The Columbia Plateau of the Northwest is founded on layer upon layer of basalt.
Our classic conception of a volcano is a lone, cone-shaped peak, which actually describes just one of several types of these surface sources of molten rock. In many cases, repeated eruptions build lava plateaus of varying sizes, with their character revealed by a cinder cone here, a hardened black-rock dike or outcrop there. Some of the world's most significant volcanic events resulted in such rolling landforms.
Summits and Plateaus
The world's largest volcanoes are shield volcanoes, so named because their repeated, relatively low-grade eruptions of magma build broad, humpbacked summits resembling an overturned war shield. The massive peaks of Mauna Loa and Mauna Kea in the Hawaiian Archipelago, among the planet's heftiest mountains, are classic examples. These often help form lava plateaus, as multiple shield volcanoes and their eruptive aprons overlap. The more explosive eruptions from composite or stratovolcanoes, which include such iconic, conical mountains as Mount Fuji and Mount Hood, may also build plateaus of lava rimming the central peak.
Flood Basalts
On occasion, vast quantities of lava issue not from distinct volcanic summits but from collections of subtle vents. These are typically associated with flood-basalt events, when low-viscosity (i.e., highly fluid) sheets of basalt flow broadly across a landscape, often in distinct, repetitive cycles. The source of the flood basalts may be a stressing of the Earth's crust: Superheated rock within the planet's mantle normally doesn't melt only because it is subject to great pressure, but a wearing thin of the overlying crust can reduce the pressure and melt rock, creating basaltic magma that rises through stress fissures to the surface. Repeated flows of flood basalts can form enormous plateaus. Some of the most notable are the Columbia River Basalts of the Pacific Northwest, the Deccan Traps of India and the Siberian Traps of Russia.
The Columbia Plateau
Around 63,000 square miles of eastern Washington, central and northeastern Oregon and western Idaho comprise the Columbia Plateau, built between 17 million and 6 million years ago by hundreds of flows of flood basalts. The source appears to be a collection of linear vents in southeastern Washington and northeastern Oregon, some of them 90 miles long. The separate lava floods manifest as discrete dark banding in roadcuts and canyons, separated by lighter-colored sediments accumulated in between inundations. Such exposures are striking in places like Hells Canyon, the Columbia River Gorge and in the broken terrain of the Channeled Scablands of eastern Washington, where the basalt layers were scoured by cataclysmic glacial outburst floods during the Pleistocene. Lava stemming from the source vents tracked all the way to the Pacific Ocean in northern Oregon, where it comprises some of that coast's striking headlands.
Flood-Basalt Impacts
The enormous basalt plateau of the Siberian Traps, which encompass an area roughly the size of the lower 48 states, represents one of the biggest-known volcanic events in Earth's history. The huge quantities of carbon, toxic gases and other materials released by the 250-million-year-old floods, which lasted hundreds of thousands of years, and the resulting climatic impacts may have played a major role in the Permian mass extinction, which killed off some 90 percent of the planet's species -- the most significant extinction event ever. Some scientists suspect a similarly devastating impact from the eruptions of the Deccan Traps 65 million years ago, which coincides with the widespread extinction of the dinosaurs. It's possible that the effects of those flood basalts were the prime culprits in that great dying.
Tags: flood basalts, Columbia Plateau, Columbia River, Deccan Traps, eastern Washington, flows flood, flows flood basalts
