Hawaiian Calderas

Abstract

Hawaiian calderas form by collapse during the last stages of growth of shield volcanoes built by frequent eruptions of tholeiitic basalt. They range from 2-12 miles across, have sunk several thousand feet, and in part have grown piecemeal by coalescence of smaller collapse craters. They may never have formed on some volcanoes, and all are partly or wholly filled by continued eruption. Toward the end of the filling activity slows, and alkalic lavas complete the filling and build a thin cap over the caldera. Gravity studies reveal masses of ultra-dense rocks only 1-2 km below the surface of several of the volcanoes-perhaps olivine-rich cumulates in the feeding pipe of the volcano, or perhaps protrusions of the mantle. The idea that these may have led to formation of the calderas by isostatic sinking of a heavy column culminating in the caldera appears to be negated by the facts that some calderas show no associated gravity high, in some the high is offset to one side of the caldera, and some highs are not associated with any apparent caldera collapse. Caldera formation probably depends on the formation of a magma reservoir within the mass of the shield volcano, with its top within a few kilometers of the summit of the shield. The Glen Coe mechanism of caldera formation seems to be ruled out by the lack of upward displacement of magma around the sinking block. Caldera collapse is probably the result of sinking of a block bounded by inward-dipping conical fractures, permitted by distension of the top of the volcano and removal of support due to drainage of magma into the rift zones, with or without flank eruption. The distension of both the summit region and the rift zones may result from a lateral spreading of the lower part of the ultra-dense core of the volcano.