Cutaway sketch of a Kīlauea showing magma rising toward the surface...

Before an eruption of Kīlauea Volcano, magma typically moves toward the surface as a long, narrow blade-like body called a dike. A single dike may extend to more than 5-10 km (3-6 mi) in length, 2-3 km (1-2 mi) in height, and a few meters (yards) in width. The process of magma moving laterally or rising toward the surface is called an intrusion. In the cutaway diagram, a hypothetical dike is shown intruding into part of Kīlauea Volcano's East Rift Zone. During the history of the East Rift Zone, many thousands of dikes have intruded into the axis of the zone, and many reached the surface to trigger an eruption.