All our evidence for changes in the Earth comes from looking at rocks. Folds and fossils in sedimentary rocks, radioactive dating and the weathering of ancient craters show that the oldest rocks are about 4000 million years old. That means the Earth must be at least as old as this.

The only thing that we have been able to observe directly is the Earth’s crust, which is the very thin outer rocky layer.

Evidence from earthquakes shows that the Earth has a very dense core surrounded by a solid mantle.

How old are the oldest rocks?

What is the earths crust made up of?

Cross section showing structure of the Earth

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

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

The Earth is just one of the eight planets orbiting the Sun, which is a star. The orbits all lie in the same plane, and the planets all go round in the same direction.

There are many other members of our Solar System:

• Asteroids are much smaller than planets, and orbit the Sun. Most of the asteroids are between the planets Mars and Jupiter, but some come close to the Earth.
• Moons orbit planets. Most are tiny. Only a few are as large as our Moon, which is nearly a sixth of the diameter of the Earth.
• Comets go round the Earth in orbits which can often be at an angle to the plane of the other planets. Comets are similar in size to asteroids, but are made of dust and ice. The ice melts when the comet approaches the Sun, and forms the comet’s tail.

Nearly all of the mass in our Solar System is in the Sun. The Sun is very large. Its diameter is over 100 times the Earth's. The Sun is the source of nearly all the energy we receive. For many years, it was a mystery as to where this came from, and this baffled the leading scientists. It is now understood that the nuclear fusion is the energy source. In nuclear fusion hydrogen nuclei are joined together to make helium nuclei. This releases enormous amounts of energy.

hydrogen nucleus + hydrogen nucleus   →   helium nuclei

In stars larger than our Sun helium nuclei can be fused together to create larger atomic nuclei. As the Earth contains many of these larger atoms, like carbon, oxygen, iron, etc, scientists believe that our Solar System was made from the remains of an earlier star.

How stars and planets are formed

As the gas falls together, it gets hot. A star forms when it is hot enough for a nuclear fusion reaction to start. This releases energy, and keeps the star hot. The outward pressure from the expanding hot gases is balanced by the force of the star's gravity. This happened about 5000 million years ago. This is quite recent in the history of the Universe, which is currently believed to be 14,000 million years old.

The Sun is 150 million kilometres from the Earth, but that’s a tiny distance compared with the distance to other stars, or other galaxies. Larger units of length are used for these measurements. One popular one is the light-year.

Light-years

A light-year is the distance light travels in a year. Light travels very fast (300,000 km per second), and takes only about eight minutes to reach us from the Sun. It takes over four years to reach us from the next nearest star (Proxima Centauri), and 100,000 years to cross the Milky Way galaxy. We say that the distance to the next nearest star is four light-years, and the diameter of the Milky Way is 100,000 light years.

The most distant galaxies observed are about 13,000 million light-years away. However, measuring distances to other stars, and to very distant galaxies, is not easy, so the data is uncertain.

Different explanations can be developed to illustrate the theory that the dinosaurs were destroyed by an asteroid impact.

Data and explanations

Data statements tell you facts, and may contain measurements. For example, look at these three statements:

• asteroids are small objects orbiting the Sun
• some asteroids have orbits close to the Earth
• the dinosaurs died out at about the same time as a large crater was made in Mexico

Explanations seek to explain the data, and formulating an explanation requires imagination and creativity. One explanation is that an asteroid collision may have killed off the dinosaurs. The asteroid impact would have created dust which blocked out the Sun.

A good explanation will explain data, and link together things which were not thought to be related. It should also make predictions.

asteroids often contain the rare metal iridium - data

a huge asteroid impact would send iridium dust throughout the world - prediction

sedimentary rocks from the time the dinosaurs died out contain iridium - data

when the asteroid crashed, the iridium came from the dust which blocked out the Sun - explanation

Data and predictions can be used to test an explanation, but you have to be careful. When an observation agrees with the prediction, it makes you more confident in the explanation, but it does not prove that the explanation is true.The opposite is also correct. When an observation disagrees with a prediction, it makes you less confident in the explanation, but it does not prove that the explanation is wrong. The data may be faulty.

