Around 200 million years ago, the breakup of Pangaea had an enormous effect on Washington and the Pacific Northwest. The North American Plate being forced westward created massive friction between the westward-moving continent and the eastward-moving floor of the Pacific Ocean. The floor of the eastern Pacific Ocean was composed of an oceanic plate known as the Farallon Plate. As North America moved westward, the eastward-moving Farallon Plate was forced beneath the edge of the continent to form an active subduction zone. This situation was entirely unstable. The subduction of the eastward-moving Farallon Plate was unable to keep pace with the westward motion of North America. The Farallon Plate was forced into fragmenting into several "microplates" which are called Intermontane Plate and the Insular Plate.

The subduction in the Pacific Northwest caused major geological phenomena. The first was the collision of exotic lands onto the ancient continental margin. In the Pacific Northwest, many terranes (for large blocks of crust that share a common geologic history) are exotic. In geological tersm they have moved great distances from where they originated) and were added to our land by plate collision and subduction. At least four major terranes of oceanic rock were added to the Pacific Northwest over the last 200 million years. These were The Intermontane Belt, The Insular Belt, The Melange Belt, and the The Crescent Belt. As these terranes collided, the edge of the continent expanded some 400 miles westward.

The other major phenomenon that shaped Washington's history was the formation of a series of continental volcanic arcs. These continental arcs are regions where molten rock (magma) intrudes and moves toward the surface, often to form volcanoes. In the Pacific Northwest, magma intruded through the added terranes formed giant plutons of granite at depth and volcanic rocks at the surface.

Scientists have organized the last 200 million years of Washington's geological history into four episodes. Each are named after the continental arc that dominated that time period.

The Omineca Episode (195-115 million years)

The Omineca Episode started with a chain of volcanic islands colliding with the western edge of North America. The islands were "welded" to the edge of the continent by molten rocks that solidified as granite. The remnants of these ancient islands, along with the granite, form much of the Okanogan Highlands of north-central Washington and southern British Columbia.

The Coast Range Episode (115-57 million years)

The Coast Range Episode began when a second chain of volcanic islands collided along the expanding western shoreline. These islands welded to the edge of the continent by molten rocks that formed the largest body of granitic rocks in North America.

The Challis Episode (57-37 million years)

During the Challis Episode large regions of the Pacific Northwest were crushed, all the while hosting a chain of volcanoes running diagonally across Washington and Idaho. At the end of this period, a large piece of ocean floor (now the Olympic Peninsula) was uplifted and forced beneath the edge of the continent, extending the continental margin to its modern western extent.

The Cascade Episode (37 million years to the present)

The Cascade Episode began as the Juan de Fuca Plate advanced into the area and was forced underneath the western edge of the continent. This gave rise to a chain of volcanoes that has been erupting here for the past 36 million years. Between 17 and 12 million years ago, great floods of molten rock erupted from cracks in the crust of Washington and Oregon to form the basalts of the Columbia Plateau. The modern Cascade Range has risen over just the last 5 to 7 million years. For the last two million years, vast continental glaciers have repeatedly scoured the Pacific Northwest, creating some of the most spectacular landscapes on the continent.