Robin T. Holcomb. U.S. Geological Survey, School of Oceanography WB-10, University of Washington, Seattle, Washington 98195. James G. Moore.
Voluminous submarine lava flows from Hawaiian volcanoes Robin T. Holcomb U.S. Geological Survey, School of Oceanography WB-10, University of Washington, Seattle, Washington 98195 James G. Moore U.S. Geological Survey, Menlo Park, California 94025 Peter W. Lipman U.S. Geological Survey, Denver, Colorado 80225 Robert H. Belderson Institute of Oceanographic Sciences, Wormley GU8 5UB, England ABSTRACT The GLORIA long-range sonar imaging system has revealed fields of large lava flows in the Hawaiian Trough east and south of Hawaii in water as deep as 5.5 km. Flows in the most extensive field (110 km long) have erupted from the deep submarine segment of Kilauea's east rift zone. Other flows have been erupted from Loihi and Mauna Loa. This discovery confirms a suspicion, long held from subaerial studies, that voluminous submarine flows are erupted from Hawaiian volcanoes, and it supports an inference that summit calderas repeatedly collapse and fill at intervals of centuries to millenia owing to voluminous eruptions. These extensive flows differ greatly in form from pillow lavas found previously along shallower segments of the rift zones; therefore, revision of concepts of volcano stratigraphy and structure may be required.
INTRODUCTION Large submarine lava flows have been discovered in the Hawaiian Trough east and south of Hawaii. The flows were found in October and November 1986 by the side-scan sonar system GLORIA (Geologic LOng Range Inclined Asdic) of the Institute of Oceanographic Sciences (IOS) of England, during a joint survey of the Hawaiian Exclusive Economic Zone by the IOS and the U.S. Geological Survey aboard the research vessel Famella. Also employed in the survey were 3.5-kHz echo-sound profiling and other geophysical tools. GLORIA produces sonar images of the sea floor that resemble subaerial images made by side-looking radar (Somers et al., 1978). The system images a swath extending out from each side of the ship track about 20 km (for the 5 km depths around Hawaii). Images are made so that areas of high and low sonar backscatter are shown by light and dark tones, respectively. Backscatter is a function of viewing geometry, microscale bed roughness, and sonar reflectivities of various materials on the sea floor; bare lava appears lighter than sediment because it backscatters sound more strongly. GEOLOGIC SETTING The Island of Hawaii (Fig. 1) is composed of five volcanoes, the exposed rocks of which are all younger than 1 Ma (Clague and Dalrymple, 1987). Mauna Loa and Kilauea (and probably also the submarine Loihi volcano) erupt frequently (Peterson and Moore, 1987). 400
The submarine east flank of Hawaii is dominated by two steep ridges (Fig. 1). The shorter northern one, here called the Hilo ridge, appears to have been built along the east rift zone of Mauna Kea; its subsidence history suggests that it stopped growing at about 130 ka (Moore and Campbell, 1987). The younger Puna ridge extends eastward from Kilauea and has grown along the still-active east rift zone of that volcano. Because the Hilo ridge is older, its distal end is probably covered by the north flank of the Puna ridge.
lighter toned than most of the surrounding sea floor (Fig. 2). On acoustic profiles the ridges have rough surfaces, whereas the adjacent sea floor has a smooth surface and is nearly flat (Fig. 3, top). The anomalous band is flat, yet even lighter toned than the ridges. This band fringing the ridges is about 110 km long and averages about 10 km wide; it forms no distinct boundary with the Puna ridge but does form a sharp, irregular boundary against the dark-toned sea floor. This band occupies the axis of the Hawaiian Trough and forms two arms joining at the east end of the Puma ridge (Fig. 1). The northern arm (Fig. 2) extends northwest about 60 km and contains small areas, 1-2 km wide, of low backscatter as well as several broader areas of slightly differing high backscatter. The major lobes of the serrate northern margin are elongate slightly west of north, in the direction of Cretaceous sea-floor structures. The southern arm of the anomalous band extends about 50 km to the southwest; its southern part is split into two light-toned lobes separated by an area of distinctly lower backscatter. Acoustic profiles show that the northern and southern arms of the anomalous band slope slightly, about 0.1°, away from their junction at the east end of the Puna ridge.
FLAT BAND OF HIGH BACKSCATTER A band of anomalous backscatter and topography fringes the distal parts of the volcanic ridges. On GLORIA imagery the ridges are
Acoustic profiles show that the edge of the anomalous band rises steeply about 10 m above the adjacent dark-toned sea floor, which slopes toward the band at an angle of about 0.4° (Fig. 3, top). Although the anomalous band is nearly flat, it contains a few broad steps 5-15 m
The volcanoes are built on Cretaceous sea floor having an age of about 80 Ma (Hilde et al., 1976). GLORIA images show that the sea floor is cut by many normal faults trending slightly west of north (Fig. 1); these faults probably developed as the sea floor was being produced along the East Pacific Rise. More than 100 km from Hawaii the sea floor appears (in acoustic profiles) to be covered by pelagic sediment only 10-15 m thick, and so the fault scarps are clearly visible in GLORIA images. Nearer the island the fault scarps are subdued, probably by thicker volcaniclastic deposits. Looping around the island is the Hawaiian Trough, where the sea floor has subsided under the weight of the volcanoes (Moore, 1987).
