Geology Reference
In-Depth Information
southcentral Oregon just before major block faulting in
the Basin and Range began. In contrast to other
volcanoes of this province, Gearhart lacks the typical
bimodal history where volcanoes initially erupt basaltic
lavas to be followed later by rhyolites. The eruption of
the Gearhart volcano began with basalt and ended with
a veneer of andesitic lavas. Just north of Gearhart, the
prominent volcanic peak, Yamsay Mountain, is signifi-
cant because it erupted during the Pliocene coinciding
with extensional stretching of the Basin and Range.
Unlike the Gearhart eruption, Yamsay has a history of
bimodal volcanism beginning with a low shield cone of
basaltic then rhyolitic lavas. Near the end of its erup-
tive history, small amounts of basalts again extruded
from its flanks. This kind of bimodal volcanic activity
may have been due to crustal extension.
Pleistocene
Continental ice sheets advancing southward
during the Pleistocene accompanied increased precipi-
tation and valley glaciers in the mountains. Rainfall
and mountain runoff filled depressions in southcentral
Oregon, creating large pluvial lakes across the region.
Pluvial lakes are dependant on rainfall, expanding or
decreasing dramatically with changing climatic condi-
tions. At the peak of the moist Pleistocene environ-
ment vast areas of Oregon were covered by freshwater
lakes, whereas today, during an interglacial warm
phase, lakes cover less than one percent of the state.
During this period nine fair-sized pluvial lakes existed
in the Basin and Range province of Oregon. The
largest of these in the western section of the province
was Lake Modoc covering 1,096 square miles, followed
by Lake Chewaucan at 461 square miles in the central
region, Lake Coleman, now Warner Lakes, spread over
Within the Owhyee Uplands of southeastern Ore-
gon a complex series of overlapping volcanic cal-
deras of immense size erupted during the Miocene
(after Rytuba, et al., 1990).
