The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
Comparison of Crater Lake with Other Calderas
AMONG shield volcanoes of Hawaiian type, calderas A result from drainage of the central conduits by copious outflow of lava from fissures far down the flanks or by widespread intrusion at depth. Support is thus removed from beneath the central, summit vent, the walls of which then founder along arcuate fractures. For instance, the pit of Halemaumau on Kilauea was greatly enlarged during the steam-blast eruptions of 1924. Yet only an insignificant fraction of the enlargement was produced by the actual eruptions; the enlargement resulted almost entirely from collapse induced by withdrawal of support as the lava level in the principal conduit was lowered by submarine flows escaping from fissures on the sides of the shield.
In composite volcanoes built of more siliceous ejecta, collapse and caldera formation may likewise be brought about by dike injection at depth, but perhaps more generally it results from drainage of the magma chamber by colossal eruptions of pumice and scoria.
The history of Crater Lake is closely paralleled by that of Krakatau. The famous outbreak of this volcano began in May 1883. At first, the eruptions were mild. Pumice was hurled above the vents and settled in showers under the control of the wind. During the culminating eruptions of August 26-28, the pumice escaped so rapidly that most of it fell in the neighborhood of the vents in the form of glowing avalanches, almost identical in character with those which swept down the canyons around Crater Lake. In all, 18 cubic kilometers of material were blown out. Of this, only about 5 per cent was composed of old lava fragments torn from the sides of the original cones. The remainder was new magma evacuated from the chamber below. This sudden drainage of the reservoir brought about collapse, and it was the foundering of the old cones into the sea that propelled the devastating tidal waves which killed 36,000 natives on the shores of Java and Sumatra. The form of the caldera floor bears the unmistakable imprint of this mode of origin. No funnel-shaped explosion vents were formed, but- in addition to the caldera itself deep submarine troughs were produced leading out from the margins along lines of tectonic weakness. Forty-four years of quiescence ensued. In 1927, a new basaltic cinder cone, Anak Krakatau, rose from the floor of the caldera, just as Wizard Island rose within Crater Lake.