Geology Reference
In-Depth Information
A view to the north of Abert Rim in central Lake County (photo by E.M. Baldwin).
were apparently extruded in an extremely short time
span. One estimate of the duration of the eruption is
only 50,000 years.
The basalt, making up the bulk of Steens
Mountain, extends into the Pueblo Mountains to the
south and to Abert Rim to the west. The upper 3,000
feet of the east scarp of the Steens as well as rock
exposed in the glacial valleys is composed of andesitic
basalt. In the vicinity of Alvord Creek, the "great flow"
of Steens Mountain basalts is nearly 900 feet thick.
Columnar joints, which mark this basalt layer, measure
as much as 5 feet across and rise 300 feet to form a
very steep scarp along the southern side of Alvord
Creek. The columns are broad at the base and narrow
toward the top. The similar Owyhee Basalts, that
erupted onto the Owyhee Plateau 13 to 12 million
years ago, attain a maximum thickness of about 1,500
feet in Owyhee Canyon but are much thinner else-
where. These basalts do not contain large plagioclase
feldspar crystals that are so common in certain areas of
the Steens flows.
About the same time as the Steens Mountain
eruption, extensive Miocene ash-flow tuffs covered
southeast Oregon and northern Nevada, originating
from more than a dozen different volcanic centers
aligned in a northeastern direction across the basin and
Owyhee Uplands. Eruptions from volcanic cones in the
Lake Owyhee and McDermitt fields resulted in im-
mense calderas. The largest of these is McDermitt
caldera which is bisected by the Oregon-Nevada line.
Actually a series of overlapping calderas, the McDer-
mitt complex has a combined diameter of 22 miles.
Three Fingers caldera west of Succor Creek,
Mahogany Mountain caldera along the Owyhee River,
Saddle Butte to the west, and Castle Peak to the north
are the four volcanoes comprising the Lake Owyhee
field in Oregon. With a north rim extending 1,000 feet
above Leslie Gulch, Mahogany Mountain is the south-
east rim of an ancient volcanic caldera 10 miles in
diameter, whereas the Three Fingers caldera, just to
the northeast, is a circular collapsed depression 8 miles
in diameter. As rhyolitic magma erupted from these
pre-caldera volcanoes, the subsequent ash, tuffs, and
continuing eruption of lava produced the two calderas.
Three Fingers rock, within the depression, is the
erosional remnant of a rhyolite plug. Saddle Butte, a
third caldera just to the southwest is 15 miles in diame-
ter. This vast structure is almost completely buried
beneath younger volcanic rocks of the Saddle Butte
field. Castle Peak caldera, part of the same field,
collapsed with the eruption of the Dinner Creek ash
flow tuff on the southwest margin of the rim. By
comparison, Crater Lake and Newberry calderas are 6
miles and 5 miles in diameter respectively.
Volcanic activity continued into the later
Miocene with the eruption of Gearhart Mountain in
