Columbia River Gorge National Scenic Area

Geology of the Columbia River Gorge

Columbia River Basalt

The Columbia River Plateau was created by a series of basalt flows. Six to seventeen million years ago, the flows covered 164,000 square kilometers -- portions of northeast Oregon, southwest Washington and western Idaho (figure 1).

Over 17 million year ago, cracks in the earth's surface began spewing molten basaltic lava. Basalt is a hard black rock. Its mineral grains are too small to see with the naked eye. Columbia River basalt is a volcanic rock composed of the black mineral--pyroxene and a white mineral--plagioclase. The total volume of Columbia River basalt is about 174,000 cubic kilometers with up to 5,000 meters thick.

The Columbia River basalt oozed in large quantities from fissures (figure 2) with moderately fluid magma. This made it impossible for volcanoes to form at the site of the fissure. Instead the lava flowed away from the fissures and covered large areas of land. Today, portions are exposed as dikes in river canyons of eastern Oregon and Washington. A dike is a tabular or sheet-like discordant igneous rock body. It forms when magma cools and crystallizes within the earth's crust.

The Columbia River basalt consists of about 300 individual lava flows within an average volume of 561 cubic kilometers per flow. Grande Ronde Basalt flows, the largest series of flows, make up 85 percent of the Columbia River basalt's total volume. The Grande Ronde Basalt flows occured from about 16.5 to 15.5 m.y. ago, originating in eastern Washington and Oregon. As it flooded the regions lowest areas, it filled canyons and permanently altered the Columbia River's path on several occasions. Today, these flows are exposed along the cliffs of the Columbia River Gorge.

"Joints" in rocks are fractures similar to faults. Unlike faults, joints occur with very tiny amounts of movement of rock on either side of the break. When basalt cools, the lava shrinks between 5 to 10 percent as it crystallizes. This causes joints to form. The Columbia River basalt flows exposed in the walls of the Gorge feature a jointing arrangement. These arrangements were created as the lava flow solidified.

Columbia River basalt flows display the different types of jointing patterns (figure 3).
1. The first is a "columnar-blocky jointing pattern" in which the entire flow consists of 5-to 6-sided columns.
2. The second type is "entablature-colonnade jointing pattern." Although the entablature and colonnade look totally different they are both parts of a single basalt flow. The entablature consists of numerous, random cooling joints that give it a rough texture. The entablature overlays a colonnade consisting of prismatic, vertical columns. The transition from entablature to colonnade is abrupt.

Another common feature is the "vesicular flow top." A flow top looks like "black soap foam" consisting of numerous sphere-shaped cavities (vesicles). These vesicles represent gas bubbles that rose through molten lava and "froze" as the lava cooled.

The uniqueness and beauty of the Columbia River Gorge is partly attributed to the Columbia River basalt flows.

Lake Missoula and Cataclysmic Floods

What really created the Channeled Scablands?

As a glacial ice sheet moved south from Canada, a large ice dam was created on the Clark Fork River. It stood about 3000 feet high and blocking the flow of water. Water rose and the lake developed. Thus, the ice dam fell.

The failure of the ice dam released a 2000-foot wall of water that rushed to the prairies of eastern Washington. The waters reached the Columbia River. The river's channel could not contain this vast flood. The waters spilled out across the Palouse region, eroding a series of intertwining river canyons (coulees) across eastern Washington. The various flood pathways converged in the Pasco Basin, where there was a narrow exit for the waters--the Wallula Gap. The narrowness of the gap caused the floodwaters to back up and form a 1,200 ft. deep lake covering over 3,500 square miles. Several other temporary lakes were created by similar events near The Dalles and Portland, OR.

The lake formed at The Dalles was more than 1,000 feet deep. Evidence of its depth can be seen on flood-scoured hillsides. In the Columbia River Gorge, floodwaters were more than 800 ft deep and submerged Crown Point.

The Missoula floods are the largest known floods on Earth in the last two million years. However this might not be a unique event. Geologic evidence indicates there could have been about 40 floods during the Pleistocene Age, with the last flood occurring 13,000 years ago.

Previous Paths of the Columbia River

About 17 million years ago:
From its origin in British Columbia, the Columbia River flowed over Washington State and turned west in northeastern Oregon. At earlier time, the river passed through the Cascade Range and met the Pacific Ocean north of Newport, OR. About 17 million years ago near the Oregon, Washington, and Idaho border, basaltic lava gushed out of long, deep fractures. The lava covered 60,000 to 65,000 square miles of eastern Washington and northeastern Oregon.

