The geologic events leading to the formation of the caldera containing Crater Lake summarized below are abstracted from the comprehensive study by Howell Williams (1942). The building up of a series of composite volcanic cones by eruptive volcanism was the final stage in the formation of the Cascade Range. The volcanos extend from Mt. Baker, just south of the Canadian border, to Mt. Shasta, in northern California. The volcano that existed before Crater Lake, called Mt. Mazama, was, like the other cones, composed chiefly of hypersthene andesites. In the final stages of eruption, the magma of the younger cones was differentiated into dacites erupted as pumice and viscous flows and basalts ejected as scoria.
Mt. Mazama built up in the depression formed by two older shield volcanos, Union Peak and Desert Ridge. The attitude of various lava flows indicates that Mazama reached an altitude probably in excess of 12,000 feet above sea level. Its formation closely resembled those of Mts. Rainier and Shasta. Glacial moraines and tills interbedded with the lava flows indicate that the mountain was glaciated at intervals during its construction, and confirm its Pleistocene age. As Mazama matured, deep glacial valleys, Munson, Sun, and Kerr Valleys, were incised on its southern slopes and the northern slope was planed by ice sheets which decapitated the former volcano of Desert Ridge. Contemporary ice sheets on Union Peak smoothed the ridges and domes depending quaverserly from the volcano and eroded away the pyroclastic cone capping the shield, and exposing the vent plug. After active construction of Mazama ceased, an arc of vents opened on the northern flank of the cone at about 7,000 to 8′,000 feet. The parasitic scoria cones developed along radial fissures and vents on the flanks of the mountain indicates that Mazama. had reached old age at the close of the Pleistocene period. Most of the scoria cones were formed after the last glacial epoch and the Mazama glaciers had retreated to about 6,500 feet. The central cone was in a state of quiescence. and a dormant period followed construction of the parasitic cones.
The dormant period was shattered by a progression of cataclysmsic explosions that culminated in the formation of the caldera. The evolution of the caldera is graphically portrayed by Williams (19L21:104, Fig. 29) Pumice was ejected by the initial series of explosions which increased rapidly in violence. Climatic explosions forced gas-impregnated pumice and later, scoria out of the cone which poured down the mountain sides at tremendous speeds in the form of glowing avalanches (nuees ardentes). At the same time, voiding of the magma chamber was materially accomplished by reaction forces that injected large quantities of magma into deep fissures underlying the mountain. The roof of the chamber, located at a relatively shallow depth and weakened by the arc of vents, fractured and foundered, permitting the cone to collapse into the chamber, forming the caldera whose floor lies about 3.,000 feet below the rim. Subsequent minor volcanic action has covered part of the floor and built up Wizard Island.
Most of the pumice ejected by the explosions was blown east and northeast of the Cascade divide by winds which apparently shifted from an easterly to northeasterly direction. The 6 inch pumice isopach extends north to the vicinity of Bend. The gas-charged avalanches of pumice and scoria fanned out for miles around. They plunged into the Rogue valley nearly to McLeod, 35 air-line miles from the point of origin; they sped a distance of ten miles beyond Diamond lake; the flows spread eastward for distances of more than thirty miles; and they filled the southeastern glacial valleys, spewing out as far as Fort Klamath, or perhaps further. Weak, dying exrplosions ejected some additional pumice and fine ash settling after the passage of the avalanches added to the mantle of pumice and scoria that blankets pre-caldera land forms from a few inches to over hundreds of feet in depth.
The glowing avalanches destroyed the forests in its path. Species identification of carbonized logs buried in the deposits showed the presence of western white pine, whitebark pine, lodgepole pine, sugar pine; western yellow pine (ponderosa), white fir, and Douglas fir, permitting the inference “that when the pumice was laid down the climate was almost if not local quite the same as that of today” (Williams, 1942 113), Charcoal samles submitted for radiocarbon analysis yielded an average date of 6453 ± 250 years B. P. (C-247, Arnold and Libby 1951,:117)., or at about 4500 B. C.