Shield volcanoes are built almost entirely of fluid lava flows. Flow after flow pours out in all directions from a central summit vent, or group of vents, building a broad, gently sloping cone of flat, domical shape, with a profile much like that of a warrior's shield. They are built up slowly by the accretion of thousands of flows of highly fluid basaltic (from basalt, a hard, dense dark volcanic rock) lava that spread widely over great distances, and then cool as thin, gently dipping sheets. Lavas also commonly erupt from vents along fractures (rift zones) that develop on the flanks of the cone. Some of the largest volcanoes in the world are shield volcanoes. In northern California and Oregon, many shield volcanoes have diameters of 3 or 4 mi and heights of 1,500 to 2,000 ft. The Hawaiian Islands are composed of linear chains of these volcanoes, including Kilauea and Mauna Loa on the island of Hawaii.
In some shield volcano eruptions, basaltic lava pours out quietly from long fissures instead of central vents and floods the surrounding countryside, forming broad plateaus. Lava plateaus of this type can be seen in Iceland, southeast Washington, eastern Oregon, and southern Idaho.