Farther south ples draping the -695-m and -925-m reefs are where the -150-m reef crosses the Hualalai rift Fig. 2) at 150-m depth to 463 ± 8 ka for the given in Table 2. Several parameters may be zone (Dive 67; Fig. 2), no lava flows were ob- deepest reef (reef 6, Fig. 2) at 1,335-m depth used to distinguish the lavas from different vol- served crossing the carbonate reef. About 20 km (Ludwig and others, 1991). Reef ages indicate canoes, but we have found that a population of farther south, near Kailua, the reef is draped by that this region has subsided at a relatively samples simply plotted on a K2O versus Si02 alkalic basalt flows from Hualalai (Moore and constant rate of 2.6 ± 0.4 mm/yr since 463 ka. diagram is generally distinctive. The data from Clague, 1987). About 8 km south of Kailua, at the different rift zones are presented in Figure 4. Keahou Bay, the lavas draping this reef change PETROLOGY AND VOLCANO The tholeiitic lavas from Mauna Loa and Huala- to tholeiitic flows from Mauna Loa Volcano BOUNDARIES lai are I0W-K2O, high-Si02 lavas indistinguish- (Moore and Clague, 1987). able from one another. The Mauna Loa glass Tholeiitic picrite lavas draping reef 2 at -430 compositions include more fractionated samples The Kohala Terrace is covered by lavas m (P5-66 and P5-70; Fig. 2) have been collected than have been sampled from Hualalai, and the erupted from Mahukona, Kohala, Hualalai, along the axis of the Hualalai rift zone, thereby Hualalai lavas include more common picritic Mauna Kea, and Mauna Loa Volcanoes. The indicating that Hualalai Volcano was still erupt- compositions that extend to higher whole-rock lavas from these different volcanoes are geo- ing tholeiitic lava as recently as 130 ka. South of MgO contents than seen for Mauna Loa. The chemically distinctive, with a few exceptions, so Kailua, alkalic lavas from Hualalai and tholeiitic tholeiitic glasses for Mauna Kea and Mahukona that lava chemistry can generally be used to infer lavas, probably from Hualalai, but possibly from are easily distinguished from those of Hualalai which volcano has been sampled, and to deter- Mauna Loa, drape the -430-m reef. These rela- and Mauna Loa by their higher K2O at a given mine volcano boundaries. The least equivocal tions suggest that Hualalai Volcano erupted tho- Si02 content, but they are not simple to distin- way to establish the compositions of lavas from leiitic basalt of the shield stage until about 125 guish from one another. In general, the Mauna each volcano is to analyze samples erupted ka, and alkalic lavas since slightly after that time. Kea lavas include compositions that have lower along the rift zones or at the summit where there Clague (1987) presented K-Ar ages of trachyte Si02 and higher K2O than those which occur in is less uncertainty as to which volcano has been from Hualalai that demonstrate that Hualalai the Mahukona lavas. The population of compo- sampled. was erupting strongly fractionated alkalic lavas sitions must be considered rather than any single Analyzed submarine samples from the rift by 105 ka. analysis to utilize this simple criterion. Unfortu- zones of Mauna Loa, Mauna Kea, Hualalai, and The next deeper reef 3 on the Kohala Terrace nately, we have no lavas from the submarine rift Mahukona Volcanoes supplement previously (-695 m; Fig. 2) is draped by tholeiitic lavas, of Kohala Volcano, and Garcia and others published whole-rock and glass analyses of some of which match lava from Hualalai, and submarine samples from near the rift zones of others match lavas from Mauna Kea. The three Mauna Loa (2 whole-rock analyses, Moore, southernmost samples are tholeiitic basalt from 1966; 12 glass analyses; Garcia and others, Hualalai (P5-76-6, 8, 9; Table 2), whereas 'Tables A, B, and C may be secured free of charge 1989), Mauna Kea (3 whole-rock analyses, by requesting Supplementary Data 9234 from the farther north, the samples from dredges M9 and Moore, 1966; 10 glass analyses, Garcia and oth- GSA Documents Secretary. M8 are transitional to tholeiitic basalt from

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TABLE 2. ANALYSES OF LAVAS DRAPING THE 925-m AND 695-m REEFS the Mauna Kea-Hualalai and Mauna Kea-Ma- hukona boundaries. Glass analyses

Sample P5-76-1 P5-76-4.5 P5-76-6 P5-76-8 P5-76-9fp P5-76-9 23-1 23-3 23-4 23-5 Sulfur Content of Sampled Lavas Depth (m) 1,245 1,005; 940 910 800 780 780 1,160 1.160 1,160 1,160 Volcano* MK MK H H H H MK MK MK MK Submarine-erupted lavas from along the rift 48.4 49.3 Si02 51.0 50.5 52.5 51.9 51.7 52.4 50.3 48.9 zones contain variable S contents. Those from AI2O3 14.2 13.2 13.4 15.1 16.0 13.9 13.7 13.3 13.7 13.3 FeO 11.3 13.3 11.2 9.81 10.5 11.5 12.0 12.0 12.2 12.10 the Mauna Loa southwest rift zone contain MnO 0.17 0.19 0.17 0.14 0.16 0.18 0.19 0.17 0.19 0.19 MgO 6.46 5.42 5.83 6.90 5.03 6.24 6.36 6.41 5.90 6.34 0.041-0.084 wt% S, whereas lavas interpreted to CaO 10.5 9.89 10.8 11.6 10.5 10.3 11.1 10.8 11.1 10.90 be subaerialiy erupted and degassed contain Na20 2.45 2.64 2.44 2.41 2.70 2.44 3.06 3.03 3.22 3.06

K2O 0.56 0.65 0.36 0.29 0.36 0.41 0.71 0.63 0.70 0.64 0.003-0.020 wt% S. On Mauna Loa, all but one 0.39 0,44 P2O5 0.40 0.41 0.22 0.19 0.32 0.28 0.33 0.40 of the lavas collected from 680 m or shallower Ti02 3.10 3.76 2.40 1.91 2.37 2.40 2.92 2.76 3.04 2.79 S 0.019 0.031 0.006 0.022 0.002 0.014 0.015 0.015 0.019 0.017 are subaerialiy degassed, whereas all but one Total 100.2 100.0 99.3 100.3 99.6 100.1 100.7 98.4 98.9 99.0 sample from 1,100 m or deeper contain higher S contents (Table A). The submarine-erupted Sample 23-8 24-2 P5-69-1.2 P5-69-3,5b 22-1 22-2 22-3 M10-3T M9-3+ Depth (m) 1,160 1,170 910:950 1,110; 980 1,130 1,130 1,130 920 835 glasses from the Hualalai northwest rift zone Volcano* MK MK MK MK MK MK MK MK MK (Table C) contain 0.023-0.063 wt% S, whereas the subaerialiy degassed samples contain 0.004- sio2 48.3 50.4 50.4 50.2 49.7 48.2 50.6 50.7 49.3

AI2O3 14.1 13.4 12.8 13.1 13.4 14.6 13.2 13.0 13.1 0.018 wt% S. All samples from shallower than FeO 11.9 13.1 13.8 13.0 13.2 11.3 13.0 12.6 12.8 MnO 0.19 0.19 0.19 0.19 0.19 0.16 0.18 0.21 0.20 860 m were subaerialiy erupted, whereas only 4 MgO 6.21 5.16 5.38 5.68 5.56 7.55 5.80 5.71 5.83 of 20 flow units from deeper than 935 m were CaO 11.2 9.60 9.75 10.1 9.90 11.2 10.2 10.4 11.0

