Why are some igneous stones floating?


30 kilometers south of Iceland, an island was born out of the sea. A young volcano has been spewing fire and ashes here since November 14th. Its lava masses have already given rise to an island 40 meters high and 500 meters long.

White-gray ash clouds hang in the sky and darken it. Fine volcanic rock patters the area, every lava discharge is accompanied by the rumble of thunder. The smoke column caused by the volcanic eruption rises 10 kilometers. And an island off Iceland's south coast continues to grow.

The eruption of the underwater volcano came unexpectedly, but not without harbingers. Seismologists had already measured smaller earthquakes in the capital Reykjavik a week earlier - signs that a lot is happening at the plate boundary of the Mid-Atlantic Ridge. In addition, a research vessel had found that the sea was warmer than usual. And residents of the nearby coastal region believed they smelled hydrogen sulfide. When the volcano erupted on the seabed at a depth of 130 meters, it initially went unnoticed. Its explosions were weakened by the water pressure. But as it grew, it approached sea level and finally broke through it, spitting wildly. That was the birth of an island in Iceland.

The new island off the south coast already has a name: it is called "Surtsey" after Surt, the giant of fire. A Nordic legend tells of him that he hurls fire and destroys all life with his glowing sword.

How Iceland came into being

Iceland is actually nothing more than the climax of a huge mountain range in the Atlantic: The mid-Atlantic ridge, which stretches from north to south through the entire Atlantic, is almost 20,000 kilometers long. At the height of Iceland, the North American and Eurasian plates drift apart, by about two centimeters every year. Where they spread, hot magma penetrates from the interior of the earth to the surface. These volcanic eruptions have been piling up mountains underwater for millions of years and caused Iceland to appear above sea level 17 to 20 million years ago. These volcanoes are still active today. And now they have born another island: Surtsey.

Volcanic mountains

The snow-covered Kilimanjaro can be admired from a great distance. As a single mountain range, it protrudes far beyond the African steppe - very different from the typical folded mountains such as the Alps or the Himalayas. Because Kilimanjaro was not created when tectonic plates collided. About one and a half million years ago, several volcanoes erupted in close proximity in the region. The mighty Kilimanjaro massif rose up into the sky from the lava masses. Today its highest peak is the Kibo at 5895 meters above sea level.

There are also volcanic mountains in Germany, for example the Siebengebirge near Bonn. Several volcanoes became active here 25 million years ago and hurled their hot rock masses to the surface of the earth. Today they are long gone, but as mountains and heights of the low mountain range they are still clearly recognizable.

Incidentally, the longest mountain range on earth is of volcanic origin - it is sunk in the sea: the mid-Atlantic ridge extends over a full 20,000 kilometers through the middle of the Atlantic Ocean. The mid-Atlantic ridge is one of the mid-ocean ridges. These lie where two oceanic plates move away from each other. A gap forms on the sea floor between the two plates, from which hot magma gushes. Long and high mountain ranges form under water at such plate boundaries. In some places their peaks protrude above sea level. This is where Iceland, the Galapagos Islands and the Azores see the light of day.

What happens in the event of a volcanic eruption?

It steams and bubbles, it smokes and hisses. Glowing hot rock shoots up from inside the earth. A cloud of ash rises, lava gushes out of the volcano and flows over the surface of the earth. When a volcanic eruption occurs, enormous forces are at work. But how does a volcano actually erupt?

In the earth's mantle, the rock layer under the earth's crust, temperatures of over a thousand degrees Celsius and very high pressure prevail. If the heat and pressure are high enough, the rock melts and becomes a viscous mass called magma. This magma expands and rises to the top. There it first collects in cavities, the magma chambers. But none of this happens overnight, but takes tens of thousands or hundreds of thousands of years.

When the magma chamber is full and cannot hold any more material, the hot magma makes its way out. It penetrates through channels and crevices to the surface and emerges there as glowing hot lava - the volcano erupts. The channel through which the magma swells up is called a chimney and its exit is called a crater.

Some volcanoes regularly spit lava, for example the Stromboli in southern Italy. One can observe its eruptions every day. Other volcanoes remain quiet for centuries but are not actually extinct. Often their craters are clogged with lava and debris. That makes them very dangerous because if they break out there can be huge explosions; well-known for this are, for example, Vesuvius near Naples or Krakatau in Indonesia. Such explosive eruptions blow up millions of tons of rock. The ash cloud that rises from the eruption can stay in the air for a long time and be widely dispersed by the wind. This cloud then only slowly settles on the earth as a fine layer of ash.