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high, and the 3.5-kHz records show that some of the steps reflect sound better than do others (Fig. 3, top). If the older sea floor projects straight beneath the band of high backscatter at an angle of 0.4°, the band must form the surface of a body that is thin and wedge-shaped in cross section. If the wedge thickens from 10 to 85 m over a distance of 10 km and is 100 km long, its volume must be about 50 km 3 . Interpretation We interpret the band of high backscatter as a cluster of elongate lava flows. Its lobate outline and the dark-toned areas enclosed by it suggest that the band consists of material that flowed across the sea floor, leaving uncovered small inliers (kipukas) of older sea floor. The length and thinness of the band on a very gentle slope indi-
cate a low viscosity (less than 103 poise) for the flowing material. Its low relief at the hectometre scale distinguishes the band from submarine debris flows in Hawaii, which are hummocky at the same scale (Lipman et al., 1988); we therefore infer that the band consists of basaltic lava flows. Multiple flows within the band are indicated by sharp lateral changes in backscatter (Fig. 2) and depth (Fig. 3). Though individual flows cannot be mapped confidently by using available data, they appear to have lengths of as much as 60 km, widths of 5 km or more, and areas of a few hundred square kilometres. If they are typically 10 m thick (suggested by heights of steps in 3.5-kHz records, as in Fig. 3), they each have volumes of a few cubic kilometres. If their mean volume is about 2 km 3 , their total volume
of about 50 km 3 requires about 25 flows, most of them being buried beneath a few surface flows (Fig. 3, bottom). The elongations and surface slopes of the flows indicate that they were erupted from the distal segment of the Puna ridge (Fig. 4). Although some flows in the northern arm could have been fed by eruptions at higher levels via the trough between the Hilo and Puna ridges, ground slopes require all of the large flows in the southern arm to have been erupted at depths greater than about 2500 m (Fig. 3); the surficial slopes of the flows suggest that much of the lava was erupted very near the end of the rift zone, at depths greater than 4500 m. The serrate margin of the northern arm suggests that spreading of the lava was influenced by the fault scarps in the older sea floor.
Figure 1. Bathymetry around Hawaii. Heavy dashed line is axis of Hawaiian Trough; exposed normal faults to east are shown diagrammatically. Solid lines bound five volcanoes that compose island: Kohala (K), Mauna Kea (MK), Hualalai (H), Mauna Loa (ML), and Kilauea (KIL). Loihi Volcano (L) has not yet grown to sea level. Dotted lines indicate rift zones that radiate from volcano summits; submarine rift zones follow axes of Puna ridge (PR) and Hilo ridge (HR). Stippled pattern shows major offshore lava fields, vented from submarine rift zones of Kilauea (a), Mauna Loa (b), and Loihi (c). Rectangles indicate areas shown in Figures 2 and 4; heavy line southeast of Puna ridge indicates location of profile shown in Figure 3. Bathymetric contour interval is 100 m.
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Ages of the flows probably range over several thousand years. Most differences in flow brightness probably arise from differences in age, because brightness seems to correlate with stratigraphic position: the brighter flows occur nearer the inferred vent area and seem to overlap the darker flows (Figs. 2 , 3 , and 4). This correlation between age and brightness probably arises from differences in sediment cover; we have no reason to suspect a correlation between age and intrinsic surface roughness or other physical properties of the lavas. If the brighter flows do have less sediment cover than the darker flows, age differences must be large, relative to rates of sedimentation. Those rates are not well known; but if they are low enough for small Cretaceous faults to still affect the spreading of lava, flows of detectably different brightness must differ in age by at least centuries or millenia. However, the oldest flows could not have spread along the north side of the Hilo ridge until the Puna ridge extended beyond the end of the older ridge. The Puna ridge now extends about 20 km beyond the Hilo ridge, and the total length of Kilauea's east rift zone is about 120 km. If the Puna ridge has grown at a steady rate of 0.6 k m / k a during the lifetime of Kilauea (about 200 ka; Easton, 1978), its oldest flows north of the Hilo ridge must be younger than about 35 ka. The corresponding recurrence interval for 25 flows of 2 k m 3 mean volume would be less than 2 ka.
LAVA F L O W S AND CALDERA COLLAPSE Geologists have long suspected that caldera collapses of Kilauea and other basaltic volcanoes are caused by voluminous submarine eruptions that drain magma chambers beneath their summits (Dutton, 1884; Macdonald, 1965; Simkin and Howard, 1970). Repeated voluminous eruptions at intervals of a few centuries to a few millenia have been inferred to cause repeated caldera collapses of Kilauea (Holcomb, 1976, 1987), but direct evidence for such voluminous submarine eruptions has not been observed until now. This discovery by GLORIA confirms the occurrence of voluminous eruptions and lends support to the idea that caldera collapse is repeated often during the growth of the shield volcanoes.
