Geology of Washington Mini-Textbook
Washington State is a great place to study geology! Here we have an active plate boundary, active volcanoes, and
towering mountains. We also have landscapes shaped by enormous eruptions, massive floods, and vast glaciers. In this
booklet, we will explore many of the state’s landforms, and learn about the immense forces that created them.
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Setting the Stage: The Convergent Plate Boundary
About 200 million years ago, the supercontinent Pangaea began to break
up. A divergent plate boundary formed between what is now North
America, Africa, and Europe causing the continents to rift apart. As the
continents drifted away from each other, the Atlantic Ocean grew
wider.
Control click to see an animation of the breakup of Pangaea:
http://www.divediscover.whoi.edu/tectonics/pangea-animation.html
As the continents moved, a convergent plate boundary formed along the northwest edge of what is now the United
States. Today, we call this plate boundary the Cascadia Subduction Zone. The oceanic Juan de Fuca plate is diving down
under the North American continental plate. As the oceanic plate sinks into the mantle, it melts and is destroyed.
Image: http://geology.com/nsta/convergent-plate-boundaries.shtml
The Cascadia Subduction Zone directly causes many of the landforms you see around us today.
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Formation of Washington State
When Pangaea broke up, most of Washington didn’t exist yet. As the oceanic plate dove under North America, it carried
small continents, island arcs, and ocean sediments with it. Over millions of years, these rocks and sediments piled up
against the west coast, building more and more land. As the land built up, the subduction zone moved westward and
stayed right along the coast. Everything on the map below that is not purple was added on to the state by the
subduction zone.
Image:
http://www.burkemuseum.org/static/geo_history_wa/New%20Lands%20Along%20an%20Old%20Coast%20v.2.7.htm
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Earthquakes & Faults
Earthquakes occur along all plate boundaries. As the oceanic plate sinks below the continent, it sometimes gets stuck.
Stress builds up and is eventually released by a sudden motion of the plates. This is a subduction zone earthquake. The
longer the oceanic plate gets stuck, the more stress builds up, and the larger the earthquake will be when it releases.
Our area experiences major subduction earthquakes every 300 to 500 years. The last large earthquake is believed to
have occurred about 300 years ago.
Image: http://www.capemeares.org/news/315th-anniversary-of-the-great-1700-cascadia-subduction-zone-cszearthquake-and-tsunami/
A subduction earthquake can cause major damage to buildings. It can also cause landslides, and generate massive ocean
waves called tsunamis. This is why having earthquake drills is so important here! The link for the image above gives a
detailed description of the last subduction earthquake to occur here.
The pressure on the continental plate from the subduction zone also causes the
continental crust to crack in places. These cracks are called faults. Sometimes
there is motion along these faults, and that causes smaller earthquakes. There
are earthquakes along small faults in the Pacific Northwest every single day.
The map to the left shows faults in the Puget Sound region.
Image: https://en.wikipedia.org/wiki/Puget_Sound_faults
Check out this interactive map, which lists and shows locations of all recent
earthquakes in our region. http://pnsn.org/earthquakes/recent
For more details about earthquakes and how we study them:
http://earthquake.usgs.gov/learn/kids/eqscience.php
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Columbia Plateau
The Columbia Plateau covers much of Eastern Washington, as well as parts of Oregon and Idaho. A plateau is a broad,
flat area of land.
About 17 million years ago, lava spilling over the land from a
series of at least 300 massive eruptions (called flood basalt
eruptions) formed the Columbia Plateau. The eruptions probably
came out of cracks or fissures that opened in the crust of the
Earth. The fissures are now buried beneath the hardened lava.
The eruptions lasted for millions of years. The lava piled up in
layers, eventually reaching a thickness of almost 6000 feet (1.8
km). That’s as tall as 10 Space Needles stacked on top of each
other! The lava covers an area of 63,200 square miles (163,700
km).
Image: http://geology.isu.edu/Digital_Geology_Idaho/Module10/mod10.htm
Where the Columbia River has cut into the plateau, you
can see some of the layers of lava stacked one on top of
another.
