Robin T. Holcomb

Large landslides from oceanic volcanoes

Abstract GLORIA sidescan sonar surveys have shown that large landslides are ubiquitous around the submarine flanks of Hawaiian volcanoes, and GLORIA has also revealed large landslides offshore from Tristan da Cunha and El Hierro. On both of the latter islands, steep flanks formerly attributed to tilting or marine erosion have been reinterpreted as landslide headwalls mantled by younger lava flows. Large landslides have also been inferred from several oceanic islands elsewhere by other workers using different evidence, and we suggest that seacliffs previously attributed to marine erosion of many additional islands may instead be headwalls of still other landslides. These landslides occur in a wide range of settings and probably represent only a small sample from a large population. They may explain the large volumes of archipelagic aprons and the stellate shapes of many oceanic volcanoes. Large landslides and associated tsunamis pose hazards to many islands.

Pliocene and Pleistocene alkalic flood basalts on the seafloor north of the Hawaiian islands

Abstract The North Arch volcanic field is located north of Oahu on the Hawaiian Arch, a 200-m high flexural arch formed by loading of the Hawaiian Islands. These flood basalt flows cover an area of about 25, 000 km 2 ; the nearly flat-lying sheet-like flows extend about 100 km both north and south from the axis of the flexural arch. Samples from 26 locations in the volcanic field range in composition from nephelinite to alkalic basalt. Ages estimated from stratigraphy, thickness of sediment on top of the flows, and thickness of palagonite alteration rinds on the recovered lavas, range from about 0.75–0.9 Ma for the youngest lavas to somewhat older than 2.7 Ma for the oldest lavas. Most of the flow field consists of extensive sheetflows of dense basanite and alkalic basalt. Small hills consisting of pillow basalt and hyaloclastite of mainly nephelinite and alkalic basalt occur within the flow field but were not the source vents for the extensive flows. Many of the vent lavas are highly vesicular, apparently because of degassing of CO 2 . The lavas are geochemically similar to the rejuvenated-stage lavas of the Koloa and Honolulu Volcanics and were generated by partial melting of sources similar to those of the Koloa Volcanics. Prior to eruption, these magmas may have accumulated at or near the base of the lithosphere in a structural trap created by upbowing of the lithosphere.

Eruption-Triggered Avalanche, Flood, and Lahar at Mount St. Helens—Effects of Winter Snowpack

An explosive eruption of Mount St. Helens on 19 March 1982 had substantial impact beyond the vent because hot eruption products interacted with a thick snowpack. A blast of hot pumice, dome rocks, and gas dislodged crater-wall snow that avalanched through the crater and down the north flank. Snow in the crater swiftly melted to form a transient lake, from which a destructive flood and lahar swept down the north flank and the North Fork Toutle River.

Dating recent Hawaiian lava flows using paleomagnetic secular variation

Hawaiian paleomagnetic secular variation (SV) is defined from samples at 67 sites on lava flows of known age. Paleomagnetic directions range through 40° of inclination and 30° of declination; angular dispersion within sites is commonly Dating precision is limited by dispersion of 4.5° among sites of apparently similar age. The main dispersion sources are in 14 C dates (3.0°), intra-flow deformations (2.0°), and local magnetic anomalies (1.5°). Results from 68 sites on undated flows show that 95% of Kilauea9s surface is younger than 1,000 yr. A hiatus in volcano growth 1,000–1,500 yr ago coincides with the filling of a large caldera. Averaging of available data yields an SV reference curve which is fairly reliable to ∼1,500 yr ago, contains gaps and ambiguities 1,500–3,000 yr ago, and remains highly uncertain 3,000–6,000 yr ago. The SV dating method can be precise but is limited by the need for a calibrated SV history. It can be a powerful correlation tool even if a history is unavailable.

South Arch volcanic field—Newly identified young lava flows on the sea floor south of the Hawaiian Ridge

Several young lava fields were imaged by GLORIA sidescan sonar along the Hawaiian Arch south of Hawaii. The largest, 35 by 50 km across, includes a central area characterized by high sonar backscatter and composed of several flow lobes radiating from a vent area. Reflection profiling and sea-floor photography indicate that the central lobes are flat sheet flowsbounded by pillowed margins; thin surface sediment and thin palagonite rinds on lava surfaces suggest ages of 1-10 ka. Vents are localized along the arch crest near bases of Cretaceous seamounts. Two dredged flows are basanite and alkalic basalt, broadly similar to rejuvenated-stage and some pre-shield alkalic lavas on the Hawaiian Ridge. Arch volcanism representsperipheral leakage of melt from the Hawaiian hot spot over much larger areas than previously recognized.

Geomagnetic secular variation from 14C-dated lava flows on Hawaii and the question of the Pacific non-dipole low

New palaeomagnetic data from 106 14C-dated lava flows ranging in age from 200 to 31000 years b.p. yield an estimated angular dispersion value of 9.5°. These data and other new geological information permit a more precise estimate of the time interval recorded by lava flow sequences previously used to measure palaeosecular variation in Hawaii. When weighted according to revised estimates of recording interval, the combined Brunhes lava sequences yield an angular dispersion of 11.21));j) degrees, still lower than that predicted by global models of the secular variation. Several of the lava flow sequences previously thought to have recorded quiet intervals of geomagnetic behaviour actually record only very short time intervals.

Scientific Rationale for Establishing Long-Term Ocean Bottom Observatory/Laboratory Systems

The oceanographic community is in a position scientifically and technologically to initiate programs leading to the installation of one or more permanently instrumented observatory/laboratory complexes on submarine spreading centers. The dynamic nature of these systems is well established. Yet, there has been no long term, inter-disciplinary effort focused on specific sites to document rates of change in system components, nor the interactions linking the physical, chemical, and biological processes involved. The ultimate goal of this natural laboratory approach would be to establish, then model, the temporal, and the spatial, co-variation among the active processes involved in generation and aging of 60 percent of the planetary surface. The technological and intellectual stimulation involved in successful implementation of natural seafloor laboratories will provide a new generation of dynamically-based, quantitatively testable models of ocean lithosphere genesis and of the biological and chemical consequences of its formation.