Lava that is not thrown into the air flows down from the crater rim as a scorching stream of molten rock. When this lava flow cools, it solidifies into lava rock. Little by little, lava flows, ash and debris build a mountain around the crater - the volcanic cone.

Consequences of volcanic eruptions

Volcanic eruptions can have dire consequences. Hail of rock, ash rain, poisonous gases and glowing lava flows have already killed hundreds of thousands of people. During the eruption of Vesuvius in 79 AD alone, during which the cities of Pompeii and Herculaneum were buried, around 5000 people died. An entire city was also wiped out in Colombia: the eruption of the icy Nevado del Ruiz volcano in 1985 triggered several mudslides. The avalanches buried the city of Armero, 47 kilometers away, and 25,000 residents.

Tsunamis can also arise from volcanic eruptions: The explosion on the volcanic island of Krakatau in 1883 caused a tidal wave that flooded regions thousands of kilometers away. Even earthquakes sometimes follow such an explosive volcanic eruption. During these quakes, built-up tensions in the earth are discharged.

In Iceland, the eruption of over a hundred volcanoes in the Laki Fissure in 1783 triggered a famine. The outbreak released toxic gases into the air. The poison settled and contaminated the sheep pastures. The animals died from the poisoned food, an estimated ten thousand people because of the famine that followed.

The “laki fires” on Iceland were followed by a cooling that could still be felt far away. The rising ash cloud darkened the sky, strong winds came up and the temperature dropped. The whole of Northern Europe then experienced an unusually cold winter. In fact, volcanic eruptions change the climate. This is mainly due to the sulfur gases emitted, which form fine sulfuric acid droplets in the air that float in the atmosphere for a long time. The sunlight is scattered by the droplets and partly reflected back. This can lower the average temperature all over the world.

Where on earth are there volcanoes?

There are not volcanoes everywhere on earth, they are very unevenly distributed. Most of them lie along the plate boundaries - where tectonic plates rub against each other, where one plate dips under the other or where they drift apart. At these fractures, hot magma can swell from the earth's interior to the surface.

A particularly large number of active volcanoes can be found around the Pacific Ocean, for example Mount St. Helens in the USA, the Popocatepetl in Mexico and the Bezymianny in Russia. They are all part of a 40,000 kilometer chain of volcanoes, the Pacific Ring of Fire. Because all around the Pacific, the Pacific plate is pushed under other plates. When the Pacific plate descends, the earth's crust is melted. Magma collects in these places and volcanoes form above them.

Volcanoes are not just above, but also below sea level - and most of them are still completely unknown to us. These underwater volcanoes are called “seamounts”. They include the volcanoes of the Mid-Atlantic Ridge, a huge underwater mountain range in the Atlantic. There plates drift apart and therefore magma rises constantly to the top. Sometimes the volcanoes also reach the surface of the sea: In 1963 a new volcanic island - Surtsey - grew out of the sea within a few months to the south of Iceland. Iceland itself was also formed by volcanism on the Mid-Atlantic Ridge.

The situation is completely different with the volcanoes in Hawaii: They are located far away from plate boundaries, in the middle of the Pacific plate. But below Hawaii the earth's mantle is particularly hot; this is called a “hotspot”, a hot spot in the earth's mantle. Hot magma rises here and can easily break through the crust - this creates a volcano. Whenever a plate in the earth's crust slides over a fixed hotspot, a new volcano continues to dig its way through the crust. This creates a whole chain of volcanoes, such as the chain of islands in Hawaii. The Kilauea volcano is currently active there because it is currently above the hotspot.

When the earth shakes

The earth trembles, cracks gape in the ground, trees sway and houses collapse - earthquakes are forces of nature with destructive power. When the earth shakes, entire districts can collapse. The earth shakes particularly often in certain areas, namely where the plates of the earth's crust adjoin one another. This is the case, for example, in Japan, on the west coast of the USA or in the Mediterranean region.