Na20 3.13 2.60 2.57 2.58 2.55 2.61 2.64 2.27 2.54 subaerialiy degassed. Many of the submarine- K2O 0.65 0.71 0.66 0.62 0.61 0.42 0.60 0.54 0.58 erupted samples contain surprisingly low S con- P2°5 0.39 0.46 0.43 0.39 0.39 0.29 0.38 0.36 0.38 Ti02 2.88 3.62 3.76 3.53 3.52 2.66 3.50 3.26 3.32 tents and may have been erupted in shallow s 0.011 0.014 0.025 0.031 0.034 0.058 0.028 Total 99.0 99.3 99.8 99.4 99,1 99.0 100.1 99,0 99.1 water. All of the glasses from the Mauna Kea east rift zone (Table B) were submarine erupted Whole-rock analyses and contain 0.034-0.123 wt% S. On Mahukona

Sample P5-69-3 P5-69-7 P5-76-I P5-76-2 P5-76-3 P5-76-6 P5-76-7 P5-76-8 24-1 Volcano, the lavas erupted below the slope- Depth (m) 1,110 945 1,245 1,180 1,085 910 865 800 1,170 break on the west rift zone contain 0.048-0.116 Volcano* MK MK MK MK MK H H H MK wt% S. Lavas draping the slope-break and the

SiOz 48.9 45.5 46.2 49.0 48.8 48.1 45.2 49.5 47.6 -1145-m reef, however, include subaerialiy de- AI2O3 12.00 13.90 7.78 13.10 12.1 9.68 6.48 11.40 10.10 gassed samples containing <0.02 wt% S, and a Fe20, 1.03 2.04 1.41 1.38 1.58 1.39 1.43 1.86 13.00 FeO " 10.41 10.13 10.70 9.65 10.19 10.09 10.23 9.57 group of samples with intermediate S contents MnO 0.17 0.17 0.17 0.16 0.17 0.17 0.17 0.17 0.17 MgO 12.50 9.61 22.90 11.00 12.1 19.30 29.30 14.10 16.50 (0.028-0.057 wt% S) indicative of eruption in CaO 10.30 11.20 7.18 11.00 10.4 7.83 5.16 9.32 8.98 NajO 1.95 2.49 1.31 2.07 2.04 1.55 1.10 1.84 1.71 shallow water.

K20 0.38 0.27 0.28 0.36 0.39 0.22 0.13 0.25 0.36 P20, 0.23 0.34 0.17 0.24 0.26 0.14 0.10 0.18 0.24 Most of the lavas draping the -695- and Ti02 2.29 2.44 1.65 2.24 2.43 1.48 0.95 1.76 2.23 H20* 0.29 0.67 0.21 0.18 0.25 0.19 0.14 0.27 -925-m reefs were subaerialiy degassed and 0.06 H20" 0.03 0.26 0.02 0.02 0.02 0.05 0.05 contain <0.022 % S. In particular, tholeiitic co2 <0,01 0.58 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 Total 100.49 99.60 99.99 100.41 100.77 100.17 100.45 100.28 100.89 flows from Hualalai Volcano that drape the -695-m reef are all subaerialiy degassed. A *H = Hualalai; MK ~ Mauna Kea. ^Analyses courtesy of James Natland. number of samples from Mauna Kea Volcano that drape the -925-m reef, however, have in- termediate S contents (0.025-0.058 wt% S) that probably reflect eruption in shallow water. Mauna Kea Volcano. Reef 4 (-925 m, Fig. 2) is the underlying reef. An example of this is the These data suggest that a west rift zone from draped by tholeiitic and transitional lavas, both Waha Pele flow (Hualalai volcano, 710 ± 150 Mauna Kea Volcano, now buried by younger apparently from Mauna Kea Volcano. These yr B.P.; R. B. Moore and others, 1987), the dis- flows and terraces, existed in the vicinity of samples include samples P5-76-4 -5; all lavas tal end of which reached only 3 km off the dredge 22 and dive 69 (see Fig. 2). from P5-69; dredges 22,23, and 24 from cruise present coast, where mapped in a series of sub- F288HW; and dredge 10 from the Melville. An mersible dives (Moore and Clague, 1987). Addi- GROWTH OF VOLCANOES additional sample from below the -925-m reef tionally, along-strike changes in the character of COMPRISING THE ISLAND near its southern end (sample P5-76-1) is also a the reefs as shown on bathymetric charts have OF HAWAII tholeiitic basalt from Mauna Kea Volcano. been utilized to define boundaries between The boundaries between the lavas originating volcanoes. Haleakala Volcano from the different volcanoes (Fig. 2) are based These non-geochemical criteria provide on the identifications using these compositional much of the data to define the Kohala- Haleakala volcano, which comprises east differences. Their positions are also dependent Mauna Kea, Kohola-Mahukona, Hualalai- Maui (Fig. 1), is the youngest volcano north of on the observation that lava flows do not flow Mahukona, Mauna Loa-Mauna Kea, and the Island of Hawaii, and data on its age are far into the sea, and that the lavas draping the Mauna Loa-Hualalai boundaries (Fig. 2). Geo- reviewed preliminary to examining the growth reefs are therefore thought to be close in age to chemical criteria are most important to define of volcanoes comprising the island of Hawaii.

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1.0 -i—•—i—<" i T • 1 • 1 1.0 + glass + -945 m glass o Mauria Kea • o whole-rock -945 m whole-rock 0.8 X -695 m glass 0.8 * Garcia etal. (1988) s + Zi 0.6 - + - & ++ I( • 0.6 ~ ì 0 + . - - + k f o 0-4 04 CM °CM O 0

1 1 1.0 + -1335 m glass o -1335 m whole-rock 0.8 X -1145 m glass • » * -1145 m whole-rock 0.6 x 0.6 ** s. + Ì O 0.4 - 0.4 o CM •Hi* 04 * 0.2 - 0.2 Mahukona Mauna Kea o I.I. o.o 0.0 43 45 47 49 51 53 47 49 53 43 45 47 49 51 53

Si02 (wt.%) Si02 (wt.%) Si02 (wt.%)

Figure 4. K20-S!02 diagrams for dry, reduced, normalized glass and whole-rock analyses of samples from the submarine rift zones of Hualalai, Mauna Kea, Mauna Loa, and Mahukona volcanoes and for lavas draping reefs on the Kohala terrace.

