Tadahide Ui | USGS Science

USGS2002

Deep-sea volcaniclastic sedimentation around the southern flank of Hawaii

Most slopes of the Hilina slump are steep, but local small benches, mantled by volcaniclastic sand and fine sediments, were sampled in 1998-1999 with ROV KAIKO and DSRV SHINKAI 6500. Most surficial glass sands on the Hilina slump have compositions of subaerially erupted Kilauea lava, which fragmented and quenched as they entered the sea. Samples from the base of the Puna Ridge contain both subaerially and submarine-erupted fragments of Kilauea composition. Some glass sands from the base of Loihi contain both subaerial Kilauea and submarine Loihi compositions in the same bed. Two piston cores, collected 120 km (P6) and 250 km (P5) southeast of Hawaii, are composed of fine sediments interbedded with volcaniclastic turbidite layers. Although some volcaniclastic fragments in the fine sediments intervals are disturbed by bioturbation, nearly continuous volcanostratigraphic sequences are preserved. P6 and the upper 2.3 m of P5 are normally magnetized: the lower part of P5 is reversed and is therefore older than 0.78 Ma. At depths of 4 m below sea floor (mbsf) in P6 and 1 mbsf in P5, the dominant glass compositions change down core from Kilauea to Mauna Loa type. In the P6, an interval of abundant submarine-erupted alkalic glasses lies between 3.3 and 1.75 mbsf and may record the ancestral alkalic phase of Kilauea volcano. Magnetic susceptibility trends and glass compositions suggest that the entire P6 core correlates with only the uppermost 1.2 m of the P5, and that the average sedimentation rate at the P6 is about 5 times greater than that at P5.

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USGS2002

Ancestral submarine growth of Kïlauea Volcano and instability of its south flank

Joint Japan-USA cruises in 1998-99 explored and sampled the previously unstudied deep offshore region south of Kilauea. Bathymetric features, dive observations, and recovered samples indicate that the 3-km-deep mid-slope bench, bounded seaward by a 2-km-high lower scarp, is underlain by massive turbidite sandstone and interbedded debris-flow breccia. Debris-flow clasts are submarineerupted (high-S) alkalic basalt, distinctive fine- to coarse-grained alkalic gabbro and nephelinite (some containing phlogopite), and subordinate transitional basalt. N o recovered clasts are similar to recent Kilauea tholeiite. Primary volcanic deposits (pillow basalt, hyaloclastite breccia) are absent. The sandstone and breccia matrix, a large fraction of the volcaniclastic apron, are mainly subaerially erupted (low-S) tholeiitic glass sand generated by shoreline processes on pre-Kilauea volcanoes. Fractures, shears, slickensided clasts, and open folds indicate widespread deformation low in the lower scarp; upward-decreasing proportions of alkalic materials define a gross stratigraphy. Alkalic high-S compositions of many basalt clasts and some sandstone glass indicate derivation from the submarine "Lo'ihi" stage of ancestral Kilauea, prior to growth of its tholeiitic shield. Slopes (3300-2800 m depth) above the mid-slope bench contain submarine-erupted (S >750 ppm) pillow lava of transitional basalt, defining the initial flank of subalkaline Kilauea. The geometry and diverse constituents of the bench and lower scarp require initial landsliding during alkalic volcanism at 200-300 ka, prior to shield growth at Kilauea and inception of Hilina faults. The active Hilina slump structures on Kilauea's south flank are in an early growth stage, thus possibly posing greater potential for future large-scale landsliding and tsunamis

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