The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Origin of the Caldera
EXCEPT Diller, all who have studied Crater Lake agree that the destruction of the summit of Mount Mazama occurred either during or immediately after the great pumice and scoria eruptions just described. Had the caldera been formed before these eruptions, the bottom and sides would be covered by ejecta and the floor would be marked by large vents. Eruptions of the magnitude of those we have discussed have never been known to originate within a large caldera, for once a caldera is formed, the energy of a volcano is largely or entirely spent. Caldera formation on composite volcanoes is usually a sign of exhaustion.