Volcanic collapses leave evidence

Discovery of so many landslide deposits was made possible because of characteristics that geologists can easily recognize in the field:

1. Hummocks, or rounded mounds of material, within landslide deposits. Formation of hummocks is not fully understood, but several mechanisms have been proposed:

Hummocks shown in the foreground 10's of kilometers away from Mt. Shasta, California (Harry Glicken, USGS 1982)

  • Landslide lateral spreading. Some hummocks are bounded by faults or slumping, which suggests that the slope of the hummock represents the slip surface of the fault or slump.
  • Individual landslide blocks with no faulting between hills; the blocks may be suspended in rock debris that has been thoroughly mixed (i.e. the original layers of the volcano are not preserved).
  • Some hummocks formed as the basal part of the landslide slowed and the faster-moving upper part either sculpted the material into elongate hills parallel to the direction of flow or piled up the material into randomly-oriented elongate hills.

2. Shattered but otherwise well-preserved volcanic rock layers within the deposits

Debris-avalanche block sizes found in different parts of a debris avalanche (Glicken, 1996)

3. Lahar deposits rich in hydrothermally-altered silt and clay particles and volcanic rocks downstream from a volcano

Lahar deposit with volcanic rock pieces and lighter clay (K.M. Scott, USGS)

4. Horeshoe-shaped avalanche scarps.

Avalanche scarp of white island volcano, New Zealand (http://wikitravel.org/en/White_Island)


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