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A major eruption took place in 1973, when a volcano on Heima Island (Heimaey) spilled lava into the town of Vestmannaeyjar, an important fishing centre. Most of the more than 5, 000 residents had to be evacuated, and—although the harbour remained intact—about one-third of the town was destroyed. Continuous eruptions took place in the Krafla area in the northeast in 1975–84, damaging a geothermal generating project in the area. Iceland’s best-known volcano, Hekla, erupted four times in the 20th century: in 1947, 1970, 1980, and 1991; it also had a series of small eruptions in 2000. There were eruptions in the Vatnajökull area in 1983 and 1996.
Eyjafjallajökull volcano, beneath an extension of Mýrdalsjökull (Mýrdals Glacier), erupted in March 2010 for the first time since 1821. It erupted again beginning on April 14 and sent wandering ash plumes into the skies that disrupted air traffic for days across northern and central Europe. Relief Iceland is largely a tableland broken up by structural faults. Its average elevation is 1, 640 feet (500 metres) above sea level, but one-fourth of the country lies below 650 feet (198 metres). The highest point is 6, 952 feet (2, 119 metres), at Hvannadals Peak, the top of Öræfajökull in Vatnajökull.
The glaciers range in size from those in small mountain recesses to the enormous glacial caps topping extensive mountain ranges. Vatnajökull covers an area of more than 3, 000 square miles (8, 000 square km) and is about 3, 000 feet (900 metres) deep at its thickest point. Much of Iceland is underlain by basalt, a dark rock of igneous origin. The oldest rocks were formed about 16 million years ago. The landscape in basaltic areas is one of plateau and fjord, characterized by successive layers of lava visible one above the other on the valley sides. The basalt sheets tend to tilt somewhat toward the centre of the country.