Why do volcanoes stop erupting?

What do volcanoes look like?

A steep mountain, flames shoot into the sky, above it a dark ash cloud: this is what a volcano looks like in a picture book. But there are also completely different volcanoes. What shape they have depends above all on the lava that penetrates from the interior of the earth.

Thin lava flows smoothly and evenly out of the crater. It cools down slowly and spreads widely. This creates extensive areas or flat mountains that look like large shields. Hence these volcanoes have their name: Shield volcanoes. Typical examples are the Hawaii volcanoes with their glowing lava lakes and diameters of up to 400 kilometers.

Viscous lava, on the other hand, does not get very far - it partially sticks inside the volcano and clogs it. Below that, magma continues to push upwards. The pressure rises until the lava plug is blasted out of the volcano in a big explosion like a cork from a champagne bottle. Lava fragments and rocks fly into the air and fall on the volcano. A layer of ash settles on the area. In the course of time, a pointed mountain of ash and rock debris piles up, increasing layer by layer with each eruption. Well-known examples of this Stratovolcanoes are Mount Etna in Sicily or Mount St. Helens in the USA. Stratovolcanoes are particularly dangerous because of their explosive eruptions.

There are also volcanic explosions that take place underground. When hot magma meets groundwater in the depths, the water suddenly evaporates. The resulting pressure is so high that the soil above is blown up. What remains is a hole in the surface of the earth, shaped like a bowl or funnel Maar. Water often collects in this crater, then a maar lake is created, such as the Laacher See in the Eifel.

If the magma chamber is empty after a volcanic eruption, the volcano can collapse over it. There is a deepening in the landscape, a Caldera. The dimensions of the collapsed magma chamber can be guessed at from the size of the caldera. Some are huge, like the caldera of Ngorongoro in Tanzania with a diameter of around 20 kilometers. When magma rises again from the depths and emerges as lava, a new volcano forms in the caldera; one then speaks of a daughter volcano. Vesuvius, for example, is such a daughter volcano: It originated in the caldera of Monte Somma.


It is the most active volcano on earth: Kilauea in Hawaii. It has been spraying lava out of craters and crevices since 1983, making for a great natural spectacle. The Kilauea measures 1,247 meters, its older siblings Mauna Loa and Mauna Kea tower more than 4,000 meters above sea level.

It was around 200,000 years ago that Kilauea broke through the earth's crust, 50,000 years ago it emerged from the sea. Due to its constantly swelling lava flows, it continues to grow. At the same time, it keeps losing mass due to landslides.

Like the entire chain of islands in Hawaii, Kilauea owes its creation to a hotspot. This hot spot in the earth's mantle melts the rock and forms a magma chamber. From here a volcano erupts from time to time. Because the hotspot always stays in the same place, but the tectonic plate slides over it, entire chains of volcanoes or volcanic islands are created over a period of millions of years.

Since the lava of Kilauea flows relatively slowly and steadily at a maximum of 10 km / h, it is not particularly dangerous for humans. Nevertheless, several villages have fallen victim to it in the past few decades, and more than 100 houses have been destroyed.

Pele's tears

Wreaths of flowers, gin and cigarettes lie on the edge of Kilauea's crater. They are offerings to the goddess Pele, who is very venerated by the Hawaiians. The name Pele means "molten lava". She is the goddess of fire and volcanoes. And sometimes the Hawaiians dance the hula for them on the edge of the volcano.

According to legend, the quick-tempered Pele is said to have created volcanoes with a magic wand; she found her residence in the crater of Kilauea. When the volcano breathes fire, Pele sometimes shows itself in the glowing lava.

Volcano researchers also seem to be impressed by Pele and named lava rock after the goddess: “Pele's hair” is what they refer to as elongated, hair-shaped volcanic glass. It forms when the volcano hurls out fountains of lava that the wind pulls apart before they get cold. “Tears of Pele”, on the other hand, arise when lava drips down and solidifies into tiny, shiny black spheres.


The Laacher See glitters quietly in the sunlight. But the idyllic tranquility is deceptive: Hell on earth raged here 13,000 years ago - this landscape is the result of a volcanic eruption. And scientists expect the Eifel volcanoes to erupt again. It is only unclear when ...

