REPORT:
Rockslide-debris Avalanche of May 18, 1980, Mount St. Helens Volcano, Washington

Table of Contents

Abstract

Introduction

Acknowledgments

Geology of the source area

General Statement

Description of rock units

Older dacite unit

Andesite and basalt unit

Modern dacite unit

Cryptodome

Pre-eruption structure

Geology of the slide blocks

Geometry of the deposit

Volume and thickness

Morphology of hummocks

Orientation of elongate hummocks

Hummock types

Formation of the hummocks

Interpretation of size data

Geologic maps of the deposit

General statement

Morphologic map of the debris-avalanche deposit

North Fork unit

Johnston Ridge unit

Spirit Lake unit

Marginal unit

Proximal unit

Distal unit

Lithologic map

Block facies units

Older dacite unit

Andesite and basalt unit

Modern dacite unit

Modern undifferentiated unit

Mixed block and matrix facies unit

Contacts between lithologic units

Other deposits of the May 18, 1980 eruption

Blast deposits

Lahar deposits

Pyroclastic flow deposits of the afternoon of May 18

Tertiary bedrock

Texture of the deposit

General statement

Terminology

Scales of study

Textural interpretation of map relations

East of the break-in-slope

West of the break-in-slope

Lateral variation in debris-avalanche block size

Textural interpretation of exposures

General statement

Description of windows

Block-facies windows

Block- and matrix-facies windows

Matrix-facies windows

Discussion of windows

Density measurements

Grain-size analysis

Methods

Results

General statement

Distribution plots

Cumulative plots

Statistical parameters

Lateral variations of statistical parameters

Conclusions

Slide and blast events

Slide block I

Slide block II and blast surge

Slide block III

Final events

Water budget

Transition to flow and disintegration of material

Fluidization and fluids in the flow

Turbulence

The base of the debris avalanche

Mobility of the debris avalanche

References cited

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