What makes fossil fuels combustible

Fossil fuels: petroleum, natural gas and coal

It is called black gold because of its color and because it is so valuable to us: We are talking about petroleum. 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 crude oil: We need it as fuel for vehicles, as heating material or as the basis for plastic.

The raw material for crude oil is plankton, which floated in the sea millions of years ago. The remains of these tiny sea creatures sank to the bottom and were hermetically buried under other layers of sediment, such as sand and clay. The remains decomposed and turned into digested sludge. More sediments were deposited above this, the weight of which pressed on the digested sludge. Under this pressure the temperature rose and the digested sludge chemically changed to a mixture of gaseous and liquid hydrocarbons: crude oil. Because it was lighter than water and the surrounding rock, it rose further and further up through pores until it hit an impermeable layer under which the viscous mass collected: an oil reservoir was created.

Natural gas was also produced under conditions similar to those of oil. This is why both fuels are often found in one deposit. Natural gas is lighter, which is why it is stored on top of oil. Because both substances are fossilized remains of marine organisms, they are referred to as "fossil" fuels.

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

4.5.2010

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 especially 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 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 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 to clean 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.

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 in the process. 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 their mining is worthwhile has yet 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.

From inside the earth: ores and solid metals

Copper was the first metal that humans discovered in the earth's crust. It could be shaped into simple tools or weapons and was so important that an entire epoch was named after it: the Copper Age. The tools got better when man mixed the copper with tin and thus invented the bronze. And when he learned to smelt iron, the triumph of metal tools finally began.

Unlike the core of the earth, the earth's crust consists largely of non-metals. Nevertheless, metals such as iron, aluminum, manganese and potassium can be found in their rock. Experts (geochemists) can determine exactly how often they occur. They found out that around seven percent of the earth's crust consists of iron.

Like most metals, iron occurs as a chemical compound with other elements, so-called ore. In order to extract iron from the ore rock, the ore rock is ground, mixed with coal and heated. Then a chemical reaction takes place that removes the other elements from the ore, leaving the pure, elemental iron.

On the other hand, some metals hardly combine with other elements. They therefore do not weather and occur in pure form in the earth's crust. These “solid metals” include gold, silver and platinum. Platinum and gold are also extremely rare: on average, gold is only contained in a ton of rock with an average of 0.001 grams. A place is only referred to as a deposit if it contains a thousand times the amount of gold - i.e. one gram of gold per ton of rock.

The "rare earth metals" are more common than gold or platinum. What sounds strange has a simple reason: These metals are considered rare because they do not form deposits of their own, i.e. they do not occur in concentrated form, but only in scattered areas. We are therefore also talking about spice metals. Their importance has increased significantly in recent years because they are required for the manufacture of electronic devices such as cell phones or computers.

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 - in relative terms - as thick as its skin. Under continents it reaches an average of 40 kilometers deep, under the oceans it is only about seven kilometers.

Below is the outer part of the earth's mantle, which extends to a depth of around 100 kilometers. 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 has broken into slabs of different sizes, which slowly drift around on the hot, viscous mantle of the earth.

Where the rock melt penetrates upwards from the hot earth's mantle, the earth's crust can break up. Then lava flows out, which becomes the new crust of the earth. This mainly happens where the plates of the lithosphere adjoin one another, such as on the mid-ocean ridges.

In Iceland, for example, these plate boundaries are easy to recognize: cracks and furrows run through the earth's crust, 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 in Italy and there are always earthquakes.

The crust is covered by the bottom. The soil of the land masses is formed from weathered rock and remains of animals and plants. The sea floor, on the other hand, develops from deposits such as clay and sunken remains of marine organisms. On the coasts, the sea floor also consists of deposited rubble that was removed from the mainland and washed into the sea.

Sedimentary rocks

Some rocks look like they're striped. In the Dolomites, for example, such transverse bands can often be clearly seen. Sandstone or limestone quarries sometimes have similarly pretty patterns.

The "stripe design" is created when the rock is formed. The starting material is weathered debris that is carried away by water or the wind. Rivers, glaciers and dust storms lose their strength at some point: rivers become slower and slower towards the mouth and finally flow into the sea or a lake. Glaciers are advancing into warmer regions and melting. Dust storms also subside at some point. Then they can no longer move dust, sand and rubble. The crushed rock that is dragged along settles out. Over time, the deposited material forms an ever higher layer - the sediment. Such sediments, including the remains of dead animals or limestone shells, collect particularly on the seabed and on the bottom of lakes, where rivers wash up a lot of material.

Gradually, different sediments are layered on top of each other. A layer can, for example, consist of sandstone: During the dry season, the wind blew desert sand here. If the sea level rises again, this layer is covered by water: the limestone shells of marine animals sink to the sea floor and deposit another layer over the sand. Over millions of years the climate changed again and again and made the sea level fluctuate. This allowed different layers to deposit.

Over time, the sediment cover becomes thicker and thicker. Under the weight of one's own weight, the initially loose sediments are compressed more and more, small cavities disappear, the mass condenses. Further layers are deposited over it, the sediment becomes more and more solid and finally becomes sedimentary rock under pressure. This process is also called diagenesis in geology. For example, if the shells of tiny marine animals are pressed into stone, limestone is created. Fine grains of sand made of quartz cement together under the high pressure to form sandstone.

In addition to rubble, dead animals also settled, for example fish on the ocean floor. Their bones and scales remained hermetically sealed and petrified. Such fossils are immortalized in the stone. Even after millions of years, they reveal a lot about the time in which the sediment was formed. Therefore, geologists can read in the rock layers like a history book.

Usually only the top layer is visible to us. However, when a river digs its way through the sedimentary rock, is lifted during mountain formation, or blasted free in a quarry, we get a glimpse of the cross-section. The individual layers of sediment can then be easily recognized as "stripes" or bands in the rock.