62 The Climax: Culminating Explosions of Pumice and Scoria

The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams

The Climax: Culminating Explosions of Pumice and Scoria


THE stage was now set for the grand finale, the destruction of the top of Mount Mazama. How long a period of quiet preceded the culminating eruptions there is no accurate means of telling. But that the interval of rest was fairly long seems to be indicated by the intensity and scale of the explosions which followed. So great was the volume of material erupted, so hurriedly did it escape, and so highly was it charged with gas that the magma had probably been undergoing differentiation for a long time, building up gas pressure as it slowly crystallized. The interval of rest may have been of the order of hundreds of years. Along the north wall of the caldera, as we have seen, the welded tuff of the Wineglass is overlain by patches of glacial till, and where the tuff is absent on the east wall, a thick and extensive sheet of till rests on coarse lump pumice. Everywhere this till is succeeded by the pumiceous products of Mazama’s culminating activity. Presumably, therefore, glaciers had advanced down the north and east slopes of the volcano, and had then retreated, leaving moraines in their wake, before the climactic explosions began.

Whatever the trigger action, the first explosions were not catastrophic. Like the great pumice eruptions which led to the formation of the calderas of Krakatau and Santorin, the explosions of Mazama gradually increased in violence as activity proceeded. As more magma escaped and deeper, more gas-charged layers of the reservoir were tapped, the violence of the explosions continued to grow until the chamber was largely exhausted. The history of all but a few volcanic eruptions in recent years shows this slow unfolding of energy to be a typical process. Only eruptions of phreatic type begin violently and then wane; normally, eruptions involving the discharge of new magma begin mildly and increase in intensity to the end. Certainly this is what happened at Mount Mazama. The initial explosions were weak and the pumice fragments at first were small. Later, the size and volume of the ejecta increased until toward the close the pumice was no longer shot high above the cone and dispersed by winds, but rose only a short distance above the crater and then, falling on the flanks of the volcano, rushed down the canyons in the form of glowing avalanches or nuées ardentes. More precisely, the early eruptions were of Vulcanian type and these increased in strength until they gave place to colossal eruptions of Pelean type.

It is impossible to remain long in the vicinity of the caldera without realizing the presence of two distinct types of pumice, radically different both in appearance and in mode of origin. One type is heavily charged with pumice lumps, some of which attain a length of more than 6 feet, set in a fine, dusty, almost flourlike matrix. This type contains many charred logs and is confined to the valleys, which it fills in places to a depth of 300 feet. The other type is much more widespread, covering hill and valley alike. In this type there is little dust and there are few lumps more than 6 inches across. Most of it consists of fragments of the size of sand or gravel, and in general it becomes finer with increasing distance from the source.