Geologic Background:
The most voluminous of the Cascade volcanoes,
Mount Shasta is a massive compound stratovolcano composed
of at least four main edifices constructed over a period
of at least 590,000 years. An ancestral Shasta volcano
was destroyed by Earth's largest known Quaternary
subaerial debris avalanche, which filled the Shasta River
valley NW of the volcano. The Hotlum cone, forming the
present summit, and the Shastina lava dome complex were
constructed during the early Holocene, as was the SW
flank Black Butte lava dome. Eruptions from these vents
have produced pyroclastic flows and mudflows that
affected areas as far as 20 km from the summit. Eruptions
from Hotlum cone continued throughout the Holocene.
Historic Activity:
- Shasta's only historical eruption was observed from
the ship of the explorer La Perouse off the California
coast in 1786.
Recent Activity:
- Thermal and hot-spring activity continues near the
summit.
- Six earthquakes near Shasta were recorded within 24
hours of the June 28, 1992, M 7.5 earthquake in southern
California (900 km SSE of Shasta).
Data Sources:
- Smithsonian's Global Volcanism Network (V. 17, No.
6).
|