Geology Reference
In-Depth Information
Coastal margin rifting model for formation of the
Oregon-Washington coast volcanics (after Snavely,
1987; Babcock, et al., in press)
Another possible explanation for the formation
to receive different formation names. From the north
of the Coast Range block is that the volcanics devel-
to south, they include the Metchosin, Crescent, Black
oped above a series of rifts or crustal rips along the
Hills, and Grays Harbor volcanics in Washington, the
coastal margin. Differential movement of large-scale
Tillamook volcanics in the northern Oregon Coast
blocks would have created a line of overlapping tears
Range, the Siletz River volcanics in the central part of
releasing lavas that became the base of the Coast
the Coast Range, and the Roseburg volcanics in the
Range. Alternately, the rifting could have been caused
south. The oldest volcanic rocks in the range are those
by a hot spot below the crust. With coastal margin
of the 64 million year old Roseburg and Metchosin
rifting, the Coast block would have been formed in
formations at the southern and northern extremes,
place and not accreted.
while the youngest are the 44 million year old Grays
Differences in age, texture, and mineralogy of
River Formation in the central region. Common to all
the core Coast Range volcanic rocks have caused them
of these volcanics are elliptical bodies called "pillows",
