Hot spots are a common term for plumes of magma welling up through the crust (Earth's outermost layer of rock) far from the edges of plates.
To understand what hot spots are and why they are important, some understanding of the theory of plate tectonics is necessary. This widely accepted theory proposed by Alfred Wegener in 1912 states that the crust is composed of huge plates of rock that drift over Earth's mantle. Where the plates separate, magma from the mantle approaches the surface and encounters decreased temperature and pressure, allowing it to solidify into new rock. At the edges of plates that crash together, trenches form, in which one plate slides under the other. In some places, such as the San Andreas Fault in California, the plates slide by each other. Most volcanoes and earthquakes occur near the edges of these plates.
Some volcanoes, however, are far from the plate margins. These volcanoes tend to be very high, in the center of raised areas, and the rock produced there is alkaline, chemically different than the theoleiite rock produced at the margins. Moreover, there are several dotted lines of extinct volcanoes (such as the chain of the Hawaiian islands) that are arranged oldest-to-youngest in a line, tipped by a young active volcano.
These are explained, in a theory proposed by J. Tuzo Wilson in 1963, as fixed spots in Earth's mantle, from which thermal plumes penetrate the crust. The lines of extinct volcanoes do not indicate that the plume is moving: rather that the plate is moving relative to the mantle. Therefore, the hot spots can be used to deduce the direction in which a plate is moving. In the case of the Hawaiian ridge, the most recent volcano (Kilauea) is southeast of the older volcanoes. The oldest volcano in this line dates back to about 40 million years ago. From this, scientists have deduced that the Pacific plate is moving northwest at about 3.9 in (10 cm) each year. However, a line of even older extinct volcanoes, the Emperor seamounts, trail northward from the end of Hawaiian ridge: the youngest are southernmost and the oldest (about 70 million years old) are northernmost. From this, we can deduce that the Pacific plate changed direction sometime between 40 and 50 million years ago.
In addition to the volcanoes, hot spots have other effects on the areas around them: they lift the areas around them and represent areas of high heat flow.
The number of hot spots in the world is uncertain, with numbers ranging from a few dozen to over a hundred. They range in age from a few tens of millions of years in age (like the Hawaii-Emperor hot spot) to hundreds of millions of years old. Some appear to be extinct.