This view south toward the core of the Olympic Mountains is from Hurricane Ridge in Olympic National Park. Bits and pieces of oceanic crust that have been bent and broken in the vicinity of a major fault are exposed at Hurricane Ridge. Everything in this picture is part of an accretionary complex, which is formed when pieces of the ocean floor get shoved into the edge of a tectonic plate along major reverse faults.

Ocean Floor Basalt Pillow

This is a "pillow" of basalt at Hurricane Ridge, so-called because of its pillow-like shape and size. If you could look more closely you could see that it has a finer-grained rind and a slightly coarser-grained center, which are caused by the way the basalt is chilled and solidified in contact with water as it erupts on the floor of the ocean. This pillow is part of a large mass of pillow basalts that wrap around the north and east sides of the Olympic Mountains and are called the Crescent Formation. In plate tectonic terms it is called either the Crescent Terrane or Siletzia. This big pile of pillow basalts was erupted onto the ocean floor and formed part of an oceanic plate. Now it has been accreted to North America and is part of the Olympic Mountains. Select the image to see a larger view. Use your browser's back button to return to this page.

Turbidites

This is a sequence of turbidites at Hurricane Ridge. Turbidites are sequences of sediment layers that form from underwater landslides. These underwater landslides flow as "turbidity currents" (mixtures of sediment and water) down slopes on the ocean floor into deeper water. A blocky sandstone layer and a fine-grained shale layer mark each turbidite in this picture. Sand is the first to settle out of a turbidity current, followed by layers of finer-grained silt and clay that form shale. Even though turbidites start out as nearly horizontal layers, the processes that made these turbidites part of the Olympic Mountains have bent and tilted them. Select the image to see a larger view. Use your browser's back button to return to this page.

Pencil Slate

This picture is a close-up of pencil slate - slate that tends to fracture into pencil-sized pieces. Slate forms from shale and is the lowest grade of metamorphic rock. This pencil slate at Hurricane Ridge indicates that the rocks there were buried deeply enough, and heated and pressurized enough, to become slightly metamorphosed. The metamorphism was slight enough that the rocks do not look very different from the shales, sandstones and basalts that they started out as. In fact, the metamorphic minerals can only be seen in a microscope, and indicate that the rocks went part way down a subduction zone before being shoved up into the Olympic Mountains along major reverse faults. Select the image to see a larger view. Use your browser's back button to return to this page.

Folded Turbidites

In this picture you can see turbidite layers at Hurricane Ridge - layers of sandstone and shale - that have been compressed into a fold called an anticline. The layers angle up steeply on the left side, bend over the top (at the "hinge" of the fold), and angle down steeply on the right side of the picture. Select the image to see a larger view. Use your browser's back button to return to this page.

Glossary terms that appear on this page: accretionary complex; tectonic plate; reverse fault pillow basalt accreted terrane turbidite slate metamorphic rock shale sandstone basalt anticline