The Geology of Crater Lake National Park, Oregon With a reconnaissance of the Cascade Range southward to Mount Shasta by Howell Williams
MOST calderas are partly, and some are almost wholly, filled by the products of later eruptions. After a period of quiescence, measurable usually in tens or hundreds of years, magma finds its way upward either through fissures on the caldera floor or through arcuate fractures along the margins. If it escapes along marginal fractures, rim volcanoes are formed, and these may continue to grow until they discharge lavas down the outer flanks of the volcano, as they did in the Medicine Lake caldera of northern California.1 If, on the other hand, magma escapes from fractures well inside the caldera, the new vents may either show a linear arrangement, as in the calderas of Tengger in Java and Aso in Japan, or they may be scattered without order. Whether the new vents are distributed at random or form a definite pattern depends primarily on the manner in which the collapse of the caldera occurs, and on the shape of the underlying reservoir.
Soundings of Crater Lake have so far failed to reveal evidence of eruption of lava from arcuate fissures at the base of the caldera walls. A lava lake may once have occupied the deepest part of the floor and may account for the surprising flatness of the area enclosed by the 1900-foot isobath. Apart from Wizard Island, however, there are no definitely proved centers of activity on the caldera bottom. Considering the recent origin of Crater Lake in relation to the size of Wizard Island and the age of its last eruptions, the period of quiet following the collapse of Mount Mazama was probably short.