The tephra produced by its eruptions is high in fluorine, which is poisonous to animals. This fissure opens along its entire length during major eruptions and is fed by a magma reservoir estimated to have a top 4 km below the surface with centroid 2.5 km lower. Hekla is situated on a long volcanic ridge of which the 5.5 km Heklugjá fissure is considered Hekla proper. The unusual form of Hekla is found on very few volcanoes around the world, notably Callaqui in Chile. Hekla has a morphological type between that of a fissure vent and stratovolcano (built from mixed lava and tephra eruptions) sited at a rift- transform junction in the area where the south Iceland seismic zone and eastern volcanic zone meet. Cumulatively, the volcano has produced one of the largest volumes of lava of any in the world in the last millennium, around 8 km 3. Approximately 10% of the tephra created in Iceland in the last thousand years has come from Hekla, amounting to 5 km 3.
The volcano's frequent large eruptions have covered much of Iceland with tephra, and these layers can be used to date eruptions of Iceland's other volcanoes. Hekla looks rather like an overturned boat, with its keel being a series of craters, two of which are generally the most active. The most active part of this ridge, a fissure about 5.5 km (3.4 mi) long named Heklugjá, is considered to be within Hekla proper. Hekla is part of a volcanic ridge, 40 km (25 mi) long. During the Middle Ages, Europeans called the volcano the "Gateway to Hell". Hekla is one of Iceland's most active volcanoes over 20 eruptions have occurred in and around the volcano since 874. Hekla ( Icelandic pronunciation: ( listen)), or Hecla, is a stratovolcano in the south of Iceland with a height of 1,491 m (4,892 ft). Eggert Ólafsson, Bjarni Pálsson, 20 June 1750
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