A volcanic hotspot is produced by a mantle plume, which pushes up into the crust, melting bedrock and, in most cases, erupting at the surface to produce lava flows or volcanoes. The hotspot itself is stationary but may resolve over millions of years.
As the crust moves over a hotspot, a chain of volcanoes or lava flows is produced; as one might expect, the oldest of these are farthest from the hotspot (in the direction of the crust movement) while the youngest is nearest (often over) the hotspot and may be active or temporarily dormant. Once a volcano moves away from the influence of the mantle plume, it becomes extinct and will not erupt again.
Hotspot volcanic fields result from a hotspot beneath continental crust while volcanic ridges form on oceanic crust, producing volcanic island chains which represent the high points of the ridge; examples of the latter are the Hawaiian and Society Island Chains in the Pacific Ocean. In North America, hotspot fields are widespread in the western U.S.; since this continental plate is gradually moving westward (as the Atlantic Ocean continues to open), the oldest, extinct volcanoes in a field are at its western end while dormant or active volcanoes are at the eastern end (near or above the hotspot). Examples include the San Francisco Volcanic Field in northern Arizona, the Snake River Plain-Yellowstone Field and the Raton-Clayton Field of northeastern New Mexico; the latter field is thought to be extinct.
