Geologic Background:
Mount Rainier, at 4392 m the highest peak in the
Cascade Range, forms a dramatic backdrop to the Puget
Sound region. Large Holocene mudflows from this massive,
heavily glaciated volcano have reached as far as the
Puget Sound lowlands. The present summit was constructed
within a large crater breached to the north during the a
mid-Holocene eruption as a result of the collapse of a
once-higher edifice. Several postglacial tephras have
been erupted from Mount Rainier; tree-ring dating places
the last recognizable tephra deposit during the 19th
century. Extensive hydrothermal alteration of the upper
portion of the volcano has contributed to its structural
weakness; an active thermal system has caused periodic
melting on flank glaciers and produced an elaborate
system of steam caves in the summit icecap.
Historic Activity:
- Historic eruptions occurred during the
mid-1800's.
Recent Activity:
- Fumarolic activity continues today in the twin summit
craters.
- An average of 30 earthquakes occur under Mt. Rainier
per year, making it the most seismically active volcano
in the Cascade Range after Mt. St. Helens.
Data Sources:
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