The asteroid theory is not the only theory about the death of the dinosaurs.

• there were huge volcanic eruptions in India at the time the dinosaurs died out - data
• big volcanic eruptions cause dust clouds which block out the Sun - data
• the big Indian eruptions could have killed out the dinosaurs by cooling the Earth - explanation

Alfred Wegener proposed the theory of continental drift at the beginning of the 20th century. His idea was that the Earth's continents were once joined together, but gradually moved apart over millions of years. It offered an explanation of the existence of similar fossils and rocks on continents that are far apart from each other. But it took a long time for the idea to become accepted by other scientists.

Before Wegener

Before Wegener developed his theory, it was thought that mountains formed because the Earth was cooling down, and in doing so contracted. This was believed to form wrinkles, or mountains, in the Earth's crust. If the idea was correct, however, mountains would be spread evenly over the Earth's surface. We know this is not the case.

Wegener suggested that mountains were formed when the edge of a drifting continent collided with another, causing it to crumple and fold. For example, the Himalayas were formed when India came into contact with Asia.

• the same types of fossilised animals and plants are found in South America and Africa
• the shape of the east coast of South America fits the west coast of Africa, like pieces in a jigsaw puzzle
• matching rock formations and mountain chains are found in South America and Africa.

Publishing and peer review

Scientists report their ideas to the scientific community, which is made up of all the other scientists. They present them at conferences and then write them up in journals or books.

At conferences, other scientists will listen and debate the new ideas. Before journals or books are published, other expert scientists read the new ideas and decide if they are sensible. This is called peer review.

• Wegener presented his ideas at a conference in 1912, and then published them in a book in 1915.
• 

Data often allows more than one possible explanation, so different scientists can have different explanations for the same observations.-Wegener’s ideas could certainly explain similar fossils in different continents, but other geologists thought that there were once ‘land bridges’ between continents, allowing animals to travel between them.

The different backgrounds of different scientists can affect their judgements, so they may have quite different explanations for the same data. Wegener was trained as an astronomer and a meteorologist. Many geologists did not think that he had the right background to judge geological theories.

A new explanation becomes accepted

A scientific explanation is rarely abandoned just because some data does not correspond to it. It is safer to stick with a theory that has worked well in the past.The old geological theory explained mountains as wrinkles made by the Earth shrinking as it cools down.There was no clear explanation of how continents could move about.

A new scientific explanation often needs new supporting evidence to convince scientists that it is correct.In the 1950s, evidence from magnetism in the ocean floor showed that the seafloors were spreading by a few centimetres each year. This showed movement of large parts of the Earth’s crust, now called tectonic plates.

The Earth’s crust, together with the upper region of the mantle, consists of huge slabs of rock called tectonic plates. These fit together rather like the segments on the shell of a tortoise.

Although the mantle below the tectonic plates is solid, it does move. This movement is very, very slow – a few centimetres every year. This means that the continents have changed their positions over millions of years.

The typical speed of seafloor spreading is slow: about 10 cm per year. When the magma oozing out of mid-ocean ridges solidifies into rock, the rock records the direction of the Earth’s magnetic field. The Earth’s magnetic field changes with time, and sometimes even reverses its direction. These changes are recorded in the rocks. The same magnetic patterns are seen on both sides of the mid-ocean ridges.

Movement of tectonic plates - higher

Volcanoes, mountains and earthquakes occur at the edges of tectonic plates - their creation depends on the direction the plates are moving.

Volcanoes

If the plates are moving apart, as at mid-ocean ridges, volcanoes are produced as molten magma is allowed to escape. This happens in Iceland.

Mountains

If the plates are moving towards each other, the edges of the plates crumple, and one plate ‘dives’ under the other. This is called subduction. It produces mountains, like the Himalayas. The friction of the movement can also melt rocks and produce volcanoes.

This is also part of the rock cycle, because the plate which dives under the other one becomes part of the mantle and emerges much later from volcanoes and in seafloor spreading.

If the plates are moving sideways, stresses build up at the plate boundary. When the stress reaches some critical value, the plates slip suddenly, causing an earthquake. It is hard to predict when such an earthquake may happen.

On the western coast of the USA, in California, the San Andreas fault at the edge of the North American tectonic plate marks the point at which two plates are moving sideways.

