The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
The Northern Arc of Vents
THE main cone of Mount Mazama had perhaps reached its full height when the centers of principal activity moved from the summit region to the northern flank. When this took place, the glaciers were thin and small and none extended far beyond what is now the caldera rim. Possibly this northward shift of the vents was brought about by enlargement of the magma chamber in the same direction, either as a result of internal assimilation of the cone or, more likely, as a consequence of ring-fracture stoping. Whatever the cause, an arcuate line of vents opened along what later became the north wall of the caldera, approximately 5000 feet below the original summit of Mount Mazama.
It is surely no mere coincidence that the rim of Crater Lake cuts across this Northern Arc of Vents. On the contrary, the position of the rim seems to have been controlled thereby, for the center of Crater Lake does not lie immediately below the former summit of Mount Mazama, but approximately a mile to the north. The eccentricity of the caldera with respect to the main cone appears to have been determined by a predisposing plane of weakness, possibly a continuous ring fracture, the surface expression of which was the Northern Arc of Vents. This eccentric relation accounts in large part for the fact that the north wall of Crater Lake is so much lower than the south wall.