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Geology of the National Parks

GEOLOGY OF OLYMPIC NATIONAL PARK:
PART I OLYMPIC GEOLOGY

Development of Olympic structure
Fig.26. Development of Olympic structure.

Colliding Plates and Olympic Rocks

The theory of plate tectonics can explain many features of the Earth, including many of those we find in rocks of the Olympics. Where two plates collide, such as at a continental margin, the rocks are likely to be squeezed and mashed to a remarkable degree. Olympic rocks offer abundant evidence of such folding, smashing, and tearing apart on a colossal scale (see fig. 23 ). The disrupted rock belts of the Olympic core (fig. 16 and geologic map) are long and thin; even though they have been highly deformed, they maintain their continuity, as might be expected where a broad oceanic plate moves under a long section of the continent's edge. Also, a crude arrangement of younger and younger rocks west-ward suggests a sequential collision of material with the continent. The youngest rocks, farthest to the west, were not so strongly smashed as those farther east, possibly because the rate of motion between colliding ocean and continental plates was slowing down.

The basalt of the Olympic horseshoe was once riding on the ocean bottom. It may have erupted not far from the continent's edge and been skimmed off the oceanic plate as it plunged under the continent; it then stuck to the margin of the continent. Rocks that had been deposited either on top of the basalt pile or on its continental side (that is, the peripheral rocks; see fig. 26) were folded along with the basalt pile, although its great mass and rigidity protected them from extreme deformation. Not so for rocks deposited seaward of the pile, some of which were just the thin oceanward edge of the same basaltic pile. They were crammed against and underneath the main basalt abutment. This material was not only folded and sliced as it collided with the pile but was also pushed down to regions of higher temperatures, where new minerals began to form in it.

Olympic rocks folded into an inside corner
Fig.27. Diagram showing how the rocks that form the Olympic Mountains may have been squeezed into the in-side corner of Vancouver Island and the Cascade Range by the opposing movement of the oceanic and continental plates.

We do not know for certain why the Crescent Formation and the arcuate belts of core rock are arranged in the horseshoe pattern. If we view the basaltic horseshoe from high above, we see that it appears to bend into an inside corner formed by the older geologic terranes of Vancouver Island and the Cascade Range. It looks as though it were squeezed into the corner (fig. 27).

However the horseshoe formed, the thick mass of mostly sedimentary rock from the continent was jammed up against and under the edge of the basalt, which in turn was jammed against the continent. When movement of the ocean floor ceased, the sedimentary rock began to rise because the sandstone and shales are lighter than the oceanic crust beneath. The spreading ocean crust acted as a conveyor belt, driving the rocks down beneath the continent; but when movement ceased, the rocks bobbed up like a cork (fig. 26). This last episode of bobbing, accompanied by additional disruption of beds and faulting, raised the rocks in a domelike fashion to produce the height of land we call the Olympic Mountains. The mountains we see today were carved by erosion; even as the land rose, streams and rivers began to carry the rocks back to the sea.


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Material in this site has been adapted from Guide to the Geology of Olympic National Park by Rowland W. Tabor, of the USGS. It is published by The Northwest Interpretive Association, Seattle.

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