Hawaiian Calderas
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1965-07
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University of Hawai'i Press
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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.
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Macdonald GA. 1965. Hawaiian calderas. Pac Sci 19(3): 320-334.
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