DESCRIPTION:
Columbia Plateau
Columbia River Basalt

Between 14 and 16 million years ago, "fissure" volcanic eruptions in eastern Washington, eastern Oregon, and western Idaho produced enormous volumes of molten Columbia River basalt that flowed like water west into the Deschutes-Columbia Plateau province in eastern Washington and northeastern Oregon, with some lava continuing to flow as far west as the Pacific Ocean via the ancestral Columbia River valley. As the basalt cooled and congealed, it formed the columnar cliffs that dominate the landscape today. Erosions by the Columbia River has exposed a particularly spectacular sequence of these rocks in the Columbia River Gorge on Oregon's northern boundary.

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Pacific Northwest Volcanics showing the Cascade Range and Columbia Plateau. Generalized subdivision of Cascade Range from Hammond (1979). Distribution of Columbia River Basalt Group from Tolan et.al. (in press).
-- Modified from: Swanson, et.al., 1989, American Geophysical Union Field Trip Guidebook T106

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Location of Columbia Plateau features
-- Modified from: Swanson and Wright, 1981, IN: USGS Circular 838

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Columbia River Flood Basalts
-- Topinka, 2002

In some shield-volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside with lava flow upon lava flow, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeastern Washington, eastern Oregon, and southern Idaho. Along the Snake River in Idaho, and the Columbia River in Washington and Oregon, these lava flows are beautifully exposed and measure more than a mile in total thickness.

17-12 million years ago (Miocene) -- During this period, 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 Gorge (Columbia River Gorge) forming layers of rock up to 2,000 feet (600 meters) deep.

Lava Flows:

During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earths surface engulfed about 63,000 square miles of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 6,000 feet. As the molten rock came to the surface, the earths crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin (Plateau). The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.

Folding of the Plateau:

With the end of the outpouring of lava, tremendous forces deep within the earth began to warp the plateau in several places. A general uplift of the mountainous region in the north caused the entire plateau to tilt slightly to the south. This tilting and associated stairstep rock folds, called monoclines, in the vicinity of Coulee City and Soap Lake, played an important role in the formation of the Grand Coulee.

The Ice Age:

With the beginning of the Pleistocene time about one million years ago, cooling temperatures provided conditions favorable for the creation of great sheets of moving ice called glaciers. Thus began the Ice Age. Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent, forming extensive ice fields. This vast continental ice sheet reached a thickness of about 4,000 feet in some areas. Sufficient pressure on the ice caused it to flow outward as a glacier. The glacier moved south out of Canada, damming rivers and creating lakes in Washington, Idaho and Montana. One especially large lake, covering a portion of northwest Montana, played an important role in the formation of Dry Falls. As this lake grew in size, it eventually broke through the ice dam, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington. Such catastrophic floods raced across the southward-dipping plateau a number of times, etching the coulees which characterize this region, now known as the channeled scablands. As the floods in this vicinity raced southward, two major cascades formed along their course. The larger cataract was that of the Upper Coulee, where the river roared over an 800-foot waterfall. The eroding power of the water plucked pieces of basalt from the precipice, causing the falls to retreat 20 miles and self-destruct by cutting through to the Columbia River valley near what is now the Grand Coulee Dam. The other major cataract started near Soap Lake, where less resistant basalt layers gave way before the great erosive power of this tremendous torrent and waterfalls developed. As in the Upper Coulee, the raging river yanked chunks of rock from the face of the falls and the falls eventually retreated to their present location. Here then is Dry Falls, the skeleton of one of the greatest waterfalls in geologic history. It is three and one-half miles wide, with a drop of more than 400 feet. By way of comparison, Niagara, one mile wide with a drop of only 165 feet, would be dwarfed by Dry Falls.

Fort Spokane area:

The dark rock that encompasses the Fort Spokane area is basalt. This is hardened lava that came from great fissures on what is now the southern part of the Idaho/Washington border. Beginning approximately 17 million years ago lava flow after lava flow poured onto the landscape of what is now central Washington and northern Oregon. After 11 million years of flows, up to 150 separate lava flows with a combine depth of over 2 miles remain. Most recently wind blown soil, called loess, from the glaciers was deposited on top of the lava flows. Massive floods which roared through central Washington washed away some of this loess. Where the loess remains, the soil is extremely fertile, proving to be excellent for growing wheat, barley, canola and many other crops which you will encounter directly south of Lake Roosevelt. The soil deposits found right along Lake Roosevelt are the result of sediments left from the gigantic Ice Age Floods that came through this area 10,000 to 20,000 years ago.

Columbia Plateau Province:

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.

Columbia River basalts:

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.

It is difficult to conceive of the enormity of these eruptions. Basaltic lava erupts at no less than about 1,100 degrees C. Basalt is a very fluid lava; it is likely that tongues of lava advanced at an average of 5 kilometers/hour -- faster than most animals can run. Whatever topography was present prior to the Columbia River Basalt eruptions was buried and smoothed over by flow upon flow of lava.

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.

With the beginning of the Pleistocene time about one million years ago, cooling temperatures provided conditions favorable for the creation of great sheets of moving ice called glaciers. Thus began the Ice Age. Over the centuries, as snowfall exceeded melting and evaporation, a great accumulation of snow covered part of the continent, forming extensive ice fields. This vast continental ice sheet reached a thickness of about 4,000 feet in some areas. Sufficient pressure on the ice caused it to flow outward as a glacier. The glacier moved south out of Canada, damming rivers and creating lakes in Washington, Idaho and Montana. One especially large lake, covering a portion of northwest Montana, played an important role in the formation of Dry Falls. As this lake grew in size, it eventually broke through the ice dam, allowing a tremendous volume of water to rush across northern Idaho and into eastern Washington. Such catastrophic floods raced across the southward-dipping plateau a number of times, etching the coulees which characterize this region, now known as the channeled scablands.

