Mount Adams was born in relatively late Pleistocene time compared to most other Cascade Range volcanoes. Adams 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 9000 feet (~2700 meters). Potassium-argon dating has identified two such eruptive periods; one 275,000 - 200,000 years ago and 150,000 - 100,000 years ago. Most of these eruptions and therefore most of the volcano, consisted 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 20 to 200 feet (~6-61 meters) thick circle the base of the volcano (they filled existing depressions and ponded in valleys). Most of the volcano is made of andesite although a handfull of dacite and pyroclastic flows 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 made 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 main cone above 7000 feet (~2100 meters) 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 6500 feet or ~1980 meters). One of the most recent flows issued from South Butte and created the 4.5 mile long by half mile wide A.G. Aiken Lava Bed. This flow looks young but has 3,500 year old Mount Saint Helens ash on it meaning it is at least that old. The last lavas known to have erupted from Adams are the 2500 to 3500 year old Muddy Fork lava flows.
The Trout Lake Mudflow was the last large debris flow from Adams and the only large one since the end of the 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 a mile (~1.6 km) and its debris covered one mile square 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 from a small steam explosion.
Since then, thermal anomalies (hot spots) and gas (including hydrogen sulfide) emissions especially on the summit plateau, 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 landladies and other debris flows.
