During the middle Pleistocene era, there were two oceanic plates: the Kula and Farallon plates. About 300,000 to 360,000 years ago, these two plates were subducted, or pushed beneath the western edge of the North American Plate and created a volcanic mountain that is now Mount Shasta Volcano in California.
During that time, a monster avalanche swept the entire north side of the original Mount Shasta volcano into what is now the Shasta Valley, a place of many mounds and hills. This is the largest known landslide in the history of the world, according to the 2001 Mount Shasta Companion and Volcano Discovery.
Mount Shasta is a composite volcano, where ash and lava have formed in various layers and at different times. Mount Shasta has an incredibly complex summit that consists of four major, overlapping volcanic cones.
There are many cones, domes and peaks in the mountain. The multiple summits and peaks came from various types of volcanic activity that was then weathered through glaciation and a host of other environmental conditions for hundreds of thousands of years.
The major cones and domes include Sargeant Ridge cone, which sits on the south side of the mountain. This is the oldest exposed, post avalanche part of the original Mount Shasta.
Holtlum Cone is a dacite cone that forms both the newest cone and the highest summit of Mount Shasta. This cone formed about 8,000 years ago, and the Holtlum glacier lies on its north face. After Holtlum cone formed, a dacite cone intruded, creating the highest point and an active volcano. The last eruption at Mount Shasta happened about 200 years ago at Holtlum cone.
Misery Hill lies just below today’s summit. Misery Hill is the second oldest cone at about 15,000 to 20,000 years old. Misery Hill is an andesitic cone.
Gray Butte lies near Panther Meadow and came late in the development of Misery Hill perhaps 15,000 years ago. It is a plug dome.
Shastina cone is a satellite cone on Mount Shasta that came after the last ice age, about 9,500 years ago. Shastina is a plug dome that was formed by pyroxene andesite lava flows. Shastina has a distinctive conical peak with a clearly defined interior crater.
Black Butte is a plug dome on the southwest base of the mountain that formed about 9,500 years ago.
Mount Shasta has seven named glaciers that lie on the north and east sides. These lie at the summit and radiate downward to below 10,000 feet. The four largest include Hotlum glacier, which is the largest glacier in volume in California. Whitney glacier is the longest glacier on Mount Shasta. Bolam and Wintun are smaller glaciers.
As a result of hundreds of thousands of years of some of the most gigantic and active volcanic or glacial activity in the history of the world, the picturesque Mount Shasta has earned its reputation as one of the natural wonders of the world.
