America's Volcanic Past -
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| "Though few people in the United States may actually experience an erupting volcano, the evidence for earlier volcanism is preserved in many rocks of North America. Features seen in volcanic rocks only hours old are also present in ancient volcanic rocks, both at the surface and buried beneath younger deposits." -- Excerpt from: Brantley, 1994 |
Learn More About
Columbia Plateau Volcanics
Learn More About
Columbia River Region and the Columbia River Gorge
Volcanic Highlights and Features:
[This list is just a sample of
various Columbia Plateau volcanic features or events and is by no means inclusive.]
The Columbia Plateau province is enveloped by one of the worlds largest
accumulations of lava. Over 500,000 square kilometers of the Earth's surface is covered by it.
The topography here is dominated by geologically young lava flows that inundated
the countryside with amazing speed, all within the last 17 million years.
Over 170,000 cubic kilometers of basaltic lava, known as the Columbia River basalts,
covers the western part of the province. These tremendous flows erupted between
17-6 million years ago. Most of the lava flooded out in the first 1.5 million years --
an extraordinarily short time for such an outpouring of molten rock.
Over 300 high-volume individual lava flows have been identified,
along with countless smaller flows. Numerous linear vents, some over 150 kilometers long,
show where lava erupted near the eastern edge of the Columbia River Basalts,
but older vents were probably buried by younger flows.
Excerpts from: USGS/NPS Geology in the Parks Website, 2001 |
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| Columbia River Basalts |
Columbia River Basalt Group:1
The Columbia River Basalt Group (CRBG) is
the youngest and most studied flood basalt.
The province underlain by the basalt is
loosely termed the Columbia Plateau. Such an
overall designation is a misnomer,
however, for the basalt has been sharply folded
and broadly warped, so that its top varies in
elevation from slightly below
sea level in the Pasco Basin to more than 2.5 kilometers
above sea level in the Wallowa Mountains of northeast Oregon.
The group has a volume of about 174,000 cubic kilometers
and covers about 164,000 square kilometers.
These figures have been revised downward from previous estimates.
It was erupted between 17.5 and 6 million years ago, as
measured by K-Ar and 40Ar-39Ar ages.
The group is formally divided into five formations, which in turn
are broken into formal and informal members.
Imnaha, Grande Ronde, Picture Gorge,
Wanapum, and Saddle Mountains Basalts:1
Early eruptions (17.5-17 million years ago)
fed the Imnaha Basalt, which is
confined to the southeast part of the province.
Most of the group was formed during a 1.5-million-year
period between about 17 and 15.5
million years ago, resulting in the Grande Ronde Basalt
and the greatly subordinate and geographically
limited Picture Gorge Basalt
Later eruptions formed the Wanapum Basalt
(about 15.5-14.5 million years ago)
and the Saddle Mountains Basalt (about 14-6 million years ago).
Between the flows:1
Relatively little erosion took place between flows, owing to the
rapid rate of accumulation, except during Saddle Mountains time.
However, a regionally extensive saprolite (fossil soil) or a
sedimentary interbed separates the Grande Ronde
and Wanapum in most places;
flows just below and above the contact typically are
normally magnetized, so that the time represented by the
break is probably less than a few hundred
thousand years, most likely less
than 100,000 years. In Saddle Mountains time, however,
interflow erosion was significant, and most contacts are erosional
unconformities.
| Columbia River |
Columbia River:3
The Columbia River and its tributaries form the dominant
water system in the Pacific Northwest Region. The mainstem of
the Columbia rises in Columbia Lake on the west slope of the
Rocky Mountain Range in Canada. After flowing a circuitous
path for about 1200 miles, 415 miles of which are in Canada,
it joins the Pacific Ocean near Astoria, Oregon. The river
drains an area of approximately 219,000 square miles in the
States of Washington, Oregon, Idaho, Montana, Wyoming,
Nevada, and Utah. An additional 39,500 square mile portion of
the basin, or about 15 percent, is within Canada.
The Columbia River Basin is bounded principally by the Rocky
Mountain system on the east and north, the Cascade Range on
the west, and the Great Basin on the south. The basin area
includes 3,000 square miles of waterways and lakes, of which
2,500 miles are within the United States.
| Columbia River Gorge |
Columbia River Gorge:2
17-12 million years ago (Miocene), unusual volcanoes, called basalt floods,
erupted in eastern Washington and Oregon. These volcanoes were cracks in the
earth's crust, several miles long, which poured out floods of liquid molten
rock. 41,000 cubic miles (170,000 cubic kilometers) of this lava spread to
cover large parts of Oregon and Washington. Out of 270 lava flows that spread
across the region, 21 poured through the Columbia River Gorge
forming layers of rock up to
2,000 feet (600 meters) deep. Look at the cliffs in the Gorge. Can you see
these layers?
As the lava
cooled it formed a dark gray rock called basalt.
Many of these lava
flows cooled into columnar basalt;
the lava cracks, forming six-sided columns.
As you look for lava layers, notice that some contain columnar basalt.
If you look closely at a columnar layer, you might notice it is divided into two
parts. At the bottom, the lava cooled slowly forming regular, widely spaced
columns. Higher up, it cooled rapidly creating a jumbled looking mass of
irregular, closely spaced columns.
Excerpts from:
Swanson, Cameron, Evarts, Pringle, and Vance, 1989, IGC Field Trip T106:
Cenozoic Volcanism in the Cascade Range and
Columbia Plateau, Southern Washington and Northernmost Oregon:
American Geophysical Union Field Trip Guidebook T106
2) U. S. Army Corps of Engineers, Portland District, and the
U. S. Department of the Interior, U. S. Geological Survey,
The Geologic History of the Columbia River Gorge: Information Brochure
3) U. S. Corps of Engineers, North Pacific Region, Technical Management Team
Website, February, 1997
4) USGS/NPS Geology in the Parks Website, Geologica Provinces of the United States, 2001
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