Geologists aren’t sure what caused the flood basalt
eruptions. One of the most popular ideas is that there is a
hot spot in the mantle under the crust of North America.
This hot spot would periodically trigger volcanic activity in
the crust above it as the North American plate slowly
moves over it. This same hot spot may have also caused
the volcanic activity at Craters of the Moon in Idaho and
the current geothermal activity in Yellowstone National
Park. Hawaii is also an example of volcanic activity that we
think is caused by a different hot spot in the mantle.
Image: http://columbiariverimages.com/Regions/Places/wallula_gap.html
Another possible cause of the Columbia flood basalt eruptions is the subduction plate boundary off the Washington
coast. It may be that oceanic plate temporarily changed direction or stopped moving. This would have released the
stress on the North American plate. As the North American plate “relaxed,” fissures could have opened up in what is
now Eastern Washington, leading to the eruptions.
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During the most recent ice age, glaciers crept down
from Canada and into what is now the northern
United States. One lobe of ice dammed a river in
Montana, the Clark Fork River. The dammed river
backed up and formed a huge lake called Glacial
Lake Missoula. Eventually, the lake grew large
enough that the water either broke through or
floated the ice dam. A massive amount of water
roared across Eastern Washington, through the
Columbia Gorge, and eventually to the Pacific
Ocean. This huge wall of water would have
destroyed everything in its path, all the way across
the state! Afterward, the ice dam would reform and
the lake would then slowly refill. Geologists think
that the floods happened about 40 times between
15,000 to 13,000 years ago. Every time a flood
occurred, the wall of water would have raced
across the land at 80 miles per hour!
Image: http://wsm.wsu.edu/s/index.php?id=472
These massive floods scoured the Columbia Plateau, carrying away the soil and carving channels into the rock. The area
where the floods flowed is called the Channeled Scablands (“scabby” because it is no good for farming). At Dry Falls,
you can see the remains of one of the largest waterfalls that ever existed, carved during the flood events. During the
floods, water would have poured over these cliffs.
Image: https://en.wikipedia.org/wiki/Dry_Falls
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Olympic Mountains
The Olympic Mountains began forming about 50
million years ago. The Cascadia Subduction Zone
dragged igneous rock from the oceanic crust,
along with sea floor sediments, against the edge
of the continent. Some of this material
subducted and melted, but some piled up
against the edge of the continent, forming a
great mountain range. This process is still
happening today, so the Olympic Mountains are
still growing.
Image: http://www.allolympicpark.com/mountains/olympic_mountains.php
The action of the subduction zone builds the Olympic Mountains up. At the same time, weathering and erosion
(especially by glaciers and rivers) are wearing them down. These processes have created the beautiful, jagged peaks we
see today.
More about the Olympic Mountains: http://www.britannica.com/place/Olympic-Mountains
Image: http://www.dnr.wa.gov/programs-andservices/geology/explore-populargeology/geologic-provinceswashington/olympic
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Cascade Mountains
The geology of the Cascade Mountains is ridiculously complex. The Cascades sit atop an area built of microcontinents
and island arcs that docked with the west coast of Washington, carried there by the subduction plate boundary.
Additionally, the subduction plate boundary has caused several ancient arcs of volcanoes to form across Washington. As
the plate boundary has shifted westward, these volcanic arcs have gone extinct and been weathered down. Because so
many processes contributed to their creation, the underlying rocks of the Cascades are a complex mix of things.
Image: http://voices.nationalgeographic.com/2014/02/24/geography-in-the-news-amazing-crater-lake/
The mountain range exists because of the subduction plate boundary. The ocean and continental plate are pushing
against one another, and the continental plate is forced upward, or uplifted. The uplift forms a jumbled array of
mountains. These forces also cause the continental plate to crack, allowing magma from the melting ocean plate to work
its way upward. Some of the magma crystallized below ground to form intrusive igneous rocks. The pressure also causes
metamorphism of existing rocks.