The oldest volcanic rock dated by K-Ar on Ha- Haleakala apparently represents eruption of lava west, and drops off steeply on the northwest and leakala is a 1100 ka tholeiitic lava flow of the in shallow water that produced littoral explo- southwest flanks of Mahukona volcano (Figs. 2 Honomanu Basalt exposed in Honomanu Gulch sions and mantled the nearshore area with glassy and 3). We interpret this to have been the west- on the north side of the volcano (Chen and oth- volcanic ejecta. This notion of post-H terrace facing shoreline of Mahukona when the island, ers, 1991). (post-850 ka) tholeiitic volcanism is contrary to which was at least 30 km in diameter, ceased K-Ar dating indicates that tholeiitic and al- the on-land dates, which indicate the final expanding when copious lava flows no longer kalic lavas of the Honomanu Basalt are interca- changeover to alkalic volcanism occurred at crossed its shoreline (Clague and Moore, lated over the period 1100 to 970 ka, after 970 ka. Possibly the shallow-water tholeiitic 1991a). Coral collected from reef 6 yields a which the lavas remained alkalic (Chen and volcanism of the east rift was not duplicated (or 234U/238U age of 463 ± 8 ka (Ludwig and oth- others, 1991). The existence of an alkalic lava not represented) at the on land site, or the U ers, 1991), which marks the end of shield build- flow beneath the dated 1100 ka tholeiitic flow series ages are imprecise because they are near ing of Mahukona volcano (Clague and Moore, reveals that the beginning of the shift from tho- the upper age limit of this technique. We provi- 1991b). Mahukona first emerged above sea level leiitic to alkalic lavas began somewhat prior to sionally assume that the U-series ages are too about 800 ka, considerably before its subaerial 1100 ka, but the timing of the initial changeover young, and that the H-terrace and end of shield shield building ceased. to alkalic lava is unknown. The volcano erupted building are somewhat older than 850 ka, here The full eastern extent of the original Mahu- both tholeiitic and alkalic lava for at least assumed to be 950-1000 ka. kona volcano is not known because of burial by 130 k.y. lava flows from Kohala and Mauna Kea. The A tilted coral reef submerged 1,400-1,800 m Mahukona Volcano -1,145-m coral terrace (Fig. 2; reef 5), about 10 on the east rift zone of Haleakala yielded a km east of the -1,335-m slope break, parallels 284U/238U Evidence for the earliest emergence of a vol- the slope break and clearly defines a 430 ka age of 750 ka (Moore and others, canic edifice above sea level adjacent to the Mahukona paleoshoreline that is 190 m higher 1990a). Extrapolation of average subsidence present island of Hawaii is a reef terrace about than reef 6 (Fig. 5). Reef 5 is draped by tholeiitic rates suggests that a major slope change 300 m 30 km long (reef 6; Fig. 2) on Mahukona sub- lavas, indicating that tholeiitic volcanism per- deeper than the reef (the H terrace of Moore, marine volcano (Clague and Moore, 1991) now sisted at least to 430 ka. 1987) formed about 850 ka (Moore and Camp- lying at a depth of about -1,335 m (Moore and The reef terraces at -925- and -695-m depth bell, 1987; Moore and others, 1990a). The H Campbell, 1987). This is the deepest abrupt (reefs 3 and 4; Fig. 2) are poorly developed terrace, which can be traced around much of slope change identified on the flanks of the is- along the northern half of their extent, where, east Maui, is regarded as marking the end of land of Hawaii and is 335 m deeper than a slope based on the bathymetric gradient, they lie on shield building of Haleakala volcano. Tholeiitic change on Kohala volcano, the oldest subaerial the western flank of Kohala volcano (Fig. 2). hyaloclastite (recovered in dredge hauls D34 volcano on Hawaii. Reef 6 is 25-40 km west of The surface on which the southern extent of and D35, Moore and others, 1990a) from the north cape of the island, is convex to the these reefs formed is either an original Mahu- slightly above the H terrace on the east rift of

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kona surface or an early Mauna Kea slope. It is slightly greater than that of the -925-m reef (340 flat-floored canyons of northeast Kohala, as well more likely a Mauna Kea surface, because of the ka) exposed best southwest of Kohala (reef 4; as the 20-km-wide indentation in the coastline north-northeast reef trend which conforms clos- Fig. 2), suggesting that the age of the end of marking the head of the slide. est to possible paleoshorelines of Mauna Kea. shield building is somewhat greater than 340 ka. The Pololu landslide formed a series of pull- The last known eruption of Mahukona built a Coral recently collected near this slope change at apart basins along its headwall near the summit large submarine cone, or cluster of cones, -950 m off the north cape of Kohala (Jones, of Kohala Volcano. The basins served as struc- (dredge site 19; Fig. 2) of transitional basalt on 1991), however, yielded a 234U/238U age of tural traps for the flows that erupted after the its west rift zone (Clague and Moore, 1991a), 259 + 6 ka (K. Ludwig and B. Szabo, 1991, landslide occurred. The younger of the pre- the summit of which is now at a depth of 1,065 written commun.) and an electric spin resonance landslide flows are part of the Pololu Basalt of m as recorded by Seabeam bathymetry from the (ESR) age of 247.5 ± 37 ka (A. T. Jones, 1991, the tholeiitic shield stage and are certainly R/V Thomas Washington, October 1991. Gar- written commun.). These data imply that the age younger than 400 ka and perhaps as young as cia and others (1990 and 1991) wrote that they of the coral and slope break correlate better with 306 ka (McDougall and Swanson, 1972; G. B. believed that this is the summit of Mahukona reef 3 (695 m deep southwest of Kohala; Fig. 2) Dalrymple, unpub. data). Other flows from the volcano primarily because they interpret a set of with a model age of 245 ka (Table 1) than with Hawi Volcanics are as old as 254 ka. The ages of ridges that apparently radiate from the cone as reef 4, and this age is tentatively accepted as the these flows bracket the age of the landslide be- reflecting rift zones radiating from the volcano date of the end of Kohala shield building. The tween 400 ka and 200 ka, and perhaps between summit. On the other hand, we (Clague and fact that the reef north of Kohala is about 230 m 306 ka and 254 ka. More detailed work deter- Moore, 1991a, 1991b) have suggested that the deeper than the reef of the same apparent age mining the ages of the oldest Hawi flows and original summit of Mahukona volcano is some southwest of Kohala is perplexing. Perhaps be- vents inside the basins and the youngest of the 30 km east, probably between reefs 4 and 5, cause the north Kohala reef site is almost di- pre-landsliding Pololu Basalt flows could more because such a location is consistent with the rectly on the axis of the Hawaiian ridge, it has tightly bracket the age of the landslide. position and age of reef 6, which is interpreted as undergone much more subsidence than the the paleoshoreline of Mahukona at the end of its southwest site, which is about 30 km west of the Mauna Kea Volcano shield building (see Fig. 2). axis. It is also possible that the coral collected off the north cape of Kohala was from a shallower The low sulfur (Clague and Moore, 1991a) Comparisons with the vertical growth rate of reef and had moved downslope prior to collec- and low CO2 (<5 ppm, D. A. Clague and J. E. Kilauea and Mauna Loa suggest that Mauna tion. This disparity in depths of reefs of about Dixon, unpub. data) content of the transitional Kea began on the ocean floor about 750 ka the same age emphasizes the need for more basaltic glass from the -1,065-m cone of Mahu- (Frey and others, 1990). Subaerial lavas belong- sampling and dating of the drowned reefs adja- kona (dredge 19; Fig. 2) suggest that eruption ing to the Hamakua Volcanics, the oldest sub- cent to Hawaii. from this cone occurred in shallow water (< 120 aerial volcanic unit on Mauna Kea, range in

