Geology
Adams is made of several overlapping cones that together form an 18-mile (29 km) diameter base which is elongated in its north-south axis and covers an area of 250 square miles (650 km2). The volcano has a volume of 85 cubic miles (350 km3) placing it second only to Mount Shasta in that category among the Cascade stratovolcanoes. Mount Adams was probably created by the subduction of the Juan de Fuca plate, which is located just off the coast of the Pacific Northwest.
Mount Adams was born in the relatively late Pleistocene and grew in several pulses of mostly lava-extruding eruptions. Each eruptive cycle was separated from one another by long periods of dormancy during which glaciers eroded the mountain to below 9,000 feet (2,700 m). Potassium-argon dating has identified two such eruptive periods; the first occurring 275,000 to 200,000 years ago and the second 150,000 to 100,000 years ago. Most of these eruptions and therefore most of the volcano, consist of lava flows with little tephra. The loose material that makes up much of Adams' core is made of brecciated lava.
Andesite and basalt flows formed a 20 to 200 feet (6 to 60 m) thick circle around the base of the Mount Adams, and filled existing depressions and ponded in valleys. Most of the volcano is made of andesite together with handful of dacite and pyroclastic flows which erupted early in Adams' development. The present main cone was built when Adams was capped by a glacier system in the last ice age. The lava that erupted was shattered when it came in contact with the ice and the cone interior is therefore made of easily eroded andesite fragments. Since its construction, constant emissions of heat and caustic gases have transformed much of the rock into clays (mostly kaolinite), iron oxides, sulfur-rich compounds and quartz.
The present eruptive cone above 7,000 feet (2,100 m) was constructed sometime between 25,000 to 10,000 years ago. Since that time the volcano has erupted at least seven times, all of which were above 6,500 feet (1,980 m) on the summit. One of the most recent flows issued from South Butte and created the 4.5-mile (7.2 km) long by 0.5-mile (0.8 km) wide A.G. Aiken Lava Bed. This flow looks young but has 3,500 year old Mount St. Helens ash on it meaning it is at least that old. The last lava known to have erupted from Adams are the 2,500 to 3,500 year old Muddy Fork lava flows.
The Trout Lake Mudflow is the youngest large debris flow from Adams and the only large one since the end of the last Ice Age. The flow dammed Trout Creek and covered 25 miles (40 km) of the White Salmon River valley. Impounded water later formed Trout Lake. The Great Slide of 1921 started close to the headwall of the White Salmon Glacier and was the largest avalanche on Adams in historic time. The slide fell about 1 mile (1.6 km) and its debris covered about 1 square mile (2.6 km2) of the upper Salt Creek area. Steam vents were reported active at the slide source for three years, leading to speculation that the event was started with a small steam explosion. This was the only debris flow in Mount Adams' recorded history, but there are five known lahars.
Since then, thermal anomalies (hot spots) and gas emissions (including hydrogen sulfide) have occurred especially on the summit plateau and indicate that Adams is dormant, not extinct. Future eruptions from Adams will probably follow patterns set by previous events and will thus be flank lava flows of andesite or basalt. Since the interior of the main cone is little more than a pile of fragmented lava and hydrothermally-altered rock, there is a potential for very large landslides and other debris flows.
The Indian Heaven volcanic field is located between St. Helens and Adams. Its principal feature are an 18-mile (29 km) long linear zone of shield volcanoes, cinder cones, and flows with volumes of up to 23 cubic miles (96 km3). The shield volcanoes, which form the backbone of the volcanic field, are located on the northern and southern sides of the field. Mount St. Helens and Mount Adams are on the western and the eastern sides.
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