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 border

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.

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.

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.

Below this lies the outer part of the Earth’s mantle, which reaches down to a depth of about 100 kilometres. It is also solid, but consists of heavier rock. The earth’s crust and this outermost part of the mantle together are also called the “lithosphere”. This solid layer of rock is broken into plates of different sizes that drift very slowly on the hot, viscous mantle.

Where the molten rock from the hot mantle penetrates upwards, the earth’s crust can break open. Lava then flows out and becomes new crust. This mainly happens where the plates of the lithosphere adjoin each other, such as at the mid-ocean ridges.

In Iceland, for example, these plate boundaries are clearly visible: Cracks and furrows run through the earth’s crust here, where the Eurasian and North American plates drift away from each other. There is also a plate boundary in the Mediterranean region. Because the African plate is pressing against the Eurasian plate here, there are many volcanoes and earthquakes in Italy.

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.

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.