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:
- 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
3. Lahar deposits rich in hydrothermally-altered silt and clay particles and volcanic rocks downstream from a volcano
4. Horeshoe-shaped avalanche scarps.