BackgroundFrom: Wood and Kienle, (eds.), 1990, Volcanoes of North America - United States and Canada: Cambridge University Press.
cited on CVO website(USGS Cascades Volcano Observatory)
East and southeast of Mount Adams, extending south to the town of Goldendale, is a broad expanse of folded and faulted basalt that is little known. A north-south line of around 24 cinder cones crosses the center of the field, and south of the 46th parallel another dozen cones extend the line to the southeast. The oldest units are thick flows of rhyolite in the eastern Simcoe Mountains. These are contemporaneous with pyroxene-olivine basalt and olivine basalt of the Simcoe Mountains. The pyroxene-olivine basalt forms short, 6-10-meter-thick flows, whereas the olivine basalts are 1-6-meter-thick flows, extruded from small shields and cinder cones. A few of the above units are dated at 4.5 to 0.9 million years ago.
East of Mount Adams are a shield volcano and the lava flows forming Lincoln Plateau; all are olivine basalt probably younger than 0.9 million years. Perhaps the youngest unit of this volcanic field is the dacite that forms Signal Peak, east of Mount Adams, that is believed to be 1.0-0.5 million years.
See also: Sentinel hub | Landsat 8