Earthquakes are common in this region. These include the Great San Francisco Earthquake of 1906.

Life at a tectonic plate edge - higher

There are frequent earthquakes and volcanic activity at the edges of tectonic plates which are moving sideways. Countries in these regions need to protect against the damage and danger of earthquakes and volcanoes as well as possible. They also need to provide the necessary help when the disasters strike.

These dangers can be very local, as in a volcanic eruption. Alternatively they may be over a large area, as happens when an earthquake produces a large tsunami, which is a tidal wave.Some of the things public authorities in these regions can do are:

• assess how vulnerable local buildings are
• draw up and enforce building regulations to limit the effect of earthquakes
• provide education and training for emergencies to make sure that the emergency services can go into action quickly and effectively
• monitor natural hazards in the local area to look for early signs of earthquakes or volcanic activity
• take part in international research and monitoring of earthquake and volcanic activity to get a better understanding of how, where and when earthquakes and volcanoes happen

Looking at the sky

Everything we know about stars and galaxies has come from the light, and other radiations, that they give out. This has become more difficult to see from the Earth’s surface, as light pollution from towns and cities interferes with observations of the night sky.

Looking at the sky with the naked eye shows the Sun, Moon, stars, planets and a few cloudy patches called nebulae. When telescopes were invented and developed, astronomers could see that some of the nebulae were in fact groups of millions of stars. These are galaxies.

Parallax

Powerful telescopes allowed astronomers to answer a question which had baffled scientists since Copernicus first suggested that the Earth moved around the Sun. If the Earth moves, you would expect to see a different view of the stars at different times of the year, in the same way as the room you are in looks slightly different if you move your head to one side. That is to say everything seems to move in the opposite direction to your head, but the objects close to you seem to move more. This effect is called parallax. So if the Earth was moving, why did the stars always look the same?

The answer to the question was revealed by more powerful telescopes. These showed that nearby stars do seem to move from side to side and back every year when compared with very distant stars, but that the amount of movement is tiny.

The second nearest star to us is Proxima Centauri. The Sun is the nearest.

It seems to move through an angle of 1.5 seconds between January and June. As one second = 1/60 of a minute, and one minute = 1/60 of a degree, this tiny movement, which is less than a thousandth of the diameter of the Moon, needed powerful telescopes and accurate measurement to observe.

In the last 200 years, it has become very difficult to make astronomical observations in industrialised countries such as Britain. This is not just because of cloudy weather or air pollution. It is due to the bright lights found in cities and towns, and on roads. This light pollution means that it is hard for many people to see more than a few of the very brightest stars at night.

Telescopes

Telescopes are now placed in the few remote, dark places left on our planet, or out in orbit around the Earth.

The Very Large Telescope is part of the Paranal Observatory which is built on top of the Cerro Paranal mountain, which is 2635 metres high, in the Atacama Desert in Chile.

Stars can be classified as different types depending on their brightness and colour. For many stars, the spectrum of light given out is an indication of how bright they are. Two stars with exactly the same spectrum should be of similar brightness. If one is much dimmer, that is because it is further away. This can be used to estimate the distance of stars that are too far away to show much parallax movement over the year.

Very distant stars gave out the light we see them by a long time ago. The light reaching us from a star which is 10,000 light-years away left that star 10,000 years ago. This fact, together with the theories of nuclear fusion in the cores of stars, has allowed astronomers to work out how stars are produced and how they eventually die out

Galaxies

Galaxies contain thousands of millions of stars. For many years, it was thought that our galaxy, which is the Milky Way, was the only one that existed, and that the blurry nebulae that could be seen were clouds of dust and gas in the Milky Way.Observations of many of these nebulae by astronomers such as Edwin Hubble showed they were in fact galaxies outside the Milky Way, and that distant galaxies are all moving away from us.

The beginning and end of the Universe

Hubble’s observations led to the ‘Big Bang’ explanation of the beginning of the Universe, and set a date for this at 14,000 million years ago.There are many questions left unanswered about the beginning and end of the Universe. Observations suggest there is a lot of ‘dark matter’ in the Universe that cannot be seen, and this is not yet clearly understood.