m based on C02 content) and therefore before Both reefs 3 (-695 m, 245 ka) and reef 4 K-Ar age from 80-240 ka (Frey and others, development of the -925-m-deep reef 4, which (-925 m, 340 ka) are not defined by existing 1990; Wolfe and others, in press) and include drowned 340 ka. Hence we estimate that bathymetry on the west flank of Kohala primarily alkalic basalt with some intercalated Mahukona volcano first emerged above the sea (Campbell, 1987), whereas reefs 1 (-150 m, 18 transitional basalts in the lower section. No sub- about 800 ka, ceased voluminous tholeiitic ka) and 2 (-430 m, 130 ka) are well developed aerial section has been found that exposes pre- shield building about 463 ka, and ceased tholei- (Fig. 2 and Campbell, 1987). This may occur alkalic tholeiitic lavas. The alkalic basalts in the itic eruptions after 430 ka. The ca. 405 ka erup- because of the post-reef 2 tilting of reefs 3 and 4 Hamakua Volcanics are as young as 80 ka and tion that produced the transitional basalt cone as a result of the greater subsidence of the axial were followed by eruption of mainly hawaiitic defines our estimated age for the tholeiitic to part of the ridge discussed above. lavas of the Laupahoehoe Volcanics. This shift alkalic transition. Kohala, as well as the other volcanoes, was from alkalic basalt to hawaiite marked a further much larger above sea level at the end of shield decline in the eruptive output of the volcano. Kohala Volcano building than at any other time in its history The volcano probably appeared above sea (Fig. 5). Its narrow dimension (in the northeast level on the flank of Kohala somewhat before Kohala volcano probably emerged above sea direction) was then more than 50 km as com- 400 ka, and by 340 ka had merged with Kohala level on the east flank of Mahukona volcano pared to the present 20 km. Slightly before the volcano and built a submerged platform sur- before 500 ka. The average K-Ar age of the end of shield building, a gravity failure on the mounted by reef 4 (-925 m; Fig. 2) onlapping three oldest lavas (from the Pololu Basalt, the northeast flank of the volcano produced a major Mahukona volcano. Reef 4 drowned at 340 ka oldest volcanic unit on Kohala) is 430 ± 20 ka landslide. This slide, the Pololu debris ava- and is draped by lavas, including transitional (McDougall and Swanson, 1972). The change lanche, was 20 km wide at the shoreline and Mauna Kea lavas (such as 23-8, Table 2). The from tholeiitic to alkalic lava occurred between traveled 130 km into the Hawaiian Deep (Fig. 5 low-S nature of these lavas indicates that they 300 and 260 ka (estimated at 280 ka), and the and Moore and others, 1989). The head of the were subaerially erupted, and as lavas do not last of the alkalic lavas was erupted about 60 ka slide reached to the summit of the volcano, flow far across a shoreline, they post-date (McDougall and Swanson, 1972; Clague and which at that time was about 2.7 km above sea formation of the reef only slightly. Hence, the Dalrymple, 1987). level. A series of deep canyons were eroded sub- volcano had already begun its tholeiitic-alkalic When the volcano stopped its active shield aerially in this landslide amphitheater, probably transition by about 340 ka. Subaerially erupted building, an abrupt sea-level slope change was because it was oversteepened and because the tholeitic and transitional Mauna Kea lavas drape created that is preserved on the northwest and protective vegetation was stripped away. Subsid- reef 3 (-695 m, 245 ka, dredges 8 and 9; Fig. 2), north submarine slopes at a depth of about ence of the volcano and alluviation of these indicating that the transition from tholeiitic to 1,000 m. The depth of this slope change is canyons have produced the present-day giant, alkalic lava was not completed by that time.

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(40-16 ka) (Porter and others, 1987; and Wolfe and others, in press). Pohakaloa glaciation oc- curred after the end of the eruption of the Hamakua volcanics, and the drift contains no Laupahoehoe lava fragments. Pohakaloa glacia- tion probably predated the end of shield build- ing when Mauna Kea had not undergone its last 375 m of subsidence. At that time, the summit probably stood about 4,500 m above sea level. About 15 eruptions of Laupahoehoe volcanics have occurred on Mauna Kea in the past 10 k.y., several since a distinctive soil (Humuula Soil) with a 14C age of 4,500 yr was formed. The most recent eruption from Mauna Kea was about 3,300 yr ago (Porter and others, 1987).

Hualalai Volcano

Hualalai volcano apparently grew above sea level somewhat before 300 ka on the southwest flank of Mauna Kea at about the same time that Mauna Loa appeared on the south flank. The growth of Hualalai is uncertain because of burial by lavas from the more vigorous Mauna Loa, and because of the gravitational failure of a large part of the southwest flank (the North Kona landslide; Fig. 2 and Moore and others, 1989). Figure 5. Six stages in the growth of the island of Hawaii over the past half million years Reef 1 (-150 m, 18 ka) and reef 2 (-430 m, 130 on present-day base map with bathymetric contours showing depth in kilometers. Island ka) drape the northwest rift zone ridge of Huala- growth is shown at 100 ka interval with shoreline and volcano boundaries (heavy shaded lai and provide some information on its growth lines), vigorous subaerial volcanic centers (solid stars), waning subaerial volcanic centers history. Profiles down the crest of the rift-zone (open stars), dormant or feebly active subaerial volcanic centers (open circles), axis ridge show a marked inflection with steeper ter- of Hawaiian Deep (heavy, dashed line), landslides (stippled pattern). Volcano notation as in rain below and west of about 430-m depth (Fig. Figure 1. 2 of Clague, 1987), indicating that shield build- ing ceased about 130 ka. The -430-m reef, al- though continuous across the rift zone, however, Hence, the transition period from tholeiitic to steep subaqueous slope increase at sea level did is draped by several flows of tholeiitic picrite alkalic stages probably began somewhat before not develop. (Table C: P5-66-3, -4, -5; P5-70-1, -6, -7) that 340 ka to approximately the age of the oldest By about 225 ka, the shoreline had receded indicate that tholeiitic eruptions continued to subaerial lavas of the Hamakua Volcanics, about on the west due to submergence, but it expanded sometime after 130 ka. 240 ka. on the east as lava crossed and extended the Lavas dredged from the northwest rift zone are The submarine east rift zone of Mauna Kea shoreline. Copious flow of lava to the west was all tholeiitic basalt (Clague, 1987; and Table C). became inactive long before the volcano died. perhaps partly blocked by a growing Hualalai Exposed subaerial lavas on the volcano, how- Similarly, the submarine rifts of Hualalai and volcano (Fig. 5). Mauna Kea reached its greatest ever, are alkalic basalt; evidence from drill holes Mauna Loa volcanoes seem presently to have subaerial extent on the east when it stopped indicates that trachyte dated at 100-105 ka oc- become inactive, or to have greatly reduced ac- vigorous shield building. This is marked by a curs between the tholeiitic and alkalic basalts tivity, perhaps foretelling the end of summit ac- prominent paleoshoreline or break in slope (Clague, 1987). Hence the transition from tho- tivity. Lavas collected from below 1,280 m on which has now subsided as a major submarine leiitic to alkalic lava occurred between 130-105 the Mauna Kea east rift zone include both tho- terrace to a depth of 375 ± 15 m depth. The age ka, estimated here at 115 ka. Hualalai is still an leiitic and transitional types (Table B), and be- of this slope break, and hence the age of the end active volcano, having last erupted alkali basalt cause virtually none of the subaerial Hamakua of shield building, is about 130 ka based on the in 1800-1801. Volcanics is tholeiitic, the submarine rift lavas similar depth of reef 2 (-430 m) on the other The North Kona slump (Fig. 2) on the probably predate the oldest dated Hamakua side of the island. The end of shield building at southwest flank of Hualalai probably occurred lavas. 130 ka, therefore, occurred considerably after during the period of active shield building prior Subaerialiy erupted lava extended the east the tholeiitic stage was completed, during the to 130 ka. The landslide head reached back in shoreline, producing a steep subaqueous slope period when the volcano was actively erupting the shield about to the axis of the northwest rift similar to that forming off the present Kilauea alkalic basalt. zone and produced a concave scarp more than and Mauna Loa coasts. On the west coast, how- Mauna Kea volcano underwent at least three 40 km wide and up to 4 km high (Fig. 2). ever, lava flows advanced over the submerged general periods of glaciation: Pohakaloa (about Northward encroachment of the slide amphi- gentle slope of Mahukona volcano, and the 150 ka), Waihu (70-65 ka), and Makanaka theater was probably inhibited by the rein-

