Although most volcanic rocks are generated at plate boundaries, there are a few exceptionally active sites of volcanism within the plate interiors. These intraplate regions of voluminous volcanism are called hotspots. Twenty-four selected hotspots are shown on the adjacent map. Most hotspots are thought to be underlain by a large plume of anomalously hot mantle. These mantle plumes appear to be generated in the lower mantle and rise slowly through the mantle by convection. Experimental data suggests that they rise as a plastically deforming mass that has a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. Such plume heads are thought to have diameters between ~500 to ~1000 km.

Decompressional melting of this hot mantle source can generate huge volumes of basalt magma. It is thought that the massive flood basalt provinces on earth are produced above mantle hotspots. Although most geologists accept the hotspot concept, the number of hotspots worldwide is still a matter of controversy.

Hotspot Tracks

The Pacific plate contains several linear belts of extinct submarine volcanoes, called seamounts. The formation of at least some of these intraplate seamount chains can be attributed to volcanism above a mantle hotspot to form a linear, age-progressive hotspot track. Mantle plumes appear to be largely unaffected by plate motions. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced.

The island of Hawaii lies above the mantle plume. It is the only island that is currently volcanically active. The seven Hawaiian Islands become progressively older to the northwest. The main phase of volcanism on Oahu ceased about 3 million years ago, and on Kauai about 5 million years ago. This trend continues beyond the Hawaiian Islands, as demonstrated by a string of seamounts (the Hawaiian chain) that becomes progressively older toward Midway Island. Midway is composed of lavas that are ~27 million years old. Northwest of Midway, the volcanic belt bends to the north-northwest to form the Emperor seamount chain. Here, the seamounts become progressively older until they terminate against the Aleutian trench. The oldest of these seamounts near the trench is ~70 million years old. This implies that the mantle plume currently generating basaltic lavas on the island of Hawaii has been in existence for at least 70 million years.

© Dr. Vic Camp, How Volcanoes Work, http://www.geology.sdsu.edu/how_volcanoes_work/ 2004