Some process other than the expulsion of magma from the feeding chamber must also have operated. Whereas 17 cubic miles of the volcano disappeared, at most 11.75 cubic miles of ejecta were laid down. Moreover, by far the bulk of these ejecta consists of vesicular glass. The equivalent volume of liquid magma was less than half as much. Accordingly, it must be concluded that during or immediately before the great eruptions, large volumes of magma were injected into fissures at depth. It was by a combination of deep-seated intrusion and explosive eruption that the magma chamber was drained to make room for the collapse of the peak of Mount Mazama. Doubtless the caldera floor also subsided by cooling and solidification of the magma left below.
The collapse of Mount Mazama was eccentric with respect to the former summit, for this lay well to the south of what is now the center of Crater Lake. This eccentric collapse was controlled by the pre-existing semicircular line of weakness along which the Northern Arc of Vents was formed.
The formation of Crater Lake took place approximately 5000 years ago. The discovery of artifacts beneath the pumice deposits shows that man already inhabited this part of Oregon and was a distant witness of the catastrophe.
After a period of quiet of unknown duration, activity commenced anew. Close to the western edge of the caldera rose the cone of Wizard Island, the final act of which was to erupt a rugged sheet of blocky lava, perhaps no more than a thousand years ago. As intra-caldera eruptions went on, rain and snow formed a lake on the floor, and this continued to gain in volume until it reached a depth of almost 2000 feet.
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