Geology of the Olympic Mountains

By | February 24, 2017

Olympic Mountains


Almost everywhere in the San Juan Islands will provide a commanding view of the Olympic Mountain Range as it towers over the southern horizon. Many people may wonder how these titans came to exist here on the relatively low-relief area of the Olympic Peninsula. Their origin story is a fascinating one.


You may be aware that there is a large fault off the coast of Washington State where the North American tectonic plate and the oceanic Juan de Fuca tectonic plate meet. Because the Juan de Fuca plate is made of much denser materials than the North American plate, the Juan de Fuca plate is slowly being shoved (subducted) underneath the North American plate as the two plates collide with one another. As the Juan de Fuca plate sinks, all of the oceanic debris sitting on the ocean floor is scraped off and accretes (accumulates) along the continental margin. The Olympic Mountains are the result of this accretion process over millions of years. That is why you can hike to the top of the Olympic Mountains and find oceanic fossils!


Photo Courtesy of Pacific Northwest Seismic Network


While the accretion process is working on increasing the height of the Olympic Mountain Range, the earthly process of erosion is steadily wearing them away. Scientists have found that the Olympics have reached a steady state of erosion and uplift since about 14 million years ago, and thus have remained roughly the same height since then. The elevation today is about 7980’ at the peak of Mount Olympus.


Much of the Olympic Mountain Range is made up of basalt, which is a fine-grained volcanic rock that originated on the ocean floor, and sedimentary deposits. The sedimentary deposits are made up of oceanic deposits, as well as material that originated from the land and were washed out to sea by rivers and streams. The most common sedimentary rock found in the Olympic Mountains is limestone. There are also deposits of manganese and copper-bearing minerals located in the Olympics that were prospected and mined at various times in the early twentieth-century, however today they are not considered ecologically significant.

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