An Introduction to the
Geologic History of
Catherine L. Townsend
John T. Figge
The lands of Washington State have evolved over more than a billion years of geologic history. (Image: A Digital Atlas of the United States)
This online exhibit is supported by the National Science Foundation, The University of Washington, The Burke Museum of
Natural History and Culture, and the Robert Frost family.
Reminders from a Restless Planet
We live in one of the most geologically active regions of the Earth.
Earthquakes, volcanic eruptions and rising young mountains all remind us that Washington State is situated along the violent boundary between ocean and continent.Â As the Pacific Northwest shudders with earthquakes and erupts with volcanoes, we directly experience the powerful geologic forces that have built our land over hundreds of millions of years.
The geologic evolution of Washington State is one of the most fascinating geologic stories ever told.Â It is a tale of the breakup of ancient giant continents, the birth and death of great ocean basins, the collision of exotic islands, the uplift and wearing away of generations of mountain ranges, enormous floods of molten lava and great continental glaciers of the Ice Ages.Â Washingtonâs geologic history stretches back in time more than a billion years, and it continues to unfold around us every day.
Modern Insights into Ancient History
We live in a remarkable time of geologic discovery.Â Over the last few decades, scientists have revolutionized our understanding the geologic processes that shape our land and lives in the Pacific Northwest.Â New technologies allow us to study our region from space, to map the adjacent ocean floor, to look deeply into the Earthâs interior, and to pinpoint the location and magnitude of earthquakes with great precision.Â We can even directly measure the motion of Earthâs great tectonic plates and reconstruct their movements through time.
With new insight, geologists can now begin to reconstruct a basic geologic history of Washington and the Pacific Northwest.Â If you are enchanted by the Pacific Northwest and have wondered about Washingtonâs most ancient natural history, we have designed this site for you.
This exhibit is organized into three chapters:
An introduction to the dynamics of our planet and the motion of Earthâs great tectonic plates. This section will introduce you to some of the basic concepts of geology to more fully appreciate Washingtonâs ancient history.Â
The creation and destruction of giant continents played a major role in the early geologic history of Washington and the Pacific Northwest.Â This section describes how the evolution of two giant ancient continents influenced Washingtonâs earliest history.
Â§ The Giant Continent of RodiniaÂ (1.2 billion to 750 million years ago)
A billion years ago, most of the landmasses of the Earth assembled into a single giant continent geologists call "Rodinia."Â This giant continent was a harsh and hostile world where some of the oldest rocks in Washington formed.Â Rodinia broke apart about 750 million years ago along a giant rupture that cut through eastern Washington.
Â§ The Western Margin of Ancient North AmericaÂ (750 to 300 million years ago)
Following the breakup of Rodinia, the original western edge of North America was located not too far west of modern-day Spokane and Pullman.Â The rocks deposited along that ancient coast in eastern Washington are still exposed there today.Â
Â§ The Giant Continent of PangaeaÂ (300 to 195 million years ago)
While sediment quietly accumulated along the passive coast of Washington, the continents were assembling into yet another giant supercontinent.Â Like its predecessor Rodinia, the giant continent of Pangaea would also fall victim to the Earthâs internal heat and rupture â this time to form the Atlantic Ocean.Â
As Pangaea ruptured to form the Atlantic Ocean about 200 million years ago, the North American plate began drifting westward.Â Exotic volcanic islands of the Pacific began to collide and weld to the northwest margin of the continent.Â The geologic evolution of Washington and the modern Pacific Northwest developed during four episodes over the past 200 million years:
Â§ The Omineca Episode â (195 million to 115 million years ago)
Named after the remote Omineca Mountains in Northeastern British Columbia, the Omineca Episode began when a chain of volcanic islands collided with the western edge of North America.Â The islands were "welded" to the edge of the continent by molten rocks that solidified as granite.Â The remnants of these ancient islands, along with the granite that welded them to the continent, form much of the Okanogan Highlands of north-central Washington and southern British Columbia.
Â§ The Coast Range Episode â (115 million to 57 million years ago)
Named after the Coast Range Mountains of British Columbia, the Coast Range Episode began when a second chain of volcanic islands collided along the expanding western shoreline.Â These islands welded to the edge of the continent by molten rocks that formed the largest body of granitic rocks in NorthÂ America.Â
Â§ The Challis Episode -Â (57 million to 37 million years ago)
The Challis Episode, named after a small town in Idaho, is the most controversial chapter in Pacific Northwest geologic history.Â Large regions of the Pacific Northwest were crushed, all the while hosting a chain of volcanoes running diagonally across Washington and Idaho.Â 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.
Â§ The Cascade Episode -Â (37 million years ago to present)
The Cascade Episode began as the modern ocean plate (the Juan de Fuca Plate) advanced into this 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.Â 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,Â creating some of the most spectacular landscapes on the continent.