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Geology and History Summary for Mount Baker

Mount Baker is the youngest volcano of a larger, multivent, volcanic field that has remained recurrently active for the past 1.3 million years. It is located in Washington State about 50 km (31 mi) east of Bellingham Bay and 25 km (16 mi) south of the international border with Canada. Volcanism at Mount Baker is related to subduction of the Juan de Fuca plate beneath the western coast of North America.

Over the past four million years volcanic activity in the volcanic field encompassing Mount Baker region has migrated southwestward in a stepwise fashion. The Hannegan caldera, about 20 km east- northeast of Mount Baker, formed approximately 3.7 Ma. The largest eruption in the field formed the Kulshan caldera at 1.15 Ma and was similar in scale to the climactic eruption of Mount Mazama, which formed Crater Lake Caldera in Oregon. Between these two calderas, both in space and time, a large body of magma was pushed into the crust from below to form the Lake Ann granodiorite about 2.8 Ma. It is uncertain whether the Lake Ann intrusion caused material to erupt on the surface, because any associated volcanic rocks were glacially eroded.

The last main southwestward shift in the magmatic focus was underway by 0.75 Ma, resulting in growth of an andesitic stratovolcano cluster from 0.5 Ma to the present. The initial growth phase of this part of the volcanic field was the of the Black Buttes stratocone from 495 to 288 ka, remnants of which are preserved off the southwest flank of present day Mount Baker . For the past 140 ka, Mount Baker has erupted andesitic lava flows, with its main pulse of activity from 40 to 12 ka. Eruptions of lesser volume took place at scattered locations throughout the volcanic field, and the magmatic system never fully shut down between main pulses. Black Buttes, Mount Baker, and their satellites, together constitute a magmatic focus comparable in extent (and probably longevity) to each of the previous periods of major activity (Hannegan, Lake Ann, and Kulshan).

Because of its northerly latitude, glacial ice has been present throughout the lifetime of the Mount Baker volcanic field and has affected most eruptions and severely eroded their products. Because ice readily removes pyroclastic and other loose deposits, most surviving volcanic outcrops are lava flows.