Heitur reitur er landsvæði sem einkennist af mikilli eldvirkni og jarðhita og stendur hátt yfir umhverfið. Ísland er einn af stærstu heitu reitum jarðar og gnæfir 2-4 km yfir venjulega hæð Norður-Atlantshafshryggjarins sem gengur í gegnum landið. Markmið fjölþjóðlega jarðskjálftaverkefnisins ÍSBRÁÐ (bráðin sem myndar Ísland) er að rannsaka þessa jarðfræðilegu uppsprettu landsins. (Ingi Þ. Bjarnason o.fl. 1996). Raunvísindastofnun leiðir þetta verkefni en samstarfsaðilar eru frá Bandaríkjunum, Bretlandi og ýmsum stofnunum hér á landi.
An international geophysical project named ICEMELT (the melt that produces Iceland), is using means of seismology to illuminate the mantle under Iceland down to approximately 650 km depth to construct a model of the mantle plume under Iceland. The plume and its interaction with the Atlantic Ridge is the main source of the excessive volcanism in this region, and hence the source of the existence of lceland. The plume is imaged by analysing the effect it has on seismic waves, that are slowed down when they traverse the part of the mantle that is affected by the heat of the plume. For this task, data were collected continuously 1993-1996, with broadband seismometers, that are sensitive to earthquakes anywhere in the world. These measurements have confirmed a remarkably narrow plume with a diameter between 150-300 km at 100-400 km depth undir Iceland. The plume probably reaches down to at least 650 km depth, and its geographic center is under central Iceland and the western part of Vatnajökull, which is also the location of the most powerful volcanic systems in Iceland. Analysis of surface waves has revealed a large reduction (approx. 10%) in S-wave velocity at approximately 50 km depth in the mantle under central Iceland and along the volcanic zone north of Vatnajökull. This velocity reversal probably marks the upper extent of a partially molten asthenosphere, the melting region or oven where the materials that make Iceland are smelted. Seismic activity associated with volcanoes under the Vatnajökull glacier was well recorded by seismometers located on the glacier and other places (1. and 3. figures). Three events of seismic tremor were recorded originating under the glacier. In July 1995 and August 1996 tremor was recorded in association with jökulhlaups from the Skaftá cauldrons, and in September-October 1996 eruption tremor was recorded from the Gjálp eruption. Comparison of these three tremor events indicate that the tremor from the Skaftár cauldrons could be associated with small volcanic eruptions, lasting approximately two days in July 1995 but only half a day in August 1996. Ingi Þ. Bjarnason and Bergþóra S. Þorbjarnardóttir (1996) have interpreted the sequence of medium size (size approx. 5.0) earthquakes in the Bárðarbunga volcano as a manifestation of increased pressure in the volcano for at least 22 years, that resulted in the Gjálp eruption in 1996. The increased pressure is due to flow of magma from the mantle into the region under the volcano. It is difficult to predict the continuation of these events, but an additional medium size earthquake in Bárðarbunga should be a likely signal for further eruptions in this sequence.
