Description

From Wood and Kienle (1990) ^[1] : "Tanada Peak is the erosional remnant of an andesitic shield volcano that contained an oval-shaped summit caldera , ~8 km long x 6 km wide. Very little of the original shield, which covered more than 400 square km, remains; rugged Tanada Peak, highest point on the volcano, is composed entirely of flat-lying intracaldera? flows. Tanada was built on a thick sequence of andesitic flows, flow breccias, and laharic deposits probably of late Pliocene to early Pleistocene age. A riftlike chain of younger basalt and basaltic andesite cones mantles part of the northern flank of the shield, and much of the volcano's southern flank is covered by flows and pyroclastic deposits from younger unmapped eruptive centers.
"Tanada shield lavas consist chiefly of thin (<10 m) andesite flows that dip 1 to 20 degrees away from the summit area. The summit caldera, of apparent non-explosive origin, is filled with >900 m of massive, flat-lying andesite flows and dacitic agglutinates. The dacitic rocks are restricted to the uppermost part of the caldera fill, suggesting a temporally related change in magma chemistry. A few post-caldera dikes are either andesitic or dacitic in composition. The caldera wall dips 20 to 50 degrees inward and is mantled locally by thin pyroclastic beds and rubbly breccias. Most of the remaining exposed wall is composed of the older andesite sequence; only on the east side of the volcano, and locally elsewhere, are shield lavas still present in the wall."