what is a hot spot?

Answer 1

In geology, a hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time. J. Tuzo Wilson came up with the idea in 1963 that volcanic chains like the Hawaiian Islands result from the slow movement of a tectonic plate across a "fixed" hot spot deep beneath the surface of the planet. Hotspots were thought to be caused by a narrow stream of hot mantle convecting up from the mantle-core boundary called a mantle plume [1], the latest geological evidence is pointing to upper-mantle convection as a cause[2][3][4]. This in turn has re-raised the antipodal pair impact hypothesis, the idea that pairs of opposite hot spots may result from the impact of a large meteor.[5] Geologists have identified some 40-50 such hotspots around the globe, with Hawaii, Réunion, Yellowstone, Galápagos, and Iceland overlying the most currently active.

Answer 2

Not much to add to the previous answer - he touched pretty much all the bases. A hot spot is a place inside the edges of a techtonic plate that experiences some sort of volcanic activity. A good example would be the hawaiian islands which are at the center of the pacific plate. Another example would be Yellowstone park which is actually just a dormant super-volcano. Early in the history of life on earth there was a super-volcano eruption in the middle of what is now Russia (also in the middle of a plate) which caused the worst mass extinction in earth's history.

As for the cause of these hot spots - just read the first answer, they said it very well. Although I believe Iceland lies along the mid-atlantic ridge - putting it at a divergent plate boundry, which would easily explain why it experiences so much geothermal activity.