The next 5 million years:
These lava flows changed the course of the Columbia River many times, gradually pushing the river north. Each new channel of the river was destroyed by the next flow. Crown Point represents a lava flow that filled an old Columbia River channel about 14.5 m.y. ago.

PATHWAYS OF COLUMBIA RIVER

15.4 million years ago:
The oldest Columbia River channel was changed by the Ginkgo flow (Columbia River Basalt Group). This flow also destroyed drainage systems throughout most of the Columbia Plateau. The rivers and streams on highlands surrounding the plateau drained onto the plateau surface. This created a system of shallow, interconnected lakes.

14.5 million years ago:
The next channel was changed by Rosalia flow. The flow overwhelmed the channel and forced the river northward.

12 million years ago:
The next channel, the Bridal Veil channel, was moved by the Pomona intracanyon flow. It existed for more than 10 million years.

Six to two million years ago:
Basalt flows from Cascade range volcanoes filled the Bridal Veil channel. It forced the river north to its former canyon.

Two million years ago: The present-day Columbia River Gorge began to form as the Cascade range "uplifted." The river cut its present-day canyon in the Cascades.

Geologic Faults

Distinguishing Characteristics

Hanging Wall and Footwall: In order to classify the types of fault, geologists have adopted the terms hanging wall block and footwall block from early miners who found mineral deposits along fault zones. Imagine this: You are climbing to the surface of the Earth in the fault plane. Your feet are on the footwall block with the hanging wall block over your head.

Dip-slip fault: All movement is parallel to the dip of the fault plane. One block moves up or down in the opposite direction of the block on the other side of the fault. Depending on the relative movement of the hanging wall and footwall blocks, two types of dip-slip faults are recognized: normal and reverse. Normal faults are characterized by the hanging wall moving downward relative to the footwall. An example of a normal fault in the Gorge is the Hood River fault. If the hanging wall moves upward relative to the footwall, it is called a reverse fault. An example of a reverse fault in the Gorge can be seen along the west side of the water gap in the Columbia Hills west of The Dalles, Oregon.

Strike-slip fault: This is a fault in which displacement is parallel to the trace (strike) of the fault. The land moves in opposite directions along itself. An example of a right-lateral strike-slip fault in the Gorge can be seen at Fairbanks Gap, east of The Dalles, Oregon.

Cascade Range

Volcanic peaks throughout the Cascades are mostly made of andesitic lava and pyroclastic debris. Andesite contains more silica than basalt and is therefore less fluid. This property of andesite allows the formation of steep composite cones and often gives rise to explosive eruptions.
Such eruptions are responsible for volcanoes such as Mt. Saint Helens (pictured prior to it's eruption), Mt. Adams, and Mt. Hood. The Cascade Range contains over two-dozen prominent peaks.

There are two parts to the Cascade Range -- the old and the new Cascades. The west side is known as the old Cascades (Western Cascade Group), while the east side is known as the new Cascades (High Cascade Group). The west side was formed 40 to 5 million years ago through plate convergence. The change in the angle of subduction of the oceanic plate has caused the location of volcanic activity in the Cascades to migrate east to form the newer range.

The Cascade Range, with its towering volcanoes and mountains, creates a substantial barrier between east and west. The climate in the east is dryer, with more extreme temperatures. Meanwhile, the west side experiences a wetter, milder climate. The Cascade Range has been a difficult barrier for people to cross. The only sea level passage is the Columbia River Gorge.

Common Geologic Terms

Aa: A type of lava flow having a rough, fragmental surface; highly viscous.

Abrasion: The grinding of mineral and rock particles against each other or against bedrock.

Andesite: A fine-grained, intermediate, extrusive igneous rock, usually found in volcanic arcs. Typically contains 53-62% SiO2 by weight.

Anticline: An upfold of layered rocks in the form of an arch.

Avalanche: A large mass of snow, ice, soil or rock or mixtures of these materials, falling or sliding very rapidly under the force of gravity.

Bar: A ridge-like accumulation of coarse sediment deposited by flowing water in the channel, along the banks or at the mouth of a stream or river.

Basalt: A fine grained, extrusive igneous rock. Typically contains 45-52% Sio2 by weight.

