Volcanoes vary greatly depending on the type of magma which emerges, how the magma reaches the surface and on how much magma reaches the surface. Iceland has a great diversity of volcanoes. There are eruptions under glaciers, in the sea, effusive eruptions, and explosive eruptions of varying size and type. Eruptions can also change mid-eruption, e.g. from tephra eruptions or explosive eruptions to effusive eruptions. This happened during the Surtsey eruption for example, which began as a powerful submarine tephra eruption and eventually, as an island was formed around the vent, lava began to flow, to the delight of geologists and others as it reinforced the island against the erosive forces of the sea. Several other islands appeared in the same eruption but have all since disappeared.
Shield volcanoes
Shield volcanoes reached peak activity at the end of the last Ice Age, 10-12,000 years ago. After the weight of the ice caps had disappeared the land rose again. Volcanic eruptions became more frequent and huge effusive eruptions occurred away from the central volcanoes. Magma appears to have come all the way from the mantle as there is no magma chamber beneath the shield volcanoes. The fissures were initially long but as eruptions continued the centre of activity became concentrated in one crater and finally formed a shield volcano. The largest such shield volcano in Iceland is Skjaldbreið and the eruption which formed the mountain probably lasted decades. Eruptions of this type decreased sharply in number once uplift had slowed down and more typical volcanic activity began to take over.
Effusive eruptions
An effusive eruption is a type of eruption in which the majority of volcanic material is in the form of lava. The most common effusive eruptions are basaltic eruptions, and lava can be classified as vicious, slow-flowing lava known by the Hawaiian term a’a, or thin, smooth lava known by the term pāhoehoe. The Skaftáeldar (Skaftá Fires) eruption is an example of an effusive eruption.
Explosive eruptions and Plinian eruptions
Strictly speaking, these terms refer to the same phenomenon. Explosive eruptions occur when magma comes into contact with water, either high up in the crust (groundwater) or at the surface. Another critical factor is how fast the magma is forced up the conduit. Eruptions beneath glaciers or the sea are most likely to become explosive eruptions but this is not always the case. The eruption at Askja in 1875 is an example of a very powerful explosive eruption.
Subglacial eruptions
Many central volcanoes in Iceland are covered by ice. In Southern Iceland, these volcanoes include Katla, Eyjafjallajökull, Bárðarbunga and Grímsvötn. When volcanoes erupt beneath glaciers the material ejected is in the form of tephra or ash in tephra eruptions (phreatomagmatic eruptions) or explosive eruptions.
Glacial outbursts
Subglacial volcanoes are dangerous due to the sometimes massive ashfall and also because of glacial outbursts or jökulhlaup which occur when eruptions cause glacial ice to melt. Vast tracts of land in Southern Iceland are uninhabitable due to regular glacial outbursts. The great sands around Mýrdalsjökull and Vatnajökull have been formed by glacial outbursts, mainly from Katla (Mýrdalssandur) and Grímsvötn (Skeiðarársandur).
Submarine eruptions
The area around the Reykjanes peninsula has been a hotspot for volcanic activity throughout the ages. The Surtsey eruption from 1963 to 1967 is one of the most famous submarine volcanoes. The location surprised scientists as the Vestmannaeyjar system had been considered dormant until the eruption. A further eruption occurred on the island of Heimaey in 1973. Submarine eruptions resemble subglacial eruptions in that the main volcanic material produced is tephra. If the eruption forms an island, lava can flow, as was the case on Surtsey. Submarine eruptions have seldom caused any damage in Iceland. They are rarely large and not all of them reach the surface and are sometimes only detected by scientific instruments.