The first volcanic eruptions occurred in the Eifel 600,000 years ago. The series began with the volcanoes in the West Eifel: ash volcanoes, craters and maars formed here. Later the volcanic area of ​​the Eifel expanded to the southeast. Then the volcanoes came to rest.

However, this calm ended with a bang, almost 13,000 years ago: A huge explosion in the interior of the earth tore a large hole in the landscape. Meter-high streams of mud rolled down into the valley. The Rhinelander were completely surprised by this catastrophe - as shown by skeletons that have been found in the strata of the earth. The Laacher See was created at the site of the volcanic explosion. The last volcanic eruption in the Eifel took place 11,000 years ago, when the Ulmen Maar was formed.

Smaller earthquakes between Laacher See and Koblenz indicate today that the earth has not yet finally come to rest. The quiet bubbling in the Laacher See also testifies to volcanic activity. The carbon dioxide bubbles that rise from the water here come from the hot volcanic subsoil. The Eifel is on the move - will it surprise us soon with a new volcanic eruption? A question that science cannot answer either.

Ant alert

Volcano researchers have observed the behavior of ants and want to use it to develop an alarm system for volcanic eruptions. They believe ants will notice an impending volcanic eruption early on and leave their nests. The reason they suspect is that ants are fleeing the poisonous gases that escape during volcanic activity. However, biologists don't really believe in this idea: They have noticed that ants move several times a year, too.

But the volcano experts continue to research and observe more than 2000 anthills in the Eifel. Because, although the Eifel is volcanic, there are only a few fixed measuring stations that could warn of a volcanic eruption. And the volcanoes in the Eifel, the researchers suspect, will not sleep forever!


At noon on August 24, 79 AD. the summit of the volcano Vesuvius explodes. The terrible volcanic eruption claimed thousands of lives. Hail of rock, lava flows and ash rain bury the inhabitants of Pompeii. The two cities on the Gulf of Naples are completely destroyed.

Until August 24th, Pompeii is a thriving trading city. The day starts out sunny, there is a crowd and noise in the alleys, and merchant ships dock at the port. The residents have no idea of ​​the approaching disaster. They know nothing about the lava stopper that has been blocking the outlet of Vesuvius like a cork has blocked a champagne bottle for centuries. A violent earthquake 17 years earlier caused this plug to loosen dangerously. When it loosens around noon on August 24, it shoots into the air along with the tip of the volcano with a huge bang - the beginning of the fall of Pompeii.

Soon there will be a hail of stones and a layer several centimeters thick will form on the roofs of the city. The rocks get bigger and with them the horror of the inhabitants. Fist-sized and red-hot pumice stones fall from the sky, smashing windows and roofs. There are the first fatalities. Violent tremors shake houses and streets. Many of the 20,000 Pompeians try to flee, others save themselves in the cellars. But their houses become a trap: the following night, Vesuvius emits deadly gases that settle on the city. Whoever breathes it in suffocates painfully. Over the course of the following day, three streams of lava burn and bury whatever is left of the city. Finally, Vesuvius spreads a thick layer of ash over the already completely devastated city.

The neighboring town of Herculaneum with 4,000 inhabitants is also razed to the ground by the eruption.

Witness to the downfall

From Misenum, a port city 30 kilometers from Pompeii, the 17-year-old Pliny followed the volcanic eruption and the fall of Pompeii. In a letter he describes the course of the catastrophe and the death of his uncle. He was a Roman fleet commander and set off with the ship to save people from Pompeii ...

He hurried to where others fled and headed straight for the danger [...] Ashes were falling on the ships, getting hotter and thicker the closer they got, soon pumice stone and black, half-charred, fire-cracked stones as well. The sea suddenly receded and a landslide made the shore impassable. For a moment he was undecided whether to turn back, then he called to the helmsman who had advised him to do so: "Fortune helps the brave, go to Pomponianus!"

Pomponianus is a friend of his uncle who lives in Stabiae. There is also panic there and people want to flee. Pliny 's uncle tries to calm her down.

In the meantime, wide flames and high columns of fire glowed from Vesuvius in several places, the radiant brightness of which was intensified by the dark night. [...] Together they discussed whether they should stay indoors or go outside, because as a result of frequent, strong tremors, the buildings shook and seemed to sway back and forth as if they had been released from their foundations. In the open air, on the other hand, the raining down of annealed, but only light pieces of pumice stone was alarming [...] They put pillows over their heads and tied them with towels; that offered protection against falling rocks.

