Olympic Mountains

The Olympic Mountain Province, located in the northwest corner of the state, is composed of a mountain range that rises from the Pacific Ocean to an elevation of 7,980 feet at the peak of Mount Olympus. The mountain range is bounded on three sides by the waters of the Pacific Ocean to the west, Puget Sound to the east, and the Strait of Juan de Fuca to the north. The higher peaks are covered with glaciers and snowfields, feeding the many rivers that radiate outward from the center of the range. The enormous amount of rain and snow that falls on this mountain range feeds the glaciers and streams as well as the lush rain forest that covers most of the western side of the province.

The Olympic Mountains are composed of a large uplifted and folded section of oceanic crust that was accreted onto the continent over the last 40 million years along the Cascadia subduction zone. The range is cored by highly folded and disrupted sedimentary and metamorphic rocks, which are flanked to the east by a horseshoe-shaped border of Crescent Formation basalt and overlying sedimentary rocks. These rocks are locally covered by young glacial and non-glacial sediments.

Geologic History

There are three main rock types in the Olympics: sandstone, shale, and basalt. Most of these rocks originated underwater in the Pacific Ocean.

Sediment (sand and mud) was carried onto the ocean floor by rivers and were deposited layer by layer onto the ocean bottom. Basalt (or volcanic rock) erupted from underwater volcanoes and flowed on top of the sediment. This underwater deposition of alternating sand, mud, and basalt happened for 40 million years (from about 55 Ma to 15 Ma). Nowadays, these rocks make up the Olympic Mountains. How did they get there?

Accretion and Uplift

The thick accumulation of oceanic crust began to move slowly towards the North American continent by a process called subduction. Subduction occurs when shifting tectonic plates causes dense rocks (like oceanic crust) to subduct, or shift beneath lighter, less dense rocks (like continental crust).

Mineral Resources

Pervasive manganese deposits are found in the northern part of the province within limestone interbedded in the Crescent Formation basalt. Some manganese deposits were large enough to be mined—the largest mine was the Crescent mine, producing over 50,000 tons of manganese ore. Red jasper may be found near the manganese deposits. The Crescent Formation basalt also hosts native copper grains and other copper minerals. Beautiful zeolite specimens may be found in the pillow basalts. Several gold placer deposits were mined historically along the coast within beach sands. Cinnabar, galena, and quartz crystals have also been reported in the province.