Of course, the Cascade Range is also studded with massive, active stratovolcanoes (see Volcanoes) that form because of
the subduction zone. The modern Cascade Range is about 7 million years old, but many of the complex rocks mentioned
above (for example, some of the ancient volcanic rocks) go back as much as 37 million years ago.
As the mountains grow upward, they are weathered and eroded by rivers, glaciers, and freeze-thaw cycles. Weathering
and erosion has exposed some of the intrusive igneous and metamorphic rocks at the surface. Rocks that formed deep
underground are now, in places, at the very tops of high mountains!
As the Cascadia Subduction Zone is still active, geologists think the Cascades are still growing higher even as they are
being worn down.
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The Columbia River, which runs through central Washington and then along the southern border with Oregon, is more
ancient than the modern Cascade Range. The river cuts straight through the mountain range. As the mountain range
was growing, the Columbia River just kept cutting down through the rising mountains.
More about the Cascade Range: http://www.nature.nps.gov/geology/education/concepts/concepts_cascadesierra.cfm
Image: "Mount Shuksan tarn" by Frank Kovalchek from Anchorage, Alaska, USA - Mt. Shuksan reflected in a small tarn on
the Artist Point trailUploaded by hike395. Licensed under CC BY 2.0 via Commons https://commons.wikimedia.org/wiki/File:Mount_Shuksan_tarn.jpg#/media/File:Mount_Shuksan_tarn.jpg
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Volcanoes
As the oceanic Juan de Fuca plate sinks into the mantle, it heats up and
begins to melt. Magma from the melting plate rises up into the
continental crust and forms intrusive igneous rocks. In certain places,
the magma reaches the surface and forms stratovolcanoes.
Stratovolcanoes are fairly explosive volcanoes that are built up from
layers of lava and ash (“strato-” means layers). Mount Saint Helens is
the most famous Cascadian stratovolcano, but there are 5 active
volcanoes in Washington. There are also volcanoes along the
subduction zone in British Columbia to the north and Oregon and
California to the south.
Image: https://en.wikipedia.org/wiki/Mount_Rainier
If Mount Rainier erupted, it would have a huge impact on the Seattle area. The biggest dangers are ash falls and
mudflows from melting snow and ice (called lahars). Lahars could endanger towns closer to the mountains, as well as
disrupting transportation and water supplies.
Control-click here to see a video of the Mount Saint Helens eruption in 1980: http://www.history.com/topics/usstates/washington/videos/mount-st-helens-erupts (please turn your sound off).
Image: https://en.wikipedia.org/wiki/Cascade_Volcanoes
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Puget Sound
Many ice ages have occurred during the Earth’s long history. The most recent ice age lasted from 110,000 to 12,000
years ago. During that period, ice sheets advanced and retreated many times. At times, massive ice sheets came down
from Canada and covered parts of Washington.
The Puget Sound area was sometimes underneath ice as much as a mile deep. The ice sheet
was so heavy that it actually made the continental crust sink lower down into the mantle. The
ice disappeared 12,000 years ago, but even after all that time, the crust of the Puget Sound
area is still rebounding upward.
Image: http://www.ecy.wa.gov/programs/sea/pugetsound/tour/geology.html
Image: http://www.michw.com/2011/05/puget-soundforearc-basin/
The glaciers (sheets of moving ice) gouged out the channels and passageways of what is now Puget Sound. As the ice
melted, melt water also eroded the land. The melting ice deposited large amounts of sediments in some areas of the
Puget Sound region. These sediments piled up in layers and over time formed new sedimentary rocks.
When the ice age ended, glaciers around the world melted. All the melted water raised
the level of the world’s oceans by more than 300 feet. Puget Sound filled to its present
level. The waters of the Sound wore away at the shoreline, exposing some of the glacial
sediments in places.
Image: http://www.ecy.wa.gov/programs/sea/pugetsound/tour/geology.html
More about Puget Sound’s geology:
http://www.ecy.wa.gov/programs/sea/pugetsound/tour/geology.html
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