Hawaiian Calderas — Macdonald 
321 
MORPHOLOGY OF THE CALDERAS 
The summits of the active shield volcanoes, 
Kilauea and Manna Loa, are indented by oval 
calderas. That of Kilauea is 4.4 km long and 
3 km wide. Mokuaweoweo caldera, at the sum- 
mit of Mauna Loa, is 4.5 km long and 2.6 km 
wide. Both are slightly excentric with respect 
to the precise summits of the mountains. Ki- 
lauea caldera is 150 m deep at its western edge, 
beneath the highest point of the mountain, but 
its walls decrease in height essentially to zero 
at its south side. It is separated from the adja- 
cent pit crater of Kilauea Iki by only a low, 
narrow ridge (the Byron Ledge). Mokuaweo- 
weo is 180 m deep at its western edge, 130 m 
deep at its eastern edge. At the south it merges 
with the pit crater, South Pit, and at the north 
its floor is continuous into the pit crater known 
as North Bay, which is bounded at the north 
by a wall only about 5 m high. At its northeast 
edge Mokuaweoweo also merges with the very 
small pit crater, East Bay; and farther north the 
pit crater Lua Poholu lies within the outermost 
faults of the caldera. It is generally agreed that 
the pit craters, like the calderas, have formed 
by collapse. 
Extending outward from the calderas are the 
rift zones of the volcanoes ( zones of fracturing 
averaging about 3 km wide ) that have served as 
the feeding conduits for most flank eruptions of 
the volcanoes. On the southwest rift zone of 
Mauna Loa, within 3 km of Mokuaweoweo, lie 
three pit craters, the central one of which has 
been formed since 1840. Along the east rift 
zone of Kilauea within 20 km of the caldera 
lies a whole series of pit craters (Stearns and 
Macdonald, 1946: PI. 1). 
In most places the boundaries of the calderas 
are steep cliffs, with an average slope of about 
75°. Layers of lava in the cliffs slope outward 
away from the summit of the mountain and 
their truncated edges project upward into space 
in the present area of the depression. The vents 
that fed them must have been located at a 
higher level, and must have dropped out of 
sight at the time of origin of the caldera. In 
other words, the caldera cannot have been pres- 
ent in anything approaching its present dimen- 
sions until the shield had reached essentially its 
present size. The cliffs bounding the calderas 
are fault scarps. Along most of the boundary 
the cliff is simple, but in places it consists of a 
series of step-fault blocks. The fault planes sep- 
arating the step blocks appear to dip toward 
the center of the caldera at about the same 
angle as the rest of the boundary scarp. Locally, 
the faults pass into monoclines (Macdonald, 
1957). 
The main calderas are partly surrounded by 
benches as much as 3 km wide that have also 
been dropped down on faults, but not as far as 
the central caldera. Viewed from an airplane a 
shori distance away, the summit areas of both 
Kilauea and Mauna Loa are seen to have sagged 
downward over a subcircular area 6-8 km 
across, with the sharply defined downfaulted 
calderas at the center. 
The floor of Kilauea caldera is a very gently 
sloping dome or cone rising to an apex at Hale- 
maumau crater, in the southwestern part of 
the caldera. The cone was formed by repeated 
overflows from Halemaumau during the long 
period of lava lake activity before 1924, with 
the minor addition of the lava flow of the 1954 
eruption (Macdonald and Eaton, 1957). The 
floor of Mokuaweoweo also slopes upward 
toward the cones of the 1940 and 1949 erup- 
tions. During the last 150 years the history of 
the calderas has been one of repeated collapse 
of the floor and refilling by eruptions on the 
floors. In 1825 the center of Kilauea caldera 
was a pit some 260 m deep, surrounded by a 
narrow "black ledge" 30 m or so below the 
present floor level. This central depression is 
presumed to have formed by collapse at the 
time of the flank eruption in 1823. By 1832 the 
central pit had been filled to overflowing, but 
in that year it was reestablished in much its 
former condition by another collapse. Again it 
was refilled, only to be reformed by collapse 
accompanying the eruption of 1840. Still again 
it was refilled, only to sag down in a less ex- 
tensive depression at the time of the 1868 erup- 
tions, and so on. From 1840 onward each col- 
lapse was smaller than the one that preceded it, 
until in the 1920’s two small collapses were 
followed by a large one (Table 1). It should 
be noted here that the collapses of the 19th 
century listed in the table represent the volume 
