How colliding plates cause earthquakes

Where plates collide

When two vehicles collide, their sheet metal is crumpled together. Something similar happens when two plates of the earth's crust collide. Then their rock is pushed together and very slowly laid into huge folds - this is how fold mountains are created. What the crumple zone is in a car accident, the mountains are in a collision of plates - only that a car accident takes place in fractions of a second, whereas a plate collision takes many millions of years.

This is exactly how the Alps came into being: Africa pressed against the Eurasian continent and unfolded the mountains. The Himalayas in Asia and the Andes in South America also owe their origins to the collision of migrating crustal plates.

In such a crash, the rock of the lighter plate is pushed upwards, the heavier plate sinks into the depths. This process is called subduction, the area in which the plate descends, the subduction zone. There are often deep gullies along these zones, which is why they are easy to see. The deepest of them is the Mariana Trench in the Pacific Ocean. This deep-sea channel lies where the Pacific plate dips under the Philippine one.

The further the earth's crustal plate disappears in the interior of the earth, the hotter it gets. The rock melts and magma forms in the depths. Due to the increasing pressure, it can be pressed up again. Where it penetrates to the surface of the earth, volcanoes spew lava and ash. There are entire chains of such volcanoes around the Pacific Plate, for example in Indonesia. Because one volcano follows the other, this plate boundary is also called the “Pacific Ring of Fire”.

Not only do volcanoes erupt at such plate edges. The earth also frequently shakes because the movement of the plates creates tremendous pressure and increasing tensions. As soon as these discharge, quakes shake the earth's surface. In Japan, for example, three plates meet: the Pacific, the Filipino and the Eurasian. It is for this reason that violent earthquakes hit Japan so often.


The seaquake that hit the seabed off the coast of Japan on Friday, March 11, 2011, has devastating consequences. The quake triggered a tsunami over 10 meters high, which raced towards the coast at several hundred kilometers an hour. The water flooded a 1,000-kilometer stretch of coastline and left an image of devastation. How many people died in the disaster is not yet known. The monster waves also hit the Fukushima nuclear power plant and destroyed the cooling system. Explosions at the reactor blocks give rise to fears of a core meltdown.

With a magnitude of 8.9, it was the worst earthquake in Japan's history. It began at 2:45 p.m. local time and its epicenter was 130 kilometers east of the city of Sendai in the northeast of the country. A few minutes later, a tsunami over ten meters high washed over the coast of the Japanese main island of Honshu. The massive tidal wave hurled ships inland, tore buildings with it and buried the coastline under a layer of mud. Tens of thousands of people were victims of the floods, countless are missing. The emergency shelters are completely overcrowded. 100,000 soldiers are now helping with the clean-up work.

At the same time, fear of a worst-case scenario is growing in Japan. The cooling of the fuel rods at the Fukushima nuclear power plant has failed. The meltdown may have already started. The government has declared a nuclear emergency. Thousands of residents have since been evacuated.

Tsunami warning for Europe?

Europe is also not safe from a tsunami. Because the Mediterranean is tectonically active, earthquakes are the result. After particularly strong seaquakes, the dreaded giant waves could pile up in the Mediterranean and even in the Atlantic. This happened around 2000 years ago, when an earthquake off Crete was followed by a tsunami that devastated the coasts of the eastern Mediterranean. In 1755, a quake off Portugal and the subsequent tsunami destroyed the city of Lisbon. And there was also a giant wave during a quake off Sicily in 1908. Because people are now more aware of these dangers again, experts are now working on a tsunami early warning system for Europe.


During a meeting of the Geological Society in Frankfurt, the meteorologist and polar researcher Alfred Wegener put forward a daring theory: In his opinion, the continents move on earth. Colleagues in geology are skeptical or even negative.

If Alfred Wegener had claimed that the earth was flat, he would hardly have caused astonishment among his listeners. According to Wegener, all the continents of our earth are said to have been united into a single land mass a long time ago. He calls this supercontinent Pangea, which moved on the Earth's mantle and split into two parts 200 million years ago. These two continents are said to have further divided and shifted. There are clear indications of the breaking and moving of the continents: They fit together like pieces of a puzzle. It is also noticeable that the same animal species occur on different continents.

So Africa and South America should have been one? To the professional world, Wegener's speech sounds as believable as a fairy tale from the Arabian Nights. One is still convinced to this day that the earth's crust is firmly connected to its subsurface. As far as we know, the continents are fixed and were once connected to each other by land bridges. Many geologists still disparagingly refer to Wegener's continental drift as the “geopoetry of a weather frog”. The main thing that remains unclear is the motor of movement: what drives the continents? But research can no longer ignore Alfred Wegener's theory. Can it also be proven?

