Abstract

The pumice of the 1915 eruption of Lassen Peak is conspicuously banded. Light bands are dacite; dark bands are andesite. They represent two distinct magmas which were imperfectly mixed at the time of eruption; the portions of different composition were drawn out into bands by flowage. Similar, but less obvious and possibly less extreme, inhomogeneity exists in the lava flow of the same eruption. In both lava flow and pumice, the phenocrysts were out of equilibrium with the enclosing magma and were reacting with it. Numerous feldspar phenocrysts that originated in the dacite portion show clouding, which resulted from fritting of their outer parts, and a more calcic outer zone; however this appears to be related only in small part to the addition of the basic magma, represented by the dark streaks in the pumice. These feldspars probably are remnants of remelted granitic rocks of the underlying sialic basement admixed with basaltic magma rising from still greater depth. Phreatic explosions in 1914 probably resulted from distension of the Lassen dome by the rise of new magma.

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