Volcanic edifice collapses produce scars that vary from shallow spoon-shaped scarps to huge horseshoe-shaped amphitheaters several kilometers wide. Some large avalanche scarps could be confused with erosionally breached calderas; there are, however, differences. Avalanche amphitheaters have subparallel sidewalls or widen in the breached direction; in contrast, erosionally breached calderas commonly narrow in the breached direction (Siebert, 1984). The sizes of avalanche scarps and calderas overlap, but calderas can be larger. Avalanche scarps commonly range from 1 to 3 km in width and have a median diameter of 2 km; calderas have a median dimater of 6 km and can be several 10's of km wide (Siebert, 1984). Volcanic edifice collapses can occur in association with magmatic eruption (like those at Bezymianny in 1956 and Mount St. Helens in 1980), or with no explosive eruptions (like that at Unzen volcano in 1972)(Siebert et al., 1987). Laterally directed blasts have significant low-angle components directed in arcs of 180 degrees of less (Crandell and Hoblitt, 1986). Lateral blasts can spread over large areas regardless of intervening topography, leaving behind only thin deposits, which have some of the characteristic of pyroclastic flows and pyroclastic surges. The Mount St. Helens blast moved at speeds in excess of 300 m/s (Kieffer, 1981), devastated about 600 km², and produced a basic three layer stratigraphy in distal areas (Hoblitt et al., 1981). The basal unit generally is fines-depleted and very friable; but may also be more massive and poorly sorted in its upper part especially near the volcano; the fine-grained surge unit is planar bedded with small-scale dunes; the accretionary lapilli unit is a very-fine-grained fall deposit containing abundant accretionary lapilli (Hoblitt et al., 1981).
Debris avalanches are the processes most commonly associated with edifice collapses. Debris avalanche is a term that connotes sudden, rapid flowage of wet or dry, incoherent, unsorted mixtures of rock and matrix in response to gravity (Schuster and Crandell, 1984). Although the debris can contain significant moisture, the bulk of it is unsaturated. A debris avalanche deposit is characterized by its irregular hummocky topography, which may exhibit mounds, closed depressions, large transverse ridges, and lateral levees. Some debris avalanches contain sufficient moisture that they dewater and form lahars. Lahars having this origin flowed several tens of kilometers downstream from the 1980 debris avalanche at Mount St. Helens (Janda et al., 1981).
Edifice collapses can also directly form lahars. Crandell (1971) noted four Holocene lahars having this origin at Mount Rainier and Vallance (in press) noted two others at Mount Adams. Rock is more likely to produce a lahar after it collapses, when it contains hydrothermal clay, which, because of its large interstial surface area, is capable of a large volume of pore-water. Volcanoes which have hydrothermally altered summit regions thus are more likely sites for lahars of this type. Avalanche induced lahars have greater mobility and may flow further than similar-sized debris avalanches.
(Vallance et al., 1988)
- Conditions that could influence the probability and direction of edifice collapse events. A. Zones of hydrothermal alteration and less competent clastic beds may form zones of weakness within the edifice; further less competent layers may form natural planes of weakness. B. The intrusion of magma to a shallow depth may deform and thus weaken the edifice. C. Possible zones of weakness may exist in the boundary between paired volcanoes. Because the youngest of four such pairs in Guatemala and El Salvador is always to the south (Halsor and Rose, 1988), and the possible zones of weakness dip to the south, the preferred failure direction is also to the south. D. The orientation of bedrock underlying a volcano may influence the direction of possible future edifice collapse. (Vallance et al., 1988)
