Upper mantle: depth of 10-400 kilometers
Solid fragments of the upper mantle have been found in eroded mountain belts and volcanic eruptions. Olivine (Mg, F e)2SiO4 and pyroxene (Mg,Fe)SiO3 have been found. These and other minerals are crystalline at high temperatures. Part of the upper mantle called the asthenosphere might be partially molten.
Oceanic crust: depth of 0-10 kilometers
The majority of the Earth's crust was made through volcanic activity. The oceanic ridge system, a 40,000 kilometer network of volcanoes, generates new oceanic crust at the rate of 17 km3 per year, covering the ocean floor with an igneous rock called basalt. Hawaii and Iceland are two examples of the accumulation of basalt islands.
Continental crust: depth of 0-75 kilometers
This is the outer part of the Earth composed essentially of crystalline rocks. These are low-density buoyant minerals dominated mostly by quartz (SiO2) and feldspars (metal-poor silicates). The crust is the surface of the Earth. Because cold rocks deform slowly, we refer to this rigid outer shell as the lithosphere (the rocky or strong layer).
Plate tectonics and earthquakes:
The world's earthquakes are not randomly distributed over the Earth's surface. They tend to be concentrated in narrow zones. Why is this? And why are volcanoes and mountain ranges also found in these zones, too?
An explanation is to be found in plate tectonics, a concept which has revolutionized thinking in the Earth's sciences in the last 10 years. The theory of plate tectonics combines many of the ideas about continental drift and sea-floor spreading.
The lithosphere covers the whole Earth. Therefore, ocean plates are also involved, more particularly in the process of sea-floor spreading. This involves the midocean ridges which, are a system of narrow submarine cracks that can be traced down the center of the major oceans. The ocean floor is being continuously pulled apart along these midocean ridges. Hot volcanic material rises from the Earth's mantle to fill the gap and continuously forms new oceanic crust. The midocean ridges themselves are broken by offsets know as transform faults.
One of the keys to plate tectonics was the discovery that the Earth's magnetic field has reversed its polarity 170 times in the last 80 million years. As new basaltic material is squeezed up into the midocean cracks and solidifies, it is magnetized according to the polarity of the Earth's magnetic field. If the field reverses its polarity, the strip of new material is magnetized in an opposite sense. As the oceanic floor continues to spread, the new strips of rock are carried away on either side like a conveyer belt.
Using these magnetic strips as evidence of movement, it became obvious that the Earth's surface consisted of a mosaic of crustal plates that were continually jostling one another. If the Earth was not to be blown up like a balloon by the continual influx of new volcanic material at the ocean ridges, then old crust must be destroyed at the same rate where plates collide. The required balanced occurs when plates collide, and one plate is forced under the other to be consumed deep in the mantle.
We now know that there are seven major crustal plates, subdivided into a number of smaller plates. They are about 80 kilometers thick, all in constant motion relative to one another, at rates varying from 10 to 130 millimeters per year. Their pattern is neither symmetrical nor simple. As we learn more and more about the major plates, we find that many complicated and intricate maneuvers are taking place. We learn, too, that most of the geological action - mountains, rift valleys, volcanoes, earthquakes, faulting - is due to different types of interaction at plate boundaries.
How are earthquakes connected with plate tectonics? In 1969, Muawia Barazangi and James Dorman published the locations of all earthquakes, which occurred from 1961 to 1967. Most of the earthquakes are confined to narrow belts and these belts define the boundaries of the plates. The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. There are notable exceptions to this. An obvious one is the 1811-1812 earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston, South Carolina. As yet there is no satisfactory plate tectonic explanation for these isolated events; consequently, we will have to find alternative mechanisms.
Plate tectonics confirm that there are four types of seismic zones. The first follows the line of midocean ridges. Activity is low, and it occurs at very shallow depths. The point is that the lithosphere is very thin and weak at these boundaries, so the strain cannot build up enough to cause large earthquakes. Associated with this type of seismicity is the volcanic activity along the axis of the ridges (for example, Iceland, Azores, Tristan da Cunha).
The second type of earthquake associated with plate tectonics is the shallow-focus event unaccompanied by volcanic activity. The San Andreas fault is a good example of this, so is the Anatolian fault in Northern Turkey. In these faults, two mature plates are scraping by one another. The friction between the plates can be so great that very large strains can build up before they are periodically relieved by large earthquakes. Nevertheless, activity does not always occur along the entire length of the fault during any one earthquake. For instance, the 1906 San Francisco event was caused by breakage only along the northern end of the San Andreas fault.
