The mountain is “a massive compound stratovolcano composed of overlapping cones centered at four or more main vents; it was constructed during a period of more than 100,000 years. Each of the cone-building periods produced pyroxene-andesite lava flows, block-and-ash flows, and mudflows originating mainly at the central vents.”
Closely associated with Mount Shasta, and providing clues to the history of the mountain’s formation, are the debris-avalanche deposits extending northward “across the floor of Shasta Valley in northern California. The debris-avalanche deposits underlie an area of about 675 square kilometers, and their estimated volume is at least 45 cubic kilometers. Radiometric limiting dates suggest that the debris avalanche occurred between about 300,000 and 380,000 years ago. Hundreds of mounds, hills, and ridges formed by the avalanche deposits are separated by flat areas that slope generally northward at about 5 meters per kilometer.” These deposits were laid down by a predecessor volcano. The current structure was created by the same subsurface forces that detonated the proto-Mount Shasta.
As noted supra, Shasta is composed of a number of overlapping cones, each of which has its own geologic history. The volcano “consists of at least five separate cones that were built at different times during the past several hundred thousand years. Each new cone has a different appearance or ‘face’ because of the different sequence of eruptions that built the cones and the varying lengths of time that erosion has had to tear away the cones. These overlapping cones were built upon the stump of an even older volcanic cone that was destroyed by an ancient landslide more than 300,000 years ago.