The seabed

The surface of the oceans glistens in dark blue. It is hard to believe that the seabed lies many kilometres deeper in places and that a spectacular underwater landscape is hidden down there. For the seabed is not as smooth as the bottom of a swimming pool: On the seabed there are high mountains, deep trenches and lava-spewing volcanoes as well as vast plains.

The water of the oceans is not equally deep everywhere. Around the continents lie the shallow shelf seas. Here the seabed slopes gently downwards from the coastline until it reaches a depth of about 200 metres below sea level. The bottom of the shelf seas consists of continental crust. Therefore, it actually belongs to the mainland, even though it is covered by seawater.

Only many kilometres from the coast, on average after 74 kilometres, does the shallow shelf area end with the shelf edge. From this edge, it descends steeply, like a slide, to a depth of about four kilometres. This steep slope forms the transition to the deep sea, where no more light penetrates. That is why no plants grow down there. Only some animal species have been able to adapt to this habitat, despite the hostile conditions.

In the middle of the oceans, mountains rise up into the air, the mid-ocean ridges. These underwater mountains stretch over long distances through all the world’s oceans. In some places they rise above sea level as islands. Iceland, for example, lies directly on the mid-Atlantic ridge, the longest mountain range in the world.

Deep trenches also run through the oceans. Most of them are in the Pacific. Among them is the Mariana Trench, the deepest trench in the world. It reaches down to 11,034 metres below sea level. Only two people have ever been down there: The marine explorer Jacques Piccard and his companion Don Walsh on their record-breaking dive in 1960.

Record Dive into the Deep Sea

No man has ever sunk so deep: with their submersible “Trieste”, Swiss oceanographer Jacques Piccard and US naval officer Don Walsh reach the Challenger Deep in the Mariana Trench – 10,910 metres below sea level. A sensation!

The metal sphere into which Piccard and Walsh squeeze themselves on the morning of 23 January is only two metres in diameter. They can barely stand upright. From 8.23 a.m. onwards, they start to descend: their submersible Trieste sinks one metre per second, 18-centimetre-thick steel walls separate them from the water masses of the West Pacific. No one knows for sure whether the submersible can withstand the enormous water pressure.

At 1.06 p.m. the two reach their ambitious goal: the seabed at the deepest point on earth. A column of water weighing more than 170,000 tonnes weighs on them. It is pitch dark down here. Nevertheless, Piccard claims to have spotted a flatfish through a Plexiglas window. Otherwise, there is little going on in the depths: no plants, no schools of fish. After a short stay, the adventurers begin the ascent.

The nerve-racking dive lasts nine and a half hours. When the two reach the top safe and sound, the jubilation is immeasurable. A milestone is reached – Piccard and Walsh throw a container with the American flag into the depths. With their sensational journey, the two will go down in the history of mankind.

Who is this Jacques Piccard?

Scientist, tinkerer and adventurer – all this applies to the Swiss Jacques Piccard. He was born on 28 July 1922 in Brussels. The spirit of discovery was in his cradle: His father Auguste was a physicist and inventor. After Auguste Piccard had set a balloon altitude 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 developed the legendary submersible Trieste together with his father. The US Navy was impressed, financed test dives and bought the boat. Jacques Piccard became a scientific advisor and, against initial resistance from the Americans, managed to get on board for the record-breaking dive to the Challenger Deep. Hard to believe: the Trieste reached its destination at the lowest point on earth. Since this spectacular event, Piccard is on top of everything else: a pioneer of the deep sea!

Mysterious holes in the earth’s crust

British researchers have discovered a hole in the Earth’s crust several thousand square kilometres in size on the seabed. According to the scientists, the Earth’s mantle lies open there. The research ship RSS James Cook is now on its way to examine the spots more closely. The expedition’s first target is a hole between Tenerife and Barbados. The high-tech robot TOBI will be used to scan the seabed and take samples.

The open spots are located at the Mid-Atlantic Ridge – there, earth plates drift apart and new ocean floor is formed. Holes and cracks are not uncommon at this location, but they usually fill up again quickly with lava from below, thus covering the mantle rock. It is still a mystery to scientists why a lava crust is missing in this case. Was it torn away or could it not have formed in the first place? The results of the investigation are expected in the coming months.

Excavator lifts mantle rock

Huge chunks of rock are shovelled out of the icy sea in the Arctic by the dredger of the German icebreaker Polarstern. Under the microscope, what the researchers had hoped for a long time is confirmed: In the samples they find pure mantle rock that has not been filled in by volcanoes. This is 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 spur of the Mid-Atlantic Ridge. There, the earth’s crust spreads more slowly than anywhere else in the world – less than one centimetre per year. That is why there is so little volcanic activity there, so that the mantle rock has remained well preserved.

Oil spill in the Gulf of Mexico

A fortnight ago, the oil rig “Deepwater Horizon” exploded in the Gulf of Mexico. Since then, millions of litres of crude oil have been leaking into the sea every day. The viscous soup now threatens the coasts in the southeast of the USA in particular. The damage to the environment can hardly be estimated.

On 20 April, the Deepwater Horizon oil platform caught fire and sank two days later. Eleven workers were killed in the explosion, 115 could be rescued. 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 metres. But all attempts to stop the escaping crude oil have failed so far. Efforts to prevent the oil spill from spreading also failed to achieve the desired success. High swells, for example, hindered the deployment of floating barriers 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 expect billions of dollars in damage. About half of the sum will have to be used to clean up the polluted coasts. Huge losses in tourism and fishing are also expected.

