As lava issuing from a vent or ﬁssure cools, it forms a surface crust, which often is broken by continued movement underneath into a rubble of sharp-edged hunks, until eventually the entire ﬂow cools to a halt. Thinner lavas ﬂow faster, with less surface breakage, sometimes hardening with ripple marks intact. A lava ﬁeld is an area of acres or a few square miles covered by such ﬂows, relatively unweathered. A lava plain is a larger land area, usually several hundred square miles, underlain by lava ﬂows—the Snake River Plain is an example. A lava plateau is a yet broader, regionally deﬁning feature. The Columbia Plateau of Oregon, Washington, and western Idaho was built by a series of voluminous basalt lava ﬂows that inundated sixty-two thousand square miles to a maximum depth of nearly three miles. The engine driving these extraordinary outpourings was the magma plume that enlivens Yellowstone today. The plume has held its place in the planet’s mantle as the continental tectonic plate has shifted overtop of it to the west for the last sixteen million years, at the rate of one or two inches a year.