The third type of earthquake is related to the collision of oceanic and continental plates. One plate is thrust or subducted under the other plate so that a deep ocean trench is produced. In the Philippines, ocean trenches are associated with curved volcanic island arcs on the landward plate, for example the Java trench. Along the Peru - Chile trench, the Pacific plate is being subducted under the South American plate, which responds by crumpling to form the Andes. This type of earthquake can be shallow, intermediate, or deep, according to its location on the down going lithospheric slab. Such inclined planes of earthquakes are known as Benioff zones.
The fourth type of seismic zone occurs along the boundaries of continental plates. Typical of this is the broad swath of seismicity from Burma to the Mediterranean, crossing the Himalayas, Iran, Turkey, to Gilbraltar. Within this zone, shallow earthquakes are associated with high mountain ranges where intense compression is taking place. Intermediate- and deep-focus earthquakes also occur and are known in the Himalayas and in the Caucasus. The interiors of continental plates are very complex, much more so than island arcs. For instance, we do not yet know the full relationship of the Alps or the East African rift system to the broad picture of plate tectonics.
How can plate tectonics help in earthquake prediction? We have seen that earthquakes occur at the following three kinds of plate boundary: ocean ridges where the plates are pulled apart, margins where the plates scrape past one another, and margins where one plate is thrust under the other. Thus, we can predict the general regions on the Earth's surface where we can expect large earthquakes in the future. We know that each year about 140 earthquakes of magnitude 6 or greater will occur within this area, which is 10 percent of the Earth's surface.
But on a worldwide basis we cannot say with much accuracy when these events will occur. The reason is that the processes in plate tectonics have been going on for millions of years. Averaged over this interval, plate motions amount to a several millimeters per year. But at any instant in geologic time, for example, the year 1977, we do not know exactly where we are in the worldwide cycle of strain buildup and strain release. Only by monitoring the stress and strain in small areas, for instance, the San Andreas fault, in great detail can we hope to predict when renewed activity in that part of the place tectonics arena is likely to take place.
In summary, plate tectonics is a blunt, but, nevertheless, strong tool in earthquake prediction. It tells us where 90 percent of the Earth's major earthquakes are likely to occur. It cannot tell us much about exactly when they will occur. For that, we must study in detail the plate boundaries themselves. Perhaps the most important role of plate tectonics is that it is a guide to the use of finer techniques for earthquake prediction.
Where is the Evidence for Plate Tectonics?
The continents seem to fit together like a giant jigsaw puzzle:
If you look at a map, Africa seems to snuggle nicely into the east coast of South America and the Caribbean sea. In 1912 a German Scientist called Alfred Wagener proposed that these two continents were once joined together then somehow drifted apart. He proposed that all the continents were once stuck together as one big land mass called Pangea. He believed that Pangea was intact until about 200 million years ago.
CONTINENTAL DRIFT
The idea that continents can drift about is called, not surprisingly, continental drift.
When Wagener first put forward the idea in 1912 people thought he was nuts. His big problem was that he knew the continents had drifted but he couldn't explain how they drifted. The old (and very wrong) theory before this time was the "Contraction theory" which suggested that the planet was once a molten ball and in the process of cooling the surface cracked and folded up on itself. The big problem with this idea was that all mountain ranges should be approximately the same age, and this was known not to be true. Wagener’s explanation was that as the continents moved, the leading edge of the continent would encounter resistance and thus compress and fold upwards forming mountains near the leading edges of the drifting continents. Wagener also suggested that India drifted northward into the Asia forming the Himalayas and of course Mount Everest.
SEA FLOOR SPREADING
It is hard to imagine that these great big solid slabs of rock could wander around the globe. Scientists needed a clue as to how the continents drifted. The discovery of the chain of mountains that lie under the oceans was the clue that they were waiting for.
VIP- PLATES ARE CREATED: In the diagram below you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen. In the diagram below you can see that the continental crust is beginning to separate creating a diverging plate boundary. When a divergence occurs within a continent it is called rifting. A plume of hot magma rises from deep within the mantle pushing up the crust and causing pressure forcing the continent to break and separate. Lava flows and earthquakes would be seen.
This is an example of a divergent plate boundary (where the plates move away from each other). The Atlantic Ocean was created by this process. The mid-Atlantic Ridge is an area where new sea floor is being created.
