Saturday, January 21, 2006
Unique volcanic eruptions in Iceland!
An Icelandic volcano that erupted in November 2004.
These flood plains, known as sandar, extend some 800 square miles. Parts of the southern coast were formed some 9,000 years ago, when meltwater spilled out from under Iceland's cloak of glacial ice and galloped forward in violent surges called jokulhlaups, or glacial outburst floods. But jokulhlaups (pronounced YOKE-uhl-howps) are no geologic remnant of the distant past. They occur with almost predictable regularity today, and they may pose great risks to life and property in Iceland.
Glacial floods occur in many regions of the world where mountaintop glaciers sit on top of volcanic regions, as they do here. Fluids, gases and steam from active volcanoes continuously melt the overlying ice, creating pools of water sandwiched by glacial ice. Some of this water drains off at intervals, at times trickling out and other times leading to floods.
But the most potent type of glacial flood is caused by an erupting volcano. Glacial ice cloaks 10 percent of Iceland, a country that straddles the mid-Atlantic ridge and is a simmering cauldron of geothermal and volcanic activity.
Nearly 60 percent of volcanic eruptions in Iceland occur beneath glacial ice.
That is what worries scientists. Katla, one of Iceland's most notorious volcanoes, has erupted five times since 1721, at intervals ranging from 34 to 78 years. The last one was in 1918, so an eruption may be overdue.
"Basically everything you see to the east of Reykjavik is a wall of mountains formed in eruptions under glaciers," said Magnus Tumi Gudmundsson, a professor of geophysics at the University of Iceland, who added, "Katla has been showing signs of unrest over the last few years."
To head off catastrophe, geologists and civil engineers here have developed an extensive, exquisitely sensitive monitoring system intended to provide early warnings of floods. It has issued 16 accurate forecasts since 2001, though it has yet to contend with a major eruption.
When the birth pains of an eruption begin, pressurized magma oozes toward the surface of the volcano, leaving boiling groundwater in its path. Glacial ice acts as a lid on a giant pressure cooker: the thicker the ice, the more force with which it presses back against the erupting lava.
When a volcano erupts, magma as hot as 2,200 degrees Fahrenheit meets ice and boiling water, sending vast plumes of steam and rock particles rocketing upward in what Matthew J. Roberts, a glaciologist with the Icelandic Meteorological Office, compares to a classic mushroom cloud.
That is not all. Steam combines with tiny particles raining out of the eruption to create high static charges, causing lightning strikes several times a second. The 1918 eruption of Katla is said to have killed hundreds of heads of livestock grazing nearby - by electrocution.
Then come the jokulhlaups. "An eruption beneath a thick glacier often leads to a hazardous glacial flood that can begin within minutes to several hours after the eruption has started," Dr. Roberts said.
Floods after a volcanic eruption are a mixture of water, ash, mud and ice; they tend to leave the surrounding countryside covered in ash.
Records from floods in the 1800's indicate that icebergs of Titanic proportions were seen drifting near farmhouses. And one flood is thought to have heaved ice blocks for miles. Geologists are still uncovering this ice, which was buried by so much insulating debris that it is still there more than 150 years later.
In 1996, an eruption beneath the Vatnajokull ice cap, Europe's largest ice mass, led to a jokulhlaup that forced sediment, meltwater and ice out along the 12-mile stretch of the glacier's edge. The flow of water out of the glacier created a river to rival the Amazon in size, at least for a few minutes. It demolished a bridge and added almost three square miles to the area of Iceland. (The flood did not reach nearby settlements, and no one was killed or injured.)
The seismic monitoring system developed in the past few years consists of a network of instruments strung along the countryside like Christmas lights. The devices are similar to those used for monitoring volcanoes like Mount St. Helens, in Washington, but because Iceland is relatively small, the network is densely concentrated.
Unlike conventional seismometers, which detect tremors greater than a magnitude of 1, Iceland's have been fine-tuned to measure magnitudes of minus 1 or even lower.
These micro-earthquakes are thought to result from fractures only 100 feet long or so; the fault lines of earthquakes felt by humans tend to be 10 times as long. "But it's these very tiny cracks which are often the sign of something larger," Dr. Roberts said.
Once an eruption is under way - a process that takes place over days or weeks - meltwater spreads out like a sheet between the underlying rock and the base of the glacier. The pressure causes ice to fracture in an "ice quake" within the glacier.
Using the new seismometers to determine the quake's location, scientists can plot the movement of water.
In addition to the threat from flooding, volcanic meltwater often contains a toxic chemical cocktail that can cause breathing difficulties when the water rises to the surface. Last July, such a flood originated from an ice caldron underneath the western flank of Vatnajokull. "The floodwater was very rich in hydrogen sulfide, to the extent that a strange haze could be seen above the water surface on the river," Dr. Roberts said.
But this toxicity is also providing scientists with real-time monitoring of the likelihood of flooding; if the water has interacted with a geothermal system, it will carry an assortment of ions and give off a higher electrical conductivity. That, too, can be measured.
From the air, scientists can monitor the ice surface to keep track of accumulating meltwater under the glaciers. Increased melting is visible from the air in the form of a "melt pit," or ice caldron; the caldron's growth indicates how much heat is being released from the volcano.
Katla is situated precariously behind the village of Vik, roughly 16 miles away. In the last few years, the volcano grew more seismically active, began inflating in the magma chamber and showed increased geothermal activity.
At the end of 2004, it became quieter again. But the authorities here are taking no chances. In May, the Icelandic Civil Protection Department plans to conduct a large-scale evacuation drill in a region to the west where a jokulhlaup might engulf inhabited areas.
In the meantime, each new field season offers a natural laboratory of visible change for scientists like Andrew Russell, a glacial geologist from the University of Newcastle Upon Tyne in Britain, who headed out to the sandur last summer in a caravan of Land Rovers and cargo trailers with volunteers from the organization Earthwatch.
On that expedition, Dr. Russell pointed to an area about halfway up Skaftafellsjokull ("jokull" means glacier) where a fresh scab of glacier had just broken off after a small ice avalanche, revealing virgin layers of ancient ice and sediment.
At another field site, Dr. Russell pointed out a "kettle hole" - a pit formed when buried ice melts away. This one was filled with dangerous quicksand.
This year, his team may need to take inflatable rafts to cross a lake of meltwater where a glacier stood just a few years ago. Still, he is happy to get fresh access to river sediments that are deposited inside tunnels within the glacial ice.
"We can see some of these melting out beautifully at the moment," he said. "You get a real feel for the forces that are in operation."