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Columbia River Gorge National Scenic Area |
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Geology of the Columbia River GorgeColumbia River Basalt 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 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). 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 FloodsTwo early ideas concerning the formation of the Channeled Scablands:1. Glaciers that moved south to the United States from Canada carved the scablands of eastern Washington. These glaciers were powerful enough to gouge out the land and carry large boulders found scattered throughout the region. 2. The scablands were carved by rivers during the Pleistocene Era (1.6 m.y. ago to 10,000 years ago). These rivers would have been much larger than the present day Columbia. What really created the Channeled Scablands?
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 RiverAbout 17 million years ago: The next 5 million years: PATHWAYS OF COLUMBIA RIVER 14.5 million years ago: 12 million years ago: Six to two million years ago: 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 FaultsA fault is a surface or zone of rock fracture, where there has been displacement of land from a few centimeters to a few kilometers in scale. Faults are classified by direction of movement. Distinguishing Characteristics
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![]() The Cascade Range was formed by the interaction of the Juan de Fuca oceanic plate and the North American continental plate. Along the area of contact, which is part of the Pacific Ring of Fire, the heavier oceanic plate is subducted below the lighter continental plate. In this process, portions of basaltic crust from the oceanic plate melts with sediments from the ocean floor. This action forms bodies of magma, some of which vent to the surface as mountains or volcanoes.
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.
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.
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.
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.
Prepared by Portland State University Geology Capstone Students for the USDA Forest Service Columbia River Gorge National Scenic Area. Columbia River Basalt Heath Thibodeau & Rob
Parker Web design modified in December 2003 to conform to Forest Service web standards.
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USDA Forest Service - Columbia River Gorge National Scenic
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