The eruptive history of the Mount Baker volcanic field is virtually continuous from 1.3 million years ago to the present, and it includes at least 25 discrete eruptive vents and >100 intruded dikes.
Early-Pleistocene Precaldera Activity, 1.3 to 1.15 Ma
A million years of erosion has left few, if any, of the volcanic products that were erupted prior to the formation of the Kulshan caldera at 1.15 Ma. Remnants of ten volcanic units scattered throughout the field were dated to be between 1.3 and 1.15 Ma.
Kulshan Caldera Eruption, 1.5 Ma
The largest eruption in the history of the Mount Baker volcanic field occurred approximately 1.15 Ma when ice covered much of the region around Mt. Baker. More than 50 km3 of rhyodacite magma exploded from a shallow magma storage region and filled the caldera and surrounding valleys with ignimbrite. That amount of material would cover the island of Manhattan to the greater than the height of One World Trade Center (541 m or 1776 ft high, construction finished in 2013). As the eruption progressed, the rock on top of the magma storage region collapsed forming the depression of the Kulshan caldera.
Although glacial erosion removed most of the tephra fallout near the source of the eruption, a 30-cm-thick (1-ft-thick) tephra deposit is preserved 200 km (125 mi) south of the caldera, which indicates the Kulshan caldera eruption was comparable volume to the eruption that produced Crater Lake caldera in Oregon (Mount Mazama).
Postcaldera lava flows and domes, 1.15 to 0.99 Ma
For 160,000 years after the caldera-forming event, dike-fed eruptions produced silicic (mostly rhyodacite) lavadomes and lava flows inside the Kulshan caldera. Additionally, at least two eruptions occurred about 3 to 4 km (1.9 to 2.5 mi) west of the caldera at Chowder Ridge and Dobbs Cleaver.
Kulshan Cone, 1.1 to 0.5 Ma
It is likely that after the period of silicic post-caldera eruptions, a large volume of more mafic material was erupted in the Kulshan caldera region. However, if a cone was once built up, it has been stripped down to basement rock during the last few glacial episodes. More than 60 andesite dikes were emplaced within this time period and cut through the silicic post-caldera deposits. No eruption products associated with these dikes have been located on the surface, but they are important because they are the only known record of a major episode of early Pleistocene eruptions that probably fed a large andesitic volcano pile now completely removed.
Initiation of the Southwestern Focus, 745 to 500 ka
Near Lava Divide and Parke Butte, are the signs of the initial shift of volcanic activity to the southwest, away from Kulshan. Located about halfway between present-day Mount Baker's summit and the western Kulshan caldera boundary, thick (at least 30 m or 100 ft) andesite lava flows erupted between 745 and 500 ka. Near the southern foot of modern Mount Baker, a basaltic lava flow (15-25 m or 50-80 ft thick) near Park Butte erupted around 716 ka. Today there is a lookout for hikers atop Park Butte, which is a former fire lookout.