Iceland consists mainly of a central volcanic plateau, with elevations from about 700 to 800 m (2,297–2,625 ft), ringed by mountains, the highest of which is Hvannadalshnúkur (2,119 m/ 6,952 ft), in the Öræfajökull glacier. Lava fields cover almost 11% of the country, and glaciers almost 12%. Among the many active volcanoes there is an average of about one eruption every five years. The largest glacier in Europe, Vatnajökull (about 8,400 sq km/3,200 sq mi), is in southeast Iceland. There are also many lakes, snowfields, hot springs, and geysers (the word "geyser" itself is of Icelandic origin). The longest river is the Thjórsá (about 230 km/143 mi) in southern Iceland. Most rivers are short and none are navigable, but because of swift currents and waterfalls, Iceland's rivers have important waterpower potential. There are strips of low arable land along the southwest coast and in the valleys. Good natural harbors are provided by fjords on the north, east, and west coasts.