Deposits of pre-1980 pyroclastic flows and lahars from Mount St. Helens Volcano, Washington

Abstract

Mount St. Helens in southern Washington has been one of the most active and most explosive volcanoes in the Cascade Range since its birth about 40,000 years ago. In this report its history is divided into four eruptive stages, the most recent of which is subdivided into six eruptive periods. Prior to 2,500 years ago the volcano erupted tephra, domes, and pyroclastic flows, but only a few lava flows that were large enough to extend beyond the base of the volcano. Most products of volcanism were dacite or silicic andesite. Eruptive behavior changed about 2,500 years ago, and since then the volcano has produced more mafic rock types as well as dacite. The initial eruptions of the volcano were preceded by repeated glaciations of much of the Mount St. Helens region, most recently during the next-to-last glaciation when the Hayden Creek Drift was formed. The initial eruptions of Mount St. Helens during the Ape Canyon eruptive stage, which began sometime before 40,000 years ago and lasted until about 35,000(?) years ago, produced lithic and pumiceous air-PRE-1980 PYROCLASTIC FLOWS AND LAHARS, MOUNT SAINT HELENS The early part of the Kalama eruptive period may be analogous to the dacitic eruptive activity that began at Mount St. Helens in 1980 with an explosive eruption followed by dome extrusion. If the current eruptive sequence repeats the events of Kalama time, future volcanic activity will include multiple eruptions of dacite in the form of domes, tephra, and pyroclastic flows, and andesite in the form of lava flows, tephra, and pyroclastic flows, and will continue intermittently for at least a century.

Directed blasts and blast-generated pyroclastic density currents: a comparison of the Bezymianny 1956, Mount St Helens 1980, and Soufrière Hills, Montserrat 1997 eruptions and deposits

“…Multiple plug domes in the structures of these volcanoes indicate previous eruptions of viscous siliceous magmas, which were able to degas and crystallize in the upper parts of conduits while slowly ascending toward the ground surface. Open conduit eruptions of the plinian type, although they occurred also (especially frequently on MSH; Mullineaux 1986), were relatively rare in the late histories of these volcanoes (Mullineaux and Crandell 1981;Crandell 1987;Wadge and Isaacs 1988;Roobol and Smith 1998;Druitt and Kokelaar 2002).…”

Field-trip guide for exploring pyroclastic density current deposits from the May 18, 1980, eruption of Mount St. Helens, Washington

“…Crandell established the general eruptive history of Mount St. Helens in the 1960s-1980s through their extensive studies of the tephra deposits found around the volcano. They recognized that the eruptive history was episodic, grouped the tephras into "sets" of similar age and composition (Mullineuax, 1996, and references therein) that they described as a series of eruptive stages and periods, and correlated some of the lava flows, domes, and fragmental deposits (Crandell, 1987). Radiocarbon dating provided some geochronologic control, but Crandell and Mullineaux's work was hindered by the lack of preserved tephras older than ~60 ka, because these had been removed by glaciation.…”

Crater glaciers on active volcanoes: Hydrological anomalies

“…The authors here propose that there simply is no pressurized drainage system conveying water along the bed of the east glacier.The crater glacier grew atop several tens of meters of rubble (mainly rock-avalanche debris) that accumulated on the crater floor following the cataclysmic 1980 eruption [Mills, 1992] .The volcanic edifice beneath this rubble is geo logically complex, consisting of multiple lava flows, pyroclastic and lahar deposits, and other fragmental deposits [Crandell, 1987] .Thus, wa ter that reaches the glacier bed probably flows out of the crater through the thick rubble layer or downward into the volcano's ground-water system, rather than moving along the glacier bed. In support of this hypothesis, it should be noted that there are no outlet streams at the glacier termini, although there are springs and seeps farther downslope.…”