Sea Stack Sunset

This photo shows a sunset off the coast of the state of Washington from Olympic National Park.

The rocks before you are known as Sea Stacks. They are created by the interaction between two geologic forces; erosion and tectonics.

The rocks of the Pacific Northwest are being gradually pushed upwards due to subduction. A small piece of oceanic crust known as the Juan de Fuca plate (a small fragment of the ancient Farallon plate) is being subducted offshore. As this plate is pushed down into the mantle, it is pushing back against the rocks of the North American continent, stressing and bending some of the rocks near the shoreline.

The sea stacks are composed of sediments that were once deposited offshore. The rocks are conglomerates and sandstones, many of which were actually turbidites. The rocks were deposited offshore in landslides; giant avalanches beneath the waters carrying rocks from the shore out to sea.

The offshore rocks are being pushed upwards by the stresses of subduction. Eventually some of them reach sea level and beyond. Once they cross sea level, they are exposed to the eroding power of the ocean waves. The waves gradually eat away at the rocks, but places that are strong and resistant to erosion continue to stand tall. Combined with large changes in sea level over the past million years (giving these rocks a break from erosion when the waves retreat), erosion and tectonics have worked to create these features. Today they are occupied by a variety of birds and plants as well.

-JBB