This road cut high on the road to Hurricane Ridge reveals the origins of the Olympic Mountains. Looking across the street from the interpretive sign you will notice grey rocks that have a dough-like appearance. These are called pillow basalt. Pillow basalt is formed when basalt flows into water - in this case, the Pacific Ocean. So how do ocean-made rock formations end up at 3000-4000 feet above sea level?
Most Olympic rocks were born in the deep waters of the Pacific Ocean, where sand and mud from the land accumulated, bed by bed, into thick, nearly horizontal layers. Adding to this pile of rock was flow after flow of basalt lava that issued from cracks or broad volcanoes on the ocean bottom. Because of this origin, almost all the rocks of the Olympics reflect the assaults and caresses of ocean currents, the buoyancy of water, and the cold and pressure of ocean deeps.
The Olympic Peninsula is a ‘young’ addition to the west coast of North America. The prime mover in the system has been the Juan de Fuca Plate of oceanic crust moving toward the east from a ‘mid-ocean rise’ of magma deep in the earth. The oceanic plate, being denser, dives under the North American continental-crust plate in a subduction zone and trench. As the oceanic crust slab slants down deeper under the continent, the process produces basalt flows and Cascade volcanoes.
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth’s mantle, as the plates converge. Subduction zones involve an oceanic plate sliding beneath either a continental plate or another oceanic plate. Subduction zones are often noted for their high rates of volcanism, earthquakes and mountain building. This is because subduction processes result in melt of the mantle that produces a volcanic arc as relatively lighter rock is forcibly submerged.
The Cascadia Subduction Zone is a very long sloping fault that stretches from mid-Vancouver Island to Northern California. It separates the Juan de Fuca and North America plates. New ocean floor is being created offshore of Washington and Oregon. As more material wells up along the ocean ridge, the ocean floor is pushed toward and beneath the continent. The Cascadia Subduction Zone is where the two plates meet.
To qualify as a "find", email (do not post online) the answers to the following questions and perform the following tasks. Failure to do that in a timely manner will result in a log deletion.
Visit the interpretive sign at the posted coordinates and answer the following questions:
1) How many volcanoes are located on the Olympic Peninsula and what are their names?
2) Name the two kinds of rocks located here.
3) Observe the pillow basalt. Do you notice bubbles or a lack of bubbles? What possible reason could there be for their presence or lack of?
4) UPLOAD a photo with either the pillow basalts in the background OR nearby interpretive signs, yourself and your GPSr. and attach it to your found log. (If you choose the sign option be sure to not give away any of the answers.)
We hope you have learned a thing or two about the origins of this area and thanks for visiting the Olympic National Park.