HOLOCENE HISTORY OF MOUNT BAKER VOLCANO, NORTH CASCADES

Following completion of edifice construction by 11,000 ¹⁴C yrs BP, Mount Baker was the site of multiple Holocene eruptions (mainly hydrovolcanic) and flank collapses. Intensely altered and water-saturated collapses mobilized to clay-rich (cohesive) lahars that traveled as far as the Puget Lowland. All significant collapses were from the SE half of the volcano, the sector between summit azimuths N 30 E and S 50 W. The main Holocene events:

Schriebers Meadow cinder cone formed near the base of the volcano (8,800 ¹⁴C yrs BP), yielding a widespread scoria fall (tephra SC) and a lava flow damming the Baker River valley, followed by flank collapse of the sector of Mount Baker upslope from the cone (8,500 ¹⁴C yrs BP).

The Mazama Park eruptive period (newly dated at 5,600-5,900 ¹⁴C yrs BP) includes collapses and eruptions previously dated throughout a range of 4,300-6,700 ¹⁴C yrs BP. Collapse occurred initially in Park Creek on the east and in the Middle Fork Nooksack River on the southwest, the latter producing the largest Holocene lahar, extending >44 km. Then, a phreatomagmatic eruption produced lithic tephra OP, synchronous with a flank collapse from near modern Sherman Crater--the active 0.5-km-wide vent south of the summit--that produced a lahar traveling >33 km. Finally, a magmatic eruption produced juvenile andesitic tephra BA, extending >34 km northeast. We interpret the events as reflecting a single intrusive cycle, with the Park Creek and Middle Fork collapses resulting from edifice destabilization by the earliest intrusion. Then, after a discrete interval (indicated by erosional stratigraphy), sequential phreatomagmatic and magmatic eruptions occurred, probably triggered directly by unloading by the collapse near Sherman Crater.

The Sherman Crater eruptive period (AD 1843 to present) began with a phreatomagmatic eruption shaping the modern crater and ejecting lithic tephra YP. Collapse (AD 1845-1847) of the east crater rim produced a lahar inundating much of the Baker River valley. Hydrovolcanic activity, including the well-documented tenfold increase in thermal activity in 1975, continues.

In AD 1890-1891, collapse from Lava Divide produced a debris avalanche/lahar extending 10.5 km; in 1927, probable seismogenic collapse of >1.0 km of stagnant ice in the distal Deming Glacier yielded a debris flow extending >10 km.