Geology Reference
In-Depth Information
Several large volcanoes and hundreds of smaller volcanic vents are present in the Cascade Range of
the Pacific Northwest. Medicine Lake Volcano and Newberry Volcano are shield volcanoes that lie
just east of the main trend of the Cascade Range. Volcanism
began in the range during the Oligocene, but the most
recent episode started about 5 million years ago, and
of course, it continues to the present.
Mount St. Helens, a composite
volcano, as it appeared from
the east in 1978.
Mount Garibaldi
BRITISH COLUMBIA
Mount Baker
Galciei Peak
Seattle
WASHINGTON
Mount Rainier
Mount
St. Helens
Mount Adams
Portland
Mount Hood
Lassen Peak today. This most southerly peak in the
Cascade Range is made up of 2 km3 of material, including
the bulbous masses of rock visible in this image.
Mount Jefferson
Three Sisters
OREGON
Newberry Volcano
Crater Lake
(Mount Mazama)
Mount McLoughlin
Mount
Shasta
Medicine Lake
Volcano
CALIFORNIA
NEVADA
Lassen
Peak
0
150
km
1. Of these volcanoes, only Lassen Peak
and Mount St. Helens erupted during
the 1900s. Mount
St. Helens began
erupting again in
October 2004
Eagle Peak 2810 m
Lassen Peak 3817 m
Ski Heil 2713 m
Broke off Mountain 2815 m
3397 m
al
3154
2790
Sulphur Works 2164 m
2. Lassen Peak is a
lava dome that
formed about 27,000
years ago on the
northeast flank of
a deeply eroded
composite volcano
known as Mount
Tehama.
2426
0
1
SW
NE
km
1915 dacite from Lassen Peak
Talus
Dacite domes
Pre-Lassen Peak dacites from the Lassen Peak vent
Breccia (vent filling) of Mount Tehama
Decomposed andesites
Andesites of Mount Tehama
650
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