Formation of the caldera about 1.5 million years ago was accompanied by the largest eruption in the history of the Mount Baker volcanic field when ice covered much of the region around Mount Baker. More than of rhyodacite magma exploded from a shallow magma chamber and filled the caldera and surrounding valleys with ignimbrite. Subsequent glacial erosion removed most of the tephra fallout near the source of the eruption, leaving behind a thick tephra deposit preserved south of the caldera. The eruption that created the caldera was comparable in volume to the one that produced Crater Lake in Oregon.