tected summit deflation of less than 0.1 km 3 , with no visible changes in the caldera; (2) moderate summit deflation of about 0.1-1 k m 3 accompanied by draining of summit lava lakes or localized collapse of small pit craters, and sometimes minor phreatic explosions (such as those that occurred in A.D. 1924); and (3) profound summit deflation of more than 1 k m 3 that en-
larges the caldera and decompresses the magma reservoir to cause major phreatomagmatic explosions (such as those that occurred in A.D. 1790). Volume of flank eruption and summit collapse is related directly to distance of eruption site from the summit and inversely to elevation of eruption site. For well-measured brief eruptions along the subaerial east rift zone, the linear relation of volume, V, to elevation E,V= 103.4 - 0.110 E, has a correlation coefficient of 0.90 (Epp et al., 1983). Extrapolation predicts eruption volumes of about 0.7 k m 3 from the distal end of the submarine rift zone, but the volumes of the observed lava flows are apparently greater than this (typically about 2 km 3 , as described above). Evidently the relation of volume to depth does not remain linear at great depths, or the flows resolved by G L O R I A are merely the largest of a population ranging widely in volume at those depths. If either alternative is correct, the depths of future eruptions may not yield reliable predictions of erupted volume or occurrence of explosive caldera collapse. Although the volumes of these submarine eruptions are large, they represent only a small fraction of the total magma supply. If a submarine eruption of 2 k m 3 occurred every 2 ka, it would require a supply rate of 0.001 k m 3 / y r , but the well-measured supply rate for Kilauea is about 0.11 k m 3 / y r (Swanson, 1972; Dzurisin et al., 1984). The large eruptions use less than 1% of the total magma supply, and more than 99% is used in building the rest of the volcanic edifice.
Caldera collapses represent one end of a range of summit subsidences related to flank eruption. These include, in order of increasing magnitude and decreasing frequency, (1) instrumentally de-
LAVA F L O W S F R O M O T H E R RIFT ZONES A particular benefit of G L O R I A is the synoptic view that this imaging system provides over large regions, permitting comparisons between different areas. This survey has revealed distinct differences among the young Hawaiian rift zones. N o deep lava flows are evident offshore from Kilauea's southwest rift zone. This is commensurate with the subdued subaerial expression of that rift zone, which is less active than the east rift zone. More notable is the lack of imaged lava fields around the base of the very large ridge extending east of Maui (Fig. 1). If a lava field is present there, it must have been covered by landslides or sediment since the last eruptions from that ridge.
Figure 2. Part of GLORIA sonograph across distal end of Puna Ridge (PR) and along base of Hilo ridge (Fig. 1), showing backscatter contrast between base of Hawaiian volcanoes (light tone) and older sea floor (dark tone).
The only other submarine lava field comparable in size to the one around the Puna ridge is a fringe of lava covering about 400 k m 2 around the base of Loihi volcano (Fig. 1). In contrast to the Kilauea flows, Loihi lavas appear to have stopped spreading on steeper slopes having gradients of as much as 11°. Because the Loihi flows do not extend far, the eruptions may have
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M i Pillow lava
Large s h e e t flows
Volcaniclastic sediment
H U Pelagic sediment L U C r e t a c e o u s basalt
Figure 3. High-resolution 3.5-kHz echogram (top) and interpretive cross section (bottom) along line A-B of Figure 4. S indicates areas interpreted as pelagic sediment, R = pillow lava and debris of ridge, Y = extensive young sheet flows, and O = older sediment-mantled sheet flows. Layers of sheet flows and pillow lava are hypothesized to lap successively across sea floor, as shown diagrammatically.
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been much smaller than those from Kilauea. Caldera collapses of Loihi may also be correspondingly smaller, perhaps because of the smaller size of this volcano. No large lava fields were clearly imaged offshore from Mauna Loa, though a reflective area projecting 3 - 1 0 km across flat sea floor from the east side of the southwest rift zone may be young lava (Fig. 1). In contrast to the lava fields of Kilauea and Loihi, this area is less reflective (perhaps because of faster sedimentation nearer the shoreline), encloses no clearly defined kipukas, and lacks distinct lobate flow fronts. This possible lava field terminates to the southwest against Cretaceous seamounts and appears to be covered to the northwest by debris of a submarine slide from Mauna Loa's southeast flank. Any Mauna Loa lava on the west side of the lower southwest rift zone is similarly covered by debris of the Ka Lae slide (Lipman et al., 1988). The absence of any obvious major lava fields offshore from Mauna Loa is especially puzzling because the volcano has had repeated caldera collapses during recent millenia (Holcomb, 1980), most recently at about 0.6 ka (Lockwood and Lipman, 1987). Several factors could be involved: (1) The lower subaerial parts of both Mauna Loa rift zones have become less active in the late Pleistocene and Holocene, probably due to buttressing by the growth of Kilauea (Lipman, 1980). Perhaps magma can no longer intrude distal segments of the rift zones, so that the volcano has grown steeper and its recent caldera collapses have been related only to large subaerial eruptions (known volumes up to 1 km 3 ). The present caldera appears to be a cluster of pits that individually are much smaller than the single caldera sink of Kilauea. (2) Large lava fields may have been erupted formerly at the GEOLOGY, May 1988
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