Perhaps the Universe will continue to expand in the way it is at the moment. Perhaps gravity will eventually win and pull all the fleeing galaxies back together again. Better observations of very distant galaxies and a better understanding of the mysterious ‘dark matter’ are needed before these will be understood.

Hubble’s Law - Edwin Hubble measured the distance to many galaxies, and also the speeds with which they are moving away from us. He found a strong correlation between these factors.

Astronomers have detected many planets moving around distant stars. By examining the light from these planets, it is possible to work out what atmospheres they have. If any planet is found with oxygen in its atmosphere, it would be a strong hint that there was life present. All the oxygen in our atmosphere was created by plants.

There are so many stars in the Universe – thousands of millions in each galaxy, and there are thousands of millions of galaxies – that many scientists think it is likely that life exists somewhere else in the Universe. None has yet been detected, although the Search for Extraterrestrial Intelligence (SETI) project has been looking at radio signals from space, hoping to detect an artificial transmission.

Alien civilisations

The Earth’s atmosphere is about 21% oxygen, as a result of photosynthesis by plants. If we found evidence of oxygen in the atmosphere of another planet, it could indicate the presence of life forms. It is possible to detect oxygen and other gases on other planets by studying the light reflected between planets.

It is thought possible that alien civilisations capable of transmitting radio signals exist. The Search for Extraterrestrial Intelligence (SETI) is a programme that uses radio telescopes to look for non-natural signals coming from space. It should be possible to detect even alien TV programmes, if they exist!

The atmosphere

Some radiation of the electromagnetic spectrum is absorbed by the atmosphere, but some is transmitted.

Light, some infrared, some ultraviolet, and microwaves, pass through the atmosphere and reaches the Earth’s surface. Gamma rays, X-rays, most of the ultraviolet and some of the infrared are absorbed by the atmosphere and do not reach the Earth’s surface.

Infrared :Infrared from the Sun reaches the Earth’s surface and warms it.

The warm Earth emits some infrared radiation, and some of this is absorbed by gases in the atmosphere. This is called the greenhouse effect. If there were no greenhouse effect, the Earth would be too cold for life as we know it.

Photosynthesis

Light from the sun reaching earth Light from the Sun reaching the Earth’s surface provides the energy for plants to produce food by photosynthesis.

Photosynthesis replaces carbon dioxide in the atmosphere with oxygen. This reverses the process of respiration.

Microwaves

The atmosphere transmits microwaves, and these can be used to communicate with satellites.

Radiation and cell damage

Any radiation absorbed by living cells can damage them by heating them. However, ionising radiations are more likely to damage living cells. This is because photons of ionising radiation deliver much more energy. They can easily kill cells, and can also cause cancer by damaging the DNA in the nucleus of a cell.

Effects of microwaves

Microwaves in the environment may be harmful, but there is no agreement on this. They are not ionising, and so cannot cause cancer in the way that ultraviolet, X-rays or gamma rays do.

Microwave ovens work because the food contains water molecules which are made to vibrate by the microwaves. This means that food absorbs microwaves and gets hot. The microwaves cannot escape from the oven, because the metal case and the metal grid on the door reflect microwaves back into the

Ultraviolet

 Umbrellas can be useful in the sun as well as the rain

One health risk which is definitely present in our environment is ultraviolet, in sunlight. Not much of the ultraviolet reaching the Earth gets to us, because the ozone layer high up in the atmosphere absorbs most of it. In the summer, it is wise to use sun-screens and clothing to absorb ultraviolet, and prevent it reaching the sensitive cells of the skin.

The ozone layer - higher only

Ozone molecule formation

The ozone layer absorbs ultraviolet because ultraviolet ionises the ozone, which then changes to oxygen. This chemical change is reversible, and the oxygen changes back to ozone.

Scientific or technological developments often introduce new risks.

• Chemicals used in aerosol spray cans and fridges, when released into the atmosphere, gradually made their way up to the ozone layer, and removed some of it. This has increased the intensity of the ultraviolet radiation reaching the Earth. These chemicals are not used any more, and the ozone layer is gradually returning to normal. However, this will take a few years.

It is important to be able to assess the size of risk in any activity. No activity is completely safe.

