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| Eyjafjallajokull volcano, Iceland, April 19, 2010, ASTER/Terra daytime image. |
(Steve Marshak, a professor of geology at UIUC wrote a substantial part of this.)
The eruption of Eyjafjallajokull volcano has captured the attention of the world during the past week, for it has disrupted air travel to or from Europe, and therefore has traumatized transportation links, business, and tourism worldwide. A worried public wonders why there, why now . . . and how much longer? A bit of background about the geology of Iceland, and of the style of eruption we are now observing may be of help.
Eyjafjallajokull is a vent along one of the major fissures or gashes that transect Iceland, for this island sits astride the Mid-Atlantic Ridge, the very active boundary between the North American plate (including North America and Greenland) to the west, and the Eurasian plate to the east. The Mid-Atlantic Ridge is, in geologic jargon, a divergent plate boundary, meaning it is a surface at which two plates move apart. This movement is accommodated by sea-floor spreading, a process by which new oceanic crust forms by the rise of magma from the mantle below.
Geologists proposed back in the 1960s, that sea-floor spreading takes place along mid-ocean ridge systems and that, through this process, ocean basins grow wider over time and the continents on either side move apart. A huge volume of geologic data from the sea-floor supports this theory. For example, the theory predicts that they youngest crust of the Atlantic Ocean occurs at the Mid-Atlantic Ridge, and that the oldest sea floor occurs adjacent to the continents. And that's exactly what researchers have found.
In the last few years, careful measurements using GPS (the same satellite-based global positioning system used in an automobile's navigation system) have allowed geologists to see the continents moving in real time—the process of sea floor spreading takes place without a shadow of doubt. On average, the distance between London and New York increases by about 2 cm (1 inch) per year, about the rate that your fingernails grow, and nothing that humans can do can change that fact. The sea-floor spreading process may seem really slow, and it is. But given the vast expanse of geologic time, slow movements can yield great distances. At 2 cm/year, the Atlantic has widened by about 2 km in the last 100,000 years (the time since the appearance of modern humans), and about 2,000 km in the last 100 million years. At this rate, the North Atlantic started to open about 180 million years ago—before that time, the Mid-Atlantic Ridge didn't exist and a dinosaur could have walked from New York to London without getting its feet wet.
Most of the volcanism along the worlds 40,000 of mid-ocean ridge occurs at depth 2 km beneath the sea, in utter darkness away from the inquiring eyes of humans. But research submarines have been able to photograph the consequences of this activity, including fresh lava flows and black smokers, remarkable jets of super-hot water heated by magma (molten rock) below the surface. Iceland is special — it sits atop a huge plateau of lava, a volume much greater than any other location along a mid-ocean ridge. For this reason, many geologists have suggested that Iceland is a hot spot, a region where a column of particularly hot rock is rising slowly from great depths in the Earth. When this rock reaches the base of the plate, it starts to melt, producing vast quantities of magma, much more than normally occurs along mid-ocean ridges. As this magma erupts, it built up the Icelandic plateau and eventually emerged from the sea as an island. But it is not a stable island—as sea-floor spreading progresses inexorably, the island splits along fissures, erupting currents of lava (molten rock at the Earth's surface). These eruptions drain the supply of magma that accumulated below the island for a while, and when enough of the magma has drained, the eruption ceases. But just for a while—inevitably, as more magma rises, and as the island slowly splits, an eruption is sure to happen again.
The last really major eruption along the fissure system of Iceland happened in 1783, when Benjamin Franklin was ambassador to France. The ash affected the climate in Europe that year, causing an overall cooling. It was Franklin, in fact, who published the first article to suggest a relation between climate and volcanic eruptions. (The effect he described, significantly, is not the same effect as caused by long-term changes in the concentration of greenhouse gases, such as CO2—volcanic eruptions of the magnitude we're seeing in Iceland, or that the world has witnessed in recent centuries, such as at Krakatoa and Pinatubo, have a fairly short-term impact. Other ashy eruptions, such as the one of Tambora in 1816, also had global climate impacts—1816 was so wet and cold in Europe that it came to be known as the "year without a summer." It's been suggested that somber climate of the last one inspired Mary Shelley to write Frankenstein)
The fissure came alive again, on March 20, when the volcanic vent called Eyjafjallajokull awakened from repose with an eruption on the northeast flank, in a narrow 2-km ice free zone between it and neighboring Katla volcano
. Low fire fountains reminiscent of Hawaiian volcanism burst from a 500 meter long fissure, and a small plume of ash less than 1 km high developed. This eruption continued intermittently until April 12.