As the rift valley expands two continental plates have been constructed from the original one. The molten rock continues to push the crust apart creating new crust as it does.
As the rift valley expands, water collects forming a sea. The Mid-Atlantic Ridge is now 2,000 meters above the adjacent sea floor, which is at a depth of about 6,000 meters below sea level.
The sea floor continues to spread and the plates get bigger and bigger. This process can be seen all over the world and produces about 17 square kilometers of new plate every year.
VIP- PLATES ARE DESTROYED (SUBDUCTION):
This is a convergent plate boundary, the plates move towards each other. The amount of crust on the surface of the earth remains relatively constant. Therefore, when plates diverge (separate) and form new crust in one area, the plates must converge (come together) in another area and be destroyed. An example of this is the Nazca plate being subducted under the South American plate to form the Andes Mountain Chain.
Here we can see the oceanic plate moving from left to right. The top layer of the mantle and the crust (all called the lithosphere) sinks beneath the continent. A deep ocean trench is formed. Water gets carried down with the oceanic crust and the rocks begin to heat up as they travel slowly into the earth. Water is then driven off triggering the formation of pools of molten rock, which slowly rises. The plate moves downwards at a rate of a few centimeters per year. The molten rock can take tens of thousands of years to then either:
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Solidify slowly underground as intrusive igneous rock such as granite.
or
Reach the surface and erupt as lava flows. Cooling rapidly to form extrusive igneous rock such as basalt.
The floor of the Easter Pacific is moving towards South America at a rate of 9 centimeters per year. It might not seem much but over the past 10 million years the Pacific crust has been subducted under South America and has sunk nearly 1000 kilometers into the Earth's interior
The floor of the Easter Pacific is moving towards South America at a rate of 9 centimeters per year. It might not seem much but over the past 10 million years the Pacific crust has been subducted under South America and has sunk nearly 1000 kilometers into the Earth's interior.
Types of Convergent Boundaries
The example above showed what happened when the dense oceanic plate subducts under a lighter continental plate (i.e., oceanic - continental). Two other types of subduction can take place:
When two oceanic plates meet each other (oceanic-oceanic) this often results in the formation of an island arc system. As the sub ducting oceanic crust melts as it goes deeper into the Earth, the newly-created magma rises to the surface and forms volcanoes. If the activity continues, the volcano may grow tall enough to breech the surface of the ocean creating an island.
The key to subduction seems to be water which acts as a kind of lubricant as the heavier plate slips underneath the lighter plate.
I must not forget to mention the Himalayas and Mount Everest because this is the third example of plate movement
Millions of years ago India and an ancient ocean called the Tethys Ocean were sat on a tectonic plate. This plate was moving northwards towards Asia at a rate of 10 centimeters per year. The Tethys oceanic crust was being subducted under the Asian Continent. The ocean got progressively smaller until about 55 million years ago when India 'hit' Asia. There was no more ocean left to lubricate the subduction and so the plates welled up to form the High Plateau of Tibet and the Himalayan Mountains. The continental crust under Tibet is over 70 kilometers thick. North of Katmandu, the capital of Nepal, is a deep gorge in the Himalayas. The rock here is made of schist and granite with contorted and folded layers of marine sediments, which were deposited by the Tethys ocean over 60 million years ago.
The fourth type of plate movement involves plates sliding past one another without the construction or destruction of crust. This boundary is called a conservation zone because plate is neither created nor destroyed. An example of such a boundary is the San Andreas fault in California. The force needed to move billions of tones of rock is unimaginable. When plates move some of the energy is released as earthquakes.
The Rock Cycle
The upper part of the earth (mantle, crust and surface) can be envisioned as a giant recycling machine; matter that makes up rocks is neither created nor destroyed, but is redistributed and transformed from one rock type to another. Petrology, the study of rocks and their origins, is essentially the formal process by which we resolve the interrelationships expressed in the rock cycle.
Liquid (molten) rock material solidifies at depth or at the earth's surface to form igneous rocks. Uplift and exposure of rocks at the Earth's surface destabilizes these mineral structures (c.f. Bowen's Reaction Series). The minerals break down into smaller grains, which are transported and deposited (either from solution or by lowering the hydraulic energy regime) as sediments. The sediments are lithified (compacted and cemented), and sedimentary rocks are formed. Changes in temperature, pressure, and/or rock or fluid chemistry can allow igneous and sedimentary rocks to change physically or chemically to form metamorphic rocks. At higher temperatures, metamorphic (or any other rock type) rocks may be partially melted, and crystallization of this melt will create igneous rocks. Uplift and erosion can expose all rock types at the surface, re-initiating the cycle.