The cause of earthquakes is the movement of the plates. These float on the viscous material of the earth's mantle, whose currents propel them like a motor. Where two plates adjoin each other, their rock masses can get stuck and come to a standstill. The problem is: the current in the interior of the earth drives them on. This creates enormous tension between the two plates. If the tension becomes too great at some point, one of the plates jerks forward. The tension discharges: the earth shakes.

Earthquakes often happen where two plates slide past each other at different speeds, such as on the coast of California. This also does not go smoothly where plates collide. For example, the African drifts towards the Eurasian plate and dives under it. Because this plate boundary runs in the Mediterranean, the earth keeps shaking in Italy and Turkey. There are also tremors where the earth's crust is being pulled apart, for example in the Upper Rhine Rift. Although these were less strong in the past centuries, there have already been violent tremors here too: In 1356, a strong earthquake caused great damage in the city of Basel.

The movement of the plates is not "to blame" for an earthquake every time. Collapses can also shake the area. This happens when natural or man-made cavities collapse. Such quakes do not extend as far and are not as strong as quakes caused by the movement of the earth's plates.

The exact point from which an earthquake emanates is the focus of the earthquake, also known as the hypocenter. From here the earthquake waves spread in all directions - comparable to the waves after a stone has plopped into the water. The greater the distance from the focus of the earthquake, the weaker the earthquake waves that cause the earth to sway.

The epicenter on the surface of the earth lies directly above the focus or hypocenter. The destruction of an earthquake is usually greatest around this epicenter. How strong an earthquake is can be measured with special devices. Usually the strength is given with values ​​on the Richter scale, which is open at the top. The strongest earthquake recorded so far was that of Valdivia on May 22, 1960, also known as the Great Chile Earthquake. It reached a strength of 9.5 on the Richter scale.

Where plates diverge

A long, deep crack gapes in the earth and is getting wider and wider. Huge forces are tearing the earth's surface to pieces: the East African Rift runs along this break through the continent. Africa began to break up here 20 million years ago. Hot magma from the interior of the earth pushed upwards and tore the earth's crust apart. Since then, the pieces of crust have drifted apart, by about an inch every year. The fact that the earth is very active here can also be seen from the many volcanoes that rise along the rift. Should seawater ever penetrate, the East African Rift will become an ocean. Something similar happened in the Red Sea. The African and Asian continental plates have been separating there for 25 million years. The resulting crack was flooded by sea water.

There where continental Crust breaks apart, one arises Rift valley. Where against it oceanic When pieces of crust move away from each other, mountains grow on the sea floor: the Mid-ocean ridges. They consist of magma that seeps up from the Earth's mantle through the oceanic crust. New sheet material is formed here. It presses itself, so to speak, between two oceanic plates and solidifies to form basalt rock that piles up further and further.

In some places the mid-ocean ridges protrude as islands above sea level. Iceland, for example, and the still young Icelandic island of Surtsey are nothing more than parts of the Mid-Atlantic Ridge. The oceanic crust is constantly growing here due to the replenishment of solidified rock. It not only grows in height, but also to the sides. The two oceanic plates are pushed outwards. Because they spread apart in the process, one also speaks of one Divergence zone.

In this way, new seabed is created and the ocean is slowly getting wider - but only a few centimeters a year. But modern satellites can measure the continents with millimeter precision. From the movement one can calculate that the Atlantic has been 25 meters wider since Columbus' crossing in 1492.

But because the earth as a whole is not getting any bigger, the increase in the seabed has to be compensated for elsewhere. This happens where the oceanic crust is submerged under the continental crust: While the Atlantic continues to grow, the Pacific slowly sinks under the plate margins of America and East Asia.

The ocean floor

The surface of the ocean glistens in a dark blue. It is hard to believe that the sea floor is sometimes many kilometers deeper and that a spectacular underwater landscape is hidden there below. Because the sea floor is not as smooth as the bottom of a swimming pool: On the sea floor there are high mountains, deep trenches and lava-spewing volcanoes as well as extensive plains.

The water in the oceans is not the same depth everywhere. The shallow shelf seas lie around the continents. Here the seabed slopes gently down from the coastline until it reaches a depth of around 200 meters below sea level. The bottom of the shelf seas consists of continental crust. Therefore it actually belongs to the mainland, even if it is washed over by sea water.

