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Stages of volcanism at Medicine Lake volcano

The eruptive activity at Medicine Lake volcano is probably driven by intrusions of basaltic magma that occur during east-west stretching of the crust in an extensional tectonic environment. Vents are typically aligned parallel or subparallel to major structural features, most commonly within 30 degrees of north. Vents for all flows typically form in lines and are within a few degrees of north-south, also the most common direction for faults.

Stage 1 - early silicic volcanism (<500 ka to 300 ka)

Volcanism began at Medicine Lake about half a million years ago with an early episode of activity lasting until about 300,000 years ago; 18 mapped units have been correlated to this eruptive stage. The earliest-dated lavas are rhyolite (~475 ka), which are both exposed at the surface and can be correlated to units found in two caldera drill holes. This single unit may cover as much as 12 km (7.5 mi) east-west by 2 km (1.2 mi) north-south, with an average thickness of 200 m (656 ft). This single rhyolite unit may therefore be equally as voluminous as the largest basalt units of the volcano and nearly five times larger in volume than the largest Holocene silicic eruptive unit, Glass Mountain. Only four of the 18 mapped Stage 1 units are basalt, two of which are dated at 445 ka.

Stage 2 - pre-dacite tuff, basalt to andesite (~300 ka to ~180 ka)

Approximately one-third of the volcano’s eruptive units (69 total) were emplaced during this interval, which was dominated by basaltic and andesitic lava flows. Dacite eruptions also occurred late in this stage and culminated with the eruption of the volcano’s only ash-flow tuff, the dacite tuff of Antelope Well (~180 ka). The tuff is the only widespread marker bed on the volcano and is the single most important stratigraphic unit at Medicine Lake volcano.

Stage 3 - caldera-rim construction (~180 ka to 100 ka)

Approximately 80 mapped Stage 3 eruptive units are exposed over about a quarter of the volcano (487 km2, 188 mi2). Eruptions in stage three were dominantly basalt, basaltic andesite and andesite. Only a single low-silica rhyolite and one dacite were mapped. Late in this time period, much of the present caldera rim was constructed by eruptions from vents along arcuate caldera ring faults. The upper parts of the north and south rim andesite units have been modified by glaciation.

Stage 4 - big basalts (<100 ka to ~13 ka)

Although stage 4 volcanism is slightly longer than stage 3 and includes far fewer units (24), its eruptive products cover 974 km2 (366 mi2), nearly half the volcano’s total area. Several of the 16 basalt eruptions in this group produced significant volumes of fluid lava that spread across low topography adjacent to the volcanic edifice; they cover 886 km2 (342 mi2. Four basaltic andesites, two andesites, and one low-silica dacite erupted early in this period. The single rhyolite has an age of ~30 ka.

Stage 5 postglacial (~12.5 ka to present)

At least 17 eruptions have occurred since the end of glaciation at Medicine Lake, or between 1 and 2 eruptions per century on average, although activity appears to be strongly episodic. A significant amount of the area and volume of this stage was generated in the Giant Crater event that occurred just after the end of glaciation. This pulse of mafic volcanism in immediate postglacial time might have been related to pressure release accompanying melting of the volcano’s ice burden. Nearly 8,000 years of quiescence followed this early mafic episode before three subsequent closely spaced sruptive episodes occurred. Between 3,000 and 900 years ago, eruptions produced approximately 2.5 km3 (1 mi2) of lava ranging in composition from basalt to rhyolite. Late Holocene lava compositions include basalt and andesite, but silicic lavas dominate.