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My Journey to Iceland During my journey to Iceland, I have seen a natural event that made that journey very important to me. In southeastern Iceland, some 4, 500 feet above sea level, lies Vatnajokull -- the largest temperate-zone ice cap in Europe. Vatnajokull, 3, 200 frozen square miles overlying Iceland's most active volcanic region, sits, as does the rest of the island, above a mantle plume -- a column of hot rock that rises from the depths of Earth and feeds volcanoes with lava. Although it is paradoxical that Iceland's hottest region boasts its biggest ice cap, it is no coincidence: the ice sheet is huge and permanent precisely because lava flowing from the mantle plume has built the mountains so high. Iceland's awesome volcanic power comes from the confluence of two processes: tectonic volcanism and the action on a mantle plume.
This island nation lies exactly on a mid-ocean ridge running down the center of the Atlantic. A global network of such ridges -- 37, 000 miles of mountain ranges that are almost entirely underwater -- has resulted from the drifting apart of tectonic plates. The ensuing gap, flanked by strings of volcanoes, continually spews molten rock that becomes new crust. The volcanic activity of underwater mountains dwarfs the output of large terrestrial volcanoes such as Mount Saint Helens, Vesuvius, and Mount Pinatubo. We are scarcely aware of it, but every year about three cubic miles of molten rock erupts underwater: an average of 1, 600 tons of magma every single second of every day, enough to fill an Olympic swimming pool twenty times a minute. Although the mantle plume lying below Iceland is not molten, up to 30 percent of it melts while it approaches the surface, because its melting temperature drops as the plume rises and the pressure on it decreases (much as the boiling point of water drops when atmospheric pressure decreases).
Solid, upwelling mantle rock begins to melt when it is still sixty miles beneath the surface, and as it rises, the pressure drops further and the rate of melting increases, generating huge volumes of molten rock that bleed upward to feed the volcanoes at the surface. Jokulhlaup is a term that describes what happens when fire and ice meet, the most recent event happened in 1996. The molten rock, at a temperature of 2, 000 degrees F ate its way up through 1, 500 feet of ice in just thirty hours, breaking through to send a plume of ash, hot gases, and superheated steam 30, 000 feet into the air. More than a trillion gallons of meltwater were created during the eruption, which released the energetic equivalent of about 15, 000 tons of high explosives (or the atomic bomb dropped on Hiroshima) every minute for ten days. Yet the 1996 event produced only one-twentieth the lava generated by a 1783 eruption in the same region of Iceland (the largest eruption on Earth in historic times). The melted ice flowed into a subglacial lake at the bottom of a three-mile-wide caldera known as Grimsvotn, which lies under the ice cap.
Held in on one side by just a dam of ice, the water rose at sixty feet per day, compared with a normal rate of sixty feet per year. A torrential flood called a jokulhlaup by Icelanders (and now by the rest of the world) finally came shortly after dawn on November 5, 1996, when the rising water, lifting the overlying cap like an ice cube in a cold beverage, began to flow south toward the sea. The flood was the largest in sixty years; the water, heated to 40 - 50 degrees F, rapidly eroded the glacier under which it was flowing, cutting large channels through the ice. Maximum flood rates reached 1. 6 million cubic feet per second. Where the water broke through the edge of the glacier, the jokulhlaup was powerful enough to break off lumps of ice thirty feet high, weighing up to a thousand tons, and to sweep them across the adjacent plain. On the sand flats of Skeidararsandur, the flood washed away sections of the only road around the island, including a large bridge, as well as power transmission cables.
Since the Icelanders were expecting such an event, however, no one was hurt.
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