Is there an hot spot beneath Iceland?

Answer

This question might more correctly be broken into two: "Is Iceland an hot spot?" and "Why is Iceland an hot spot?"

Hot spots are places on the Earth's surface that (a) are exceptionally volcanically active and (b) rise high above their surroundings. Clearly, Iceland is the most volcanically active part of the Mid-Atlantic ridge in the region. This is reflected in the exceptional thickness of the basalt crust in Iceland, about 30 km as compared to 6 km to the north and south. And equally clearly, Iceland rises high above sea level unlike the Mid-Atlantic ridge to the north and south.

Consequently Iceland is undisputably an hot spot and has remained so for at least 60 million years. This is shown most clearly by the Greenland-Faroe Ridge which may be regarded as a submarine extension of Iceland toward WNW and ESE, made of 25-30 km thick basaltic crust. The basalt successions in E-Greenland and the Faroe Islands were formed 60-65 million years ago when these places were located where Iceland is now.

For over 30 years it has been generally agreed that hot spots form where mantle plumes are rising from depth. These plumes are 200-300°C hotter than the surrounding mantle and for that reason less dense. The Iceland plume, in particular, can be "seen" seismically down to about 450 km depth but in order to "see" it further down a much more extensive network of geophones would be required than now exists.

There is, on the other hand, other evidence indicating that mantle plumes - at least the more powerful ones - reach all the way down to the core-mantle boundary at 2900 km depth and that the heat they bring to the surface is largely derived from the core. It has also been shown that when rock having the physical properties of mantle material is heated enough for it to start convecting, like porridge in a pot, that convection will be in the form of narrow plumes rather than in the form of the entire mass convecting.

In many ways mantle plumes, as we imagine them, behave like salt diapirs which are well known for example in the Middle East and elsewhere: a layer of salt resting beneath denser strata rises towards the surface as plumes - salt diapirs - and can in some instances breach the surface and flow out like glacial ice.

The mantle plume hypothesis provides powerful means for explaining the properties of hot spots. They (the hot spots) rise above their surroundings because they are underlain by relatively light mantle. The exceptional volcanic activity, resulting in a thick basaltic crust, is the consequence of the high temperature of the plume: As the solid mantle rock rises in a plume towards the surface, basaltic magma is formed when the mantle material starts melting due to pressure release, and the hotter the material, the more melt will it produce. The 200-300°C difference in temperature between the plume and the surrounding mantle suffices to explain the difference in crustal thickness.

Geochemical analyses of basalt samples from Iceland indicate about 30% melting beneath the country, corresponding to 25 km thick basaltic crust. And seismic evidence indicates that the boundary between the basaltic crust and the ultrabasic mantle beneath is at 25-30 km depth. Finally it should be noted that mantle plumes appear to be relatively constant features - well rooted in the mantle - relative to the crust which is, as well known, divided into crustal plates that are drifting to and fro across the Earth's surface. Thus the angular distance between the Hawaii and Iceland hot spots has remained constant for at least 40 million years.

At present, therefore, the answers are: Iceland is an hot spot, and beneath Iceland is a mantle plume, about 200 km in diameter, which probably reaches all the way down to the crust-mantle boundary.