Keith A. Howard¹*

1 U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025-3591, USA

Abstract

The 1968 trapdoor collapse (1.5 km³) of Fernandina caldera in the Galapágos Islands developed the same kinds of structures as found in small sandbox-collapse models and in concentrically zoned sinks formed in desert alluvium by fault subsidence into underground nuclear-explosion cavities. Fernandina’s collapse developed through shear failure in which the roof above the evacuating chamber was lowered mostly intact. This coherent subsidence contrasts to chaotic piecemeal collapse at small, rocky pit craters, underscoring the role of rock strength relative to subsidence size. The zoning at Fernandina implies that the deflated magma chamber underlay a central basin and a bordering inward-dipping monocline, which separates a blind inner reverse fault from an outer zone of normal faulting. Similar concentric zoning patterns can be recognized in coherent subsidence structures ranging over 16 orders of magnitude in size, from sandbox experiments to the giant Olympus Mons caldera on Mars.

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Manuscript received 29 Sept. 2009; accepted 18 Feb. 2010

DOI: 10.1130/GSATG82A.1