|Aerial view from the south of snow-covered Mokuaweoweo, the summit caldera
of Mauna Loa Volcano, and several pit craters of its southwest rift zone, Mauna Kea
Volcano, which last erupted about 3,000 years ago, can be seen in the distance.
(Photograph by Donald W. Peterson.)
||Hawaiian and other shield volcanoes characteristically have a broad summit,
indented with a caldera, a term commonly used for a large depression of volcanic
origin. Most calderas form by collapse because of removal of magma from the
volcano's reservoir by eruption and/or intrusion. Kilauea's summit caldera is
about 2.5 miles long and 2 miles wide. Mokuaweoweo, the summit caldera complex of
Mauna Loa is more elongate, measuring about 3 by 1.5 miles. The terms crater or
pit crater are applied to similar but smaller collapse features.
Rift zones radiate from the summit calderas of both Mauna Loa and Kilauea and extend down the volcanic flanks into the sea. They are elongate tapering ridges expressed by prominent open fissures, pit craters, cinder and spatter cones, and small volcanic shields. The orientation of rift zones is influenced by the gravitational stresses and buttressing effects of pre-existing neighboring volcanoes. Most Hawaiian eruptions take place either within summit calderas or along rift zones.
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