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forcement of the volcanic pile imparted by the Eruptive activity on the northeast rift zone of on Kohala volcano. Younger Mauna Loa lava dikes and other intrusions in the rift zone. Exten- Mauna Loa has also retreated back toward the has largely filled the canyons and covered much sive fractured and fragmented lava encountered summit relatively recently. Prehistoric eruptive of the area between canyons. on the rift zone crest in dive 71 (Fig. 2) may be vents within the city limits of Hilo 55 km from related to failure at the head of this slump. Rub- the summit caldera may be the most distal of Kolauea Volcano ble is not apparent at the base of the scarps near Mauna Loa's northeast rift zone vents mapped the axis of the Hawaiian Deep, perhaps because on land. In contrast, historic eruptive vents of Kolauea volcano grew on the south flank of post-slide subsidence of the Hawaiian Ridge had the northeast rift extend only 15 km from the Mauna Loa and probably first appeared above depressed the toe of the slide, permitting burial summit. Retreating of these rift zones as the vol- sea level about 200 ka (Fig. 7). The volcano is by the lower reaches of the Alika phase 2 slide, cano matures may result from the upward currently erupting tholeiitic basalt in the vigor- the age of which is indirectly estimated at 105 ka growth of the magma chamber, which causes ous shield-building stage, and 90% of the sub- (Moore and others, 1989). Post-Alika slide sub- the deep part of the rift zone to die when the aerial surface lavas of the volcano are younger sidence has been sufficient to slightly tilt the base of the magma chamber moves above it. than 1,100 yr (Holcomb, 1987). Palagonite and broad Alika slide toe 1-1.5 m/km east and to A series of giant landslides on the west flanks manganese-oxide studies indicate that dredged cause the axis of the Hawaiian Deep (as defined of Mauna Loa (Figs. 2 and 6; and Lipman and lavas from the submarine east rift zone are by the 4,800-m depth contour; Fig. 2) to migrate others, 1988; Moore and others, 1989) include younger than 25 ka (D. A. Clague, 1991, writ- east of the toe to the base of the landslide scarp. the North Kona slump, the South Kona debris ten commun.). avalanche, and the Alika phases 1 and 2 debris Drill holes on the east rift zone of Kilauea 40 Mauna Loa Volcano avalanches. The South Kona slump is perhaps km east of the summit encounter submarine- the oldest, because an integrated drainage sys- erupted lava at a depth of about 300 m, with an The oldest exposed lavas of Mauna Loa, the tem has developed between the blocky seaknolls estimated age of 100 ka (Moore and Thomas, subaerial tholeiitic Ninole Volcanics have of its lower part which attain 10 km in diameter 1988). These holes were drilled at about 200 m yielded K-Ar ages of 100-200 ka, but resolution (Moore and Normark, 1990). above sea level and 1,000 m below the 1,200-m is poor because of low contents of K and radio- The youngest large gravity failure on the west summit of the volcano. Hence, at 100 ka, assum- genic Ar in the weathered rocks (Lipman and flank of the volcano, the Alika phase 2 debris ing a constant slope angle from summit to sea others, 1990). The age of this sequence suggests avalanche, has been correlated with a wave level, the summit of the volcano was about 700 that the volcano probably emerged above sea deposit on Hawaii and adjacent islands (Lipman m above sea level. level about 300 ka and grew very rapidly there- and others, 1988; Moore and others, 1989). The A large part of the south flank of the volcano after. Mauna Loa's robust activity has pro- wave attained heights of 326 m on Lanai where is presently undergoing intermittent seaward duced sufficient tholeiitic basalt in the past 4,000 it deposited coral fragments dated at 105 ka by movement as part of the Hilina slump (Moore yr to cover about 90% of its subaerial surface the U-series methods (Moore and Moore, 1984, and others, 1989). The Hilina slump is about (Lockwood and Lipman, 1987). Despite rapid 1988). 100 km wide and 80 km long, and it involves subsidence, this lava output has built the vol- The West Ka Lae and East Ka Lae debris much of the south flank of Kilauea volcano. It is cano probably higher (4,160 m above sea level) avalanches on the southwest submarine flank of moving intermittently seaward at a rate of about now than ever before. Available data indicate Mauna Loa are perhaps somewhat younger than 0.1 m/yr, apparently around Loihi seamount. In that coastal Mauna Loa, like the rest of Hawaii, the Alika landslides because they appear fresher 1975, accompanied by a M 7.1 earthquake, the is subsiding at about 2.6 mm/yr. The volcano, in GLORIA sonograms. The upper part of the slump moved southward as much as 8 m (Lip- now in its vigorous shield-building stage, shows East Ka Lae landslide is fed from a well-defined man and others, 1985). the prominent slope change at sea level along amphitheater, the east wall of which is the sub- In addition to this giant slump, several most of its coastline (Mark and Moore, 1987) aerial Kahuku fault scarp and its offshore exten- medium-sized landslides, including the Papa'u typical of a volcano that has not yet completed sion (Fig. 6). Exposed in this submarine scarp, and Loihi slides, ride on top of the Hilina slump shield building. there are rift zone parallel dikes (Fornari and and are poised for repeated failure. The 40 km3 All lavas collected from the submarine part of others, 1979b). Hence, the gravity failure that Papa'u slide failed as a unit about 5 ka (Fornari the southwest rift zone of Mauna Loa are tholei- produced the East Ka Lae debris avalanche cut and others, 1979a). itic basalt. The rift zone is capped by a coral reef and steepened the west flank of the submarine In 1988-1989, small-scale coastal slumping that drowned about 18 ka, and it also shows a rift zone ridge and formed the subaerial part of occurred where lava entered the sea on the flat, subaerially formed crest down to depths of the Kahuku fault scarp on land after the southeast coast of Kilauea Volcano (K. A. Hon, 500 m and perhaps as deep as 750 m (Moore southwest rift zone ridge was established in its 1990, written commun.). The shifting lava and others, 1990b). Considering the present sub- present position (Fig. 6). streams flowed over the sea cliff into the sea and sidence rate of the island, this indicates that the Numerous faults cutting prominent erosional built up a submarine wedge of lava rubble that flat submarine part of the rift zone has under- remnants of older lava on the southeast flank of eventually produced a subaerial lava delta sea- gone little volcanic activity since 170 ka and southern Mauna Loa have led Lipman and ward of the former sea cliff. Beginning in May possibly since 270 ka. Additional evidence for others (1990) to postulate the presence of a 1988, part or all of this newly formed lava delta inactivity of the submarine part of the rift zone is major landslide (Punaluu landslide) on this flank repeatedly and dramatically slumped into the the presence of S-poor lavas, presumably between the south cape and Loihi seamount. In sea. By August 1989, many (31) of these slump- erupted above or close to sea level, that extend this view, the erosional remnants (Ninole Hills) ing events had occurred. These collapses each down to 680-m depth, the current extended in- represent the interfluves between large canyons involve as much as -50,000 m2 of rock above activity of the offshore southwest rift zone per- cut in the amphitheater of the landslide in much sea level and an unknown area and volume haps indicates that the subsurface magma the same manner as canyons were cut in the below sea level. Despite the coastal slumping, reservoir has now migrated above sea level. amphitheater at the head of the Pololu landslide however, flow of lava into the sea caused a net

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extension of the shoreline and growth of the volcano of a few square kilometers during the period May 1988 to May 1991.

