Glacial History Geologic History

Ancestral Mount Shasta formed during the Middle Pleistocene as a result of subduction of the former Kula and Farallon oceanic plates beneath the western margin of the North American plate. Between 300,000 and 360,000 years ago a large debris avalanche swept much of this STRATO-VOLCANO into the Shasta Valley (Brantley and Glicken, 1986; Crandell et al., 1984), forming several of the numerous hills and mounds that are such a notable feature of Shasta Valley.

Mount Shasta from the Shasta Valley

Mount Shasta is a COMPOSITE VOLCANO with four major cones that formed during the QUATERNARY (Hirt, 1995). The oldest cone is the Sargents Ridge Cone on the south side of Mount Shasta. Two periods of PLEISTOCENE glaciation have affected Sargents Ridge (Christiansen, 1976; Harris, 1988).

The Misery Hill Cone, just below the present-day summit, is an ANDESITIC cone (as is the Sargents Ridge Cone) that formed between 15,000 to 20,000 years ago (Harris, 1988). Gray Butte, near Panther Meadow, is the PLUG DOME which occurred near the end of the Misery Hill stage (Hill, 1984). Pleistocene glaciation has left its mark on this region.

The final episodes of vulcanism on Mount Shasta occurred during the HOLOCENE (Hoblitt, Miller, and Scott, 1987). Volcanic deposits from these events covered much of the evidence of Pleistocene glaciation.

Shastina, the secondary peak of Mount Shasta, formed about 9,500 years ago (Christiansen, 1976; Miller, 1980; Harris, 1988). Black Butte, a PLUG DOME on the southwest base of Mount Shasta, formed around the same time.

Black Butte
Photo by Bill Hirt, October 15, 1999

The summit of Mount Shasta is a DACITE plug dome that formed on the summit of the Hotlum Cone, the last cone to form on Mount Shasta about 8,000 years ago (Harris, 1988). The Hotlum Cone has erupted about seven times in the last 4,000 years (Myers), with the last eruption occurring about two hundred years ago (Aune, 1970; Hoblitt, Miller, and Scott, 1987). During Holocene times Hotlum and Shastina have produced LAHARS and ash deposits. Both the towns of Weed and Mount Shasta City are built on these deposits.

For a more thorough discussion of the geologic history of Mount Shasta as well as a description of glacial processes see the geology section of the Mount Shasta Companion.