Why are volcanoes usually close to the coast?

The ocean floor

The surface of the oceans glitters 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.

So the water of the oceans is not equally deep 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 more light penetrates. 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.


No one has ever sunk this deep: With their submersible “Trieste”, the Swiss marine researcher Jacques Piccard and the US naval officer Don Walsh reach the Challenger Deep in the Mariana Trench - 10,910 meters below sea level. A sensation!

The metal ball Piccard and Walsh squeezed into on the morning of January 23 was only two meters in diameter. You can just stand up straight. From 8.23 ​​a.m. it goes down: Her submersible Trieste sinks one meter per second, 18 centimeter thick steel walls separate her from the water masses of the Western Pacific. Nobody knows for sure whether the submersible can withstand the enormous water pressure.

At 1:06 p.m., the two of them reach their ambitious goal: the ocean floor at the lowest point on earth. A column of water weighing more than 170,000 tons weighs on them. It's pitch black down here. Nevertheless, Piccard claims to have spotted a flatfish through a plexiglass window. Otherwise there is little going on in the depths: no plants, no schools of fish. After a short stay, the adventurers begin their ascent.

The nerve-wracking dive lasts nine and a half hours. When the two arrive safely back upstairs, the jubilation is immeasurable. A milestone has been reached - Piccard and Walsh throw a container with the American flag into the depths. With their sensational journey, the two will go down in human history.

Who is this Jacques Piccard?

Scientists, inventors and adventurers - all of this applies to the Swiss Jacques Piccard. He was born on July 28, 1922 in Brussels. He was born with a spirit of discovery: his father Auguste was a physicist and inventor. After Auguste Piccard had set a balloon height record in 1931, he devoted himself to exploring the deep sea after the Second World War. His son Jacques jumped on the bandwagon: After studying economics and history, he and his father developed the legendary Trieste submersible. The US Navy was impressed, funded test dives and bought the boat. Jacques Piccard became a scientific advisor and, against the initial opposition of the Americans, managed to be part of the record-breaking dive to the Challenger low. It's hard to believe: the Trieste reached its destination at the lowest point on earth. Since this spectacular event, Piccard has also been - on top of everything else - a pioneer of the deep sea!


British researchers have discovered a hole several thousand square kilometers in the earth's crust on the ocean floor. According to the scientists, the earth's mantle is exposed there. The research vessel RSS James Cook is now on its way to investigate the sites more closely. The first destination of the expedition is a hole between Tenerife and Barbados. The seabed is to be scanned and samples taken with the high-tech TOBI robot.

The vacancies are on the Mid-Atlantic Ridge - there, tectonic plates drift apart and a new ocean floor is created. Holes and cracks are not uncommon at this point, but they usually fill up again quickly with lava from below and thus cover the mantle rock. It is still a mystery to scientists why there is no lava crust in this case. Was it torn away or was it not even able to form? A test result is expected in the coming months.

Excavator lifts mantle rock

The excavator of the German icebreaker Polarstern shovels huge boulders out of the icy sea in the Arctic. Under the microscope, what the researchers had been hoping for for a long time is confirmed: in the samples they find pure mantle rock that was not filled in by volcanoes. A significant find, because the earth's mantle is difficult to access and is usually covered by a thick crust. The mantle rock was discovered on the Gakkel Ridge - a northern extension of the Mid-Atlantic Ridge. There, the earth's crust spreads more slowly than anywhere else in the world - less than one centimeter per year. That is why there is so little volcanic activity there that the mantle rock has been well preserved.


Two weeks ago, the Deepwater Horizon oil rig exploded in the Gulf of Mexico. Since then, millions of liters of crude oil have been running into the sea every day. The chewy soup is now particularly threatening the coasts in the southeastern United States. The damage to the environment can hardly be estimated.

On April 20, the Deepwater Horizon oil rig caught fire and sank two days later. Eleven workers were killed in the explosion and 115 were saved. What threatens after this disaster is a devastating oil spill in the Gulf of Mexico. For days, diving robots have been trying to seal the leaks at a depth of 15,000 meters. But all attempts to stop the outflow of crude oil have so far failed. Efforts to prevent the oil spill from spreading also remained unsuccessful. For example, high waves hindered the use of floating barriers that were supposed to contain the spreading oil slick: the oil continues to drift towards the coast. A state of emergency has already been declared in the US states of Louisiana, Mississippi, Florida and Alabama.

Experts anticipate damage amounting to billions of dollars. Around half of the sum will have to be used for cleaning up the polluted coasts. Huge losses in tourism and fishing are also expected.

The danger of deep sea drilling

A hundred years ago, rich oil deposits were comparatively easy to discover and the oil was easy to extract. Today, however, many of these oil wells have already been exploited. But because our energy requirements are constantly increasing, oil fields that are difficult to access are now being developed. They include oil deposits in the deep sea that are more than 500 meters deep. In order to get to the oil, floating drilling platforms are being set up. Raw material is extracted from these drilling rigs - also known as "offshore extraction". However, this type of oil production means a lot of effort and carries high risks, as the disaster of the Deepwater Horizon has shown. But as long as the demand increases, the search for oil must go deeper and deeper - now in water up to 3000 meters deep.

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 even up to 80 kilometers in depth. 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 earth's crust.

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 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 whole 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 trembles 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.

Treasures on the ocean floor

Hidden treasures rest deep down on the ocean floor. What is meant here is not the sunken prey of predatory seafarers; we're talking about raw materials that occur on the ocean floor.

One of these raw materials is methane hydrate. This flammable ice is stored on the sea floor at a depth of more than 500 meters. It was formed at low temperatures and under high pressure from water and methane, which are produced by certain single cells during the metabolism. In the estimated deposits of methane hydrate, more than twice as much carbon is bound as in all oil, natural gas and coal reserves on earth. However, whether it can contribute to our energy supply in the future is controversial. It is difficult to break down because it decomposes easily at higher temperatures, releasing methane. The danger here is that methane is a greenhouse gas. If too much of it gets into the atmosphere, it affects our climate and temperatures rise.

Another peculiar substance lies at the bottom of the Pacific at a depth of around 5000 meters: manganese nodules. These black lumps can be about the size of potatoes, and some even as large as heads of lettuce. As a raw material, they are of interest to humans because they contain large amounts of the metals manganese and iron. However, there are also high proportions of copper, nickel and cobalt in the wrinkled structures - metals that are required in the electrical industry and for steel production. Whether it is worth mining them still has to be researched: Although they have a much higher metal concentration than ore mines on land, the mining of manganese nodules is particularly complicated because of the great depths of the sea in which they occur.