Effects of the July 10, 1996, rock fall at Happy Isles in Yosemite National Park, California, were unusual compared to most rock falls. Two main rock masses fell about 14 s apart from a 665-m-high cliff southeast of Glacier Point onto a talus slope above Happy Isles in the eastern part of Yosemite Valley. The two impacts were recorded by seismographs as much as 200 km away. Although the impact area of the rock falls was not particularly large, the falls generated an airblast and an abrasive dense sandy cloud that devastated a larger area downslope of the impact sites toward the Happy Isles Nature Center. Immediately downslope of the impacts, the airblast had velocities exceeding 110 m/s and toppled or snapped about 1000 trees. Even at distances of 0.5 km from impact, wind velocities snapped or toppled large trees, causing one fatality and several serious injuries beyond the Happy Isles Nature Center. A dense sandy cloud trailed the airblast and abraded fallen trunks and trees left standing. The Happy Isles rock fall is one of the few known worldwide to have generated an airblast and abrasive dense sandy cloud. The relatively high velocity of the rock fall at impact, estimated to be 110–120 m/s, influenced the severity and areal extent of the airblast at Happy Isles. Specific geologic and topographic conditions, typical of steep glaciated valleys and mountainous terrain, contributed to the rock-fall release and determined its travel path, resulting in a high velocity at impact that generated the devastating airblast and sandy cloud. The unusual effects of this rock fall emphasize the importance of considering collateral geologic hazards, such as airblasts from rock falls, in hazard assessment and planning development of mountainous areas.
PCclK is not prominently visible from any major metropolitan centers, and so its attractions, as well as its hazards, tend to be overlooked.Yet, Glacier Peak has produced larger and more explosive eruptions than any other Washington volcano except Mount St.
Augustine Volcano is a 1250-meter high stratovolcano in southwestern Cook Inlet about 280 kilometers southwest of Anchorage and within about 300 kilometers of more than half of the population of Alaska. Explosive eruptions have occurred six times since the early 1800s (1812, 1883, 1935, 1964-65, 1976, and 1986). The 1976 and 1986 eruptions began with an initial series of vent-clearing explosions and high vertical plumes of volcanic ash followed by pyroclastic flows, surges, and lahars on the volcano flanks. Unlike some prehistoric eruptions, a summit edifice collapse and debris avalanche did not occur in 1812, 1935, 1964-65, 1976, or 1986. However, early in the 1883 eruption, a portion of the volcano summit broke loose forming a debris avalanche that flowed to the sea. The avalanche initiated a small tsunami reported on the Kenai Peninsula at English Bay, 90 kilometers east of the volcano. Plumes of volcanic ash are a major hazard to jet aircraft using Anchorage International and other local airports. Ashfall from future eruptions could disrupt oil and gas operations and shipping activities in Cook Inlet. Eruptions similar to the historical and prehistoric eruptions are likely in Augustine's future.
