Less ash is potentially good news for stranded travelers, but scientists who are monitoring the mountain's explosion warn the eruption is not finished, and may still set off other eruptions at nearby volcanoes.
The first sighting of glowing magma in the Eyjafjallajokull volcano was made on Monday, though the lava is not flowing down the mountain, Icelandic geologists said.
"It is sputtering and bubbling and will probably create a cone formation" as the lava spills over and freezes into rock, said Kristin Vogfjord, geologist at Iceland's Met Office in Reykjavik.
The volcanic eruption has been particularly explosive because it has surged under a 200 yard-thick glacier. Melting ice pouring into the crater helped create plumes of ash that rose as far as 5 and a half miles into the air.
Now that the crater ice has mostly melted away, the ash cloud has decreased to below 1.8 miles in height, though the eruption continues. "The plume is lower but the tremors are slowly increasing, which means more magma is flowing," said Vogfjord.
She is one of several dozen geoscientists and meteorologists who are monitoring Eyjafjallajokull's violent moods.
Seismometers and GPS stations are planted close to and around the volcano to measure tremors and land movement that can herald eruptions.
The GPS units - plastic cylinders on short poles - show the land around Eyjafjallajokull has swollen as much as 3 inches in recent months and then contracted slightly following the eruption, much like a bubble popping.
The seismometers, all of which are connected to computers and relay information automatically to a central data center in Reykjavik, check for tremors which indicate that magma is breaking through the crust to surface at the crater.
The sound of these tremors can now be heard up to 12 miles from the mountain.
Vogfjord said some the instruments are vulnerable to the ash, however, and may break down.
The ash, made up of sand and tiny abrasive glass-like particles, is very fine and can penetrate machines like computers and cameras - and, officials fear, jet engines.
That makes flying dangerous because the ash tends to stick to a jet engine's interior parts, such as the turbines, where it melts to form a glassy coating.
Vogfjord's team also monitors the volcano with the help of satellite imaging, a radar beam from the airport that sees anything above an altitude of 1.8 miles, as well as Coast Guard flights when weather permits.
(AP Photo/Brynjar Gauti)
(Left: A plume of ash from the volcano under the Eyjafjallajokull glacier covers the farm of Pall Eggert Olafsson in Thorvaldseyri, Monday, April 19, 2010.)
While the current eruption may be stabilizing, geologists warn that any further ones on Iceland could again bring European aviation to a standstill. Even a volcano that is not covered by a glacier can shoot the same abrasive ash to altitudes used by commercial airliners.
That happened in 2000 at Mount Hekla and in 2004 in Grimsvotn, both located north of the current eruption. The difference then was that the wind carried the ash to unpopulated polar regions northeast of Iceland, rather than southeast to Europe's main air travel hubs.
Besides Mount Hekla, which is typically active every 10 years, scientists are also closely watching the Oraefajokull volcano and the massive Katla, both of which are under glaciers.
"The activity of one volcano sometimes triggers the next one, and Katla has been active together with Eyjafjallajokull in the past," said Pall Einarsson, professor of geophysics at the Institute of Earth Sciences at the University of Iceland.
The glacier over Katla is more than twice the thickness of Eyjafjallajokull's, so its eruption would produce a vastly larger plume of ash and possibly ground flights for a much longer period of time.
So far, Katla shows no signs of activity, but it explodes roughly once a century - and its last eruption was in 1918, causing massive flooding, and lasted for a year.
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By Associated Press Writer Carlo Piovano