Geology Reference
In-Depth Information
and eroded by ice. Lava as well as ash, steam, and
gasses from the stratovolcanoes and small vents mixed
with packed ice and snow causing lahars or mud flows
down the sides of the mountains. Ice Age glaciation,
which brought a massive continental ice sheet south to
the northern Washington border, was responsible for
extensive valley glacier complexes in the Oregon and
Washington Cascades beginning about 2 million years
ago. A glacier system on Mt. Hood was situated at the
northern end of a nearly continuous ice field that
capped the High Cascades and extended south to Mt.
McLoughlin as late as 15,000 years ago. Glacial ice
stretched from the peak of Mt. Hood down the Sandy
River almost to the Columbia. Even the Western
Cascades had small ice masses in the valleys where
glaciers from the higher peaks reached into the lower
canyons, advancing and retreating repeatedly to carve
out valleys, steepen walls, and leave behind thick
deposits of till.
In the valley floors, moraines of gravel, sand,
silt, and clay blocked streams which backed up to
create small lakes. Many of the higher Cascade lakes
occupy semi-circular basins called cirques, formed by
glacial erosion, while some of the lower lakes, such as
Suttle Lake near Santiam Pass in Deschutes County,
ponded up behind moraines. Virtually all of the
naturally occurring lakes in the Cascades owe their
origin to glaciation, lava dams, or landslides.
Although the last cold, wet climatic interval
ended 10,000 to 12,000 years ago, small scattered
glaciers persist today throughout the range. Mt. Hood
has nine glaciers with a total volume of 12.2 billion
cubic feet of ice and snow. If it were melted, this ice
would provide enough water to enable the Columbia
River to flow at a normal rate for 18 hours. The
volume of ice above Crater Rock alone is 47 million
cubic feet. Of the five major glaciers on the volcanic
cones of the Three Sisters, Collier Glacier, extending
from the north slope of the Middle Sister across to the
base of the North Sister, is the largest with a volume of
700,000 million cubic feet. Even though it has been
disappearing very rapidly in recent years, 300 feet of ice
make this glacier the thickest of those on the Three
Sisters. The Middle Sister's steep eastern face is
covered by the Hayden and Diller glaciers, while
glaciers on the North and South Sisters are very small.
Should an eruption occur today in the Cascades, as
happened in the case of Mount St. Helens, extensive
floods and mudflows would be a by-product of the
melting ice and snow.
Geometry of the Juan de Fuca slab below Oregon
and Washington (after Michaelson and Weaver,
1986; Duncan and Kulm, 1989)
represents the trailing edge of the older Farallon plate,
and the descending slab dips up to 65 degrees beneath
the North American plate. As this slab plunges into
the upper mantle at about 60 to 75 miles depth, its
leading edge begins to melt partially creating the
magma chambers that feed Cascade volcanoes. Geo-
physical evidence suggests that the subducting Juan de
Fuca plate splits around 30 miles depth into three
eastbound descending tongues. Of these, the Washing-
ton portion dips at a shallow angle while the tongues
beneath Oregon and British Columbia are much
steeper.
Pleistocene
Quaternary volcanic activity in the High
Cascades was accompanied by extensive Pleistocene
glaciation when the major peaks were being scoured
Structure
The Cascades are scarred by numerous north-
northwest fault zones and lineaments, enormous lines
