October 11, 1996
A weekly feature provided by scientists at the Hawaiian Volcano Observatory.
Iceland's subglacial eruption
While scientists in Hawaii have been investigating the recent activity of Loihi Volcano beneath 3000 ft of seawater, in Iceland they're watching an eruption that is taking place beneath 2500 ft of glacial ice!
Since Iceland is well-endowed with both volcanoes and glaciers, subglacial eruptions are not unusual. The current eruption in southeastern Iceland is taking place beneath the country's largest glacier, the Vatnajokull ice cap, which is three-quarters the size of the Big Island.
The first sign of activity brewing beneath the ice cap was a magnitude 5 earthquake on September 29. This was followed closely by a swarm of quakes that persisted for the next day-and-a-half. In Iceland, where an eruption occurs on average every five years, scientists have plenty of experience in interpreting such events. On the afternoon of the 29th, they issued a warning to aviation authorities that an explosive eruption was likely to begin in the near future and that the resulting ash cloud could interfere with the heavy air traffic over the Atlantic.
This prediction proved correct on the morning of October 1, when aerial observers discovered that the glacier had begun to subside over the location of the 1938 Loki eruption. The bowl-shaped depression in the surface of the ice was caused by lava flows melting the base of the glacier. More depressions formed through the day, indicating that the eruption was originating from a fissure 3.5 miles long.
Simultaneously, nine miles to the south, the icecap began to bulge over the subglacial caldera of Grimsvotn volcano as meltwater from the eruption rapidly filled this depression. The Grimsvotn caldera, which is more than three times the size of the caldera at the summit of Kilauea, last erupted in 1983. The ground remains hot enough to cause continual melting of the ice, forming a subglacial lake. Periodically, this lake empties along channels beneath the ice, creating large floods when the water emerges from the edge of the glacier near the south coast of Iceland. These frequent glacial-outburst floods prevented the Icelanders from building a permanent highway along this coast until 1974.
By October 2, the new fissure eruption had broken through the ice. The i nteraction of lava and meltwater guarantees that subglacial eruptions will be explosive, and this one was no exception. Rhythmic explosions threw black ash over 1500 ft into the air, and the eruption plume reached an altitude of 13,000 ft. This activity continued as of October 11, though it appeared that the eruption might be abating slightly.
The icecap over Grimsvotn caldera continued to rise due to the accumulating meltwater, and a catastrophic flood appears inevitable. Preparations were being made to divert the floodwaters from vulnerable highway bridges south of the glacier. Aside from the highway, the eruption does not threaten any other structures or inhabited areas.
The current eruption site is part of a fissure system that bears the distinction of producing the largest lava flow erupted on earth in the last 1000 yrs. In the summer of 1783, the 15-mile-long Laki fissure erupted over its entire length. The results were disastrous for Icelanders. The lava flow covered over 200 square miles, including inhabited areas southwest of the Vatnajokull ice cap. Volcanic fumes shrouded the island the entire summer, poisoning the grass crop and triggering a famine that killed 75% of the sheep and horses in the country and ultimately a fifth of the human population.
For more information on the Grimsvotn eruption, check the eruption web page of the Science Institute at the University of Iceland.
Kilauea eruption status
Meanwhile, here on the Big Island, the eruption of Kilauea continues with little change as lava enters the ocean at the Lae`apuki site in Hawaii Volcanoes National Park. No one reported feeling an earthquake in the past week.
The URL of this page is http://hvo.wr.usgs.gov/