The structure of the Earth

The Earth

The Earth is almost a sphere. These are its main layers, starting with the outermost:

1. crust - relatively thin and rocky
2. mantle - (shown here as dark red) - has the properties of a solid, but can flow very slowly
3. outer core - (shown as orange) - made from liquid nickel and iron
4. inner core - (shown as yellow) - made from solid nickel and iron

Note that the radius of the core is just over half the radius of the Earth. The core itself consists of a solid inner core and a liquid outer core.

Plate tectonics

The Earth's crust and upper part of the mantlemantle: The mantle is a zone of hot rock in the interior of the Earth, located between the outer crust and the molten core. are broken into large pieces called tectonic plates. These are constantly moving at a few centimetres each year. Although this doesn't sound like very much, over millions of years the movement allows whole continents to shift thousands of kilometres apart. This process is called continental drift.

The plates move because of convection currents in the Earth's mantle. These are driven by the heat produced by the decay of radioactive elements and heat left over from the formation of the Earth.

Where tectonic plates meet, the Earth's crust becomes unstable as the plates push against each other, or ride under or over each other. Earthquakes and volcanic eruptions happen at the boundaries between plates, and the crust may ‘crumple’ to form mountain ranges.

It is difficult to predict exactly when an earthquake might happen and how bad it will be, even in places known for having earthquakes.

Alfred Wegener

Alfred Wegener (1880 - 1930)

The theory of plate tectonics and continental drift were proposed at the beginning of the last century by a German scientist, Alfred Wegener. Before his time it was believed that the planet's features, such as mountains, were caused by the crust shrinking as the Earth cooled after it was formed.

It took more than 50 years for Wegener’s theory to be accepted. This was because it was difficult to work out what the mechanism was that could make whole continents move, and it was not until the 1960s that enough evidence was discovered to support the theory fully.

Evidence for plate tectonics

So what was the evidence for Wegener's theory?

1. Plate tectonics explained why earthquakes and volcanoes were concentrated in specific places - around the boundaries of moving plates.
2. The match in shape between the east coast of South America and the west coast of Africa suggests both were once part of a single continent. There are similar patterns of rocks and similar fossils [fossils: hardened remains or impressions of an organism that existed in the geological past ] on both sides of the Atlantic - including the fossil remains of land animals that would have been unable to swim across an ocean.

The animation shows how the original single continent - Pangaea - is thought to have broken up and drifted apart.

Earth around 200 million years ago, at the time of Pangaea

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Composition of the Earth's atmosphere

The composition of air

You need to know the proportions of the main gases in the atmosphere.

The Earth's atmosphere has remained much the same for the past 200 million years. The pie chart shows the proportions of the main gases in the atmosphere.

It is clear that the main gas is nitrogen. Oxygen - the gas that allows animals and plants to respire [respire: to engage in respiration - the energy-producing process inside living cells ], and fuels to burn - is the next most abundant gas. These two gases are both elements and account for about 99% of the gases in the atmosphere.

The remaining gases, such as carbon dioxide, water vapour and noble gases such as argon, are found in much smaller proportions.

Oxygen in the air

The percentage of oxygen in the air can be measured by passing a known volume of air over hot copper, and measuring the decrease in volume as the oxygen reacts with it. Here are the equations for this reaction:

copper + oxygen    →    copper oxide

2Cu + O₂    →    2CuO

Gas syringes are used to measure the volume of gas in the experiment. The starting volume of air is often 100cm³ to make the analysis of the results easy, but it could be any convenient volume. In the simulation, there is 100cm³ of air at the start.

Note that there is some air in the tube with the copper turnings. The oxygen in this air will also react with the hot copper, causing a small error in the final volume recorded. It is also important to let the apparatus cool down at the end of the experiment, otherwise the final reading will be too high.

Noble gases

Group 0

Argon makes up about 0.9 per cent of the air. It is one of a group of elements called the noble gases. The noble gases are in Group 0 of the periodic table.

The noble gases - Group 0

Properties and uses of the noble gases

The noble gases are all chemically unreactive gases. The table describes their main uses.

The main uses of the noble gases

Noble gas	Main use
Helium Helium balloons	Used in balloons and airships. It is much less dense than air, so balloons filled with it float upwards.
Neon Neon tube	Used in advertising signs, it glows when electricity is passed through it. Different coloured neon lights can be made by coating the inside of the glass tubing of the lights with other chemicals.
Argon Blue light bulb	Used in light bulbs. The very thin metal filament inside the bulb would react with oxygen and burn away if the bulb were filled with air instead of argon. Argon stops the filament burning away because it is unreactive.
Krypton Laser eye surgery	Used in lasers. Krypton lasers are used by surgeons to treat certain eye problems and to remove birthmarks.

Evolution of the atmosphere

The early atmosphere

Scientists believe that the Earth was formed about 4.5 billion years ago. Its early atmosphere was probably formed from the gases given out by volcanoes. It is believed that there was intense volcanic activity for the first billion years of the Earth's existence.

The early atmosphere was probably mostly carbon dioxide, with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans.

It is thought that the atmospheres of Mars and Venus today, which contain mostly carbon dioxide, are similar to the early atmosphere of the Earth.

Changes in the atmosphere

So how did the proportion of carbon dioxide in the atmosphere go down, and the proportion of oxygen go up?

The proportion of oxygen went up because of photosynthesis by plants.

The proportion of carbon dioxide went down because:

• It was locked up in sedimentary rocks, such as limestone, and in fossil fuels.
• It was absorbed by plants for photosynthesis.
• It dissolved in the oceans.

The burning of fossil fuels is adding carbon dioxide to the atmosphere faster than it can be removed. This means that the level of carbon dioxide in the atmosphere is increasing. Check that you understand the effects of this by going to Combustion of fuels

Now try a Test Bite