• The consequence of too much ultraviolet – skin cancer – often does not appear until much later in life, so it doesn't seem a real risk to young people.
• It is difficult to assess how much ultraviolet you are receiving when you are sunbathing. If you feel hot, that is because of the infrared, not the ultraviolet.
• Weather forecasts now inform you of the intensity of ultraviolet radiation. See for example BBC weather.

Benefits

For most risky activities, there are benefits as well as risks:

• sunbathing produces a sun tan, which many people find more attractive
• some ultraviolet is good for you, as it produces vitamin D in the skin

To make a judgement about a possible bad outcome you need to consider two factors:

• What is the chance of the outcome happening?
• What is the consequence of that outcome?

The precautionary principle

The ‘precautionary principle’ tells you to avoid any activity if serious harm could arise.

• Parents may insist that their children are not allowed out on the beach at all in the summer months.

The real risk may be very different from the perceived risk ie the risk that you think is there.- You can’t see ultraviolet, and the word ‘radiation’ sounds frightening to many people. This makes the risk seem worse than something you can see, and which is more familiar.

Some parents may assume that summers are no different from when they were young, so there is no danger to their children.Other parents may be very alarmed by stories of increases in skin cancer, and not let their children out in sunny weather at all. This is the precautionary principle.

The ALARA principle

The ALARA principle is to make any risk As Low As Reasonably Achievable. This usually applies to an organisation which is responsible for its employees.

• a company employing lifeguards on the beach may insist that they wear lycra sun-suits and sun-screen cream to absorb ultraviolet when they are on duty
• the company may also arrange that lifeguards take turns at covering the hottest part of the day, when the intensity of ultraviolet is greatest

 Refraction from a prism

The pattern produced when white light shines through a prism is called the visible spectrum.

The prism separates the mixture of colours in white light into the different colours red, orange, yellow, green, indigo and violet.

In fact, visible light is only part of the electromagnetic spectrum. It’s the part we can see.

The electromagnetic spectrum and its uses

FrequencyType of electromagnetic radiationTypical useWavelength highest gamma radiation killing cancer cells shortest X-rays medical images of bones ultraviolet radiation sunbeds visible light seeing infrared radiation optical fibre communication microwaves cooking lowest radio waves television signals longest

Photons and ionisation

Electromagnetic radiation comes in tiny ‘packets’ called photons.

The photons deliver different quantities of energy, with radio photons delivering the smallest amount, and gamma photons delivering the greatest amount of energy.

If the photons have enough energy, they can break molecules into bits called ions. This is called ionisation. These types of radiation are called ionising radiation.

In the electromagnetic spectrum only the three types of radiation, which have the photons with most energy, are ionising. These are ultraviolet, X-rays and gamma rays.

Damaging to health - higher only

The ions produced when ionising radiation breaks up molecules can take part in other chemical reactions. If these chemical reactions are in cells of your body, the cells can die or become cancerous. This is the reason that ionising radiation can be damaging to health.

Energy and intensity

The intensity of electromagnetic radiation is the energy arriving each second. This depends on two things: the energy in each photon, and the number of photons arriving each second.

To have the same intensity, a beam of red light would need ten times as many photons as a beam of ultraviolet, and a beam of microwaves would need a million times as many.

Energy of 1 ultraviolet photon   =   Energy of 10 red photons   =   Energy of 1 000 000 microwave photons

Absorption of radiation -

All forms of electromagnetic radiation deliver energy. This will heat the material that absorbs the radiation. The amount of heating depends on the intensity of the radiation, and also the length of time the radiation is absorbed for.

Electromagnetic radiation

An object which gives out electromagnetic radiation is called a source of radiation.Something which is affected by the radiation is a detector.

Lower intensity of radiation

Further from the source, the detector receives a lower intensity of radiation. Intensity of beam

As the photons spread out from the source, they are more thinly spread out when they reach the detector.

Ionising radiation can break molecules into smaller fragments. These charged particles are called ions. As a result, ionising radiation damages substances and materials, including those in the cells of living things. The ions themselves can take part in chemical reactions, spreading the damage.

