William E. Scott

Reinterpretation of the exposed record of the last two cycles of Lake Bonneville, Western United States

A substantially modified history of the last two cycles of Lake Bonneville is proposed. The Bonneville lake cycle began prior to 26,000 yr B.P.; the lake reached the Bonneville shoreline about 16,000 yr B.P. Poor dating control limits our knowledge of the timing of subsequent events. Lake level was maintained at the Bonneville shoreline until about 15,000 yr B.P., or somewhat later, when catastrophic downcutting of the outlet caused a rapid drop of 100 m. The Provo shoreline was formed as rates of isostatic uplift due to this unloading slowed. By 13,000 yr B.P., the lake had fallen below the Provo level and reached one close to that of Great Salt Lake by 11,000 yr B.P. Deposits of the Little Valley lake cycle are identified by their position below a marked unconformity and by amino acid ratios of their fossil gastropods. The maximum level of the Little Valley lake was well below the Bonneville shoreline. Based on degree of soil development and other evidence, the Little Valley lake cycle may be equivalent in age to marine oxygenisotope stage 6. The proposed lake history has climatic implications for the region. First, because the fluctuations of Lake Bonneville and Lake Lahontan during the last cycle of each were apparently out of phase, there may have been significant local differences in the timing and character of late Pleistocene climate changes in the Great Basin. Second, although the Bonneville and Little Valley lake cycles were broadly synchronous with maximum episodes of glaciation, environmental conditions necessary to generate large lakes did not exist during early Wisconsin time.

Character, mass, distribution, and origin of tephra-fall deposits of the 1989–1990 eruption of redoubt volcano, south-central Alaska

Abstract The 1989–1990 eruption of Redoubt Volcano spawned about 20 areally significant tephra-fall deposits between December 14, 1989 and April 26, 1990. Tephra plumes rose to altitudes of 7 to more than 10 km and were carried mainly northward and eastward by prevailing winds, where they substantially impacted air travel, commerce, and other activities. In comparison to notable eruptions of the recent past, the Redoubt events produced a modest amount of tephra-fall deposits − 6 × 10 7 to 5 × 10 10 kg for individual events and a total volume (dense-rock equivalent) of about 3–5 × 10 7 m 3 of andesite and dacite. Two contrasting tephra types were generated by these events. Pumiceous tephra-fall deposits of December 14 and 15 were followed on December 16 and all later events by fine-grained lithic-crystal tephra deposits, much of which fell as particle aggregates. The change in the character of the tephra-fall deposits reflects their fundamentally different modes of origin. The pumiceous deposits were produced by magmatically driven explosions. The finegrained lithic-crystal deposits were generated by two processes. Hydrovolcanic vent explosions generated tephrafall deposits of December 16 and 19. Such explosions continued as a tephra source, but apparently with diminishing importance, during events of January and February. Ash clouds of lithic pyroclastic flows generated by collapse of actively growing lava domes probably contributed to tephra-fall deposits of all events from January 2 to April 26, and were the sole source of tephra fall for at least the last 4 deposits.

August 2008 Eruption of Kasatochi Volcano, Aleutian Islands, Alaska—Resetting an Island Landscape

Abstract Kasatochi Island, the subaerial portion of a small volcano in the western Aleutian volcanic arc, erupted on 7–8 August 2008. Pyroclastic flows and surges swept the island repeatedly and buried most of it and the near-shore zone in decimeters to tens of meters of deposits. Several key seabird rookeries in taluses were rendered useless. The eruption lasted for about 24 hours and included two initial explosive pulses and pauses over a 6-hr period that produced ash-poor eruption clouds, a 10-hr period of continuous ash-rich emissions initiated by an explosive pulse and punctuated by two others, and a final 8-hr period of waning ash emissions. The deposits of the eruption include a basal muddy tephra that probably reflects initial eruptions through the shallow crater lake, a sequence of pumiceous and lithic-rich pyroclastic deposits produced by flow, surge, and fall processes during a period of energetic explosive eruption, and a fine-grained upper mantle of pyroclastic-fall and -surge deposits that probably reflects the waning eruptive stage as lake and ground water again gained access to the erupting magma. An eruption with similar impact on the islands environment had not occurred for at least several centuries. Since the 2008 eruption, the volcano has remained quiet other than emission of volcanic gases. Erosion and deposition are rapidly altering slopes and beaches.

The geomorphology of an Aleutian Volcano following a major eruption; the 7-8 August 2008 eruption of Kasatochi Volcano, Alaska, and its aftermath.