Loihi and Keikikea Volcanoes

Loihi volcano is an active submarine volcano centered on the middle south submarine flank of Kilauea volcano (Fig. 1). The volcano is rooted on the south flank of Kilauea and rises to 1 km below sea level. Present-day activity of the vol- cano is indicated by the presence of swarms of earthquakes centered near the summit that occur every few years (Klein, 1982). Dredge hauls (Moore and others, 1982) and submersible dives (Malahoff, 1987) have revealed fresh lava and active thermal springs on the volcano's summit. Loihi is itself scarred by several landslides which involve more than three-quarters of the volcanic slopes above 2,500-m depth (Malahoff, 1987; Moore and others, 1989). Samples of dredged lavas from Loihi indicate that the volcano is presently in transition from eruption of alkalic lava to the eruption of tholei- itic lava (Moore and others, 1982). Extrapola- tion of the end of the shield-building curve of Figure 8 suggests that Loihi will reach the sea surface in about 250 k.y. and will complete its shield building in about 500 k.y. The southeastern Hawaiian volcanoes gener- ally occur at a spacing of 40 to 60 km on one of two loci lines 30 to 40 km apart (Fig. 1), origi- nally called the Loa and Kea ridges by Dana (1890). Loihi volcano is the submarine volcano on the Loa loci line, but no incipient volcano appears to be present on the Kea loci line. Ho- honu seamount, at 154°42'W long., an apparent volcanic edifice 5 km in diameter which rises about 1,500 m above the foot of the Hilina slump, is a possible candidate (see Fig. 1 and Moore and others, 1989, their Fig. 10). It lies about 20 km east of a reasonable projection of the Kea loci line, old-appearing weathered ba- salt has been dredged from it (Moore and Fiske, 1969), it is not the locus of recurrent seismic activity as is present at Loihi, and subbottom profiles taken aboard the R/V S. P. Lee in 1976 indicate that its flank is covered by about 50 m of sediment, suggesting that it is a Cretaceous volcanic seamount (W. Normark, 1991, oral commun.). Figure 6. Bathymetry and topography of south cape of the island of Hawaii and offshore The hypothetical next volcano of the Kea region (contour interval, 200 m; see Fig. 1 for location). Bathymetry west of I55°45' is from line, here called "Keikikea" (child of Kea), is NOAA multibeam survey; east, is from Moore and others, 1990, and NOAA hydrographic presently due to begin forming in a position 40 chart no. H-9813,1980; topography is from USGS 1:100,000 topographic map of the island of to 60 km S40°E of Kilauea summit on the south Hawaii. Onshore historic lava flows are shown by thin solid lines, faults by solid lines with ball submarine flank (Fig. 1) in the region currently on downthrown side, and eruptive vents by cross-marked lines (from Lipman and Swenson, dominated by the Hilina slump (Moore and oth- 1984). Offshore landslides, largely part of East Ka Lae Slide, are bounded by dashed lines. ers, 1989). The position of Loihi seamount and

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the postulated site of Keikikea volcano on, or 15000 adjacent to, the slump suggests that major slump activity may be an important element in initiat- / Kila uea ing the succesive volcanic centers on the Ha- waiian Ridge. The conduits feeding the present « 10000 Hualalai active volcanoes become increasingly elongate 9 O O o and inclined as the volcanic systems are steadily , o Maun a Loa o „0 o^rTSL carried to the northwest away from the mantle- > O k °o .o _o - O 90 « suaJSo®^ rooted hot spot by movement of the Pacific plate. 5000 Ö QJ^ 0 o\ Faulting or fracturing associated with major hHauna K ea gravity failures may provide new conduits that -^Koiiala 0 ° °o jiS^r- can deliver deep magma to new sea-floor >00 Mahuttona q 0—- volcanoes. 1 00 200 300 400 500 600 LIFE HISTORY OF HAWAIIAN Age, ka VOLCANOES Figure 7. Growth and decline in area of the six volcanoes that contribute to the island of Hawaii. Period of tholeiitic basalt eruption is shown by pattern. Volcanoes of Kea line are in The early history of Hawaiian volcanoes from italic type and those of Loa line in regular type. birth on the sea floor to emergence as an island is poorly known. Because Loihi is the only rec- ognized active submarine volcano atop the hot spot, it provides our primary information on the pre-subaerial history of a Hawaiian volcanic center. Loihi began growth on the southeastern submarine flank of its older neighbor, Mauna Loa, and geometric constraints suggest that most Hawaiian volcanoes begin their growth not on the flat sea floor, but on the side of a pre-existing edifice. The S40°E progression of the end of shield building has a rate of about 13 cm/yr in the section of the Ridge from Haleakala to Mauna Kea (Fig. 8). This is also regarded as a measure of the rate of initiation of volcanism and of the movement of the Pacific Plate in the opposite direction. This rate provides some limits on the maximum time required for a volcano to reach the size of Loihi. If Keikikea, the next volcano to form on the Kea line, were to pierce the ocean floor at the present time 50 km from Kilauea, it would be about 100 k.y. younger in development than Loihi, suggesting that the early alkalic stage just ending at Loihi lasted for 100 k.y. (Fig. 8). If it starts later, then the early alkalic stage would be less than 100 k.y. in duration. This general model, outlined in Figure Distance N40°W from Kilauea, km 8, provides a basis for estimating the rate of volcano growth, provided that the rate of vol- Figure 8. Estimated ages for events in the life histories of volcanoes (letter symbols as in Fig. canic progression and spacing of volcanoes re- 1) on or adjacent to the island of Hawaii. Volcano positions are projected and measured on a mains relatively constant. N40°W line passing through Kilauea. Squares indicate timing of transition from eruption of Hence, using this method of crude estimate, tholeiitic to alkalic lava (two symbols where two transitions were dated, see text). Solid line is we suggest that the time period from the begin- least-squares regression line through points representing the end of shield building of five ning of volcanism on the sea floor to the end of volcanoes (solid circles); its slope is 13 cm/yr, the apparent rate of progression of volcanism subaerial shield building at a Hawaiian volcano in this segment. Upper dashed line (birth) is drawn parallel with lower one through the is about 600 k.y. (Fig. 8). Of this 600 k.y., al- postulated position of Keikikea (large circle) assuming zero age of birth; second dashed line kalic basalt is erupted during the first 100 k.y., (end of early alkalic stage) is drawn through position of Loihi at zero time; third dashed line followed by shield building, which is probably (emergence) is drawn midway between birth and end of shield building. dominated by eruption of tholeiitic basalt for the

