In 1987, geologists Dwight Crandall and Donald Nichols determined that Mount Shasta has erupted at least three times in the last 750 years, and that the likelihood of an eruption within a person's lifetime is one in three or four. Mt. Shasta does not show signs of erupting in a regular, predictable pattern. Fumaroles (small plumes of steam and smoke) show that Shasta is still active beneath its surface.
Geologic evidence suggests that the most recent eruption was at least 200 years ago. For a long time, a French explorer named Jean-François Galaup de Lapérouse was credited with having witnessed a Mt. Shasta eruption in 1786, but that account is now doubted. Lapérouse's map shows Mt. Shasta in a coastal region, when the actual mountain is more than 60 miles inland.
Most of the major distribution of debris and lava flow occurred 9,500 years ago, when pyroclastic flows (the dangerous molten rock rivers that are responsible for most volcanic eruption damage on the ground nearby) covered the forests within 30 kilometers of the peak, including where the modern-day towns of Mt. Shasta and Weed are located. The most recent eruption flowed in two directions: one flowed more than seven miles down Mt. Shasta's east flank, while the other went 12 miles down Mud Creek.
The currently active cone of Mount Shasta also happens to be its peak, and is called Hotlum Cone. This is where the most recent eruption originated. The cone formed about 8,000 years ago and is now covered by a rock dome that has been warped and cratered by sulfurous hot springs and fumaroles. The cone is a popular destination for hikers and back-country skiers.
