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-Iast 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,OOO(?) years ago, produced lithic and pumiceous air-fall tephra and pumiceous pyroclastic flows. The following Cougar eruptive stage began shortly before 20,000 years ago during the Fraser glaciation and produced tephra, pyroclastic flows, and a large lava flow of silicic andesite on the south side of the volcano. Lahars during the early part of the stage were followed by a large avalanche of rock debris from the south side of the volcano that extended as far as the Lewis River valley. Two pumiceous pyroclastic flows later crossed the Lewis River valley near the site of Swift Dam. During this eruptive stage, lahars and fluvial deposits formed a thick fill in the Lewis River valley outhwest of the volcano. Subsequently, during the Fraser glaciation, alpine glaciers developed on the flanks of the volcano and in the adjacent mountains. At that time the largest glacier in the Mount St. Helens area extended down the North Fork Toutle River to a point about 27 km downvalley from Spirit Lake.
The Swift Creek eruptive stage began about 13,000 years ago, after the alpine glaciers had retreated or disappeared. Eruptive products of this stage include pumiceous air-fall tephra, both lithic and pumiceous pyroclastic flows, and lahars that partly filled valleys heading on the volcano. The Swift Creek stage ended sometime before 10,000 years ago, after which the volcano was dormant until about 4,000 years ago when explosive eruptions of the Smith Creek eruptive period initiated the Spirit Lake eruptive stage. These eruptions produced large volumes of air-fall tephra as well as pumiceous and lithic pyroclastic flows. The Smith Creek eruptive period ended about 3,300 years ago and after a dormant interval of about 300 years was followed by the Pine Creek eruptive period which lasted until about 2,500 years ago. Small volumes of tephra were erupted during Pine Creek time, and repeated pyroclastic flows and lahars formed thick fills in valleys around the volcano. The Castle Creek eruptive period followed an apparent dormant interval of about 300 years. This period is distinguished by the initial appearance of significantly more mafic rock types than those of earlier stages and periods. Castle Creek time included, from older to younger, eruptions of andesitic lava flows, dacitic pyroclastic flows and tephra, andesitic lava flows, dacitic tephra, and basaltic tephra and lava flows. The period ended before 1,600 years ago and was followed by a dormant interval that apparently lasted about 600 years. It was interrupted by the extrusion of a dacite dome during the Sugar Bowl eruptive period. The extrusion of this dome at the north base of the volcano about 1,150 years ago was accompanied by lateral blasts, small pyroclastic flows, a small volume of air-fall tephra, and lahars.
The Kalama eruptive period began late in the 15th century, probably in A.D. 1480, with an explosive eruption of dacite tephra. This was accompanied and followed by lithic and pumiceous pyroclastic flows on the west side of the volcano and by other tephra eruptions. These early events were followed by the eruption of andesitic tephra, lava flows, and pyroclastic flows, after which the extrusion of a dacite dome formed a new summit of the volcano. Avalanches of rock debris from the dome spilled down all sides of the volcano to form lithic pyroclastic flows and lahars. The Kalama eruptive period ended about 350 radiocarbon years ago after a small pumiceous pyroclastic flow descended the northwest flank of the volcano. A subsequent dormant interval of about two centuries ended in A.D. 1800 when an explosive eruption of pumiceous dacite tephra initiated the Goat Rocks eruptive period. The next recorded eruption of that period produced an andesite lava flow northwest of the volcano, followed by the extrusion of the Goat Rocks dacite dome probably during the 1840's. The last significant eruption of the period evidently occurred in 1857, but several small steam explosions were recorded between 1898 and 1921.
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.
