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 The islands of the Atlantic
Ocean created by the volcanism of the Middle Atlantic Ridge are
The Azores, Bermuda,
Madeira, The
Canary Islands,
Ascension,
St. Helena,
Tristan da Cunha, and others. The
divergence of the ridge started in the north about 150 million years ago
and 90 million years ago in the south.
Iceland is the largest island on the ridge because of the
additional volcanism caused by the hot spot under the country, which
moves slowly towards the northwest across it.
These unique
circumstances on the ridge probably are the reason for the country’s
existence, because the tectonic movement of the plates and the related
volcanism would not have sufficed.
The country probably would have looked very differently if the
hot spot were the sole reason for its creation, probably something
similar to Hawaii. The
plate tectonics move the plates apart, towards east and west, and both
the American- and Eurasian systems move to the northwest across the hot
spot.
It is possible to trace the tracks of such hot spots because
the plates move across them like steel plates across a fixed blowtorch
flame. On top of the hot spot is a 20-100% molten layer at a depth of
5-20 km, which supplies more than sufficient material for eruptions. This hot spot causes eruptions within the southern volcanic
zone including volcanic systems such as
Mt.
Hekla, the
Westman
Islands,
Katla,
Mt. Eyjafjallajokull, Mt. Tindfjoll, the
Laki
Area, the
Fishing
Lake Area, and the western sub-glacial
part of the
Vatnajokull
area.
This
volcanic zone has gradually moved to the southeast from the present
Graben, or the continuation of the ridge across the country.
The
lateral rift system across the Snaefell Peninsula and the Huna Bay
ceased being active about seven million years ago.
About 60 million years ago, when it was active, it was situated
near the present Faeroe Islands and Eastern Greenland where it left
basaltic regions and submarine ridges in both directions.
 The present centre of the hot spot lies under the Trolladyngja
shield volcano, where it reaches depths of 275-375 km and probably
stretches underneath the mainstay of the Vatnajokull and Hofsjokull
regions. The largest volcano and caldera of the country are probably
contained under the ice cap of the Hofsjokull glacier.
Magma reservoirs with 100% molten lava and areas of 100-1000 km³
are present at a depth of 10 km under the volcanic zones (the Graben),
and they feed the shallower and smaller magma chambers under the central
volcano systems. The shape
of those magma reservoirs is rather obvious on the surface, such as
under the Krafla system, where it is situated at a depth of 3 km and
above it the magma chamber at a depth of about 700 m.
Mt. Hekla does not have any magma chamber, but the magma
reservoir under the volcanic system is estimated to be at a depth of 8
km and about 40 km long. Chambers
usually are 10% of the size of reservoirs.
When the Krafla system erupts, the magma chamber feeds it, but
eruptions of the Gjastykki system, further north, are fed by the deeper
reservoirs.
Approximately
10% of the structure of the country consist of rhyolite, dasite and
other acid rock formations. The
most common type of basaltic rock is thoelite, which is divided into
many subtypes. The Ice Age
ended about 9000 years ago. The
3 million years long Ice Age was divided into 30 glaciation epochs of
almost 100.000 years, and about 10.000 years long warmer epochs between
them.
The
age of the basaltic strata from west to east is 16 – 10 million years.
Eight central volcano systems are recognizable by light coloured
rock (gabbro) and high temperature systems.
The structure of the island Hrappsey on the Breidafiord Bay is
anorthosite, the light coloured type of rock on the moon (the other type
on the moon is norite).
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