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Vulcanism is very common in Iceland, but glaciers cover 11 percent of its surface area. This is why volcanic eruptions quite often occur under glaciers, as one did in October 1996 under Vatnajokull, Europe’s largest glacier. At the site of the eruption, the glacier was 500 m thick and more or less smooth and flat. After a day’s activity, the visible sign of the eruption was a deep crater-like depression on the surface of the ice cap, in the form of an upside-down cone with a depth of 100 m and a diameter of 1 km. Explain the formation of depression. What would have been found under the ice crater at this time? Try to predict the subsequent events.

Step-by-step

The molten magma came into contact with the ice, and huge volumes were melted at the base of the 500-m-thick ice cap. As the density of water is greater than that of ice (i.e. the volume of meltwater is smaller than that of the ice from which it was formed), and moreover some part of the water could flow away, a huge conical hole would have formed under the ice. But the extremely heavy ice above the hole sank, leaving the depression at the surface. Under the ice crater, we could have found the recently solidified magma intrusion, the meltwater in a conical cavity, and the ice cover (see figure). The amount of meltwater depends on the quantity of magma, but the shape of the ice crater is determined by the hydrostatic pressure of the ice and water.
The eruption broke through the ice cap on the second day and a hot, blackash-cloud, 500 m high, was formed. This was carried up to an altitude of 3000 m by the buoyancy force of the cold air. At the end of two weeks, the cloud column had become white and reached a height of 10 km. Altogether the eruption melted 3 {{km}^{3}} of ice in two weeks. The meltwater flowed under the ice of the glacier into a lake situated within a nearby volcanic depression, the Grimsvotn caldera. A deep depression, 8 km long and a few hundred metres wide was formed in the ice surface at the eruption site. The rate of melting was extremely high, 0.5 {{km}^{3}} per day for the first four days. At the same time a new mountain, 0.6 km long and 150–300 metres high, was built under the ice by the eruption.
The meltwater was held in Grimsvotn caldera under the glacier for five weeks before it escaped. A gigantic wave swept across part of the south-east lowlands, destroying everything (roads, bridges, etc.) in its path.

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