Approximately 1,000 to 3,000 earthquakes occur each year in the Yellowstone area; most are not felt. The 1959 Hebgen Lake earthquake (M7.3) caused significant damage to park roads, show here near Gibbon Falls.
mantle definition. The region of the interior of the Earth between the core (on its inner surface) and the crust (on its outer). Note: The mantle is more than two thousand miles thick and accounts for more than three-quarters of the volume of the Earth.
asthenosphere. n. 1. (Geological Science) a thin semifluid layer of the earth (100-200 km thick), below the outer rigid lithosphere, forming part of the mantle and thought to be able to flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement. See also isostasy.