USGS/Cascades Volcano Observatory, Vancouver, Washington
REPORT:
Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula
-- Richard B. Waitt, Jr., 1985,
Case for periodic, colossal jökulhlaups from Pleistocene glacial Lake Missoula:
IN: Geological Society of America Bulletin, v.96, p.1271-1286, October 1985
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 was 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-cubic-kilometer 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 Grimsvotn, Iceland, Lake
Missoula remained sealed as long as any segment of the glacial dam remained
grounded; when the lake rose to a critical level around 600 meters 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 jokulhlaup were thus
re-established scores of times during the 2,000- to 2,500-year 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 jokulhlaups now accords Bretz's catastrophe with
uniformitarian principles.
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04/16/01, Lyn Topinka