Only many kilometers away from the coast, on average after 74 kilometers, does the flat shelf area end with the shelf edge. From this edge it goes down steeply like a slide to a depth of about four kilometers. This steep slope forms the transition to the deep sea, into which no light can penetrate. That's why no plants grow down there. Only a few animal species were able to adapt to this habitat, despite the hostile conditions.

In the midst of the oceans rise mountains, the mid-ocean ridges. These underwater mountains stretch across the world's oceans for long stretches. In some places they protrude as islands above sea level. Iceland, for example, lies directly on the mid-Atlantic ridge, the longest mountain range in the world.

Deep trenches also crisscross the oceans. Most of them are in the Pacific. One of them is the Mariana Trench, the deepest trench in the world. It reaches down to 11,034 meters below sea level. Only two people have ever been down there: the oceanographer Jacques Piccard and his companion Don Walsh on their record dive in 1960.

Igneous rocks

Biting granite means that something is hopeless. Because of its great hardness, granite can not only be used as a phrase, but also as a paving stone or for building walls. Granite is a rock that lies over two kilometers below the earth's surface and is common in the earth's crust.

Granite is formed when glowing magma solidifies when it cools. The dark spotted gabbro or the monzonite are also formed from slowly cooling magma. If this process takes place deep inside the earth, geologists speak of Deep rock, also Plutonite called.

If, on the other hand, the hot rock slurry penetrates outwards during a volcanic eruption and pours over the surface of the earth, it is from Effluent rock or Volcanite the speech. The volcanites include the light pumice stone, the porous tuff or the rhyolite, which was formed from the same material as granite, but has a different structure and is less hard because it cools faster on the surface of the earth than the granite in the depths. Basalt is also a volcanite. Sometimes it freezes into hexagonal, closely spaced columns that look as if they have been cast into shape. Basalt forms on the surface of the earth from the same mass as the gabbro in the depths.

Vulcanites weather immediately after their formation, plutonites only when the overlying rock layers have been eroded. Because both volcanites and plutonites became rock from cooled magma, both are classified as igneous rocks.


After the eruption of Eyjafjallajökull on March 20, 2010, air traffic was suspended in large parts of Europe. The eruption of the Icelandic glacial volcano had thrown a huge ash cloud kilometers high into the atmosphere. From April 15, 2010, the volcanic eruption resulted in the longest traffic ban in European airspace to date.

Tens of thousands of flights were canceled, freight containers remained on the ground, hundreds of thousands of passengers were stranded. Mattress camps were set up, and calm returned to the airports. All of Europe seemed paralyzed after the eruption of Eyjafjallajökull. The airlines suffered billions in losses from the failures. Weeks earlier, slight tremors indicated that the volcano would soon become active. Until then, volcano researchers classified the Eyjafjallajökull as rather harmless. Its eruptions were rare and not particularly violent, but when the thousand-degree hot magma shot up, it hit the 200-meter-thick glacier. The ice evaporated suddenly, the magma from the interior of the earth was atomized into powder and an ash cloud was thrown seven kilometers into the air. At this point, of all times, the wind turned over Iceland: dust contained in the air and sharp-edged grains of rock drifted towards Europe. This made them particularly dangerous for aviation. If an airplane flies through such a cloud, the dust and stone grains act like sandpaper. In the worst case, the engines are so badly damaged that they fail and the aircraft crashes. In order to avoid such a catastrophe, air traffic in Europe was suspended. After five days, the amount of ash in the air had dropped so much that most of the planes could take off again.

Sleeping brother

Eyjafjallajökull seems to have calmed down, but its big brother is slumbering nearby. Katla volcano is only thirty kilometers away and has erupted much more violently than Eyjafjallajökull in the past. Experts warn of an impending eruption of the Katla.

The last happened in 1918. At that time, the volcano hurled chunks of ice the size of multi-story houses into the air, which when hit left huge holes in the ground. A wave of meltwater flooded an area of ​​200 square kilometers - the size of a small German town.

In the past, the Katla spat fire about every 50 years, so it would be time for an outbreak again long ago. In addition, volcanic researchers have found that both volcanoes are connected: During its last two eruptions, the Eyjafjallajökull “fired” the Katla underground and thus brought about its eruption.

You don't need to worry too much, however. The Katla volcano is more explosive than its little brother. Fortunately, there are no people living in its immediate vicinity.