During the last Ice Age, very nearly all of Iceland was buried under an ice sheet more than a mile thick. There were outcroppings of rock here and there where volcanoes managed to melt their way through to the surface, producing distinctive rounded shapes called tuyas, but in general it would have been a pretty boring place.
Herðubreið – a tuya formed by a volcanic eruption in the middle of a mile-thick ice sheet. Image credit Thrainn Vigfusson.
The most recent Ice Age ended about 10,000 years ago. The ice retreated to the north, carving vast and beautiful valleys on its journey. Eventually, the glaciers disappeared almost entirely, clinging only to the highest spots on Iceland’s landscape.
The departure of the glaciers took a lot of pressure off of Iceland, in a very literal sense. A cubic foot of ice weighs just under 53 pounds; a mile-thick ice sheet exerts a pressure of over 150 tons per square foot. The ice sheet slowly crushed Iceland under trillions of tons of pressure, compacting the soil and rock beneath it. It takes a long time to compress rock, even at those kinds of pressures. But the ice sheet had tens of thousands of years to do the job.
When the glaciers finally retreated, the land began to expand in a process called isostatic rebound. The pressure in the rocks gradually eased, and as it did, pressure on fluids contained in cracks and hollow spaces in the rocks eased, too. For Iceland, the most important subterranean fluid is magma; and as the pressure in the magma subsided, a number of things happened.
Generally speaking, a rock that’s being compressed has a higher melting temperature than it would without the compression. When the pressure dropped because of the departure of the ice, rocks that surrounded the magma but had remained solid because of the pressure began to melt. Gases that had been trapped in the magma began to expand — sort of like what happens when you crack open a can of soda. Both the gases and the newly-molten rock worked their ways into cracks and crevices that had been squeezed together by the trillions of tons of ice, but now had more room to expand. They forced the cracks open wider, eventually spraying out of the ground in fountains of molten rock hundreds and sometimes thousands of feet high.
Holuhraun lava fountain, September 2014. Image credit Guidetoiceland.com.
The process didn’t happen overnight, but starting with the end of the Ice Age, lava poured out of the rift zone that slashes across Iceland from the southwest to the northeast, and from the mantle plume in the southeast part of the country. The end of the Ice Age ushered in Iceland’s golden age of the volcano. No matter where you look when you first get to Iceland, the landscape is probably no more than 10,000 years old — in some cases a lot less. The newest landscape in Iceland is about 33 square miles of lava erupted in the southeast between the end of August 2014 and the end of February 2015.
Map of Iceland showing rock ages. The light blue areas are oldest; purple are newest. The island is being slowly ripped in two and the gash filled in by lava. Image credit Icelandic Institute of Natural History.
Volcanic eruptions have not only shaped Iceland, but the people who live here. One volcano in south-central Iceland, Mount Hekla, has erupted so frequently, and with such force and lethal effect, that it earned the nickname “the Gateway to Hell.”
In 1783, a volcanic eruption north of the town of Kirkjubæjarklaustur spewed not only lava but toxic gases that killed half the livestock on the island and produced mass starvation that wiped out about a third of the people. Other, less toxic gases from eruption (known in Iceland as the Skaftáreldur (Skaftár fires) and sometimes referred to as the Laki eruption) reached the upper atmosphere and produced cooler weather throughout the Northern hemisphere, leading to crop failures in Europe and beyond. Some historians believe that the eruption was a contributing factor to the famines that helped trigger the French Revolution.
Volcanoes are probably the key defining part of Iceland’s geology, and one of the keys to its future — something I was reminded of in a field trip to a place called Bitra. I’ll write more about that next time.
Cover image credit Iurie Belegurschi.