Eyjafjallajökull consists of a volcano completely covered by an ice cap. The ice cap covers an area of about 100 square kilometers, feeding many outlet glaciers. The main outlet glaciers are to the north: Gígjökull, flowing into Lónið, and Steinholtsjökull, flowing into Steinholtslón. In 1967, there was a massive landslide on the Steinholtsjökull glacial tongue. On 16 January 1967 there was an explosion on the glacier. It can be timed because the seismometers at Kirkjubæjarklaustur monitored the movement. When about 15.000.000 cubic meters of material hit the glacier a massive amount of air, ice, and water began to move out from under the glacier into the lagoon at the foot of the glacier.

The mountain itself, a stratovolcano, stands 1.651 meters at its highest point, and has a crater 3 to 4 kilometers in diameter, open to the north. The crater rim has three main peaks (clockwise from the north-east): Guðnasteinn, 1.500 meters; Hámundur, 1.651 meters; and Goðasteinn, 1.497 meters. The south face of the mountain was once part of Iceland’s coastline, from which, over thousands of years, the sea has retreated some 5 kilometers. The former coastline now consists of sheer cliffs with many waterfalls, of which the best known is Skógafoss. In strong winds, the water of the smaller falls can even be blown up the mountain. The area between the mountain and the present coast is a relatively flat strand, 2–5 kilometers wide, called Eyjafjöll.

The Eyjafjallajökull volcano last erupted on 14 April 2010 in Iceland. It left behind vast ash clouds so large that in some areas daylight was entirely obscured. The cloud not only darkened the sky but also interfered with hundreds of plane flights. However, the people of Iceland were not concerned about the ash clouds: they were more concerned about flooding. That year all the residents close to the volcano had to evacuate in case the area flooded. When the volcano erupted, all the melted ice had to go somewhere. It did not flood that much: most of it went into rivers, but if it had flooded down the farm valleys it could have swept away all the farms in the valley. The farms in the valley were however covered in a soft layer of ash, which the farmers thought would give bad crops, but the warmth and nutrition from the ash enabled the crops to grow rather well.