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next 500 k.y. Within 200 k.y., either before or in part from its growth on a relatively deep sea stratified so that different parts or levels can be after the termination of shield building, the vol- floor rather than on the flank of a pre-existing alternately tapped. cano commonly shifts from tholeiitic to alkalic volcano. Mahukona grew following a period of The transition from tholeiitic to alkalic lavas volcanism. The volcanoes breach sea level prob- low eruptive activity resulting in a low segment may occur either before or after the end of shield ably 300 ±100 k.y. after birth on the sea floor. of the Hawaiian Ridge. The Ridge crest is about building, as defined by the abrupt change in Maps of the growth of the island of Hawaii (Fig. 3 km below sea level on the Loa line between slope on the volcano flank. The change in lava 5) assume a time period of about 200-300 k.y. Kahoolawe and Mahukona, and 2 km below composition apparently occurs when a mag- from appearance above sea level to the end of sea level on the Kea line between Haleakala and matic system begins to cool, causing the degree shield building. Kohala (Fig. 1). This period of reduced output of partial melting to decrease and eruption rates The volcanoes attain their greatest size occurred about 1000 ka and was accompanied to wane (Clague, 1987). Mauna Kea and Ha- above sea level at the end of shield building, and by a shift in the azimuth of the Hawaiian Ridge leakala began shifting from tholeiitic to alkalic thereafter they shrink as their rate of subsidence from about S20°E to about S40°E. lavas 100 k.y. or more before the end of shield exceeds their rate of continued growth. The pe- The period of active growth of individual vol- building. On the other hand, Mahukona, Ko- riod of dominant landsliding apparently occurs canoes above sea level apparently ranges from hala, and Hualalai accomplished the shift near at about the end of shield building, at which about 200 to 400 k.y., after which the volcanoes or after the end of shield building (Fig. 8). time the volcano is growing at a maximum rate, undergo a steady decline in area (Fig. 7). The Mauna Kea and Haleakala both continued to and the overall slopes are steepest, highest, and time when a given volcano first appeared above erupt so vigorously after the transition to alkalic least stable. Landsliding is probably most preva- sea level is speculative, and the estimates of Fig- basalt that shield building was able to continue lent off the leading (southeast) end of the ure 5 are based on the data of Figure 8. Early in as alkalic lavas poured into the sea and extended Ridge, where the volcanoes are youngest and their periods of existence while growing below the shoreline. Attesting to these high eruption most active. Continued growth of the Ridge sea level, these volcanoes probably grew upward rates, both of these volcanoes stood more than 4 toward the southeast, however, will override rapidly because lava would have been distrib- km above sea level at the end of their shield these landslides, so that only the lateral landslide uted over a much smaller, steeper edifice as building (Fig. 9). In contrast, Kohala and Huala- deposits on the sides of the Ridge will remain compared to the giant structures of later life. lai ceased shield building while in the tholeiitic stage, or shortly after completing it, indicating uncovered. The overall growth of the island of Hawaii in that during the waning stages of tholeiitic activ- the past 600 k.y. as determined by the estimated ity, flow of lava to the coast was insufficient to GROWTH OF THE ISLAND position of past shorelines is about 10,000 km2 extend the shoreline and steepen the offshore OF HAWAII (Fig. 7), equivalent to a rate of area increase of slope. Both of these volcanoes stood 2-3 km 0.02 km2/yr. This annual increment of new above sea level at the end of their shield building The growth of the island of Hawaii resulted area is about 5 km thick based on the current (Fig. 9). Dating of the end of shield building and from the cumulative effect of growth and shrink- eruption rate of the Hawaiian hot spot of 0.1 of the magma transition is incomplete for older ing of the six volcanoes which comprise the edi- km3/yr (Swanson, 1972). Therefore, the new volcanoes to the northwest, but probably West fice that emerged as an island (Figs. 5, 7). Each material is apparently distributed over the sub- Maui, East Molokai, and Waianae (west Oahu), volcano grows subaerially as lava flows extend marine volcano flank down to the level of the which have alkalic caps, are also of the Kohala- its shoreline and partially bury adjacent volca- ocean floor at about 5-km depth. Initially mate- Hualalai type. If so, then all of the volcanoes noes, but also by intrusion and gravitational rial is distributed downslope as fragmental that stopped shield building before the transition spreading. Each volcano shrinks (above sea quenched lava produced where lava flows pour to alkalic eruptions stood from 2.3 to 3.5 km level) primarily as a result of the steady isostatic into the sea, and by the collapse and fragmenta- above sea level at the end of shield building. subsidence of the Hawaiian Ridge, but also by tion of the coastal benches discussed earlier. erosion, landsliding, and burial by lava from ad- Eventually, however, the unstable material man- Other volcanoes that either lack an alkalic jacent volcanoes. Volcanic centers on each of tling the submarine slope may fail as a major post-shield stage (Mahukona, Koolau, and the two loci lines have contributed similar areas slump or debris avalanche, thus reducing the Lanai) or have only minor alkalic stage lavas to the island (Fig. 7). slope to one of greater gravitational stability. (West Molokai and Kahoolawe) are corre- The maximum area attained by individual spondingly smaller and stood near or less than 2 volcanoes above sea level is not uniform. Mauna RELATION OF SHIELD BUILDING km above sea level at the end of their shield Kea and Mauna Loa have exceeded 4,000 km2 TO THOLEIITIC VOLCANISM building (Fig. 9). Hence, the larger volcanoes, in subaerial size during part of their growth (Fig. fed from more robust volcanic systems, make 7). Kohala has attained about 3,000 km2. The shift from eruption of tholeiitic to alkalic the transition to alkalic lavas before the end of Kilauea, Hualalai, and Mahukona never ex- lava at some volcanoes is not abrupt. Lava flows shield building; the intermediate sized volcanoes ceeded about 2,000 km2. Kilauea is smaller of the two types are commonly interbedded, in- change near the end of shield building; and the probably because it has not yet grown to its dicating that the change takes place over an ex- smaller volcanoes never erupt, or erupt only a maximum size. The small apparent subaerial tended time interval. Additionally, lavas of an tiny suite of, alkalic lavas (Fig. 9). The three size of Hualalai (Figs. 5, 7) may result from intermediate or transitional composition are youngest volcanoes (Loihi, Kilauea, and Mauna extensive burial on the east by Mauna Loa lavas, sometimes erupted before well-defined alkalic Loa) cannot be classified because they have and perhaps, because more of the western flank lavas appear. At Haleakala and Mauna Kea, the neither ended their shield-building stage nor may have been removed by landsliding than is shift apparently occurred over a period of more shifted to eruption of alkalic basalts. The great inferred from the distribution of sea-floor depos- than 100 k.y. (Fig. 8). These prolonged transi- size of Mauna Loa, however, indicates that it is its (Fig. 5). tions indicate either that magmas of differing of the robust type and will probably erupt al- The smaller area of Mahukona and its con- compositions are stored in different chambers, kalic lava considerably before the end of shield siderably lower height (Fig. 8) apparently result or that chambers are compartmentalized or building.

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Figure 9. Elevation and mag- matic character of volcanoes rela- E tive to their position along the Hawaiian Ridge. Volcano letter c symbols are the same as those used •— in Figure 1. Those volcanoes at ® which the shift from tholeiitic to al- a> kalic volcanism began before the W end of shield building are shown by £ crossed square, those where the Q- shift began after shield building are Q shown by circle within square, and those that have little or no alkalic cap are shown by open square.

Apparently, the volume of magma derived from the hot spot is not equal for the various volcanic centers. Those volcanoes that stood 4 km above sea level at the end of shield building would have a volume above the level of the prevolcanic sea floor about double that of those that stood 2 km above sea level. The most ro- been used to refine our view of the island's evo- cano, Keikikea, apparently has not yet been bust centers not only are supplied sufficient melt lution and the history of the seven volcanoes born. to build the highest and largest edifices, but also which form its volcanic pedestal. This work in- 4. Individual volcanoes take -600 k.y. to are supplied it rapidly enough to enable them to dicates the following. grow from birth on the ocean floor to full matur- continue shield building even when smaller de- 1. Individual volcanoes each grow subaer- ity at the end of subaerial shield building. They grees of partial melting at the source causes tran- ially as lava flows extend their shoreline and probably arrive at sea level about halfway sition from tholeiitic to alkalic lava. In contrast, cover lavas from adjacent volcanoes, but also by through this period. the weaker systems never are supplied enough intrusion and gravitational spreading. They 5. Large volcanoes (whose summits are more magma to enable any, or only minor amounts shrink by isostatic subsidence, but also by ero- than 4 km above their major slope break) tend of, alkalic lava to reach the surface. sion, landsliding, and burial by lava from adja- to undergo the tholeiitic-alkalic transition before The unusual size and magmatic history of the cent volcanoes. The island of Hawaii has grown the end of shield building, whereas smaller vol- three most robust volcanic centers (Haleakala, at an average rate of about 0.02 km2/yr for the canoes make the transition after the end of shield Mauna Kea, and Mauna Loa) cannot be related past 600 k.y. and presently is close to its maxi- building or not at all. to any special features in their placement along mum size. the Hawaiian Ridge, except that they all lie 2. Mahukona was the first volcano which ACKNOWLEDGMENTS toward the southeast end of the ridge. Compar- emerged at the site of the present island of Ha- able volcanoes apparently did not develop since waii about 600 ka. It was followed in order by Recent dredging around Hawaii from the Re- the time that Oahu grew. The robust volcanoes Kohala, Mauna Kea, Hualalai, Mauna Loa, Ki- search Vessels Melville in 1987 and Farnella in occupy that part of the ridge where the propaga- lauea, and the still submarine volcano, Loihi. 1988 is the latest phase of a systematic offshore tion rate of volcanism has accelerated to about 3. Because of continuing relatively uniform sampling program that began in 1965. We are 13 cm/yr (Fig. 8) compared to the overall aver- subsidence of the cumulative volcanic pile, indebted to the scientists and crews of these age for the chain of about 8 cm/yr (Clague and comparison between the depth of the abrupt ships for their valuable service. The use of the Dalrymple, 1987). Hence, the robust volcanic slope change associated with the end of shield Pisces V submersible in 1988 was supported by centers seem to coincide with the zone where building and the depth of the dated drowned NOAA through the Hawaii Underseas Research volcanic propagation rates are especially high. coral reefs provides an age estimate of the end of Laboratory. We thank Terry Kerby, program shield building. Using this method, Mahukona manager, and the pilots, SCUBA divers, and CONCLUSIONS completed shield building about 465 ka; Ko- crew for their skillful work in this program. We hala, 245 ka; Mauna Kea, 130 ka; and Hualalai, thank Richard Fiske, Christina Neal, Donald New age data, particularly 234U/238U ages 130 ka. The remaining three volcanoes, Mauna Peterson, and J. M. Rhodes for their critical re- on submerged coral reefs adjacent to northwest Loa, Kilauea, and Loihi, have not yet completed views, which substantially improved the accu- Hawaii, have provided a chronology that has shield building, and the next hypothetical vol- racy and readability of the manuscript.

