USGS scientists are currently working on this question. Mount Shasta doesn’t erupt on a regular timescale. Research indicates that the volcano erupts episodically with ten or more eruptions occurring in short (500-2,000 year) time periods separated by long intervals (3,000-5,000 years) with few or no eruptions.
Evidence suggests that magma most recently erupted at the surface about 3,200 years ago. However, small eruptions where magma nearly reached the surface, interacted with groundwater, and caused small explosions that redistributed old rocks and debris might have occurred since then. These events are difficult to study and date because the deposits are poorly preserved and do not contain materials that can be dated.
Eruptions during the last 11,000 years produced lava flows and domes on and around the flanks of Mount Shasta. Pyroclastic flows from Shasta’s summit and flank vents extended as far as 20 km (12.4 miles) from the summit. Most of these eruptions also produced large mudflows, many of which travelled several tens of kilometers from Mount Shasta.
Learn more: USGS California Volcano Observatory