The danger of deep-sea drilling

Just 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 needs are constantly increasing, oil fields that are difficult to access are now also being developed. These include oil deposits in the deep sea, which lie at depths of more than 500 metres. In order to reach the oil, floating drilling platforms are set up. Raw material is extracted from these drilling platforms – also called “offshore extraction”. This type of oil extraction, however, involves a great deal of effort and carries high risks, as the Deepwater Horizon accident showed. But as long as demand continues to rise, oil must be sought ever deeper – now at depths of up to 3,000 metres.

Oceanic and continental crust

The earth’s crust is not the same everywhere. The land masses of the Earth consist of continental crust, the ocean floor of oceanic crust. One of the differences is that the continental crust contains mainly silicon and aluminium in addition to oxygen. The oceanic crust, on the other hand, also has a high proportion of magnesium. But that is far from the only difference:

Oceanic crust forms at the bottom of the sea, where magma rises and solidifies along the mid-ocean ridges. As crust constantly grows back here, the two lithosphere plates are pushed outwards. Towards the coasts, the oceanic crust thus becomes older and older. Some of the oldest pieces are around 200 million years old. They lie in the Atlantic off North America and east of the Mariana Trench in the Pacific. However, the oceanic crust, which is about five to eight kilometres thick, does not get any older: because it is heavier than the continental crust, it submerges when it collides and is melted again in the Earth’s interior.

Although the continental crust is lighter, it is thicker than the oceanic crust: on average, it reaches 40 kilometres, under mountains even up to 80 kilometres in depth. When exactly it formed is still a mystery even to scientists. The oldest known rock on Earth provides clues: it was found in northern Canada, is over four billion years old and is probably a remnant of the very first crust.

Where slabs diverge

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

Where continental crust breaks apart, a rift valley forms. In contrast, where oceanic crust pieces move away from each other, mountains grow on the sea floor: the mid-ocean ridges. They consist of magma that penetrates upwards from the Earth’s mantle through the oceanic crust. New plate material is formed here. It squeezes between two oceanic plates, so to speak, and solidifies into basalt rock that continues to pile up.

In some places, the mid-ocean ridges rise above sea level as islands. Iceland, for example, and the still young Icelandic island of Surtsey are nothing other than parts of the Mid-Atlantic Ridge. Due to the supply of solidified rock, the oceanic crust here is constantly growing. It not only grows upwards, but also to the sides. The two oceanic plates are pushed outwards. Because they spread apart in the process, this is also called a divergence zone.

In this way, new seabed is created and the ocean slowly widens – albeit only a few centimetres a year. But modern satellites can measure the continents with millimetre precision. From the movement, it can be calculated that the Atlantic has already become 25 metres wider since Columbus’ crossing in 1492.

But because the Earth as a whole is not getting any bigger, the increase in seabed must be compensated for elsewhere. This happens where oceanic crust submerges under continental crust: While the Atlantic continues to grow, the Pacific is slowly sinking 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 folded into enormous creases – this is how fold mountains are formed. What the crumple zone is in a car accident, the mountain range is in a plate collision – except that a car accident takes place in a fraction of a second, whereas a plate collision takes many millions of years.

The Alps were formed in exactly the same way: Africa pressed against the Eurasian continent and folded up the mountains. The Himalayas in Asia and the Andes in South America also owe their origin to the collision of moving crustal plates.

In such a crash, the rock of the lighter plate pushes upwards, the heavier one sinks into the depths. This process is called subduction, and the area where the plate dives is called a subduction zone. Along these zones there are often deep gullies, which is why they are easy to recognise. The deepest of these is the Mariana Trench in the Pacific Ocean. This deep ocean channel is located where the Pacific Plate dips below the Philippine Plate.

The further the earth’s crustal plate disappears into the earth’s interior, the hotter it gets. The rock melts and magma forms in the depths. The growing pressure can force it upwards again. Where it reaches the earth’s surface, volcanoes spew lava and ash. There are whole chains of such volcanoes around the Pacific Plate, for example in Indonesia. Because one volcano follows another here, this plate boundary is also called the “Pacific Ring of Fire”.

Not only do volcanoes erupt at such plate edges. Often the earth also shakes because the plate movement causes enormous pressure and growing tensions. As soon as these are discharged, quakes shake the earth’s surface. In Japan, for example, three plates meet at once: the Pacific, the Philippine and the Eurasian. This is why Japan is so often hit by violent earthquakes.

Treasures at the bottom of the sea

Deep down at the bottom of the ocean lie hidden treasures. We are not talking about the sunken booty of predatory seafarers here; we are talking about raw materials found on the ocean floor.

One of these raw materials is methane hydrate. This combustible ice is stored on the seabed at a depth of more than 500 metres. It has formed at low temperature and under high pressure from water and methane, which is produced by certain single-celled organisms during metabolism. The estimated deposits of methane hydrate contain over twice as much carbon as all the oil, natural gas and coal reserves on earth. Whether it can contribute to our energy supply in the future, however, is controversial. It is difficult to extract because it decomposes easily at higher temperatures, releasing methane. The danger here is that methane is a greenhouse gas. If too much of it enters the atmosphere, it will affect our climate and temperatures will rise.

At a depth of about 5,000 metres, there is another strange substance at the bottom of the Pacific Ocean: manganese nodules. These black lumps can grow to the size of potatoes, some even to the size of heads of lettuce. They are interesting to humans as a raw material because they contain large amounts of the metals manganese and iron. But the wrinkled structures also contain high amounts of copper, nickel and cobalt – metals that are needed in the electrical industry and for steel production. Whether their extraction is worthwhile still needs 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 ocean depth at which they occur.