Icelandic Volancism

Geology of Iceland

Diverging plates and hot-spot

Plate tectonics, seafloor-spreading, hot-spot and volcanism are some keywords regarding the geological processes in Iceland The North-Atlantic Ocean began opening about 60 million years ago. During that period the North American plate and the Eurasian plate has moved east and westwards, approximately by 1 cm per year in each direction. This process is still lasting today. At the plate boundaries magma has welled up and constantly created new basaltic sea floor.

But this flow of magma at the Mid-Atlantic-Ridge does not build up islands in the several thousands of meters deep ocean. The existence of Iceland points to another important fact: A so called hot-spot, the Iceland plume, is located near the Mid-Atlantic-Ridge in the North Atlantic and it is believed to have caused the formation of Iceland itself. A hot-spot (also “mantle plume”) is an upwelling of abnormally hot rock within the Earth’s mantle.

Because of the plates, drifting east and westwards, it is plausible, that Iceland has a larger spreading to these directions than to the North-South. So we can infer that the east and the west are Iceland’s oldest parts, where we find the oldest rocks. In the middle of the island the the country is a geologically young land and extremely active, having many volcanoes. And of course, there are also many solfataras, hot springs and geysers in Iceland, including Geysir, from which the English name is derived.

 

Volcanism

 

 

 

 

Effusive eruption

Iceland itself has an area of 103,100 km² and rises from a large submarine platform to an altitude of more than 2,100 meters. Most parts of the island consist of numerous overlapping lava flows. The country has a very high concentration of active volcanoes and that’s why volcanism is of  great importance in regard of Iceland’s geology. Over a period of the past 500 years, volcanoes in Iceland have erupted a third of the total global lava output!

If we compare the lava texture, there are three different types of volcanic eruptions:

•             effusive (mostly lava with low viscosity)

•             explosive (tephra only)

•             or mixed (both lava and tephra)

The tendency to explosive activity increases as magma becomes more acid. Water can enhance explosive volcanism, too. In general we can differ volcanoes into the following two caterories:

Volcanic fissures

These are more or less long rows of craters like maars (Example: Valagjá), tephra rings (Example: Vatnaöldur), but also spatter cones and scoria craters. One of the best known fissures in Iceland is Eldgjá, a volcanic canyon nearby Laki craters in South Iceland. Both Eldgjá and Laki are part of the same volcanic system as Katla under Mýrdalsjökull glacier in the south of the country

The lava of these fissure eruptions has a temperature of about 1,200 °C. The lava is very liquid in this case and the lava flows can spread out on a large area and long distances.

Central volcanoes

This kind of volcanoes have the typical profile with one crater at the top of the mountain. One example for a central volcano is a lava shields. They are relatively flat lava domes with a central crater (e.g. “Ok” in West Iceland). Shield volcanoes are formed by low-viscosity lavas. Other types are a stratovolcanoes (e.g. Hekla) and calderas (e.g. Askja). They are located above a magma chamber and erupt several times, so that the volcanoes build up a high mountain massifs. A well known type of  volcanoes in Iceland are table-mountains (e.g. Bláfell, Herðubreið), which were created during the last ice age.

In Iceland about 30 volcanic systems with hundreds of volcanoes are known active at the present. The most commonly found types in Iceland are composite volcanoes (stratovolcanoes) which are usually part of a chain and spread in vast areas.

As mentioned above, the age of Iceland’s rocks and lava flows depend on their position on the island. Because of the sea-floor spreading, in the Northwest and in the East of Iceland rocks are older than along the Mid-Atlantic ridge which is situated in the middle. The following map gives you an idea of Iceland’s profile.

 

The age of rock formations in Iceland

 

Here you will find a list of volcanic eruptions in Iceland in recorded history. http://www.eldey.de/English/geology/geology.html

 

 

 

GEOLOGY ICELAND

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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When oxygen was first produced as a waste product by certain plant cells some three billion years ago, it created a crisis for all other life on earth. Those plants were polluting the environment, exhaling a lethal gas. Earth eventually had an atmosphere incompatible with life. Nevertheless, life on earth took care of itself. In the thinking of the human being a hundred years is a long time. Hundred years ago we didn’t have cars, airplanes, computers or vaccines. It was a whole different world, but to the earth, a hundred years is nothing. A million years is nothing. This planet lives and breathes on a much vaster scale. We can’t imagine its slow and powerful rhythms, and we haven’t got the humility to try. We’ve been residents here for the blink of an eye. If we’re gone tomorrow, the earth will not miss us.

Michael Crichton, “Jurassic Park”

http://www.nat.is/travelguideeng/geology_iceland.htm

http://geology.com/world/iceland-satellite-image.shtml