If extensive collapse and insliding had taken place around the summit vent while the glowing avalanches escaped, large blocks would have been caught in the upsurging, frothy magma and have been swept along in the dense clouds of pumice and compressed gas. It does not seem likely, therefore, that crumbling and caving of the sides of the crater and conduit occurred in the manner observed at Kilauea during the steam blast eruptions of 1924. Perhaps coarse detritus may have slid into the vent between the major eruptions, to be finely triturated by intervening eruptions of moderate intensity. Perhaps, on the other hand, the vents never were clogged with blocky detritus and the small lithic fragments found in the pumice-scoria deposits were torn from the sides of the conduit by the abrasive action of the upsurging, effervescing magma. This interesting problem must be left unsolved.
The air of mystery that still hangs over Crater Lake may never be wholly dispelled. Concerning these conclusions, however, there is reasonable assurance:
- The caldera was not formed by the explosive shattering of the top of Mount Mazama. Of the 17 cubic miles of old rock that disappeared, approximately 1.5 cubic miles were removed by explosion, and even this fraction was mainly erupted in small fragments.
- The volume of pumice and scoria blown out during the culminating eruptions, when recalculated as bubble-free liquid plus crystals, amounts to approximately 5 cubic miles.
- The summit of Mount Mazama was engulfed by withdrawal of magmatic support from below. This withdrawal was caused partly by rapid eruption of pumice and scoria from the upper part of the magma chamber, but principally by injection of magma at greater depths, probably into swarms of fissures.
- Prior to the formation of the caldera, conditions for engulfment were at hand, for a semicircular ring fracture had already been formed on the northern flank of the cone, and the roof of the magma chamber lay at shallow depth.
- When the collapse took place, it was eccentric with respect to the former summit of the volcano. Hence, the center of the caldera lies well to the north of the former peak. This eccentric engulfment was determined by the pre-existence of the Northern Arc of Vents.
- The collapse was probably as cataclysmic as that of Krakatau, and presumably it occurred in a few gigantic steps, the inner parts of the cone foundering first.
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