3. A third argument was advanced against Diller’s collapse theory, namely, the absence of lava flows commensurate in volume with the part of the volcano which has disappeared. The absence of such flows cannot, however, be held to substantiate the opposite view of explosion. Removal of support necessary to cause foundering of the summit of a volcano may be brought about in other ways, as by deep-seated intrusion, or by rapid draining of the underlying reservoir by colossal eruptions of pumice. What is important is that the reservoir be so far evacuated that its roof can no longer stand.
4. Kilauea, which all admit to be a collapse caldera, differs from Crater Lake in several respects, notably in the presence of arcuate fault blocks along the margins. This difference should not be construed as an argument that the two depressions are of different origin. At Kilauea, the lavas involved are fluid and have built a low, broad shield. Repeated rise and fall of magma within the central conduit and related fissures is responsible for the familiar curved step faults on the margins of the sink. Mount Mazama, on the contrary, was a steep-sided composite cone, and its collapse was almost certainly a single paroxysmal event. Hence it displays features comparable with those of Krakatau, Santorin, Aso, and other calderas on composite cones. Around these, circumferential fault blocks are conspicuous by their absence.
5. Until recently it has generally been stated, though without adequate discussion, that most calderas of the Pacific region are of explosive origin. Elsewhere,1 in a detailed review of the caldera problem, prepared as a basis for the present study, arguments have been presented to show that few volcanic depressions more than a mile across. are caused simply by explosion. Volcanologists in New Zealand, the East Indies, and Japan are now unanimous in the opinion that large calderas are primarily the result of engulfment.
6. Smith and Swartzlow regard the shape of Crater Lake and the nature of the materials in its walls as support for the idea of explosion. They point to the presence of a quantity of pyroclastic ejecta interbedded with the lavas as proof that Mount Mazama was built largely by explosive activity. Not only has the volume of such debris been exaggerated, but even if the entire cone were composed of such ejecta, it would not follow that the caldera itself was a product of explosion. Many composite volcanoes rich in pyroclastic deposits and some cones wholly composed of fragmental debris have collapsed to form calderas.
Comparison of the form of calderas 5 or 6 miles across with forms caused by artificial explosions has little value. To produce a caldera of the size and shape of Crater Lake by explosion, the explosive charge must be of exceptional strength. To produce the necessary gas pressure, the magma generating the gas must lie at great depth. If the magma lies close to the surface it cannot hold much gas in solution; the roof of the chamber may then be shattered by mild explosions, and the crater will probably have the shape of a wide-flaring funnel. In order to blow out 17 cubic miles of solid rock and leave a vast steep-walled caldera, the magma must lie far below. Eruptions would then blow out, not only much of the old cone, but great quantities of the underlying basement rocks. Yet at Crater Lake practically all the lithic fragments were derived from the andesite and dacite flows of Mount Mazama itself. A few pieces of olivine-bearing basic andesite and basalt and still fewer chips of pre-Pliocene lava are the only fragments torn from the basement. Accordingly, the top of the magma chamber lay not far beneath the surface. Presumably, it lay above the present floor of Crater Lake.