If we examine the rock cycle in terms of plate tectonics, as depicted in the figure above, we see that mafic (tholeiitic) igneous rocks form at sea floor spreading ridges. Fluid intrusion of these rocks, both during and after formation, results in some low grade metamorphism. As the rocks cool, and more magma is introduced from below, the plate is forced away from the spreading ridge, and acquires a sediment cover. As shown in the figure, in this case, the plate is eventually subducted under a continental plate. In the trench of the subduction zone, at relatively shallow depths, high pressure – low-high temperature metamorphism of the plate and its sediment cover occur. As the plate travels deeper, high temperature conditions cause partial melting of the crustal slab. Fluid intrusion plays a key role in partial melting. As the partial melt rises, and intrudes into the continental plate, the surrounding country rock is contact metamorphosed at high temperature conditions. This melt is either driven to the surface as volcanic eruptions, or crystallizes at depth to form plutonic igneous rocks. Sedimentary rocks form from the weathering, erosion, transport and deposition of arc material onto the continental platform and shelf.
Weathering and Erosion
Rocks of every sort and shape are worn away over time. Weathering is the process, which breaks rocks into smaller bits. There are three main types:
- Physical weathering is a physical action, which breaks up rocks: An example of this is called freeze-thaw weathering when water gets into tiny cracks in rocks. When the water freezes it expands, if this is repeated the crack grows and bits eventually break off.
- Chemical weathering is when the rock is chemically attacked: An example of this is the breakdown of limestone by acid rain.
- Biological weathering is when rocks are weakened and broken down by animals and plants. An example would be a tree root system slowly splitting rocks.
Erosion is a type of physical weathering, which involves wearing down rocks.
There is an important point to remember. Rocks are weathered at different rates. Dartmoor is an upland area of 241 square miles reaching up to 2,000 feet in height making it the largest and highest area of moorland in the South of England. It is also the largest granite surface in England. Granite is made up of large interlocking crystals (igneous rock) that give it a granular texture and make it one of the toughest rocks on Earth. Sedimentary rocks such as sandstone tend to be much weaker.
Transportation
The rock cycle goes round and round, taking hundreds of millions of years. Once the rock has been broken down into smaller bits it's got to somehow move. Streams and rivers carry the small bits towards the sea (continually wearing down as the they progress). Big rivers such as the Humber and the Severn carry millions of tonnes of sediments out to sea each year.
Deposition
Deposition simply means that the sand and sediments in the sea eventually settle to the bottom.
Sedimentary Rocks
Sedimentary rocks are formed in three steps:
- Layers of sediment are deposited at the bottom of seas and lakes.
- Over millions of years the layers get squashed by the layers above.
- The salts that are present in the layers of sediment start to crystallize out as the water is squeezed out. These salts help to cement the particles together.
How can you spot a Sedimentary rock?
- Sedimentary rock will often have layers or bands across them.
- It will often contain fossils, which are fragments of animals or plants preserved within the rock. Only sedimentary rocks contain fossils.
- The rock will tend to scrape easily and often crumble easily.
SOME COMMON SEDIMENTARY ROCKS:
Sandstone
Sandstone is one of the most common sedimentary rocks. It is made from sand grains eroded from older rocks, cemented together and then hardened into new rock. Here we see a picture of a Jurassic sandstone from the USA, notice the layers. Each layer is a record of an event in the past
Conglomerate
This is made from pebbles and smaller stones stuck together in a matrix.
Limestone
Limestones are made from fragments of sea creatures that sank to the bottom of ancient tropical seas. Many limestones from Southern England are made from dissolved lime, which builds up around sand grains to form tiny spheres called oolites. Limestones frequently contain fossils. Here we see a stalactite from the limestone cave system a few miles away in Ingleton.
Mudstone or Shale
These are simply just mud hardened into rock. They consist of much finer particles than sand. They often contain fossils.
Heat and pressure make Metamorphic Rocks
Earth movements can push all types of rock deeper into the Earth. These rocks are then subjected to massive temperatures and pressures causing the crystalline structure and texture to change. They do not melt. The high pressures involved are often associated with mountain building processes.
Slate
This is formed from mudstone or clay and is the most common kind of metamorphic rock in Britain. Pressure causes new minerals to grow in parallel sheets - which makes slate split easily to make roofing tiles.
