Information in this reading has been adapted from: http://www.burkemuseum.org/geo_history_wa/

Two important geologic phenomena have accompanied subduction in the Pacific Northwest. The first was the collision of exotic lands onto the ancient continental margin. Geologists use the term “terrane” for large blocks of crust that share a common geologic history. In the Pacific Northwest, many terranes are “exotic” – they have moved great distances from where they originally formed and were accreted (added) to our land by plate collision and subduction.

At least four major terranes of oceanic rock, including the remains of two volcanic island chains and two extensive belts of ocean-floor rocks, were added to the Pacific Northwest over the last 200 million years. As these terranes collided, the edge of the continent expanded some 400 miles westward. These 4 terranes are named:

The Intermontane Belt
The Insular Belt
The Melange Belt
The Crescent Belt

A sketch map showing our interpretation of the original extent of the Terrane Belts extending into Washington. The southern end of the belts is covered by younger rock of the Columbia Plateau. This is a “best guess” at the original southern extent of the terranes.

The second phenomenon that shaped Washington’s history was the formation of a series of continental volcanic arcs. Remember that 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 accreted terranes to form giant plutons of granite at depth and volcanic rocks at the surface. You can think of the continental arcs as huge, elongate burned zones where magma intruded the older lands to form enormous bodies of granitic rocks at depth and volcanoes on the surface.

Four distinct continental arcs have formed through our region over the last 200 million years. From oldest to youngest, these include:

The Omineca Arc
The Coast Range Arc
The Challis Arc
The Cascade Arc

The four great continental arcs burned through the lands of Washington and British Columbia. The arcs are shown here superimposed over the outlines of the terrane belts they intruded. The Omineca Arc is the oldest, followed by the Coast Range, Challis, and Cascade Arcs.

The combination of these two processes – the accretion of four exotic terranes and the subsequent intrusion of four belts of molten rocks -- are the benchmarks along which Washington’s geologic history is organized over the last 200 million years. To unravel Washington’s complex geologic history, you must first understand each of the four terrane belts and each of the four continental arcs.

The Omineca Episode (180 -115 million years ago)

The name “Omineca” is a native American word meaning “slow moving water.” It is the name given to a remote range of mountains in north - central British Columbia. We also use the name Omineca for a continental arc that extended from Alaska to Washington State over Mid-Jurassic to Mid-Cretaceous time.

The Omineca Episode began when a chain of volcanic islands collided with the western edge of North America. The islands were “welded” to the edge of the continent by molten rocks that cooled and solidified as granite. The remnants of these ancient islands, along with the granite that welded them to the continent, outcrop today in the Omineca Highlands of north-central Washington and British Columbia. They extend all the way north to the Yukon Territory.

Today, rocks of the Intermontane Islands form a “superterrane” -- an enormous block of crust composed of smaller terranes, all sharing a distinctive geologic history and bounded by faults. The Intermontane superterrane is built of four smaller terranes, each named after locations in British Columbia. These include the Stikine, Cache Creek, Slide Mountain, and Quesnel (“ken-el”) terranes. These terranes originated as either volcanic islands or slabs of seafloor.

The Intermontane terranes make up thousands of square miles in a large plateau region in British Columbia and extend all the way to the Yukon Territory. Along the southern end of the Intermontane Belt, the Quesnel Terrane makes up most of the Okanogan region of north-Central Washington, extending as far west as the Methow Valley.

The approach of the Intermontane Volcanic Islands to the ancient margin of the Pacific Northwest. The Farallon Plate has fragmented into two smaller “microplates,” each with their own subduction zone. Molten rock from subduction of the Intermontane Plate intruded into the old rocks of North America. At the same time, the Insular Plate fed molten rock to build the Intermontane Islands offshore. When the islands finally collided with the continent, the Intermontane subduction zone jammed and shut down entirely.

