Ancestral Mount Shasta formed during the Middle Pleistocene as a result of subduction of several small oceanic plates beneath 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), forming several of the numerous hills and mounds that are such a notable feature of Shasta Valley.

Mount Shasta is a composite volcano with four major cones which have been formed in the last 250,000 years (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) which 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 (image to right), a plug dome on the southwest base of Mount Shasta, formed around the same time. It is possible that the gaping V-shaped Diller Canyon is a result of hot avalanches from this time period (Harris, 1988).

The summit of Mount Shasta is a dacite plug dome which 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 are built on these deposits.

There are several glaciers extant on Mount Shasta, but all of these were formed during the last 4,000 years (Harris, 1988) after the late Cenozoic ice age (or Pleistocene Ice Age). Since Mount Shasta formed during the Pleistocene, and has undergone several episodes of vulcanism, it is difficult to know which glacial features were formed during the Pleistocene and which were glaciated in the Holocene. I will briefly discuss Pleistocene glaciation, Holocene glaciation, and the Existing Glaciers of Mount Shasta in the next three sections. If you have information which can contribute to the overall picture, I'd love to hear from you.