Definition: An opening, or rupture, in the Earth's surface or crust, which allows hot, molten rock, ash and gases to escape from deep below the surface.
Definition: An isolated hill with steep sides and a small flat top, smaller than mesas and plateaus. Buttes are formed by erosion when a cap of hard rock, usually of volcanic origin, covers a layer of softer rock that is easily worn away. This hard rock avoids erosion while the rock around it wears down.