Northwest Origins
An Introduction to the Geologic History of Washington State
Catherine L. Townsend John T. Figge |
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Copyright
©
2002 The Burke
Museum of Natural History and Culture, University of Washington except as
noted. All rights reserved.
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.
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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. 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. 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. Washington’s
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. Over the
last few decades, scientists have revolutionized our understanding the
geologic processes that shape our land and lives in the Pacific Northwest. New technologies allow us to study our region
from space, to map the adjacent ocean floor, to look deeply into the
Earth’s interior, and to pinpoint the location and magnitude of earthquakes
with great precision. We can
even directly measure the motion of Earth’s 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. If you are enchanted by the Pacific Northwest and have wondered about Washington’s most ancient natural history, we have designed this site for you. This exhibit is organized
into three chapters: (I)
The Restless Earth: A Geologic Primer An introduction to the dynamics of our planet and
the motion of Earth’s great tectonic plates. This section will introduce
you to some of the basic concepts of geology to more fully appreciate
Washington’s ancient history. (II) Dance of the Giant Continents:
Washington’s Earliest History The creation
and destruction of giant continents played a major role in the early
geologic history of Washington and the Pacific Northwest. This section describes how the evolution
of two giant ancient continents influenced Washington’s earliest history. §
The Giant Continent of Rodinia (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." This giant continent was a harsh and hostile
world where some of the oldest rocks in Washington formed. Rodinia broke apart about 750 million years
ago along a giant rupture that cut through eastern Washington. §
The Western Margin of Ancient North America (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. The rocks deposited
along that ancient coast in eastern Washington are still exposed there
today. §
The Giant Continent of Pangaea (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. Like its predecessor
Rodinia, the giant continent of Pangaea would also fall victim to the
Earth’s internal heat and rupture – this time to form the Atlantic Ocean. (III) New Lands along
an Old Coast: Building the Pacific
Northwest As Pangaea ruptured to form the Atlantic Ocean about 200
million years ago, the North American plate began drifting westward. Exotic volcanic islands of the Pacific began
to collide and weld to the northwest margin of the continent. The geologic evolution of Washington and
the modern Pacific Northwest developed during four episodes over the
past 200 million years: §
The Omineca Episode – (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. 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 that welded
them to the continent, form much of the Okanogan Highlands of north-central
Washington and southern British Columbia. §
The Coast Range Episode – (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. 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 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. 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
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. 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. Continue to:
§ The Dynamic Earth: A Geological Primer §
Dance of the Giant Continents: Washington’s
Earliest History §
New Lands along an Old Coast: Building the Pacific
Northwest §
References and Recommended Readings Return to:
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