The remarkable volcanism of Shastina, a stratocone segment of Mount Shasta, California

Abstract

Mount Shasta, a 400 km 3 volcano in northern California (United States), is the most voluminous stratocone of the Cascade arc. Most Mount Shasta lavas vented at or near the present summit; relatively smaller volumes erupted from scattered vents on the volcano’s flanks. An apron of pyroclastic and debris flows surrounds it. Shastina, a large and distinct cone on the west side of Mount Shasta, represents a brief but exceptionally vigorous period of eruptive activity. Its volume of ∼13.5 km 3 would make Shastina itself one of the larger Holocene Cascade stratovolcanoes. Its andesite-dacite lavas average 63 wt% SiO 2 and have little compositional or petrographic variation; they erupted almost entirely from one central vent, although a single vent below Shastina’s north side erupted a flow of the same composition. Eruptions ended with explosive enlargement and breaching of the central crater and successive emplacement of four, more-silicic dacite domes within the crater and pyroclastic flows down its flank. Black Butte, a large volcanic dome and pyroclastic complex below the west flank of Shastina, is petrographically and chemically distinct but only slightly younger than Shastina itself, part of a nearly continuous Shastina–Black Butte eruptive episode. Shastina overlies the widespread pumice of Red Banks, erupted from the Mount Shasta summit area and 14 C dated at ca. 10,900 yr B.P. (calibrated). Shastina and Black Butte pyroclastic deposits have calibrated 14 C ages indistinguishable from one another at ca. 10,700 cal. yr B.P. A cognate granitic-textured inclusion in a late Shastina lava flow yields a 238 U- 230 Th date on zircons within error of those ages. Our conclusion that the entire, voluminous Shastina–Black Butte episode lasted no more than a few hundred years is confirmed by almost identical remanent magnetic directions of all of the lavas and pyroclastic deposits. Although extremely similar, the remanent magnetic directions do reveal a short path of secular variation through the eruptive sequence. We conclude that the entire Shastina–Black Butte eruptive episode lasted no more than ∼200 yr. The magmas that produced the Shastina and Black Butte eruptions were separate individual bodies at different crustal levels. Each of these eruptive sequences probably represents magma approximating a liquid composition that experienced only minimal differentiation or crustal contamination and remained separated from the main central conduit for most eruptions of Mount Shasta. The probability of another rapidly developing, brief but voluminous eruptive episode at Mount Shasta is low but should not be ignored in evaluating future possible eruptive hazards.