Sequence of Strata Depositions

Up to 600 feet thickness of new strata have formed since 1980 at Mt. St. Helens.

Causes of depositions

1. Primary air blasts 2. Landslide 3. Wave on the lake 4. Pyroclastic flows: "a dense, turbulent avalanche or ground-hugging slurry of gases and volcanic fragments created at the neck of a volcano by an explosive, magmatic eruption."

There were 6 eruptions in 1980 with pyroclastic flows: May 18, May 25, June 12, July 22, August 7, and October 18-19. 5. Mudflows 6. Air fall (ash fallout) 7. Stream water The nature of the deposits Extensive strata with very thin laminae and cross-bedding Course and fine sediments were separated into distinct strata by the flow process

Causes of erosion

1. Steam blasts: Energy equivalent to 20,000 tons of TNT in north direction caused hot gas and rock abrasion 2. Debris avalanche: rock, ice and debris movement caused abrasion 3. Water waves: one-quarter of the avalanche landed in the lake causing a wave 860 feet high to erode the adjacent slopes 4. Pyroclastic flows: volcanic ash and steam traveled along the ground at temperatures estimated at 950o C 5. Mudflows: melted glacier ice pouring down the mountain collected volcanic ash and eroded soft volcanic deposits as well as hard rock 6. Jetting steam: glacier ice buried by hot ash exploded forming pits 7. Water in channels: overflowing caused rill and gully patterns, even on level slopes

Specific areas:
Toutle River cross-section
Loowit Canyon cross-section
Steam explosion pits

Reference

Austin, Lumsden, Morris, and Vardiman. Mt. St. Helens Tour Guidebook. Institute for Creation Research, 1997.