Abstract Analysis of satellite images of Kasatochi volcano and field studies in 2008 and 2009 have shown that within about one year of the 7–8 August 2008 eruption, significant geomorphic changes associated with surface and coastal erosion have occurred. Gully erosion has removed 300,000 to 600,000 m3 of mostly fine-grained volcanic sediment from the flanks of the volcano and much of this has reached the ocean. Sediment yield estimates from two representative drainage basins on the south and west flanks of the volcano, with drainage areas of 0.7 and 0.5 km2, are about 104 m3 km−2 yr−1 and are comparable to sediment yields documented at other volcanoes affected by recent eruptive activity. Estimates of the retreat of coastal cliffs also made from analysis of satellite images indicate average annual erosion rates of 80 to 140 m yr−1. If such rates persist it could take 3–5 years for wave erosion to reach the pre-eruption coastline, which was extended seaward about 400 m by the accumulation of erupted volcanic material. As of 13 September 2009, the date of the most recent satellite image of the island, the total volume of material eroded by wave action was about 106 m3. We did not investigate the distribution of volcanic sediment in the near shore ocean around Kasatochi Island, but it appears that erosion and sediment dispersal in the nearshore environment will be greatest during large storms when the combination of high waves and rainfall runoff are most likely to coincide.

Mount St. Helens: A 30-Year Legacy of Volcanism

The spectacular eruption of Mount St. Helens on 18 May 1980 electrified scientists and the public. Photodocumentation of the colossal landslide, directed blast, and ensuing eruption column—which reached as high as 25 kilometers in altitude and lasted for nearly 9 hours—made news worldwide. Reconnaissance of the devastation spurred efforts to understand the power and awe of those moments (Figure 1). The eruption remains a seminal historical event—studying it and its aftermath revolutionized the way scientists approach the field of volcanology. Not only was the eruption spectacular, but also it occurred in daytime, at an accessible volcano, in a country with the resources to transform disaster into scientific opportunity, amid a transformation in digital technology. Lives lost and the impact of the eruption on people and infrastructure downstream and downwind made it imperative for scientists to investigate events and work with communities to lessen losses from future eruptions.

Quaternary volcanism in the United States

Publisher Summary This chapter describes the concept of volcanic loci––vents or groups of vents that define logical units of volcanism in space and time––and focuses on advances in the understanding of the Quaternary history of volcanic areas in the United States and key processes in the evolution of volcanoes. Large flank failures and caldera-forming eruptions catastrophically change the form of some edifices; subsequent eruptions rebuilt many of them. Some of the long-lived fields show evidence of migration of active vent areas through time as the North American Plate moved relative to a deeper fixed zone of magma genesis. Quaternary eruptions in the Hawaiian Islands reflect the long-lived volcanism related to the movement of the Pacific Plate across the Hawaiian hot spot. Each major Hawaiian volcano follows a general evolution from submarine volcano to rapidly growing emergent shield volcano. The detailed mapping, geochronologic, geophysical, and petrologic investigations of Quaternary volcanic centers in the U.S. reveal new insights about long-term eruptive history; magma-genesis, -transport, and -eruption processes; and influence of tectonics on volcanic systems.

Field-trip guide to Mount Hood, Oregon, highlighting eruptive history and hazards

Mount Hood, Oregon, an archetypal subduction zone stratovolcano, is dominated by extrusive eruptions of lava flows and domes, coupled with a high degree of homogeneity in erupted lava compositions. Over the last ~500,000 years— the age of the current edifice—the volcano has repeatedly erupted crystal-rich andesites and low SiO2 dacites, with SiO2 contents largely between 55 and 65 weight percent. Lavas also show similar phenocryst mineralogy, compositions, and textures, and are dominated by plagioclase together with pyroxene, amphibole, and occasional olivine. The presence of quenched mafic inclusions, bimodal populations of plagioclase and amphibole, mineral zoning, and a range of other evidence also shows that Mount Hood magmas are produced by quasi-binary mixing between relatively mafic (basaltic) and silicic (rhyodacitic-rhyolitic) parental magmas. Mineral zoning shows that magma mixing occurred very late in the petrogenetic history, within weeks to months of eruption. Mount Hood is a volcanic system driven by mafic recharge, where hot mafic magmas ascending from the mantle or lower crust interact with silicic magmas to produce mixed intermediate compositions. Evidence suggests that the silicic parental magma is stored within the shallow crust (3–6 kilometers) beneath the volcano as cool, crystalrich mush for long periods (>>10 ka) prior to eruptions. Mafic recharge provides both the impetus to erupt and produces the intermediate compositions, resulting in the long-term eruptive output of a homogeneous series of intermediate magmas.