Ionising radiation includes:

• ultraviolet radiation, which is found in sunlight
• X-rays, which are used in medical imaging machines
• gamma rays, which are produced by some radioactive materials

Non-ionising radiation

Not all types of electromagnetic radiation are ionising. Radio waves, light and microwaves are among them.

Microwaves

Microwaves are used to heat materials such as food. The molecules in the material absorb the energy delivered by the microwaves. This makes them vibrate faster, so the material heats up.

The heating effect increases if:

• the intensity of the microwave beam is increased
• the microwave beam is directed onto the material for longer

So you need to cook food for longer in a less powerful microwave oven. This is why they have power ratings, and food labels recommend different cooking times depending on this.

Transmitting information

Infrared light, microwaves and radio waves are all used to transmit information such as computer data, telephone calls and TV signals.

Infrared light

Information such as computer data and telephone calls can be converted into infrared signals and transmitted by optical fibres. Optical fibres are able to carry more information than an ordinary cable of the same thickness. In addition the signals they carry do not weaken so much over long distances. Television remote controls use infrared light to transmit coded signals to the television set in order to, for example, change channels or adjust the volume.

Microwaves

Mobile phone base station

Microwave radiation can be used to transmit signals such as mobile phone calls. Microwave transmitters and receivers on buildings and masts communicate with the mobile telephones which are in their range.

Certain microwave radiation wavelengths pass through the Earth’s atmosphere and can be used to transmit information to and from satellites in orbit.

Radio waves

Radio waves are used to transmit television and radio programmes. Longer wavelength radio waves are reflected from an electrically charged layer of the upper atmosphere. This means they can reach receivers that are not in the line of sight because of the curvature of the Earth’s surface.

Greenhouse gases

Some gases in the Earth’s atmosphere absorb infrared radiation. One of these is carbon dioxide. Even though carbon dioxide is only about 0.04 per cent of the atmosphere, it is a very important greenhouse gas because it absorbs infrared well.

Greenhouse effect

The Sun’s rays enter the Earth’s atmosphere. Heat is emitted back from the Earth’s surface. Some heat passes back out into space. But some heat is absorbed by carbon dioxide, a greenhouse gas, and becomes trapped within the Earth’s atmosphere. The Earth becomes hotter as a result.

Water vapour and methane - higher only

Other greenhouse gases are water vapour, and also methane. Even though methane is present in trace amounts only, it is a very efficient absorber of infrared.

The carbon cycle

The amount of carbon dioxide in the atmosphere is controlled by the carbon cycle.Processes that remove carbon dioxide from the air:

• photosynthesis by plants
• dissolving in the oceans

Processes that return carbon dioxide from the air:

• respiration by plants, animals and microbes
• combustion ie burning wood and fossil fuels such as coal, oil and gas
• thermal decomposition of limestone, for example, in the manufacture of iron, steel and cement

CelluloseAll cells contain carbon, because they all contain proteins, fats and carbohydrates. For example, plant cell walls are made of cellulose, a carbohydrate.

Decomposers Decomposers, such as microbes and fungi, play an important role in the carbon cycle. They break down the remains of dead plants and animals and, in doing so, release carbon dioxide through respiration.

 For thousands of years, the processes in the carbon cycle were constant, so the percentage of carbon dioxide in the atmosphere did not change. Over the past 200 years, the percentage of carbon dioxide in the atmosphere has increased steadily.

• burning more and more fossil fuels as energy sources
• burning large areas of forests to clear land, which means that there is less photosynthesis removing carbon dioxide from the air

Global warming

Although the changes have been gradual, most, but not all, scientists agree that the climate is getting gradually warmer. This is called global warming.

Again, most, but not all, scientists lay the blame for this on human activities increasing the amount of carbon dioxide in the atmosphere.

Global warming could cause:

• climate change
• extreme weather conditions in some areas

Climate change may make it impossible to grow certain food crops in some regions. Melting polar ice, and the thermal expansion of sea water, could cause rising sea levels and the flooding of low-lying land.

Computer climate models - higher only

One piece of evidence which supports the view of scientists who blame human activities for global warming has been provided by supercomputers. Computer climate models, based on different amounts of carbon dioxide in the atmosphere, produce the same changes as have been observed in the real world.