It was already day elsewhere, but there it was night, blacker and thicker than any other night, […] It was decided to go to the beach and see for oneself up close whether the sea would allow one to leave. But it still remained harsh and hostile. There my uncle lay down on a spread out blanket, asked for a sip of cold water every now and then, and took it to himself. Then flames and the scent of sulfur as a harbinger chased the others to flight and startled him. Leaning on two slaves, he got up and immediately collapsed dead, presumably because the thicker smoke took his breath away and closed his throat [...]

Translation from Latin: G / history, Bayard-Media


The eruption of the Krakatau volcano completely destroyed the volcanic island of the same name between Sumatra and Java. 36,000 people were killed in this natural disaster. Most of the victims drowned in the destructive tidal waves caused by the exploding volcano. Thousands of kilometers away, the monster flood wreaked havoc.

The Krakatau volcano, located in the Sunda Strait, which is busy by ships, had been relatively quiet for the past 200 years. On August 22, 1883, a series of violent eruptions began, the last of which completely destroyed the island. The August 27 explosion was so massive that it blew up almost the entire volcanic island. Several cubic kilometers of rock shot up over 30 kilometers into the sky. The thunderclap of the explosion could be heard as far as the Australian city of Perth, which is a good 3,000 kilometers away.

But the greatest damage was caused by the tidal waves caused by the volcanic eruption. Several tidal waves up to 30 meters high raced towards the coasts of Java and Sumatra. There they killed thousands of people and completely devastated the islands.

Immediately after the explosion, it got dark and ash was raining down. The dust spread high in the atmosphere and moved around the whole earth. The sky darkened, the temperatures dropped by an average of 0.5 - 0.8 degrees Celsius. The disaster was followed by a cool, rainy summer and poor harvests.

Child of Krakatau

The volcanic island of Krakatau was almost completely torn apart by the explosion of 1883. Yet she lives on: Her child, “Anak Krakatau”, is growing up in the middle of the former crater.

Anak Krakatau was born in 1927. A violent eruption caused the daughter volcano of Krakatau to work its way up to the surface of the sea. Since then it has continued to grow, by several meters every year. His outbursts are becoming more violent. At 450 meters above sea level, the volcano is already about half the size of Krakatau before its explosion. Is the Anak just as dangerous as its predecessor?

To prevent a disaster like that of 1883, the Indonesian authorities are constantly monitoring Anak Krakatau. They measure the temperature on the surface of the volcano and the viscosity of the magma. Because the more viscous the magma, the more likely it is that the volcano's chimney can clog. That in turn would increase the risk of an explosion that rocked the whole world in 1883.


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 dormitories were set up, and the airports were quiet. 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 earth's interior was atomized to 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 rock grains 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 left huge holes in the ground when they hit. 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.

Volcanic mountains

The snow-covered Kilimanjaro can be admired from a great distance. As a single mountain range, it towers 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 is the difference between lava and magma?

Magma and lava actually refer to the same thing, just in different places: Magma is inside the earth, lava is on the surface of the earth.

Magma arises where the heat and pressure in the earth's interior are very high. There the rock melts and a viscous rock slurry is created, the magma. The magma collects in underground cavities and flows up to the surface of the earth when the pressure rises. As soon as the magma swells out of the earth during a volcanic eruption, it is called lava. Gases that were trapped in the magma can then escape into the air. Therefore, lava and magma differ in their chemical composition.

As long as the lava is hot, it is soft and malleable. On the surface of the earth, the lava slowly cools down and solidifies. Then it can look very different, depending on where and how it flowed out of the earth: For example, if a volcano erupts underwater, the lava cools down very quickly. It forms into structures that look like lumps or pillows. This is why one speaks of pillow lava. Other lava flows look like long balls of wool and are therefore called knitted lava.

Over time, various rocks are formed from lava. Particularly thin lava turns into dark gray basalt after cooling. This rock is often used as a paving stone for roads and paths. When lava is thrown into the air during a volcanic eruption and puffs up like foam, it creates pumice stone. The trapped air makes pumice stone so light that a piece of it can float on the water. Volcanic ash and volcanic dust that solidify turn into tufa. Many houses in the Vulkaneifel, for example, are made of tuff.

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. An ash cloud 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. None of this happens overnight, however, 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.