The geology of Jan Mayen
The Jan Mayen shelf was formed during the opening of the Atlantic Ocean about 40 million years ago, in the Eocene epoch, and is a remnant of continental crust. However, the oldest rocks which extend above sea level are only around half a million years old. Jan Mayen as an island has, thus, hardly experienced more than the last three Quaternary ice ages.
The island rises from a subsea volcanic mountain chain called the Jan Mayen Ridge, which forms the boundary of the 1500-metre-deep Iceland Plateau in the northeast. North of Jan Mayen, the sea floor slopes fairly steeply down to a depth of more than 2000 metres in the Jan Mayen Rift Zone.
This rift zone is an active transform fault zone, in which the mid-ocean ridge between Europe and America is offset by approximately 200 km. This displacement is still taking place at an average rate of a couple of centimetres a year.
A small segment of the spreading zone probably underlies the northeastern tip of Jan Mayen. The volcanism and earthquake activity on and around the island are due to this location.
The landscape on Jan Mayen is influenced by the volcanic origin of the island. The northernmost active volcano in the world which rises above sea level is located on the northern part of the island. Beerenberg is a majestic volcano, towering to a height of 2277 metres, and having regular sides which slope at about 30° around the Central Crater and continue downwards to the main massif at around 15°. This break of slope is a result of a young stratovolcano, built of an alternation of lava and tephra, having formed on an older shield volcano, mainly composed of lava. Numerous parasite craters built of tuff, slag and tephra pockmark the lower slopes.
Beerenberg has more than twenty glaciers, which cover some 30 % of the area of the island. Some of their snouts reach right down to sea level.
The bedrock on the island consists of basalt lava flows alternating with pyroclastic rocks. The latter are rocks of volcanic origin flung out during eruptions and more or less consolidated in the air. The youngest volcanic eruptions in 1970 and 1985 took place on the lower slopes on the northeast side of Beerenberg. The Central Crater and Egg Island, a crater remnant on the south side of the mountain, have permanent fumarole activity (emission of hot steam). There is thought to be an average of one volcanic eruption on Jan Mayen every 100-130 years.
The volcanic eruption in 1970
The largest known volcanic eruption occurred in September-October 1970, when a plume of smoke up to 11 km high was observed from a passing passenger plane.
The eruption took place from several craters along a 5 km-long zone of fissures close to the northeast tip of Jan Mayen. Lava production lasted more than 4 weeks, and about 0.5 km3 of lava was produced. Over 4 km2 of new land was formed, spread out on the former seabed for up to 300 metres. However, more than half of the new land surface was completely eroded by the sea in the matter of a few years.
A more detailed account of the geology of Jan Mayen, including all the known volcanic eruptions, can be found on pages 29-45 in Meddelelser no. 144: Natur- og kulturmiljøet på Jan Mayen: med en vurdering av verneverdier, kunnskapsbehov og forvaltning.