DESCRIPTION:
Lava Plateaus and Flood Basalts

Lava Plateaus: 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.

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.

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. 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.

MORE Columbia Plateau Information

[Map,35K,InlineGIF]
Pacific Northwest Volcanics showing the Cascade Range and Columbia Plateau
-- Modified from: Swanson et.al., 1989, American Geophysical Union Field Trip Guidebook T106

The Columbia River Basalt Group is characterized by most features considered typical of flood-basalt provinces. Flows are voluminous, typically 10-30 cubic kilometers with a maximum volume of 700 cubic kilometers, and many cover large areas, as much as 40,000 square kilometers. They generally advanced as sheetfloods, rather than as channelized or tube-fed flows, and form thick cooling units composed of one or more flows. ...

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. ... Flood-basalt provinces by definition contain flows of huge volume, on the order of 5-10 cubic kilometers or more; this is the major distinction between flood-basalt and plains-basalt provinces, such as the Snake River Plain and Iceland, in which flows are generally much less than 1 cubic kilometer. Recent work by Tolan et.al., (1987,in press) suggest that about 300 great flows were erupted on the Columbia Plateau, with an average volume per flow of about 580 cubic kilometers. ...

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The Snake River Plain represents a style of volcanism between flood basalt eruptions and Hawaiian volcanism. Like Hawaiian volcanism, plains volcanism involves multiple, thin (3-5 meters) flow units erupted from central vents, and minor fountaining to produce cinder cones. However, like flood basalt eruptions, the vents are often aligned on rift zones, and some of the flows are fissure fed. The surface of flow accumulation is planar, because the vents are spread over a wide area, not focused in a central zone. Typical of plains volcanism, most flows on the Snake River Plain accumulate as (1) small, low shields, (2) fissure flows, and (3) or large tube-fed flows. All were probably emplaced relatively slowly, often advancing only a few meters per hour, forming "toey" lava flows with hummocky surfaces of several meters relief. Pressure ridges and collapse craters are common.

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