STYLES OF CATASTROPHIC COLLAPSE IN VOLCANIC TERRAINS AND THEIR COMMON LINK TO SUBVOLCANIC MAGMA SYSTEMS

Catastrophic structural slope failures of large volcanic landforms, notably volcanoes, are among the most rapid agents of landscape change, producing large-scale landslide features. Some of these landslides rank among the largest and most devastating natural hazards on Earth. Following the 1980 volcanic landslide of Mount St. Helens, there has been an increased awareness and study of collapse features of stratovolcanoes worldwide. Subsequent studies have recognized collapse events on more than 400 prehistoric volcanoes (Siebert et al., 2006). Collapse of parts of volcanoes is now widely accepted as a common phenomenon during their evolution. Although most studies of structural failure have concentrated on volcanoes, our work in Oligocene-Miocene volcanic terrains of southwest Utah has documented similar slope failures from two additional sources: 1) intrusive laccoliths, and 2) volcanic fields. Landslides from these less familiar sources share most morphological and textural similarities to landslides from volcanoes, which makes them easily mistaken as volcano-derived deposits. The largest laccolith-derived slide mass (Big Mountain slide, off the Bull Valley Mountains) is ~50 to 60 km³, covers >150 km², is more than 550 m thick in places, and extends more than 25 km from its parent laccolith dome, far larger than the Mount St. Helens volcano event. And on a far greater scale, the largest partial collapse of an entire volcanic field comes from the Marysvale volcanic field (Markagunt gravity slide) that produced a slide mass reaching ~3000 km³, covering >5000 km², with a length of >95 km. Partial collapses of volcanic fields thus represent the “supervolcanoes” of the volcanic landslide world, and although rare and least recognized, they have produced the largest subaerial volcanic landslides on Earth. Subvolcanic magma systems play an integral part in the collapse process from the three volcanic source types (volcanoes, laccoliths, and volcanic fields) by rapidly creating elevated landforms with steep slopes, by aiding in destabilization of the slopes, and by triggering the slope failure. We therefore introduce the concept that volcanic collapse landslide features should be viewed as involving multiple sources within volcanic terrains instead of from only volcanoes.