Geology Reference
In-Depth Information
ago, the western belt, through Mount St. Helens and
ending near Portland, is younger. The eastern belt,
which cooled between 50 and 25 million years ago,
stretches approximately 45 miles east under Mt. Adams
and south through Mt. Hood and into southern Ore-
gon.
The eruptive history of Cascade volcanoes has
followed a loose pattern over the past 20 million years.
Since the middle Miocene, major events, each continu-
ing for short periods of only 1 to 2 million years, have
occurred at 5 million years intervals. There is evidence
that these volcanic episodes in the Pacific Northwest
correspond with global volcanism taking place at the
same time. World-wide, shorter periods of volcanism
were interspersed with longer periods of relative
inactivity. The magnitude of volcanism and volume of
extrusions has been decreasing steadily since the middle
Miocene.
Recent Volcanism
Dramatic eruptions have been absent in
Oregon in the 20th century, although at least eleven
volcanoes in the entire Cascade range, where it extends
north into Washington and south into California, have
been active historically. In Oregon Mt. Hood and the
South Sister have signalled volcanic activity recently.
As in the past, Cascade volcanism today is due to
colliding tectonic plates. The Juan de Fuca plate
Block diagram of the Western and High Cascades at
the latitude of Eugene (after Volkes, Snavely, and
Myers, 1951; Gandera, 1977; Taylor, 1990)
