Joel E. Robinson

Morphology, volcanism, and mass wasting in Crater Lake, Oregon

Crater Lake was surveyed nearly to its shoreline by high-resolution multibeam echo sounding in order to define its geologic history and provide an accurate base map for research and monitoring surveys. The bathymetry and acoustic backscatter reveal the character of landforms and lead to a chronology for the concurrent filling of the lake and volcanism within the ca. 7700 calibrated yr B.P. caldera. The andesitic Wizard Island and central-platform volcanoes are composed of sequences of lava deltas that record former lake levels and demonstrate simultaneous activity at the two vents. Wizard Island eruptions ceased when the lake was ∼80 m lower than at present. Lava streams from prominent channels on the surface of the central platform descended to feed extensive subaqueous flow fields on the caldera floor. The Wizard Island and central-platform volcanoes, andesitic Merriam Cone, and a newly discovered probable lava flow on the eastern floor of the lake apparently date from within a few hundred years of caldera collapse, whereas a small rhyodacite dome was emplaced on the flank of Wizard Island at ca. 4800 cal. yr B.P. Bedrock outcrops on the submerged caldera walls are shown in detail and, in some cases, can be correlated with exposed geologic units of Mount Mazama. Fragmental debris making up the walls elsewhere consists of narrow talus cones forming a dendritic pattern that leads to fewer, wider ridges downslope. Hummocky topography and scattered blocks up to ∼280 m long below many of the embayments in the caldera wall mark debris-avalanche deposits that probably formed in single events and commonly are affected by secondary failures. The flat-floored, deep basins contain relatively fine-grained sediment transported from the debris aprons by sheet-flow turbidity currents. Crater Lake apparently filled rapidly (ca. 400–750 yr) until reaching a permeable layer above glaciated lava identified by the new survey in the northeast caldera wall at ∼1845 m elevation. Thereafter, a gradual, climatically modulated rise in lake level to the present 1883 m produced a series of beaches culminating in a modern wave-cut platform, commonly ∼40 m wide, where suitable material is present. The new survey reveals landforms that result from intermediate-composition volcanism in rising water, delineates mass wasting and sediment transport into a restricted basin, and yields a more accurate postcaldera history leading to improved assessment of volcanic hazards.

Low‐productivity Hawaiian volcanism between Kaua‘i and O‘ahu

The longest distance between subaerial shield volcanoes in the Hawaiian Islands is between the islands of Kaua‘i and O‘ahu, where a field of submarine volcanic cones formed astride the axis of the Hawaiian chain during a period of low magma productivity. The submarine volcanoes lie ∼25–30 km west of Ka‘ena Ridge that extends ∼80 km from western O‘ahu. These volcanoes were sampled by three Jason2 dives. The cones are flat topped, <400 m high and 0.4–2 km in diameter at water depths between ∼2700 and 4300 m, and consist predominantly of pillowed flows. Ar-Ar and K-Ar ages of 11 tholeiitic lavas are between 4.9 and 3.6 Ma. These ages overlap with shield volcanism on Kaua‘i (5.1–4.0 Ma) and Wai‘anae shield basalts (3.9–3.1 Ma) on O‘ahu. Young alkalic lavas (circa 0.37 Ma) sampled southwest of Ka‘ena Ridge are a form of offshore secondary volcanism. Half of the volcanic cones contain high-SiO2 basalts (51.0–53.5 wt % SiO2). The trends of isotopic compositions of West Ka‘ena tholeiitic lavas diverge from the main Ko‘olau-Kea shield binary mixing trend in isotope diagrams and extend to lower 208Pb/204Pb and 206Pb/204Pb than any Hawaiian tholeiitic lava. West Ka‘ena tholeiitic lavas have geochemical and isotopic characteristics similar to volcanoes of the Loa trend. Hence, our results show that the Loa-type volcanism has persisted for at least 4.9 Myr, beginning prior to the development of the dual, subparallel chain of volcanoes. Several West Ka‘ena samples are similar to higher SiO2, Loa trend lavas of Ko‘olau Makapu‘u stage, Lāna‘i, and Kaho‘olawe; these lavas may have been derived from a pyroxenite source in the mantle. The high Ni contents of olivines in West Ka‘ena lavas also indicate contribution from pyroxenite-derived melting. Average compositions of Hawaiian shield volcanoes show a clear relation between 206Pb/204Pb and SiO2 within Loa trend volcanoes, which supports a prominent but variable influence of pyroxenite in the Hawaiian plume source. In addition, both Pb isotopes and volcano volume show a steady increase with time starting from a minimum west of Ka‘ena Ridge. The entrained mafic component in the Hawaiian plume is probably not controlling the increasing magma productivity in the Hawaiian Islands.

Tsunami-generated boulder ridges in Lake Tahoe, California-Nevada

An array of east-trending ridges 1-2 m high and up to 2 km long occurs on the Tahoe City shelf, a submerged wave-cut bench <15 m deep in the northwest sector of the lake. The shelf is just north of the amphitheater of the giant subaqueous 10 km 3 McKinney Bay landslide, which originated on the west wall of Lake Tahoe. Images from a submersible camera show that the ridges are composed of loose piles of boulders and cobbles that lie directly on poorly consolidated, fine-bedded lake beds deposited in an ancestral Lake Tahoe. Dredge hauls from landslide distal blocks, as well as from the walls of the reentrant of the landslide, recovered similar lake sediments. The McKinney Bay landslide generated strong currents, which rearranged previous glacial-derived debris into giant ripples creating the boulder ridges. The uncollapsed part of the sediment bench, including the Tahoe City shelf, poses a hazard because it may fail again, producing a landslide and damaging waves.

Whole-edifice ice volume change A.D. 1970 to 2007/2008 at Mount Rainier, Washington, based on LiDAR surveying

Net changes in thickness and volume of glacial ice and perennial snow at Mount Rainier, Washington State, have been mapped over the entire edifice by differencing between a high-resolution LiDAR (light detection and ranging) topographic survey of September–October 2007/2008 and the 10 m lateral resolution U.S. Geological Survey digital elevation model derived from September 1970 aerial photography. Excepting the large Emmons and Winthrop Glaciers, all of Mount Rainier9s glaciers thinned and retreated in their terminal regions, with substantial thinning mainly at elevations

Origin of Meter-Size Granite Basins in the Southern Sierra Nevada, California

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