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Geology and History

Mount Adams, Washington as seen from Mount St. Helens (west). Tree...
Mount Adams, Washington as seen from Mount St. Helens (west). Trees are covered in frost in the foreground. (Credit: Pallister, John. Public domain.)

Ice-capped Mount Adams sits astride the Cascade Range crest 50 km (30 mi) north of the Columbia River and 60 km (37 mi) east of Mount St. Helens. The volcano is 3,742 m (12,276 ft) in elevation and towers about 3 km (9,800 ft) above the surrounding lowlands. The mostly andesitic volcano stands near the center of a volcanic field that erupted in the same time range and includes more than 120 smaller volcanoes, the majority of which are basalt. About 25 of these, all within 6 km (4 mi) of the summit, are considered flank vents that erupted lavas similar to those from Mount Adams. The rest, scattered between 6 and 25 km (4 and 15 mi) from the summit, erupted different types of rocks and probably have independent conduits that originate from magma reservoirs in the deep crust or mantle. The greatest concentration of these peripheral volcanic vents defines a north- trending belt only 6 to 7 km (about 4 mi) wide but more than 50 km (30 mi) long.

Glaciers readily erode high-standing stratovolcanoes, and Mount Adams was reduced to lower than 2,500 m (8,200 ft) at least twice in its 520,000-year lifetime. Although the present-day volume is about 200 km3 (50 mi3), the total volume it erupted is probably closer to 300 km3 (70 mi3). One third of the total erupted material has been removed by erosion over the life of the volcano.

The upper flanks of Mount Adams consist of glaciers, jagged ridges and cliffs, glacial moraines, and alpine meadows, but below an elevation of about 2,000 m (6,500 ft) most of the landscape is densely forested. The summit icecap and ten glaciers only cover about 2.5% of volcano's surface area, but during the last glacial maximum as much as 80% of Mount Adams was under ice. The U.S. Forest Service administers the western half of Mount Adams. The eastern half of Mount Adams is part of the reservation of the Yakama Nation, and except for a few designated recreational areas, it is closed to the public.