Thundereggs and Supervolcanoes

Hot spots are a form of intraplate volcanism. Isolated hot-spot volcanoes result from the presence of mantle plumes, columns of magma rising in the upper mantle. If the overlying plate is weak, some of the magma breaks through to form a volcano. Composition of the material in the eruption depends on the composition of the plate through which the magma rises; along with the composition of the magma source in the mantle. From studies of hotspots, the basalt is different from the basalt that forms from the upper mantle at spreading centers.

hot spots
Hawaiian islands formed successively over a common source of magma called a hot spot
volcano.und.edu/vwdocs/vwlessons/hotspots.html

The idea of hotspots was established in 1963 from observations concerning the geology of the Hawaiian Islands. It was known that the Islands are progressively older from the southeastern most island of Hawaii to the northwest. Mantle plumes (which are models, theories, that have been postulated from indirect evidence) are columns of molten or hot rock that originate deep inside Earth's mantle and rise slowly toward the surface, lifting the crust and forming volcanoes. They may consist of hot mantle material rising as blobs rather than in a continuous column. They are thought of as having diameters of 100-200 km and rising at rates of perhaps 2 m per year. They have a variety of shapes and sizes, and they originate at various depths between 700 km and the core mantle boundary (2900 km). Melting of occurs as the pressure drops when the material rises to the surface. Their position appears to be relatively stationary, so the lithospheric plates drift over them. The plumes are therefore independent of the major tectonic elements of the crust and they rise up under continents and ocean plates alike.

hotspots
USGS hotspot discussion and figures from The Dynamic Earth

The plumes may form over regions of locally high concentrations of radioactive elements in the mantle (heat producing) or rise over anomalies in the outer core. They are long lived sources of magma in the mantle which can serve to mark the passage of an overlying lithospheric plate. Mantle plumes appear to be temporary features that form and ultimately fade and die with a typical life span on the order of 100 million years. Bathymetric data in the Pacific Ocean show an L-shaped chain of volcanic islands and submerged volcanoes (the Emperor-Hawaiian chain) that show a progression of ages when dated radiometrically. Dates run from 75 million years at the northwestern end of the chain to 40 million years at the bend, and still active volcanoes in Hawaii . From the distance and orientation, the direction of plate movement and rate can be determined. Hot spots occur under continents as well as beneath ocean basins. They are more easily detected in the oceans, perhaps because the magmas can more readily work their way up through the thinner oceanic lithosphere. The use of hot spots to mark plate motion depends upon a presumption that movement of the hot spots is slight. Other lines of study of plate motion and position of hot spots indicates that the hot spots do move, but this movement is very slow in relation to the movement of plates.

Hotspot vulcanism Hawaii
movie of hotspot pattern vulcanism on the islands of Hawaii

A high concentration of hot spots is found in the African plate . Africa has apparently come to rest over a concentration of hot spots. This has developed uplift and a unique topography. Other plates (Antarctica, China, southeast Asia) which are moving slowly, have higher numbers of hot spots. In rapidly moving plates such as the North and South American plates, hot spots are rare.

Hot spots not only mark the movement of plates, but they also play a part in the movement of plates. When a continent comes to rest, the dome that swells up over a hot spot is subject to fracturing and producing a three armed rift. These may initiate a zone of divergence. Typically, two arms of the rift open to form an ocean basin and the third arm fails and remains as a fissure in the continental landmass. By restoring the margins of the Atlantic Ocean to their Pangaea position, an abundance of three-armed rifts is revealed. The successful arms merged to form the mid ocean spreading zone and the unsuccessful ones remained as rifts extending into the continents.