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The Hawaiian-Emperor volcanic Ludwig, K. R., Szabo, B. J„ Moore, J. G., and Simmons, K. R., 1991, Crustal complex of Molokai, Maui, Lanai, and Kahoolawe: Journal of Volca- 234 238 chain, Part I: U.S. Geological Survey Professional Paper 1350, p. 5-54. subsidence rate off Hawaii, determined from U/ U ages of nology and Geothermal Research, v. 7, p. 339-355. Clague, D. A., and Moore, J. G., 1988, Volcanic history of Mahukona sub- drowned coral reefs: Geology, v. 19, p. 171-174. Porter, S. C., Garcia, M. O., Lockwood, J. P., and Wise, W. S„ 1987, Guide- marine volcano, Hawaii [abs.]: Eos (American Geophysical Union Macdonald, G. A., and Katsura, T., 1964, Chemical composition of Hawaiian book for Mauna Loa-Mauna Kea-Kohala field trip, Hawaii Sympo- Transactions), v. 69, p. 1445. lavas: Journal of Petrology, v. 5, p. 82-133. sium on How Volcanoes Work: International Association of Volcanol- Clague, D. A., and Moore, J. 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G., and Calk, L., 1979a, A large submarine sand- logical society of America Bulletin, v. 83, p. 3731-3738. the rate of late Pleistocene subsidence of the island: Geology, v. 14, rubble flow on Kilauea volcano, Hawaii: Journal of Volcanology and Moore. G. W.. and Moore. J. G.. 1988, Large-scale bedforms in boulder gravel p. 967-968. Geothermal Research, v. 5, p. 239-256. produced by giant waves in Hawaii: Geological Society of America Wilde, P., Chase, T. E, Normark. W. R., Thomas, J. A., and Young, J. D., Fornari, D. J., Peterson. D. W., Lockwood. J. P., Malahoff, A., and Heezen, Special Paper 229, p. 101-110. 1980, Oceanographic data off the southern Hawaiian Islands: Lawrence B. C., 1979b, Submarine extension of the southwest rift zone of Mauna Moore. J. G., 1966, Rate of palagonitization of submarine basalt adjacent to Berkeley Laboratory, University of California, Berkeley, LBL Publica- Loa volcano, Hawaii: Visual observations from U.S. Navy Deep sub- Hawaii: U.S. Geological Survey Professional Paper 550-D, p. 163-171. tion 359. mergence vehicle DSV Sea Cliff: Geological Society of America Bul- Moore, J. G., 1987, Subsidence of the Hawaiian Ridge: U.S. Geological Survey Wilson, J. T., 1963, A possible origin of the Hawaiian islands: Canadian letin, v. 90, p. 435-443. Professional Paper 1350. p. 85-100. Journal of Physics, v. 41, p. 863-870. Frey, F. A., Wise, W. S„ Garcia, M. O., West, H„ Kwon, S.-T., and Moore. J. G., and Campbell, J. F., 1987, Age of tilted reefs, Hawaii: Journal of Wolfe, E. W., Wise, W. S., and Dalrymple, G. B., in press, The geology and Kennedy, A., 1990, Evolution of Mauna Kea volcano, Hawaii: Petro- Geophysical Research, v. 92, p. 2641-2646. petrology of Mauna Kea volcano, Hawaii—A study of postshield vol- logic and geochemical constraints on postshield volcanism: Journal of Moore. J. G., and Clague, D. A., 1987, Coastal lava flows from Mauna Loa canism: U.S. Geological Survey Professional Paper. Geophysical Research, v. 95, p. 1271-1300. and Hualalai volcanoes, Kona, Hawaii: Bulletin of Volcanology, v. 49, Wright, T. L., and Clague, D. A., 1989, Petrology of Hawaiian lava, in Garcia, M. O., and Kurz, M. D.,1991, Reply to Comment on "Mahukona: The p. 752-764. Winterer, E. L., Hussong, D. M„ and Decker, R. W., eds., Eastern missing Hawaiian volcano": Geology, v. 19, p. 1045-1051. Moore, J. G., and Fiske, R. S., 1969, Volcanic substructure inferred from Pacific Ocean and Hawaii: Geology of North America, Volume N: Garcia, M. O.. Muenow, D. W., Aggrey, K. E., and O'Neil, J. R„ 1989, Major dredge samples and ocean-bottom photographs, Hawaii: Geological So- Boulder, Colorado, Geological Society of America, p. 218-237. element, volatile, and stable isotope geochemistry of Hawaiian sub- ciety of America Bulletin, v. 80, p. 1191-1201. marine tholeiitic glasses: Journal of Geophysical Research, v. 94, Moore, J. G., and Fornari, D. J., 1984, Drowned reefs as indicators of the rate p. 10,525-10,538. of subsidence of the island of Hawaii: Journal of Geology, v. 92, Garcia, M. O., Kurz, M. D., and Muenow, D. W., 1990, Mahukona: The p. 752-759. missing Hawaiian volcano: Geology, v. 18, p. 1111-1114. Moore, J. G., and Moore, G. W., 1984, Deposit from a giant wave on the Holcomb, R. T., 1987, Eruptive history and long-term behavior of Kilauea island of Lanai, Hawaii: Science, v. 226, p. 1312-1315. volcano: U.S. Geological Survey Professional Paper 1350, p. 261-350. Moore, J. G., and Normark, W. R., 1990, Giant submarine landslides on Jones, A. T., 1991, Drowned reefs in the Alenuihaha Channel: Evidence for Mauna Loa volcano, Hawaii, displayed by multibeam bathymetry island subsidence and low stands of Quaternary sea level [abs.]: Pacific [abs.): Conference on Intraplate Volcanism; The Reunion Hot Spot, Science, v. 45, p. 92. Nov. 12-17, 1990, Reunion, Program, 2 p. Klein, F. W„ 1982. Earthquakes at Loihi submarine volcano and the Hawaiian Moore, J. G., and Thomas, D. M., 1988, Subsidence of Puna, Hawaii, inferred hot spot: Journal of Geophysical Research, v. 87. p. 7719-7726. from sulfur content of drilled lava flows: Journal of Volcanology and Lipman. P. W., 1980, The southwest rift zone of Mauna Loa: Implications for Geothermal Research, v. 35. p. 165-171. MANUSCRIPT RECEIVED BY THE SOCIETY NOVEMBER 25, 1991 structural evoluton of Hawaiian volcanoes: American Journal of Moore, J. G., Clague, D. A„ and Normark, W. R., 1982, Diverse basalt types REVISED MANUSCRIPT RECEIVED MAY 4,1992 Science, v. 280-A, p. 752-776. from Loihi seamount, Hawaii: Geology, v. 10, p. 88-92. MANUSCRIPT ACCEPTED MAY 6,1992

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