Alfred Wegener - an airship?

The meteorologist Alfred Wegener became famous for a record he set in balloon flight: On April 5, 1906, he ascended with his brother Kurt and stayed in the air for over 52 hours. This exceeded the previous world record by 17 hours. But the balloon flight not only served for fame, but above all for science: The Wegener brothers wanted to explore the atmosphere and test methods of location determination. Alfred Wegener's interest is not only in the weather and aviation, but also in the eternal ice. In the year of his world record, he set out to explore Greenland. He returned from this Greenland expedition in 1908. Since then, the 32-year-old scientist has been a lecturer in meteorology, astronomy and physics at the University of Marburg.

Continents on the move

For a long time it was thought that the land masses of the earth would stand rigidly in place. It later turned out that the opposite is the case. The continents of our planet are moving! Like huge ice floes, they drift in different directions, albeit not very quickly. Their speed corresponds roughly to the growth of a fingernail. But why is it that the continents are constantly on the move?

The earth's crust that envelops our planet is brittle and cracked. It resembles a cracked egg shell and is made up of seven large and many smaller plates. Some of them make up the continents, others make up the ocean floor. These plates of the earth's crust drift around on a hot, viscous rock slurry and are driven by movements in the earth's interior, more precisely: by currents in the earth's mantle. Experts also say: you are drifting. All these processes related to the movement of the earth's plates are called plate tectonics, and the movement itself is also known as plate drift.

The earth is particularly active where the individual plates adjoin one another. At some of these plate boundaries, hot rock penetrates upwards from the earth's mantle and cools down. Here new earth crust forms: the two plates grow and are thereby pushed apart. On the other hand, where two plates collide, the lighter one of them - the continental crust - is crumpled up and unfolded to form mountains. The heavier of the two - the oceanic crust - is slowly disappearing into the depths. Due to the heat in the earth's interior, their rock is melted again. As the edge of the plate sinks into the depth, it pulls the rest of the plate behind it and thus additionally drives the plate movement.

Volcanic eruptions, earthquakes, long mountain ranges and deep ocean trenches accumulate along such plate edges. Most of the unrest on the earth's surface brings with it the largest of its plates: it is the Pacific plate, which is moving northwest at a rate of about 10 centimeters per year. Most of the world's active volcanoes can be found at their edges, and violent earthquakes shake the region. Because of the frequent volcanic eruptions and earthquakes, this plate boundary is also called the “Pacific Ring of Fire”.

Mountains in motion

Mountains rise up mighty and rigid. It seems as if nothing and nobody can move them. But that's not true: mountains are constantly in motion - albeit so slowly that we cannot see the change with the naked eye.

The reason for this: the plates of the earth's crust move. And when two of these plates collide, the rock is compressed, pushed and piled up. Similar to a car accident, mountains fold up on the edge of the slab on impact. Mountains and valleys are thus a “crumple zone” of the slabs colliding. However, this does not happen suddenly like in a car accident, but much more slowly than in slow motion. The result is fold mountains like the Andes in South America. There the oceanic Nazca plate slides under the South American plate and squeezes the rock with incredible force. The elongated mountains of the Andes piles up, stretching over a distance of 7,500 kilometers. The Andes are the longest unearthly mountain range in the world.

However, there are also huge mountains below sea level. They run through the middle of the oceans. They too owe their existence to the movable plates. Where two plates move away from each other on the ocean floor, magma penetrates from the mantle through the oceanic crust. The hot rock slush cools on the sea floor and piles up to form mountains that are thousands of meters long: the mid-ocean ridges. Where the lava reaches sea level and swells beyond it, islands like Iceland arise. These mountains, which are born in the sea, are the longest on earth. The Mid-Atlantic Ridge stretches from north to south through the entire Atlantic - about 20,000 kilometers long.

Folded and reshaped - the creation of the Alps

Every year Munich and Venice come half a centimeter closer. It's not a lot, but it's measurable. The fact that the German and Italian cities are slowly moving closer together has to do with the formation of the Alps.

Compared to other mountains, the Alps are relatively young. Its story begins “only” around 250 million years ago when a shallow sea formed between the continents of Eurasia and Africa: the Tethys. Rock debris and remains of living things settle on the sea floor over a long period of time and turn into limestone.

