Hawaiian Calderas 1 
Gordon A. Macdonald 
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 piece- 
meal 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 sur- 
face 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 associ- 
ated 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. 
The origin of Hawaiian calderas has been 
discussed several times before (Stearns and 
Clark, 1930; Stearns and Macdonald, 1946; 
Macdonald, 1956), but with the increase in 
knowledge of the physical properties of Ha- 
waiian rocks and magmas and the geophysics of 
Hawaiian volcanoes during the last decade it is 
desirable to review the former conclusions, to 
see whether they need modification. It may be 
assumed as a starting point that the calderas 
are formed by collapse. They obviously are not 
formed by erosion, and the very small amount 
of pyroclastic debris at and near the summits of 
the shields clearly eliminates any possibility of 
their having been blasted out by explosion. 
There is general agreement that they have been 
1 Hawaii Institute of Geophysics Contribution No. 
92. 
formed by the sinking in of the summit regions 
of the shield volcanoes. The term "volcanic 
sink” used for them by R. A. Daly (1914:144- 
147) is an appropriate one, though it has not 
been widely used, at least partly because of po- 
tential confusion with other types of geologic 
"sinks.” 
Though the generality of the collapse origin 
of Hawaiian calderas is unquestioned, the cause 
of the sinking is still uncertain. The purpose of 
this paper is to describe briefly the calderas, to 
sketch their spatial and time relationships to 
the volcanic structures and history, to reexam- 
ine the suggested causes of collapse, and to 
suggest a new mechanism that may be of im- 
portance in bringing about the distension of the 
mountain that is indicated both by the sinking 
of the caldera and by the rift zones. 
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