Volcanologists have recognized and named four stages of volcanic activity—Ape Canyon, Cougar, Swift Creek, and Spirit Lake—separated by dormant intervals.
The youngest stage of eruptivity at Mount St. Helens, Spirit Lake, is further subdivided into six eruptive periods. Each stage of volcanism covered, reworked, or destroyed some of the evidence for previous stages; therefore, the youngest stages are both the best preserved and best understood.
Ape Canyon Stage (275 to 35 ka)
The early history of Mount St. Helens is poorly known, and the initial stage, called Ape Canyon, covers a long timespan. During this stage, lava domes erupted just west of the present volcano in two distinct periods—one from 275 to 250 thousand years ago (ka) and a second from 160 to 35 ka. It is possible that these two stages were separated by a long hiatus, or that evidence for eruptive events between 250 and 160 ka has either not yet been recognized or been buried by younger volcanic deposits.
Volcanism during the Ape Canyon Stage produced a cluster of lava domes with maximum elevations of about 1,200 m (4,000 ft). Ash layers correlating to the two eruptive periods have been found as far east as central Washington, indicating that explosive eruptions also occurred. Much of the Ape Canyon Stage history is recorded in a Cougar-age debris avalanche, glacial deposits, and lahars in the Lewis River Valley. Many Ape Canyon-age rocks were altered hydrothermally (by volcanically heated ground water), indicating that an extensive hydrothermal system existed during the latter part of the stage.
[Dormant Interval 35 to 28 ka]
Cougar Stage (28 to 18 ka)
The Cougar Stage was probably the most active eruptive stage in Mount St. Helens' history before the Spirit Lake Stage. During this time the volcano produced explosive eruptionsthat ejected large volumes of ash, lava domes, lava flows, pyroclastic flows, a debris avalanche, and lahars.
The Cougar debris avalanche was the most devastating event of the Cougar Stage, and was probably larger than the huge debris avalanche that triggered Mount St. Helens' 1980 eruption. The deposit is primarily made up of reworked Ape Canyon-Stage rocks. It originated near Butte Camp in the southwest part of the present-day edifice and left a 180–to 270–m (600– to 900–ft) thick, 17–km long (11–mi) deposit extending from the south flank of the volcano, into the Lewis River, which was temporarily dammed. Downcutting of the dam caused flooding downstream as far as the Columbia River and filled the lower Lewis River Valley with volcanic debris at least 60 m (200 ft) thick.
The Cougar debris avalanche was immediately followed by a large explosive eruption producing pyroclastic flows that buried the avalanche deposits with up to 90 m (300 ft) of dacite pumice in ancestral Swift Creek. The Cougar–stage debris avalanche probably initiated onset of this explosive eruption. About midway through the stage, continued explosive activity deposited two sets of tephra ("M" and "K") and more pyroclastic flows.
At about 18 ka, the Cougar Stage culminated with the eruption of the largest lava flow in the history of Mount St. Helens. Known as the Swift Creek flow, its thickness was up to 200 m and it reached nearly 6 km (3.7 mi) down the Swift Creek drainage, where it presently forms the divide between the West Fork and main stem of Swift Creek. The vent for this andesite lava flow, at an elevation of 1,830 m (6,000 ft) on the south flank of Mount St. Helens, marks the location of the volcano's summit at the end of the Cougar stage.
[Dormant Interval 18 to 16 ka]
Swift Creek Stage (16 to 12.8 ka)
The Swift Creek stage was relatively short lived and dominated by the construction of dacite domes on the edifice and corresponding fans of fragmental material on the flanks of the volcano. Volcanism during this stage occurred in two phases, beginning about 16 ka and ending about 12.8 ka.
During the first phase, one tephra deposit (set "S") and three extensive fans of volcanic debris were emplaced from the collapse of growing and unstable dacite lava domes.
- The Swift Creek fan, composed of pyroclastic flows and lahars, buried Cougar-age deposits on the south flank of Mount St. Helens.
- The Crescent fan consists of pyroclastic flows at least 180 m (600 ft) thick on the west flank of the volcano, which were derived from the Crescent lava dome.
- The Cedar Flats fan completely filled the ancestral valley of Pine Creek and spilled into the Lewis River at Cedar Flats. It was at least 90 m (300 ft) thick in the Cedar Flats area. The outer reaches of this fan is dominated by lahar deposits, but closer to its source, it contains mostly pyroclastic material that originated from collapsing lava domes.
During the second phase of the Swift Creek Stage, one explosive eruption produced a widespread andesitetephra layer (set "J"), but no pyroclastic flows or domes have been correlated with this activity.
At the end of Swift Creek time, Mount St. Helens consisted of a cluster of dacite domes with a summit altitude as high as about 2,100 m (7,000 ft).
[Dormant Interval 12.8 to 3.9 ka]