By 160 million years ago, the original continental margin was crushed by the Intermontane Islands (shown in brown) to form the Kootenay Fold Belt. The Insular Plate took over as the active subduction zone along the margin of the continent. As it did, a new continental arc –the Omineca Arc - sent molten plutons of granite rising into the crust to “weld” the Intermontane Belt to the continent. Sediment eroded from the Omineca Arc accumulated along the eastern Methow Shelf (shown in yellow above). Further offshore, yet another chain of volcanic islands slowly approached the Pacific Northwest.

The approach of the Insular Volcanic Islands is recorded by sediment deposited along the margins of the Bridge River Ocean. For 60 million years, sand eroded from eastern highlands in the Omineca Arc. About 130 million years ago, sediment eroded from new lands to the west was deposited (shown above by the yellow sand patterns). These sediment derived from the west are the first evidence of the approach of the Insular Volcanic Islands.

The Coast Range Episode is named after the Coast Range Mountains of British Columbia. The episode began 115 million years ago when a second chain of volcanic islands collided with the western shoreline of the Pacific Northwest. These islands welded to the edge of the continent by molten rocks that cooled to form the Coast Range “Batholith”—the largest single body of granitic rocks in America.

As the Insular Volcanic Islands collided with the Pacific Northwest, the old subduction zone of the Insular Plate jammed and shut down entirely. The descending Farallon Plate took over as the active subduction zone. As molten rock from the Farallon Plate rose upward through the accreted Insular Belt, the Coast Range Arc intruded molten granitic rocks to form the Coast Range Mountains from Washington to Alaska.

Molten rock ascending from the Farallon Plate under the newly accreted edge of the continent gave rise to a new volcanic arc along the coast known as the Coast Range Arc. It was among the largest continental volcanic arcs in the world, extending from Washington to Southeast Alaska. Plutons of hot magma burned their way upward through the newly accreted Insular Belt, injecting vast quantities of granite into rocks of the old Insular Islands. At the surface, new volcanoes built along the margin of the continent.

One of the thickest blankets of non marine sediment in North America accumulated along the passive margin of Washington during the Challis Episode (in the region shown in orange above). When the western block of the Kula Plate broke free, the old continental margin moved northward about 90 miles along the Straight Creek Fault.

The last remnant of the Farallon Plate, now called the Juan de Fuca Plate after a Spanish explorer, began subducting along the western margin of Washington. Melting of the Juan de Fuca Plate at depth intruded magma into the continental margin to form the Cascade Arc. The volcanic rock riding on the top of the Juan de Fuca plate is the Crescent Basalt, unsuccessfully trying to subduct beneath the continent and these rocks have been uplifted to form the Olympic Mountains.

The Columbia River Basalt

The most dramatic events of the Cascade Episode occurred not along the Cascade Arc, but to the east on what is now the Columbia Plateau. From fissures located in northeast Oregon and (probably) southeast Washington, vast quantities of very fluid volcanic rock, basalt, erupted out onto the surface. Between 17 and 13 million years ago, immense floods of basalt repeatedly inundated what is now eastern Washington and Oregon, flowing west in some cases all the way to the Pacific Ocean. These are the Columbia River Basalt flows, and they are the largest flood basalts in North America.

Undoubtedly, the basalt covered an even larger region before erosion removed large portions at higher elevations. Moreover, the Columbia River Basalt dips eastward along the eastern flank of the Cascade Range. This indicates that the Columbia River Basalt are older than the Cascade Range itself and were uplifted and eroded as the modern range was uplifted 4 to 5 million years ago.

Geologists have debated the origins of the Columbia River Basalt for decades. The eruptive center for these flows appears at the coincidence of two geologic features. One of these is the eastward-migrating Yellowstone "Hot Spot" which traverses through this area between 17 and 13 million years ago, before continuing to burn its way across the Snake River Plain and onto its present location at Yellowstone. This is a mantle-plume, much like in the Hawaiian Islands, except that this one now lies under our continent!