Eruptions from Mount Adams began about 520 ka, although scattered basaltic eruptions from the surrounding volcanic field began by about 940 ka. Early edifice building was focused 5 km (3 mi) southeast of the modern summit. By about 460 ka the eruption focus shifted to a position beneath the modern volcano, and subsequent eruptions constructed an encircling apron of lava flows that extend 15 km (9 mi) radially in all directions. Although the cone grew principally in three eruptive episodes centered at 500, 400, and 50-30 ka, it remained recurrently active between pulses, never shutting down. Eruptions from both the central volcano and the peripheral vents have coexisted since 520 ka. The modern summit cone was built primarily during the last major pulse from 50-30 ka, with a small cone of tephra capping the modern summit about 15 ka.
The bulk of Mount Adams and many of the cones and shields in the surrounding volcanic field predate the last major Pleistocene glaciation, but ten lava units are known to have erupted after glaciers retreated to close to their present locations by about 15,000 years ago. Nine separate vent systems erupted on or near Mount Adams and the tenth occurred at the summit and may have remained sporadically but weakly active well into the Holocene. When taken as a whole, the volume of the ten postglacial units represent a much lower average eruption rate during the past 15,000 years than over the life of the volcano.
The top 150 to 250 m (490 to 820 ft) of Mount Adams forms a broad summit plateau that covers at least 2 km2 (about 1 mi2). The largely glacier-covered plateau exposes thin and glassy lava flows and tephra that were erupted when magma encountered ice-cap melt water and produced steam explosions. These andesite eruptions occurred about 15,000 years ago, filled an older summit crater and in places flowed down the outer flanks. Much of the summit rock and debris is hydrothermally altered and weakened.Avalanches of this altered debris can form far-traveled lahars.
FLANK AND VOLCANIC FIELD
Nine flank and volcanic field eruptions occurred after the end of the last glacial maximum, though three have unknown ages. Between 15 and 14 ka, two cinder cones fed lava flows on upper and lower flanks of Mount Adams, and a short fissure eruption occurred within the surrounding field. From 7.7 to 3.8 ka three substantial vents at the base of the steep central cone effused andesite lava flows and produced at least 14 explosive eruptions of tephra when magma encountered groundwater water as it reached the surface. These units include both the most extensive and most voluminous eruptions to have occurred during the Holocene history of Mount Adams. Between about 10 and 8 ka, four explosive eruptions produced andesite tephra, which do not correlate to known lava effusions.