The history of the formation of the Cascade Mountains dates back millions of years ago to the late Eocene Epoch. With the North American Plate overriding the Pacific Plate, episodes of volcanic igneous activity occurred. Mount Adams, a stratovolcano that is southeast of Jumbo Peak, began forming in the Pleistocene. Due to Mount Saint Helens' proximity to Jumbo Peak, volcanic ash is common in the area. Jumbo Peak is composed of Pliocene-Miocene andesitic magma that intruded up into older volcanic rocks more than five million years ago and is now surrounded by a forest of old-growth Douglas fir and mountain hemlock.