Skip to Main Content
Book Chapter

Origin, behavior, and sedimentology of prehistoric catastrophic lahars at Mount St. Helens, Washington

By
Kevin M. Scott
Kevin M. Scott
Search for other works by this author on:
Published:
January 01, 1988

A series of four lahars (volcanic debris flows and their deposits) occurred in rapid succession about 2,500 radiocarbon yr ago in the river system that drains the northwest sector of Mount St. Helens. The huge initial lahar had an instantaneous peak discharge near that of the Amazon River at flood stage, and the third in the series was the second largest in the history of the watershed. The deposits of the flows form a widespread terrace underlain by as much as 12 m of deposits, but its distribution reflects the fact that the lahars were the middle segments of flood waves beginning and ending as streamflow surges. These flood waves originated as lake-breakout surges analogous to those that would have been released, without engineering intervention, from the outlet-blocked Spirit Lake and several new lakes formed in 1980.

The lahar deposits are granular and noncohesive, reflecting the origin of the entrained sediment as stream alluvium beyond the base of the volcano. Megaclasts consisting of blocks probably derived from an ancient debris avalanche are abundant in the initial flow and locally form a diamicton with the laharic diamicton as matrix. Although fine-pebble angularity resulting partly from cataclasis during flow is diagnostic of the lahars, a high content of rounded clasts and a channel facies with zones of clast support may explain why similar deposits have not been more widely recognized. The lahar channel facies superficially resembles alluvium.

The depositional record of lahars and their distally evolved, hyperconcentrated lahar-runout flows many tens of kilometers from their source can provide a remarkably detailed history of volcanic activity. It may, in fact, provide the only evidence of some important events, of which the magnitude, frequency, and behavior are vital to the assessment of future volcanic hazards. Lake breakouts, meltwater surges caused by large pyroclastic flows, and forms of catastrophic volcanic ejection are examples of events that can create large downstream lahars but may leave little evidence of their true magnitude preserved on and near a volcano. The largest lahar in the river system, for example, began as a streamflow surge that bulked to a debris flow only after more than 20 km of flow.

You do not currently have access to this article.

Figures & Tables

Contents

GSA Special Papers

Sedimentologic Consequences of Convulsive Geologic Events

H. Edward Clifton
H. Edward Clifton
Search for other works by this author on:
Geological Society of America
Volume
229
ISBN print:
9780813722290
Publication date:
January 01, 1988

References

Related

Citing Books via

Close Modal
This Feature Is Available To Subscribers Only

Sign In or Create an Account

Close Modal
Close Modal