The Westman Islands’ volcanic system comprises 70-80 volcanoes and their remainders above and below sea level. It is about 38 km (23 miles) long and 30 km (18 miles) wide. Between 20 and 30 of those volcanoes were active during Holocene, and the others during the latter part of Pleistocene. The volcanic activity of this area probably started during the last 100-200 thousand years and eruptions during that period have been relatively frequent. Some of them left submarine craters and crater rows behind, others islands. During historical times the volcanic activity has been relatively infrequent, but unrecorded submarine eruptions most likely took place without people even noticing them. One such eruption is mentioned in the proximity of Island Hellisey in a paper in Reykjavik in 1896 and another in annals from 1637. The Surtsey Island and Heimaey Island eruptions last century might confirm the constant activity of the area.
This volcanic system follows the same direction as most others of the country, i.e. southwest-northeast. A central volcano probably was created during the eruption, which left most of the Heimaey Island behind. Earthquakes and tremors indicate a magma chamber at the depth of 10-30 km. Future developments of the system might lead to a land bridge connecting the archipelago to the mainland.
Mt Heimaklettur and a few other spots on the northern part of Island Heimaey can be traced to the last part of the Ice Age. Their structure is mainly hyaloclastites (Moberg), but there are traces of basaltic rock as well.
Island Alsey and probably Island Hellisey might be about 8,000 years old.
The headland Storhofdi on Island Heimaey was probably created about 6,000 years ago and the Islands Ellidaey and Bjarnarey are almost equally old.
Island Heimaey was mostly created 5,200 years ago. At that time a large crater started very explosively on the sea floor. A few hills (Saefell) and the Holy Mountain (Helgafell) were created.
Island Surtsey was created during the longest historical eruption in Iceland 1963-1967.
A 2-3 km long eruptive fissure, partly submarine opened up across the eastern part of Island Heimaey at two o’clock in the morning of the 23rd of January 1973. The last traces of the eruption were noticed on the 26th of July the same year.
ISLAND SURTSEY. The eruption, which created this and other islands around it during a period of a little more than three and a half years, is considered to be among the longest during historic times. The eruption was first noticed early in the morning of November 14 1963 some 18 km southwest of the Heimaey Island. It probably started a few days earlier on the ocean floor, some 130 metres below the surface. It was very effusive during the time, when seawater had access to the craters, and on The 15th of November, the island started developing. At the end of January 1964, the island was 174 metres high.
Next February another island, Surtur, the younger, started developing. This eruption ceased next April and the island disappeared shortly afterwards. Lava started flowing from the western crater of Surtsey on the 4th of April 1964. Its main flows ran towards south and east and the lava shield by the crater grew 100 metres thick. This lava effusion stopped on the 17th of May in 1965. At that time the island was 2,4 km² in area. At the end of May 1965, evidence of another submarine eruption east north east of Surtsey was seen on the surface. On the 28th, the island Syrtlingur started developing and the eruption stopped in October the same year. This island had disappeared on the 24th of October.
The island Jolnir developed 0,9 km southwest of Surtsey during Christmas 1965. This eruption lasted until the 10th of August 1966 and the island had disappeared at the end of October the same year. Lava started flowing again from the eastern craters of Surtsey on the 19th of August 1966. The flows ran towards southeast and east until the beginning of June 1967, when the Surtsey-eruption officially came to an end. During the period between December 1966 and January 1967, five of the eastern craters emitted mostly ash and very little lava.
THE SURTSEY ERUPTION had lasted a little more than three years and a half, when it stopped and the island had grown to 2,7 km² in area. The total volume of tephra was estimated to be 1,1 cubic kilometres, thereof 60-70% ashes and 30-40% lava. The Westman Island archipelago consists mostly of islands created the same way. Erosion has demanded its toll of the Surtsey Island and eventually only one cliff is going to remain in the ocean. The island was immediately declared inviolate for the scientists to explore and up to this date, it is extremely difficult to get a permission to land there. It is, however, possible to sail around the island and watch it from the boat.
THE HEIMAEY ERUPTION. This eruption started at 02:00 o’clock on the 23rd of January 1973 and lasted for 155 days. The latest signs of it were noticed in the crater Eldfell (Fire Mountain) on the 26th of July. The volcanic fissure on dry land was originally 1½ km (1 mile) long and situated about 300 m (1,000 feet) to the east of the houses in town. The winds were blowing from the west during the first night and day, which made it both easier and safer to evacuate the population, as the ashes were carried away. On the 25th and 27th of January, easterly winds carried the ash plume over the town and many houses were totally buried and others caught fire. Gradually the activity of the fissure was concentrated within a 200 m area, where the crater “The Fire Mountain” was created and reached the height of 220 m.
The only pre-warnings of a pending eruption were two waves of earthquakes and no one connected them with what was to come. During the first days the northern part of the fissure, closest to town, was most active and the lava flow reached about 100 m³/sec. The plume reached the height of 9 km and was clearly visible from the capital area and elsewhere. The lava flow threatened to close the entrance of the harbour and on the 6th of February powerful pumps started pumping seawater on the lava’s edge. This had never been tried before and it probably saved quite a few houses and the harbour entrance. The maximum output of the pumps reached 2,000 l/sec.
The majority of the population of Island Heimaey was evacuated during the first night and day and in the wake of that all kinds of relief work started. The Icelandic and foreign governments and individuals spent and donated billions of Icelandic kronur for that purpose. The damage was, however, much greater than all that money could reimburse. At the beginning of the eruption about 5,300 people lived on the island and on December 1st 2007 their number was about 4.070.