But the notion that such a volume as the contents of Monte Somma was left by the single evacuation of the Pompeiian eruption, is as fantastic as that Niuafoou crater was completely made in 1886.
The total volume of everything poured out and blown out of Vesuvius from 79 A.D. to the present would scarcely change the profile of the volcano.
Imposing as they may be, the eruptions accompanying caldera formation may be of quite secondary importance as compared with the underground flow of magma into dikes and sills.
Unfortunately, few attempts have been made to compare the volumes of calderas with those of the surrounding ejecta, and it is therefore seldom possible to say how much engulfment was caused by explosion and how much by concealed intrusion. The volume of the great depression containing Lake Toba in Sumatra is, as van Bemmelen has shown, approximately equal to that of the tremendous sheet of pumice which encircles it. Hence, as he suggests, the collapse may properly be attributed to expulsion of magma at the surface. At Krakatau, Katmai, and Santorin, much of the ejecta fell out to sea, and accurate estimates of the volume are therefore impossible. At Crater Lake, fortunately, the conditions for measurement are ideal.
Obviously, to dispose of 10.5 cubic miles of magma to make room for the part of Mount Mazama not accounted for by engulfment into the space provided by eruption of pumice and scoria, far-reaching fissures must have opened at depth. If these fissures had opened near the roof of the magma chamber, the consequent intrusions would have caused a broad domical uplift of the entire volcanic edifice. Physiographic evidence of such an uplift has not been recognized. If, on the other hand, dike fissures opened at much greater depth, perhaps parallel to the general trend of the Cascade Range, there may have been little or no surface effect in the surrounding region. Swarms of subparallel rifts undoubtedly underlie the High Cascade chain of volcanoes, and it requires no stretch of the imagination to suppose that these may have been reopened or that new ones may have been formed to drain magma from the chamber beneath Mount Mazama. Steams’4 careful mapping of the denuded volcanoes of Waianae and Koolau on Oahu has shown the presence of literally thousands of subparallel dikes elongated along the major axes of those inverted-canoe-shaped domes. Presumably similar swarms of dikes, following lines of tectonic weakness, underlie the shields of Kilauea and Mauna Loa.
Is there any evidence that the formation of Crater Lake was preceded by extensive internal assimilation of the cone, like that which took place at Katmai? Enlargement of a magma chamber by solution of the walls and roof cannot of itself produce a caldera, but it must accentuate any tendency to collapse by thinning the roof and widening its span.