Deposits between M and B tephras 24Lagoon debris-avalanche deposit Pyrosclastic-flow and lahar deposits Southeast Beach debris-avalanche and lahar(?) deposit Deposits younger than B tephra 24 Grouse Point debris-avalanche deposit West Island debris-avalanche deposit Rocky Point debris-avalanche deposit North slope lava flow Beach and eolian deposits 26 Historic deposits 27 1883 eruption 27 Burr Point debris-avalanche deposit Pyroclastic-flow and surge deposits Discussion 1935 eruption 28 Lava dome Flank deposits 1963-64 eruption 28 Lava dome Pyroclastic-flow and lahar deposits Ballistics 1976 eruption 29 Lava dome Pyroclastic-flow, surge, and lahar deposits 1986 eruption 30 Lava dome Pyroclastic-flow and lahar deposits Pumiceous flows Lithic pyroclastic flows Hybrid flows and lahars Eolian deposits 31
Research Article| October 01, 1985 Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula RICHARD B. WAITT, JR. RICHARD B. WAITT, JR. 1U.S. Geological Survey, David A. Johnston Cascades Volcano Observatory, 5400 MacArthur Boulevard, Vancouver, Washington 98661 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (10): 1271–1286. https://doi.org/10.1130/0016-7606(1985)96<1271:CFPCJF>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation RICHARD B. WAITT; Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula. GSA Bulletin 1985;; 96 (10): 1271–1286. doi: https://doi.org/10.1130/0016-7606(1985)96<1271:CFPCJF>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Two classes of field evidence firmly establish that late Wisconsin glacial Lake Missoula drained periodically as scores of colossal jökulhlaups (glacier-outburst floods). (1) More than 40 successive, flood-laid, sand-to-silt graded rhythmites accumulated in back-flooded valleys in southern Washington. Hiatuses are indicated between flood-laid rhythmites by loess and volcanic ash beds. Disconformities and nonflood sediment between rhythmites are generally scant because precipitation was modest, slopes gentle, and time between floods short. (2) In several newly analyzed deposits of Pleistocene glacial lakes in northern Idaho and Washington, lake beds comprising 20 to 55 varves (average = 30–40) overlie each successive bed of Missoula-flood sediment. These and many other lines of evidence are hostile to the notion that any two successive major rhythmites were deposited by one flood; they dispel the notion that the prodigious floods numbered only a few.The only outlet of the 2,500-km3 glacial Lake Missoula was through its great ice dam, and so the dam became incipiently buoyant before the lake could rise enough to spill over or around it. Like Grímsvötn, Iceland, Lake Missoula remained sealed as long as any segment of the glacial dam remained grounded; when the lake rose to a critical level ∼600 m in depth, the glacier bed at the seal became buoyant, initiating underflow from the lake. Subglacial tunnels then grew exponentially, leading to catastrophic discharge. Calculations of the water budget for the lake basin (including input from the Cordilleran ice sheet) suggest that the lakes filled every three to seven decades. The hydrostatic prerequisites for a jökulhlaup were thus re-established scores of times during the 2,000- to 2,500-yr episode of last-glacial damming.J Harlen Bretz's "Spokane flood" outraged geologists six decades ago, partly because it seemed to flaunt catastrophism. The concept that Lake Missoula discharged regularly as jökulhlaups now accords Bretz's catastrophe with uniformitarian principles. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The rhythmic Touchet Beds in the Walla Walla and lower Yakima valleys resulted from many separate backfloodings by hydraulically ponded glacial Lake Missoula water. At least once this episodic lake briefly contained half the $$2130 km^{3}$$ of water that catastrophically drained the largest glacial Lakes Missoula. Evidence that the Touchet Beds rhythmites originated from brief backfloodings includes up-valley thinning and fining of locally derived bedload, upvalley paleocurrents, and upvalley transport of erratics derived from Cordilleran ice. Evidence that a lengthy nonflood environment followed the emplacement of each of about 40 Touchet Beds rhythmite includes inferred eolian and slopewash sediment overlying many rhythmites, uncontaminated Mount St. Helens "set S" tephra couplet atop one rhythmite as much as 220 m below the maximum level of backflooding, filled semiconsolidated rodent burrows throughout the 30 m of the thickest section, and dispersed skeletons of mammals. The lack of weathering or soil within the Touchet Beds suggests that all rhythmites are late Wisconsin. Bottom sediment of glacial Lake Missoula in Montana consists of rhythmites each interpreted as the record of a gradually deepening lake. Forty superposed rhythmites record about 40 late-Wisconsin fillings and emptyings of glacial Lake Missoula. The complementary records of about 40 separate glacial Lakes Missoula and about 40 great floods in southern Washington and in the Willamette Valley, Oregon indicate that the Missoula floods were great jokulhlaups. The last several floods were smaller than earlier ones because the controlling dam of Cordilleran ice thinned during deglaciation.