Cast: A mineral deposit that fills a cavity (creating a mold) within a sediment or rock and preserves the external features of a plant or animal fossil.

Cinder Cone: A volcano made entirely of tephra.

Colonnade: In columnar cooling joints, the lower portion of a solidified lava flow that consists of typically well-formed, prismatic columns. The columns formed as the molten lava slowly cooled and solidified.

Columnar Jointing: Parallel columns, either hexagonal or pentagonal, formed during the cooling of basaltic flows.

Composite Cone: A steep-sided volcano composed of alternating layers of lava and tephra.

Earthquake: A sudden motion or trembling in the Earth caused by the abrupt release of slowly accumulated stress.

Entablature: Irregular to random pattern of cooling joints in the upper portion of a solidified lava flow.

Eruption: The ejection of volcanic materials onto the Earth's surface.

Fault: A fracture along which there has been movement.

Feeder (Conduit): A cylindrical channel through which magma rises to the Earth's surface.

Fissure: A distinct fracture or crack in the Earth's surface.

Fissure eruption: An eruption that takes place along a fissure instead of a central vent, (such as a volcano).

Flood Basalt: An extensive, thick and smooth basaltic lava flow, or successive flows, from fissure eruptions that cover a vast region.

Igneous Rock: Rock formed by solidification of molten rock.

Intracanyon Flow: A lava flow which has been largely confined within a river canyon.

Joint: A fracture along which no appreciable movement has occurred.

Lahar: A flowing mixture of water and rock debris that forms on the slopes of a volcano, sometimes referred to as a debris flow or a mudflow.

Lava: Molten rock material that has extruded onto the Earth's surface.

Lava Dome: A dome shaped mountain of solidified lava. Generally created by the eruption of very viscous lavas.

Lavaflow: A flow of lava behaving much like rivers or streams.

Magma: Molten rock beneath the Earth's surface. When magma reaches the surface, it's called lava.

Mineral: A naturally occurring, inorganic crystalline solid having characteristic physical properties and a narrowly defined chemical composition.

Mud Slide: Slow moving flow of mud sharing similar characteristics to an avalanche.

Pahoehoe: A type of lava flow having a glassy, smooth, undulating surface.

Pillow Lava: Globular masses of volcanic rock formed when lava flows into a body of water.

Plate tectonics: A model for Earth behavior in which the outer crust is divided into a smaller number of individual masses called plates, which move independently of one another. Separation and collision of plates results in the formation of ocean basins and mountains, causing earthquakes and/or volcanic eruptions.

Plateau: An extensive flat area that stands prominently above its surroundings.

Pyroclastic Flow: A hot, fast moving and high-density mixture of ash, pumice, rock fragments and gas formed during an eruption.

Scabland: Produced by cataclysmic Pleistocene floods (Missoula Floods) which removed soil and gouged out bedrock, leaving a scab-like bedrock surface in eastern Washington and the Columbia River Gorge.

Shield Volcano: A volcano with very gentle slopes, built mostly of fluid, basaltic lava flows.

Stratovolcano: See composite cone.

Subduction Zone: The region in which one crustal plate (oceanic or continental) is thrust beneath another plate.

Syncline: A downfold of layered rocks in the form of a trough.

Tephra: All solid particles formed from material erupted into the atmosphere during volcanic eruptions.

Vent: An opening at the surface where volcanic materials are erupted.

Vesicle: Small holes created by the release and expansion of gas while the lava was still molten.

Volcano: A vent in the surface of the Earth through which magma erupts and also the land form that is constructed by the erupted material.

Volcanic Plug: The solidified material filling the vent of a dead volcano, (i.e. Beacon Rock)

Waterfall: A very steep drop in elevation where streams or rivers fall freely.

Columbia River Basalt Heath Thibodeau & Rob Parker
Missoula Floods David Straus, Windy Heesacker & Elton Whittaker
Previous Paths of the Columbia River Hsiu-wen "Joyce" Wang, Sharon Beaty-Lassen & Anne Donelson
Geologic Faults Jeff Schick & Marwan Adjaj
Cascade Range Remus Repta, Kevin Luu & Nate Miller
Common Geologic Terms David Hall
Original web design: David Hall

Web design modified in December 2003 to conform to Forest Service web standards.

USDA Forest Service - Columbia River Gorge National Scenic Area
Last Modified: Wednesday, 02 February 2005 at 15:15:29 EST