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Why are the surfaces of flood-basalt plateaus usually very flat?
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The Columbia Plateau is uniformly covered with basaltic lava flows and spans an area of about 100,000 square miles in Idaho, Washington, and Oregon.
Flood-basalt plateaus, also known as traps, are typically very flat because they are formed by the eruption of voluminous amounts of highly fluid lava from long fissure vents rather than from a single volcanic center.123
Because the lava is highly fluid, it is capable of traveling long distances from its source, allowing it to spread out and bury the preexisting terrain across a broad area. As these successive flows accumulateâreaching thicknesses of tens to hundreds of metersâthey create a level, expansive surface. While the top surface of these plateaus is characteristically flat, it is common for them to be marked by sharply incised canyons and valleys that develop over time.1