The ash spewing from the Eyjafjallajokull volcano in Iceland is disrupting air traffic throughout Europe. Volcanic ash is essentially sharp-edged glass with particles ranging from sand-sized to microscopic. These particles can wreak havoc with jet engines and your lungs. The last time this volcano erupted, the eruption lasted more than a year, from December 1821 until January 1823.
Iceland sits astride the Mid-Atlantic Ridge, a boundary between two tectonic plates, where new crust is being formed by volcanic eruptions as the plates diverge, i.e., they are moving away from each other. Movement on this structure over the last 180 million years or so has separated Europe from North America, and Africa from South America. The island of Iceland is engaged in a geologic race between the spreading motion which is ripping the island apart, and the volcanoes which are building the island up.
Other islands of the Atlantic Ocean created by the volcanism of the Mid-Atlantic Ridge are The Azores, Bermuda, Madeira, The Canary Islands, Ascension, St. Helena, and Tristan da Cunha. But, you probably haven’t heard about any volcanoes erupting in Bermuda because that island group lacks one other geologic phenomenon: the “hot spot.” Iceland also sits above a mantle plume or “hot spot” where magma from deep in the mantle forces its way to the surface. The Hawaiian Islands were formed, and are being formed, by such a hot spot. In Hawaii, the westward movement of the Pacific plate passes over this hot spot and eruptions produce new islands. Iceland, however, is not moving in such a manner.
The general geology of Iceland is shown on the map below from the Nordic Volcanology Institute.
The pink area on the map represents the rifts along the Mid-Atlantic Ridge where the crust is separating and the volcanoes are most active. Of Iceland’s 100 most active volcanoes, 25 have erupted in recent history, and 35 volcanoes have erupted in the last 10,000 years. Eleven volcanoes have erupted between 1900 and 1998. Most of the eruptions were from fissures or shield volcanoes and involve the effusion of basaltic lava.
The 1783 to 1784 eruption at Laki fissure and the adjoining Grímsvötn volcano was the largest single historic eruption of basaltic lava (12 cubic km). Benjamin Franklin, who at the time was serving as ambassador to France remarked on this eruption. The ash cloud caused the “year without summer.” That eruption of basalt lava and clouds of poisonous hydrofluoric acid/sulfur-dioxide compounds killed over 50% of Iceland’s livestock population, leading to famine which killed approximately 25% of the population. It remains to be seen if the current eruption will be as long lasting. So far, the ash cloud from the current eruption has risen to 30,000 feet which affects airline travel, but it has stayed below the stratosphere, so the climate effects are not likely to be as drastic as those in 1783.
The Boston Globe has a series of photographs of the volcano and flooding it caused. Boston Globe photos: http://tinyurl.com/y6vslk4