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.

Daredevil drilling on the high seas

Near the Mexican Pacific coast, four propellers fight against the waves. They try to hold the drilling ship, the Cuss I, in place. For 3500 metres below it, its drilling rig is supposed to turn into the seabed. After several weeks and persistent attempts, the US researchers have now succeeded in drilling a 183-metre deep hole.

Oceanic crust is much thinner than continental crust. This is why it was originally believed that at a depth of 10 to 15 kilometres they would be able to hit the boundary to the Earth’s mantle – the so-called Moho boundary. With their 183 metres, the researchers did not get very far, but the project is nevertheless a success: it shows that deep drilling on the high seas is possible. It will only be a matter of time before it is possible to reach the Earth’s mantle.

The discovery of the Moho boundary

In 1909, the earth shook near the Croatian city of Zagreb. Andrija S. Mohorovičić – a Croatian scientist who researched the inner structure of the Earth – took a closer look at the records of the earthquake waves. After many calculations, he found that some waves are refracted at a denser layer of rock about 30 to 50 kilometres below the earth’s surface – as if they were hitting a hard rock. He thus discovered the transition between the Earth’s crust and mantle: the “Moho boundary”, which was named after him.

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 midst 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.

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.

The outermost shell of the earth

Like an egg from an eggshell, the Earth is also surrounded by a hard shell. This outermost layer surrounds the Earth’s mantle and is called the Earth’s crust. If you compare the earth to a peach, the earth’s crust is – relatively speaking – as thick as its skin. Under continents, it reaches an average depth of 40 kilometres, under the oceans even only about seven kilometres.

The crust is covered by the soil. The soil of the land masses forms from weathered rock and the remains of animals and plants. The seabed, on the other hand, develops from deposits such as clay and the sunken remains of marine organisms. On the coasts, the seabed additionally consists of deposited debris that has been eroded from the mainland and washed into the sea.

How is the earth structured?

In the beginning, the young Earth was a hot ball of molten matter. All the components were initially well mixed, just as they were distributed when the earth was formed: Metals, rocks, trapped water and gases and much more – a big mess.

But over time this changed: the heavier substances sank downwards to the centre of the earth – especially metals. Rocks, on the other hand, were somewhat lighter and rose upwards, the lightest ones to the earth’s surface. There they slowly cooled and solidified.

This is how the earth’s material separated into the three spherical layers we know today. You can imagine the structure of the Earth like a peach: on the outside is a wafer-thin “shell” of light, solid rock – the Earth’s crust. On average, it is only 35 kilometres thick.

Beneath the crust is the “flesh” – the almost 3000 kilometre thick mantle of heavy viscous rock. And inside the earth lies the earth’s core made of the metals iron and nickel.

The Earth’s core itself initially consists of an outer layer about 2200 kilometres thick, the outer core. It is over 5000 degrees Celsius hot there, so the metal is molten and as thin as mercury.

On the very inside is the inner core, somewhat smaller than the moon. At over 6000 degrees Celsius, it is even hotter than the outer core – but surprisingly solid. This is because with increasing depth, not only the temperature rises, but also the pressure. The outer layers that weigh on the Earth’s core compress its material so unimaginably strongly that it cannot liquefy.

How do we know how the earth is constructed?

We can fly to the moon, but a journey to the centre of the earth will always remain science fiction. At a depth of just a few kilometres, any drilling rig becomes soft because it cannot withstand the enormous pressure and high temperature. Nevertheless, researchers know very precisely how the earth is structured – but how?

Similar to an X-ray machine, geologists can look inside the earth without having to cut it open. Their “X-rays” are earthquake waves: When there is a strong tremor in one place, the vibrations spread through the entire body of the earth, similar to sound waves in the air.

However, these waves do not always travel at the same speed: in dense and hard material, the vibrations are transmitted faster than in lighter and softer material. If they encounter a layer of rock with a higher density, they can also be refracted or reflected back, like rays of light on a pane of glass. And some waves can only travel in solid or viscous materials and cannot pass through liquids at all.

When the earthquake waves finally arrive on the other side of the world, they are recorded by a worldwide network of highly sensitive measuring devices called seismographs. From the patterns in these diagrams, researchers can read the type of waves and their speed, and trace the waves’ path through the globe.

In this way, the researchers learn a lot about the Earth’s interior – for example, at what depth there are layers of rock or metal and whether they are solid, viscous or thin.