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 waves, 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.

Fossil fuels: oil, natural gas and coal

It is called black gold because of its colour and because it is so valuable to us: we are talking about crude oil. The raw material was created 150 million years ago when dinosaurs still inhabited our planet. Today, it is hard to imagine our everyday life without petroleum: we need it as fuel for vehicles, as heating material or as the basis for plastics.

The starting material for petroleum is plankton, which floated in the sea millions of years ago. The remains of these tiny sea creatures sank to the bottom and were buried airtight under other sediment layers, such as sand and clay. The remains decomposed and became putrid mud. Other sediments were deposited on top, their weight pressing down on the digested sludge. Under this pressure, the temperature rose and the digested sludge changed chemically into a mixture of gaseous and liquid hydrocarbons: Petroleum. Because it was lighter than water and the surrounding rock, it continued to rise through pores until it encountered an impermeable layer under which the viscous mass collected: a petroleum deposit had been created.

Natural gas was also formed under similar conditions as crude oil. That is why both fuels are often found in one deposit. Natural gas is lighter, which is why it is stored above crude oil. Because both substances are fossil remains of marine organisms, they are called “fossil” fuels.

Coal is also one of the fossil fuels. It owes its origin to the remains of dead marsh plants. These formed ever thicker layers of peat, over which sediments accumulated. Under their weight, water, oxygen and other gases were pressed out of the peat layer, and the proportion of carbon increased. Over thousands of years, the peat thus turned into lignite. If the sediment cover and the pressure continued to grow, lignite became fat coal or hard coal. In order to be able to use their stored energy, the coal deposits – also called coal seams – are extracted in mines.

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, into which 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-
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, which drift very slowly on the hot, viscous mantle of the Earth.

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.

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.

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.