Located in the double ranched of the Olympic and Cascade Mountains, the Columbia Basin is the warmest and driest region of the state. Originally carved by glaciers and slowly eroded over time by the Columbia River, the basin was further sculpted about 12,000 years ago by a series of violent flows known as the Spokane Floods. The floods left behind strange basaltic rock formations known as the Channeled Scablands. The basin is characterized by native shrub-steppe vegetation and thousands of acres of ponds and marshes created by irrigation water from the Columbia Basin Project. Southwest Washington includes the vast wheatfields of the Palouse Hills and the rugged Blue Mountains which straddle the border between Washington and Oregon.

Glaciations, Ice Sheets, and Ice-Age Floods Menu

During the early stages of the Columbia Basin formation, granite rock was slowly created by heat and pressure deep in the crust of the earth. Then the crust was uplifted, exposing the granite, creating mountains similar to the Okanogan Highlands north of Grand Coulee Dam. Forty to sixty million years ago the formation of the outline of the Columbia Basin was complete. The land had subsided below sea level, and a large inland sea had formed. The land was again uplifted and then, 10-15 million years ago, was flooded with volcanic lava. The boundaries of the flood lava were located in almost the same position as the former seashore. Many layers of lava were needed to build up to a 5,000 feet (1500 meter) thickness and form the smooth surfaced Columbia Plateau.

During the Ice Age, the old Cascade Mountains were also formed. Their outline still remains on the western slopes of the Cascades. The uplifting mountains were not able to block the flow of the Columbia River completely, and a deep Columbia River gorge was formed. Near the end of the Ice Age the volcanoes of the high Cascades rose to elevations of 14,000-15,000 feet (4000-4500 meters). Older volcanoes, such as Mt. Hood and Mt. Rainier, were sculpted by glaciers of the Ice Age; others such as Mt. St. Helens remained unsculpted, retaining their original volcanic form.

Eighteen thousand years ago the Columbia Basin was nearly covered by floodwaters when an ice dam at Lake Missoula in western Montana broke. Large boulders were strewn near the outlet of the Lower Coulee (Lake Lenore). Other boulders were carried in icebergs as far as western Oregon. The floodwaters were 800 feet (250 meters) deep near Pasco and 400 feet (125 meters) deep at Portland. After the Ice Age, the Columbia River returned to its former channel. The channeled scab lands and large coulees that had been formed were left stranded 500-1600 feet (150-500 meters) above the present river floor and serve as a constant reminder of some of the most unusual episodes in geologic history.

During late Miocene and early Pliocene times, one of the largest basaltic lava floods ever to appear on the earths surface engulfed about 63,000 square miles of the Pacific Northwest. Over a period of perhaps 10 to 15 million years lava flow after lava flow poured out, eventually accumulating to a thickness of more than 6,000 feet. As the molten rock came to the surface, the earths crust gradually sank into the space left by the rising lava. The subsidence of the crust produced a large, slightly depressed lava plain now known as the Columbia Basin (Plateau). The ancient Columbia River was forced into its present course by the northwesterly advancing lava. The lava, as it flowed over the area, first filled the stream valleys, forming dams that in turn caused impoundments or lakes. In these ancient lake beds are found fossil leaf impressions, petrified wood, fossil insects, and bones of vertebrate animals.

MORE about the Columbia River

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Stratigraphic subdivision of Columbia River Basalt Group
-- Modified from: Swanson, et.al., 1989, American Geophysical Union Field Trip Guidebook T106

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 is formally divided into five formations, which in turn are broken into formal and informal members (Swanson and others, 1979a; Camp, 1981; Beeson et.al., 1985; Reidel et.al., in press; Bailey, in press). ...

The group has a volume of about 174,000 cubic kilometers and covers about 164,000 square kilometers (Tolan et.al., in press). 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 (Long and Duncan, 1983; McKee et.al., 1977, 1981; Swanson et.al., 1979a). Early eruptions (17.5-17 Ma) fed the Imnaha Basalt, which is confined to the southeast part of the province (Hooper et.al., 1984). Most of the group was formed during a 1.5-m.y. period between about 17 and 15.5 Ma, resulting in the Grande Ronde Basalt (Mangan et.al., 1986; Reidel et.al., in press) and the greatly subordinate and geographically limited Picture Gorge Basalt (Waters, 1961; Bailey, 1986). Later eruptions formed the Wanapum Basalt (about 15.5-14.5 Ma) and the Saddle Mountains Basalt (about 14-6 Ma) (Swanson et.al., 1979a; Camp, 1981; Beeson et.al., 1985). 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.

Glaciations, Ice Sheets, and Ice-Age Floods Menu

Glacial-outburst waters that crossed the Channeled Scablands during the Spokane floods (Missoula Floods) were channeled through Wallula Gap. For several weeks, as much as 200 cubic miles of water per day were delivered to a gap that could discharge less than 40 cubic miles per day. Ponded water filled the Pasco Basin and the Yakima and Touchet valleys to form temporary Lake Lewis.

Glaciations, Ice Sheets, and Ice-Age Floods Menu

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