Geology Reference
In-Depth Information
western mountains than in the eastern, younger Cas-
cades. This pronounced decrease coincides nicely with
a slowing in the convergence rate of two tectonic plates
beneath this area during the same time interval.
Initially 35 million years ago the Farallon plate was
being subducted at the rate of 3 inches per year,
decreasing to about 1/2 inch per year today. In addi-
tion, with the clockwise rotation of a portion of the
North American plate, the convergence angle between
the North American and Farallon plates became
progressively more oblique. The more oblique collision
resulted in less subduction and consequently less
volcanic activity. This trend, which began in the Mio-
cene, has continued into the present.
The Western Cascades were subjected to
additional uplift, mild folding, and faulting between 4
and 5 million years ago. Near the center of the range,
major north-trending faults and a pronounced McKen-
zie-Horse Creek fault to the east, mark the boundaries
of the uplifted Western Cascade block. What has been
called the Coburg Hills fault creates a sharp front
range to the Western Cascades just north of Eugene.
High Cascade Development
Two events, the westward tilting of the West-
ern Cascades and the sinking of the Cascade graben or
depression signal the beginning of High Cascade
volcanism. Numerous, overlapping shield volcanoes and
cinder cones built up a platform of High Cascades
aligned in a north-south direction on the eastern flank
of the older Cascades. Basalts, tuffs, and ash-flows form
the base of the range during the early phase of devel-
opment. With the late High Cascade volcanic episode,
from 4 million years ago to the present, a variety of
lavas and tuffs again dominated by basalts were ex-
pelled from a number of small composite cones con-
structed on the sides of the earlier, large shield volca-
noes. Basalt makes up as much as 85% of the Quater-
nary High Cascades by volume, although locally silica-
rich dacite and rhyodacite lavas have been erupted.
Where the Cascades extend south into California and
northward into Washington, Quaternary activity
decreased, and more andesite was produced. Virtually
all of the High Cascade peaks are Quaternary or
younger in age, and the eruptions of lavas during this
final episode of volcanism are responsible for most of
the familiar, present day topography.
Westward tilting of the Cascade block, a
process that began in the Miocene, created the Blue
River ramp sloping gently upward to the east. The
ramp made a rainshadow by trapping the flow of humid
oceanic air in western Oregon. As it was elevated over
the Western Cascades, the moisture-laden air cooled
bringing precipitation of rain and snow. With this
Columnar jointing in basalts of the Little Butte Forma-
tion along Wiley Creek (photo Oregon Dept. Geology
and Mineral Industries)
active volcanoes had narrowed to an area only slightly
wider than the present High Cascade Range.
The intensity and focus of Cascade volcanism
is the by-product of tectonic forces at work deep in the
crust. Sitting atop the North American plate, the West-
ern Cascades were rotated clockwise into their present
position by the time the earlier eruptions of the
Columbia River basalts began approximately 17 million
years ago. This was well before the Pliocene episodes
of High Cascades volcanism. As the Cascades rotated
and the angle of the descending Farallon slab became
flatter, the linear centers of volcanic activity simulta-
neously swept from west to east. The oldest rocks
produced in the Western Cascades are approximately
42 million years old, while the youngest at 10 million
years are found at the eastern edge of the High Cas-
cades. This change geographically is found within a
very narrow range of a single degree of longitude.
Along with a change in volcanic focus, there
was a profound difference in amounts of volcanic
material erupted across the Cascades, with significantly
more lava in the west and less in the eastern portion.
In comparing the volcanic activity between the two
ranges, there has been six times more volcanism in the
