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plateau basalt An extensive, thick, smooth flow or succession of flows of high-temperature, fluid basalt erupted from fissures, flooding topographic lows, and accumulated to form a plateau. The Deccan Traps in India covers 260 000 km2 and the Columbia River Plateau basalt, in Washington State, USA, covers 130 000 km2 and is more than 1800 m thick. Individual flows may have volumes of the order of 100 km3. The original area of the Thulean Plateau which formed in the north-eastern Atlantic Ocean region 30 Ma ago, was 1.8 × 106 km2. The lava plains are built up by many thousands of individual flows from numerous, coalescing, shield-type volcanoes with extremely low angles of slope.