About 100 million years ago, the African plate set out on a journey: It drifts north, pressing violently against the Eurasian continent. The rock is compressed by the pressure, it folds up in a wave-like manner. The individual folds can reach a few millimeters or hundreds of meters. In some places the folded layers slide over one another like roof tiles and form what are known as rock ceilings. Eventually magma also rises; at the moment when the African plate dips under the Eurasian plate. The rock is melted in the interior of the earth and rises upwards, but still cools below the surface of the earth. For this reason, the central Alps consist of the igneous rock granite - in contrast to the limestone of the northern and southern Alps.

The folded area eventually rises above sea level under the great pressure. At first, the folds appear as elongated islands in the sea. But the archipelago is pressed further upwards and slowly pushes up to a high mountain range in which the rivers cut deep valleys. Large amounts of rubble are piled up in the foothills of the Alps. During the cold periods, huge glaciers carve deep trough valleys and steep mountain slopes into the rock. Only now is the typical high mountain landscape of the Alps forming, which attracts us to hiking or climbing in summer and skiing in winter.

The African plate continues to drift north to this day. That is why the Alps are still being lifted and compressed. This compression is the reason that Venice and the entire area beyond the Alps move a tiny bit closer to us every year.

Cycle of rocks

No rock on earth is made to last. It weathers on the surface, is removed and redeposited. When two plates collide, layers of sediment are compressed and unfolded to form high mountains. The rock of submerged plates melts in the earth's interior and forms the source of volcanoes. Lava that spits out from a volcanic crater cools down and solidifies again into rock.

It is an eternal cycle that ensures that even the hardest rock is constantly changing and new things are created from it. The transformation does not happen overnight, of course, but over millions of years. "Players" in this cycle are three groups of rocks, each of which is formed under different conditions:

When magma cools, the hot mass solidifies igneous rock. This can happen both on the surface of the earth and inside the earth. On the other hand, where layers of excavated rock pile up, the sediments are compressed under the weight of their own weight. This pressure causes them to solidify Sedimentary rock. In turn, high pressure and great heat in the earth's interior ensure that rock is transformed and another is created. Then geologists speak of transformation or of metamorphic rock.

These three types of rock are closely related: each type can transform into any other. This rock cycle will continue as long as the earth exists.

Oceanic and continental crust

The earth's crust is not built up in the same way everywhere. The earth's land masses consist of continental crust, the sea floor of oceanic crust. One of the differences is that in addition to oxygen, the continental crust mainly contains silicon and aluminum. The oceanic crust, on the other hand, also has a high proportion of magnesium. But that is by no means the only difference:

Oceanic crust forms on the sea floor, where magma rises and solidifies along the mid-ocean ridges. Since the crust is constantly growing back here, the two lithospheric plates are pressed outwards. The oceanic crust is therefore getting older and older towards the coasts. Some of the oldest pieces are around 200 million years old. They are located in the Atlantic off North America and east of the Mariana Trench in the Pacific. The five to eight kilometers thick oceanic crust does not get any older: because it is heavier than the continental one, it submerges in the event of a collision and is melted again in the interior of the earth.

The continental crust is lighter, but thicker than the oceanic crust: on average, it extends 40 kilometers, under mountains it can even be up to 80 kilometers deep. When exactly it was formed is still a mystery even to science. Evidence of this is provided by the oldest known rock on earth: It was found in northern Canada, is over four billion years old and is believed to be a remnant of the very first crust of the earth.

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 one another, 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 an approximately 40,000-kilometer-long 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 of 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 beneath 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 reach 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 already 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.

Where plates scrape past each other

The residents of San Francisco and Los Angeles live on a powder keg: at any moment an earthquake can shake the California coast. The region has already experienced many quakes, one of which was particularly devastating. On April 18, 1906, the earth trembled so badly that entire neighborhoods of San Francisco collapsed, killing around 3,000 people. But why is the danger of earthquakes so great on the west coast of the USA in particular?

Two plates of the earth's crust move past each other along the California coast: the North American and Pacific plates. Both are drifting northwest, but the Pacific plate is a little faster. It is therefore slowly "overtaking" the North American record. So it happens that Los Angeles and San Francisco get closer and closer, by about 6 centimeters every year. If they move at the same pace, Los Angeles will be on the Pacific plate north of San Francisco, which is on the North American plate, in around 12 million years.

A long crack runs through the land where the plates meet. This San Andreas trench is over 1100 kilometers long. Here, the different speeds of the tectonic plates cause extremely strong stresses in the rock. Because the two plates don't just slide past each other, they hook into each other. At some point the tension between the rock masses is so great that the faster Pacific Plate moves forward with a jolt. Such jerky movements of the plate are expressed in more or less strong earthquakes. Because of this, California will continue to be shaken by tremors. Some researchers even claim that a tremendous quake would be imminent in a few years. But no one can predict exactly when that will be.