Marble
Marble is limestone that has been squashed and heated .The shells of the limestone breakdown and recrystallise into tiny crystals. Marble is chemically the same as limestone but it is much harder and far more expensive. Some of the finest marble comes from Italy and it is used for sculptures and as a fine building material.
Schist
Formed from mudstones subjected to great heat over long periods of time. It looks to have layers of banded crystals (It cannot be igneous because igneous rocks don't have layers)
Igneous Rocks
Igneous rocks form when molten rock (Magma if it is below the surface or lava if it has erupted from a volcano) solidifies. These rocks can be identified by the following tell-tale clues:
- Igneous rocks contain a minerals randomly arranged in crystals (Remember CRYSTALS !!!!!!)
- If the rock has small crystals this means that it had rapidly cooled, possibly because it was erupted into the ocean. We call it an extrusive igneous rock. If the rock has large crystals it means that it slowly cooled, the molten rock solidifies deep down within the crust without ever reaching the surface via an eruption. We call it an intrusive igneous rock.
- The rocks are usually tough and hard (With the most famous exception being pumice stone).
This bit is worth remembering:-
COMMON IGNEOUS ROCKS
Basalt
This is the most common form igneous rock, which makes up most of the ocean floors. It is smooth and velvety-black in appearance and very hard. Basalt is formed when magma is erupted onto the sea-bed, as soon as it hits the cold sea water it cools quickly - it's got tiny crystals.
Pumice
This rock floats on water. Carbon dioxide and water dissolved in the molten rock is released with the decrease in pressure as it reaches the surface. Lava cools quite quickly in the air so the bubbles of gas get trapped.
Granite
If molten rock doesn't reach the surface via a volcano and cools underground instead, it solidifies very slowly. This is because overlying layers of rock insulate the magma keeping it warm, this only allows gradual cooling. Some crystals grow to a much bigger size giving granite a speckled appearance. Granite is the most common form of igneous rock in the UK.
Earthquakes, Folding and Faulting
Sedimentary rocks are often found tilted, folded, fractured and twisted. This indicates that the Earth has moved with enormous force (obviously over huge timescales). Large scale movements of the Earth's crust can push up whole mountain ranges. 'What goes up must come down' as the old saying goes, weathering will ensure that the rock cycle starts all over again.
The Earth's Atmosphere
The atmosphere is a thin layer of gas, which surrounds the Earth. The two most important layers are known as the troposphere and the stratosphere. The air gets thinner and thinner the higher you go, 90% of all the molecules in the atmosphere are in the troposphere. Air is a mixture of various gases.
The present composition of the atmosphere is:
Besides water vapor, several other gases are also present in much smaller amounts:
- Carbon monoxide (formula CO)
- Neon (Ne)
- Oxides of nitrogen
-
Methane (CH4)
- Krypton (Kr)
Concentrations of these gases are measured in parts per million (ppm)
The atmosphere has changed a lot compared to the Earth's early atmosphere, but for the last billion years it has remained pretty constant. We now need to look at 3 very different atmospheric problems:
1) The Greenhouse effect
The earth is surrounded by a blanket of gases. This blanket traps energy in the atmosphere, much the same way as glass traps heat inside a greenhouse. This results in an build up of energy, and the overall warming of the atmosphere. The greenhouse effect is a natural process, which made life on Earth possible. Without naturally occurring greenhouse gases such as water vapor, carbon dioxide, methane and nitrous oxide, the Earth's surface temperature would be 33°C cooler, a chilly -18°C rather than the tolerable 15°C.
When we talk about the greenhouse effect we mean the enhanced effect, which is caused by the increase of greenhouse gases from human sources. Since the beginning of industrialization, 200 years ago, concentrations of these gases have increased. It is estimated that the Earth's average temperature has risen by 0.6°C since 1880 because of emissions of greenhouse gases from human activity.
The main sources of these emissions, particularly carbon dioxide, methane and nitrous oxide, are:
-
The combustion of large amounts of fossil fuels (producing CO2)
-
Deforestation (less trees mean that less CO2 is being mopped up)
A increase in global temperatures may seem great, you might even think of 'Costa del Black pool'. Unfortunately global warming will probably result in big swings in weather patterns across the world. Summers will become dryer and hotter. Winters will be wetter and colder. Other things will start to happen:
- Thermal expansion of the water and melting of continental glaciers would cause sea levels to rise, possibly as much as two feet, by the end of next century.
