Geologically speaking, the Vestmannaeyjar are quite young. Volcanic activity in the area probably started about 100-200,000 years ago. However, it is believed that the islands of the archipelago are considerably younger than this, from the last 10-20,000 years.
The islands were all created by the volcanic activity originating on the seabed. Nonetheless, Heimaey, the main island, is probably the only island that has been formed by the combination of several eruptions.
The volcanic belt on which the islands lie covers 700-900 square kilometres, and there are about 80 known craters in this area, of which only 17 or 18 are above sea level. Most of these volcanoes have erupted in recent times (geologically-speaking), that is to say during the last 10-12 thousand years.
Observations indicate that that at a depth of 10-30 kilometres below this volcanic area, there is a magma chamber of molten lava which supplies the central volcanoes with raw materials. As mentioned above, Heimaey was formed from several eruptions that took place during the past few thousand years.
The oldest parts of the island are the so-called "Norðurklettar" (Northern cliffs), that is Dalfjall, Klif and Há, and on the other side of the island, the cliffs Heimaklettur, Miðklettur and Ystiklettur. These cliffs are thought to have been formed by an eruption during the last ice age, at least 10 to 12 thousand years ago.
This eruption led to the formation of two separate islands which the sea then eroded and shaped for the next 4,000 years. About 6000 years ago, another eruption created a third island, Stórhöfði, which is where the lighthouse and weather station are today, at the south end of Heimaey.
The two large outer islands to the east of Heimaey (Elliðaey and Bjarnaey) were probably formed at about the same time. After this activity, there were a few hundred years of peace, as can be seen in the layer of plant fossils and other remains that lie under a younger layer of soil on top of Stórhöfði.
Radio carbon dating shows that these fossils date back about 5,400 years.
Then about 5000 years ago an explosive eruption started at Stakkabótagígur, a crater on the north-east side of Stórhöfði. This formed the largest crater in the area which is one kilometre in diameter. During this eruption, the island of Stórhöfði increased in size, and the volcanic craters on Heimaey itself were formed shortly thereafter, when Helgafell, the mountain behind the present township erupted.
At this time a lava shield was formed from thin layers of scoria lava (Hawaiian: aa), and a regularly shaped scoria-crater developed on top of the lava, about 227 metres above sea level. The lava flow from Helgafell connected Sæfell, Dalfjall and Klif.
It also closed the seaway south of the Norðurklettar and provided the conditions for Eiðið, the natural breakwater between Klif and Heimaklettur, to be formed by erosion. Eiðið "grew" in the shape of a gravel isthmus running from Kliff to Heimaklettur, gradually connecting the two islands. The youngest and the most obvious addition to the present island was formed during the Heimaey eruption that began on the night of January 23rd, 1973.
It started when a 1600 metre long fissure opened on the east side of town, spewing forth more than 30 million tons of ash and lava within the first 12 hours.
The eruption lasted a little less than 5 months and the total amount of volcanic materials that accumulated were 0,25 km², as compared to 1 km² during the earlier Surtsey eruption. As the fissure closed, the eruption became concentrated on one spot and a new scoria-crater similar to that of Helgafell formed.
This new mountain has been named Eldfell, Fire Mountain. During the Eldfell eruption, Heimaey increased in size by about 2,2 km². The lava field covers a total area of about 3,3 km².