After a very brief repose, magma worked its way into the central glacier-covered crater and a new eruption started just after midnight on the 14th of April. A series of vents opened up along a fissure nearly 2 km long. The intruded magma provide heat to melt the glacial ice, producing floods of water, known as Jokulhlaups, that flowed under the ice toward the southern coast of Iceland. These reached the coast around noon on April 14, destroying roads, infrastructure and farmlands. Icelandic geologists have done a wonderful job of monitoring and forecasting the eruptions, and 800 people were safely evacuated before the devastation hit.
When daylight broke, an eruption plume was observed, reaching more than 8 km height, carrying ash high into the atmosphere where the jet stream was parked over Iceland. During the first three days, some 70-80 million cubic meters of magma were discharged, at an average rate of about 750 tonnes/second.
The change in eruptive style between the March and April phases of this eruption can be attributed to the availability of water to the magma. The March eruptions were “dry”, driven only by the fairly minor amounts of gases dissolved in the magma, and by the pressure that squeezes magma beneath upward like bubbly toothpaste being squeezed out of a tube. In the most recent phase, water from the melting glaciers sank downwards through the ground and came into contact with the magma and surrounding hot rocks. Where the water only contacts hot rocks, it vaporizes and rises to form billows of white steam. But where it contacts the magma, it abruptly cools (is "quenched") and the magma, causing it to solidify almost instantly, into glass. The result produces vast quantities of fine volcanic ash that are then blown out of the volcano in dark roiling ash clouds. In places where the water has contacted the magma, the production of ash exposes more magma to more water and so the process feeds on itself creating more material to that erupts into the plume.
What does the ash consist of? Viewed under a microscope, it looks like tiny, jagged flakes and slivers of glass. Chemically, the ash consists of the same elements that make up the magma—mostly silica (SiO2), magnesium oxide, and iron oxide. Silica is the same chemical compound that comprises the familiar mineral quartz, which when melted and quenched produces window glass. Ashy, steamy eruptions such as the one now occurring in Iceland are called phreatomagmetic eruptions, meaning that the eruption is driven not only by magma with its dissolved gases, but surface waters (the “phreato” part of the word). In this case, the surface waters are being produced by melting of the ice cap on top of Eyjafjallajokull. The combination produces very explosive eruptions and is one reason that Iceland carries the nickname “Land of Fire and Ice”.
Winds carried the ash toward Europe where it arrived on April 15, causing the closure of air traffic throughout Scandanavia and northern Europe. Unusual stagnant high-pressure conditions in Europe have prevented the ash cloud from dispersing, causing continued air transport and economic problems in Europe. The ash plume activity Ha continued to the present, with an average height of 5 km and pulses to 8 km.
When the magma fragments into fine ash particles, static electricity builds up on the particles and is then discharged in magnificent displays of lightning. Recent research suggests that volcanic plumes rotate around their axis, like super-cell thunderstorms. Due to the rotation, electrically charged ash particles are spun out away from the axis to form a sheath on the exterior of the plume, causing dramatic lightning displays to be concentrated in the sheath.
How long will the eruption occur? No one can predict exactly, but careful monitoring will minimize the unexpected. What will the weather be like when the next eruption occurs? No one can predict at all! This eruption was a very small one by global standards, but it occurred just when the atmospheric conditions were right to make conditions in Europe miserable. It is a volcano-weather pattern that is statistically very small. Seismic activity called “volcanic tremor” now occurring in Iceland suggests that the area is remaining active, at least underground Only time will tell if 2010 will be another year without a summer.