- Rising temperatures could lead to changes in regional wind systems, which would influence global rainfall distribution and lead to the redistribution and frequency of floods, droughts and forest fires.
- Increased sea temperatures would cause the destruction of coral reefs around the world.
- Climate change would create favorable conditions for growth in insect populations. This would likely have a bad effect on agriculture and human health and result in a spread of malaria and other tropical diseases.
- Water supplies would become disrupted and droughts would be more common
There is a lot of controversy surrounding global warming, views range from those who believe that there is nothing to worry about to those who believe that the world is heading for a global catastrophe.
2) Damage to the ozone layer
Ozone is oxygen that contains molecules that have 3 oxygen atoms (O3). The molecule is triatomic instead of the usual O2 molecule, which is diatomic. There is a layer of ozone high up in the atmosphere which shields the Earth from the sun's harmful UV rays, these rays can lead to an increase in skin cancer. The ozone is present in very small quantities but it is enough to absorb the UV rays preventing them reaching the surface.
Scientists began to investigate the ozone layer in the 1970's, it wasn't until the mid 1980's that alarm bells started to ring. Concentrations of ozone appeared to be dropping in certain areas of the world (the layer was starting to thin-out). The cause of this reduction was thought to be man-made.
In 1985 over 60 countries pledged to phased out a group of chemicals called CFC's. These very stable chemicals were once widely used in aerosols and refrigerators. It was thought that their release into the atmosphere produced chlorine radicals, which reacted with O3 to produce O2. The emission of CFC's into the environment is now greatly reduced, unfortunately the damage has already been done and the CFC molecules, thanks to their stability, are still causing ozone depletion.
3) Acid rain
Rainwater is naturally acidic due to carbon dioxide, which partially reacts with water to give carbonic acid (H2O + CO2 -> H2CO3). When we talk about acid rain we mean the enhanced effect, which is caused by other gases released when fossil fuels are burnt. Two gases are the main culprits:
-
Sulphur dioxide - Fossil fuels often contain a lot of sulphur impurities, which burn to give sulphur dioxide. The SO2 reacts with water in the atmosphere to from a weak solution of sulphuric acid.
- Nitrogen oxides - Under normal conditions nitrogen and oxygen don't react together. At very high temperatures (in an engine) a small proportion of oxygen reacts with nitrogen to give nitrogen oxides. These oxides react with water in the atmosphere to from a weak solution of nitric acid acid.
The dilute acid falls to ground as acid rain, which causes the following problems:
- Lakes become acidic and plants and fishes die as a result
- Tree growth is damaged, whole forests can die as a result
- Acid rain attacks metal structures and also buildings made of limestone
One method of reducing the amount of SO2 that gets pumped into the atmosphere is to remove the sulphur impurities from the fuel.
Don't get global warming, ozone depletion and acid rain confused. They are all different.
FOSSILS INTO FUELS
Crude oil, natural gas and coal are fossil fuels. Fossil fuels are very precious resources because they are non-renewable (once they're used, that's it!). We can also make lots of organic chemicals from them, needed to make products such as paints, detergents, polymers (including plastics), cosmetics and some medicines.
Fossil fuels were formed from the fossilized remains of dead plants and animals that once lived millions of years ago. Oil and natural gas are the products of the deep burial and decomposition of dead plants and animals. Heat and pressure, in the absence of oxygen, transform the decomposed material into tiny pockets of gas and crude oil. The oil and gas then migrates through the pores in the rocks to eventually collect in reservoirs.
Coal comes mainly from dead plants, which have been buried and compacted beneath sediments. Most coal originated as peat in ancient swamps created many millions of years ago.
What is crude oil?
Crude oil is a complex mixture of hydrocarbons with small amounts of other chemicals such as sulphur. The crude oil is useless as a mixture and must be sent to an oil refinery to be separated. Crude oils from different parts of the world, or even from different depths in the same oilfield, contain different mixtures of hydrocarbons and other compounds. This is why they vary from light colored volatile liquids to thick, dark oils.
What is natural gas?
Natural gas is a mixture of hydrocarbons with small molecules. These molecules are made of atoms of carbon and hydrogen. For example, natural gas used in the home is mainly methane, CH 4.
What is a hydrocarbon?
Hydrocarbons only contain hydrogen and carbon atoms. There are two main chemical families of hydrocarbons - the alkanes and the alkenes. Thousands of synthetic products can be manufactured from hydrocarbons with many different properties.