Mount Shasta is definitively an active volcano. Rising to 14,179 feet, it is the second-highest peak in the Cascade Range, surpassed only by Mount Rainier. This massive mountain is situated in Siskiyou County, California, dominating the horizon along the Interstate 5 corridor. It remains a significant geological structure with a history of powerful, explosive activity.
Defining Mount Shasta: A Stratovolcano
Mount Shasta is classified as a stratovolcano, or composite cone. This structure results from the accumulation of alternating layers of hardened lava flows, volcanic ash, and tephra. The erupted material, primarily thick and viscous andesite and dacite, contributes to the explosive nature of its eruptions and its towering height. Mount Shasta is the most voluminous stratovolcano within the Cascade Volcanic Arc, containing an estimated 85 cubic miles of material. The mountain includes several overlapping volcanic cones. A notable feature is Shastina, a prominent satellite cone on the western flank, which indicates the complex and dynamic growth of the system.
A History of Explosive Activity
Mount Shasta’s eruptive history is characterized by episodic activity separated by long periods of quiet. Over the last 10,000 years, the volcano has erupted on average about once every 600 to 800 years. The last unambiguous eruption occurred approximately 3,200 years ago, producing block-and-ash flows on the northern flank. These flows are a type of pyroclastic flow, consisting of hot rock fragments and gases that rush down the mountainside.
The most recent activity is difficult to pinpoint; some historical accounts mention a small eruption in 1786. Although observed by a French explorer, geological evidence confirming it as a magmatic eruption remains inconclusive. The recurrence interval suggests Mount Shasta is currently in a prolonged quiet period, not an extinct one. The presence of hot springs and steam vents, known as fumaroles, near the summit indicates a relatively young and still-hot magmatic system is active beneath the surface.
Current Status and Monitoring
Mount Shasta is considered a dormant volcano, meaning it is currently quiet but retains the potential for future eruptions. The U.S. Geological Survey (USGS) and the California Volcano Observatory (CalVO) maintain continuous monitoring efforts to detect any signs of magmatic unrest. The volcano is currently ranked as a “Very High Threat” due to its history and surrounding population centers.
Primary monitoring involves an extensive network of seismometers that track earthquake activity. An increase in the frequency or intensity of small earthquakes often signals the movement of magma or volcanic fluids at depth. Scientists also use GPS and tiltmeters to measure ground deformation, looking for subtle swelling of the volcano’s flanks that would indicate rising magma. Tracking gas emissions, such as sulfur dioxide, from the summit fumaroles provides additional data on the subterranean magma reservoir.
The Potential Hazards
The most widespread and destructive hazard associated with a Mount Shasta eruption is the formation of lahars, or volcanic mudflows. Because the mountain is capped with extensive ice and snow, including seven named glaciers, even a small eruption or subsurface heating can rapidly melt this cover. The resulting torrent of water mixes with loose volcanic ash and debris, creating a dense, fast-moving mudflow that travels down river valleys. A lahar could travel tens of miles from the summit, posing a severe threat to communities along the Sacramento, McCloud, and Shasta Rivers.
Pyroclastic flows, the destructive mixture that rushed down the flanks 3,200 years ago, represent a direct threat to areas closest to the summit and upper slopes. These are high-temperature currents of gas and rock that move at extreme speeds, making them impossible to outrun. A less immediate but far-reaching hazard is the dispersal of volcanic ash, which can travel hundreds of miles downwind. Ash fall can cause widespread infrastructure failure and present a major public health and economic challenge for Northern California by:
- Disrupting air travel.
- Contaminating water supplies.
- Damaging crops.