Pit crater formation on Kilauea volcano, Hawaii.

Abstract

Most of the known pit craters in Hawaii occur along the East and Southwest Rift Zones of Kilauea volcano. The pit craters typically are either astride a single rift zone fracture or between a pair of rift zone fractures. These fractures are prominent in the pit crater walls. The pit craters are elliptical in plan view, with their major diameters ranging from 8 to 1140 m. They range in depth from 6 m to 186 m. They typically develop with initially steep, locally overhanging walls, but as the walls collapse, the craters fill with talus and become shaped like inverted elliptical cones. None of the craters apparently formed as eruptive vents, although some have been subsequently filled by lava. Devil's Throat is the best-exposed pit crater along the East Rift Zone. It is sited at a `waist' between two east-striking zones of ground cracks; the spacing between the crack zones decreases towards Devil's Throat. East-striking fractures are also prominent in the pit crater walls. Pit craters along the Southwest Rift Zone typically are elongate in plan view along the direction of the rift, have large caves at their bases along the long axes of the craters, and are smaller than those of the East Rift Zone. Some closely spaced pits there have coalesced to form a trough. Based on our observations and mechanical considerations, we infer that pit craters form by stoping over an underlying large-aperture rift zone fracture, and not by piston-like collapse over broad magma bodies or voids. Flow of magma along the underlying fracture may remove stoped blocks and prevent the fracture from being choked with debris. This mechanism is consistent with pit crater location, ground crack patterns, the preferred orientation of fractures in pit crater walls, and pit crater geometry (both in map view and cross-section). The mechanism also fits with observations of stoping into a gaping rift fracture that conducted lava from Kilauea caldera during the 1920s. Additionally, the ratio of pit crater width to depth of 0.5 to 2 is consistent with pit craters forming over a nearly vertical opening mode fracture.