Sub-surface structures and collapse mechanisms of summit pit craters

Summit pit craters are found in many types of volcanoes and are generally t hought to be the product of collapse into an underpressured reservoir cause d by magma withdrawal. We investigate the mechanisms and structures associa ted with summit pit crater formation by scaled analogue experiments and mak e comparisons with natural examples. Models use a sand plaster mixture as a nalogue rock over a cylinder of silicone simulating an underpressured magma reservoir. Experiments are carried out using different roof aspect ratios (roof thickness/roof width) of 0.2-2. They reveal two basic collapse mechan isms, dependant on the roof aspect ratio. One occurs at low aspect ratios ( less than or equal to1), as illustrated by aspect ratios of 0.2 and I. Outw ard dipping reverse faults initiated at the silicone margins propagates thr ough the entire roof thickness and cause subsidence of a coherent block. Co llapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface (aspect ratio of 0.2) or by the creation of inward dipping normal faults delimiting a terrace (aspect ratio of 1). At an aspect ratio of 1, overhanging pit walls are the surface expr essions of the reverse faults. Experiments at high aspect ratio (>1.2) reve al a second mechanism. Tn this case, collapse occurs by stopping, which pro pagates upwards by a complex pattern of both reverse faults and tension fra ctures. The initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it. An intermediate mechanism occurs at aspect ratios of 1.1-1.2. In this case, stopping leads to the formation of a cavity with a thin and unstable roof, which collapse s suddenly. The newly formed depression then exhibits overhanging walls. Su rface morphology and structure of natural examples, such us the summit pit craters at Masaya Volcano, Nicaragua, have many of the features created in the models, indicating that the internal structural geometry of experiments can be applied to real examples. In particular, the surface area and depth of the underpressured reservoir can be roughly estimated. We present a mor phological analysis of summit pit craters at volcanoes such as Kilimanjaro (Tanzania), Son Cristobal, Telica and Masaya (Nicaragua), and Ubinas (Peru) , and indicate a likely type of subsidence and possible position of the for mer magma reservoir responsible for collapse in each case. (C) 2001 Elsevie r Science B.V. All rights reserved.