Hawaii's Volcanoes Revealed

Summary

Hawaiian volcanoes typically evolve in four stages as volcanism waxes and wanes: (1) early alkalic, when volcanism originates on the deep sea floor; (2) shield, when roughly 95 percent of a volcano's volume is emplaced; (3) post-shield alkalic, when small-volume eruptions build scattered cones that thinly cap the shield-stage lavas; and (4) rejuvenated, when lavas of distinct chemistry erupt following a lengthy period of erosion and volcanic quiescence. During the early alkalic and shield stages, two or more elongate rift zones may develop as flanks of the volcano separate. Mantle-derived magma rises through a vertical conduit and is temporarily stored in a shallow summit reservoir from which magma may erupt within the summit region or be injected laterally into the rift zones. The ongoing activity at Kilauea's Pu‘u ‘O‘o cone that began in January 1983 is one such rift-zone eruption. The rift zones commonly extend deep underwater, producing submarine eruptions of bulbous pillow lava.

Once a volcano has grown above sea level, subaerial eruptions produce lava flows of jagged, clinkery ‘a‘a or smooth, ropy pahoehoe. If the flows reach the ocean they are rapidly quenched by seawater and shatter, producing a steep blanket of unstable volcanic sediment that mantles the upper submarine slopes. Above sea level then, the volcanoes develop the classic shield profile of gentle lava-flow slopes, whereas below sea level slopes are substantially steeper. While the volcanoes grow rapidly during the shield stage, they may also collapse catastrophically, generating giant landslides and tsunami, or fail more gradually, forming slumps. Deformation and seismicity along Kilauea's south flank indicate that slumping is occurring there today.

Loading of the underlying Pacific Plate by the growing volcanic edifices causes subsidence, forming deep basins at the base of the volcanoes. Once volcanism wanes and lava flows no longer reach the ocean, the volcano continues to submerge, while erosion incises deep river valleys, such as those on the Island of Kaua‘i. The edges of the submarine terraces that ring the islands, thus, mark paleocoastlines that are now as much as 2,000 m underwater, many of which are capped by drowned coral reefs.

Mapping the sea floor around Hawaii

The Japan Marine Science and Technology Center (JAMSTEC) funded and led a four-year collaborative survey of the underwater flanks of Hawaii's shield volcanoes. This exploration, involving scientists from the U.S. Geological Survey (USGS) and other Japanese and U.S. academic and research institutions, utilized manned and unmanned submersibles, rock dredges, and sediment piston cores to directly sample and visually observe the sea floor at specific sites. Ship-based sonar systems were used to more widely map the bathymetry from the sea surface.

The state-of-the-art multibeam sonar systems, mounted on the hull of GPS-navigated research vessels, convert the two-way travel times of individual sonar pings and their echoes into a line of bathymetry values across the ship track. The resulting swaths across the ocean bottom, obtained along numerous overlapping ship tracks, reveal the sea floor in stunning detail. The survey data collected by JAMSTEC form the basis for the bathymetry shown on the map, augmented with bathymetric data from other sources. Bathymetry that is predicted from variations in sea-surface height, observable from satellites, provides the low-resolution (fuzzy) bathymetry in between ship tracks. Subaerial topography is from a USGS 30-m digital elevation model of Hawaii. Historical lava flows are shown in red.

Prominent terraces (shown in orange and yellow) illustrate the larger size of the islands in the past; O‘ahu and the Maui-Nui complex (Maui, Moloka‘i, Lana‘i, and Kaho‘olawe islands, and Penguin Bank), in particular, are mere vestiges of their former extent. Lo‘ihi, the youngest volcano in the chain, has not yet reached the sea surface. Fields of blocky debris, such as Ko‘olau's Nu‘uanu Slide, were created by catastrophic landslides, which carried large parts of some volcanoes as much as 200 km across the sea floor. Slower-moving, sediment-blanketed slumps, in contrast, typically develop ridges that parallel the paleocoastlines, such as Haleakala's Hana Slump. Eruptions along the submarine part of a volcano's rift zone produce a rugged morphology, as at Kilauea's Puna Ridge. Numerous seamounts of Late Cretaceous age (approximately 80 Ma) are scattered across the deep sea floor and are unrelated to the hot spot that supplies Hawaii's volcanoes.

First posted October 24, 2003

Last modified April 16, 2007 (mfd)