An Extraordinary Period of Dome Collapse at Mount Shasta, California, USA

Mount Shasta in northern California is the most voluminous (~400 km³) and, at 4,322 m (base 1,100 m), the second tallest stratovolcano in the Cascade Range. Over the past 700 ka, the volcano has grown incrementally in five major cone-building episodes—the youngest three are the Misery Hill (~50-11ka), Shastina (~10.7), and Hotlum (10.7 to present) eruptive episodes. Cone building has been dominated by eruption of lava domes and lava flows. During the youngest three eruptive episodes dome-collapse events have covered the lower flanks of the volcano from the southwest to southeast in lithic (dense juvenile) pyroclastic-flow (pf) deposits. Paleomagnetic directions from ten sites in five mapped Misery Hill pf units on the southwest, northwest, north, east, and southeast flanks of the volcano are remarkably similar (~D=350°, I=60°, alpha₉₅=1.5). The distribution and similar directions suggest a summit eruption of relatively short duration. Pf clast densities range from 1.6 to 2.4 g/cc (average 2.1 g/cc) attesting to a dome-collapse origin. Dome collapse units have H/L values from 0.2 to 0.25. Geochemical compositions are similar for these deposits (63.5-64.2 percent SiO₂) with broader (674-913 ppm Sr) variations in minor elements that appear sector dependent. Mafic mineral assemblages are also broadly similar with ortho- and clinopyroxene and variable amounts of hornblende. The age of these Misery Hill pf deposits is unknown, but their distribution beyond the latest Pleistocene glacial limit and weak soil formation between them and overlying ~11 ka Red Banks fall deposits suggest that they are latest Pleistocene in age. Total volume ranges from 0.5 to ≥3 km³ depending on thickness and original extent. The distribution and ages of these deposits suggests that they are products of a late glacial eruption that affected at least three quarters of the volcano likely in less than a century. A similar-sized event today would have devastating consequences to nearby communities and infrastructure. The data also suggest that most of the surface of Mount Shasta (except the south flank) is latest Pleistocene to Holocene in age with two events (this Misery Hill event and the ~10.7 ka Shastina-Black Butte event)—each lasting less than a century—covering on the order of 40-60% of the volcano's surface.