In 1963, the Canadian geophysicist who discovered transform faults came up with an original idea that became known as the "hotspot" theory. His name is John Tuzo Wilson and he claimed that not all volcanic activity could be related to present day active tectonic plate borders. Wilson noted that in certain locations around the world, such as Hawaii, volcanoes have been active for very long periods of time. This could only happen, he reasoned, if relatively small, long lasting, and exceptionally hot regions - called hotspots - existed below the plates that would provide localized sources of high heat energy (thermal trails) to sustain volcanism.
Although Hawaii is perhaps the best-known hotspot, others are thought to exist beneath the oceans and continents. More than a hundred hotspots beneath the Earth's crust have been active during the past 10 million years. Most of these are located under plate interiors (for example, the African Plate), but some occur near diverging plate boundaries. Some are concentrated near the mid-oceanic ridge system, such as beneath Iceland, the Azores, and the Galapagos Islands.
These islands are all volcanoes built up of basaltic lavas; the basalt that makes up these islands contains significantly more potassium and sodium than the basalts that make up the ocean crust. This suggests that while the mantle forms the source for both these rock collections, there must be a different mechanism involved in their formation. The formation of these volcanic islands is related to the occurrence of long-lived, stationary hotspots within the mantle.
The hotspots create localized thermal trail of hot, rising mantle material. Some geologists consider that mantle plumes are generated in the lower levels of the asthenosphere by the decay of concentrations of radioactive isotopes. However, many believe that they are generated at much deeper levels in the lower mantle, perhaps even at the boundary of the mantle with the outer core.
