P1.2 What do we know about the Earth and how it is changing?
Rocks provide evidence for changes in the Earth:
- Erosion – the Earth’s surface is made up of LAYERS of rock, one on top of the other with the oldest at the bottom. The layers are made of compacted SEDIMENT, which is produced by weathering and EROSION. Erosion changes the surface of the planet over long periods of time.
- Fossils – plants and animals trapped in layers of sedimentary rock have formed FOSSILS, providing evidence of how life on Earth has changed over millions of years.
- Folding – some rocks look as if they have been FOLDED like plasticine. This would require a big force to be applied over a long period of time – further evidence that the Earth is very old.
- Mountain formation – if MOUNTAINS were not being formed, the whole Earth would have been worn down to sea level by erosion.
Rock processes seen today can account for past changes.
Scientists estimate that the Earth is around 4500 million years old – it has to be older than its oldest rocks – and when it was first formed it was completely molten (hot liquid) and would have taken a very long time to cool down. The oldest rocks that have been found on Earth are about 4500 million years old.
The age of the Earth must be greater than the age of it oldest rocks.
Alfred Wegner (1880-1930) was a meteorologist who put forward a theory called CONTINENTAL DRIFT. He saw that the continents fitted together like a jigsaw, with the MOUNTAIN RANGES and SEDIMENTARY ROCK patterns matching up almost perfectly. There were also FOSSILS of the same land animals on different continents. Wegner proposed that the different continents were once joined together but had become separated and drifted apart.
Wegner also claimed that when two continents collided, they forced each other upwards to MAKE MOUNTAINS.
| Evidence For | Evidence Against |
| ● 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 American and Africa ● Closely matching coastline |
● He was not a geologist and was therefore was considered as an outsider
● It was a big idea but he was not able to provide much evidence ● The evidence could be explained more simply by a land bridge connecting the continents that has now sunk or been eroded ● The movement of the continents was not detectable. |
Just below the Earth’s crust the mantle is very solid – further down it is liquid and able to move. CONVECTION CURRENTS in the mantle cause magma (molten rock) to rise to the surface. The force is strong enough to move the solid part of the mantle and the tectonic plates. When the magma reaches the surface, it hardens to form new areas of oceanic crust (SEAFLOOR), pushing the existing floor outwards. This process is called SEAFLOOR SPREADING. It results in seafloor spreading by a few centimetres each year.
New oceanic crust is continuously forming at the crest of an oceanic ridge and old rock is gradually pushed further outwards.
The Earth has a MAGNETIC FIELD which changes polarity (reverses) every million years or so. Combined with seafloor spreading, this produces stripes of rock alternating polarity. Geologists can work out how quickly new crust is forming from the widths of the stripes. This occurs at CONSTRUCTIVE PLATE BOUNDARIES, where the plates are moving apart.
The Earth’s crust, together with the upper region of the mantle, consists of huge blocks of rock called TECTONIC PLATES.
Volcanoes, mountains and earthquakes occur at the edges of tectonic plates – their creation depends on the direction the plates are moving.
If the plates are MOVING APART, as at mid-ocean ridges, VOLCANOES are produced as molten magma is allowed to escape
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.
This is also part of the ROCK CYCLE, because the plate that 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.
EARTHQUAKES produce wave motions on the surface and inside the Earth which can be detected by instruments located on the Earth’s surface.
There are two types of shock waves: P – waves and S – waves. Differences in the speed of P – and S- waves can be used to give evidence for the structure of the Earth.
| P – waves | S – waves | ||||
| Longitudinal waves where the ground is made to vibrate in the same direction as the shock wave is travelling, i.e. if the shock wave is travelling from left to right the ground also vibrates from left to right. | Transverse waves where the ground is made to vibrate at right angles to the direction the shock wave is travelling from left to right the ground vibrates up and down | ||||
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| ● Pass through solids and liquids
● Faster than S-waves ● Speed increases in denser material |
● Pass through solids only
● Slower than P-waves ● Speed increases in denser materials |
Waves are regular patterns of disturbance that transfer ENERGY in the direction the wave travels without transferring matter- there are two types of LONGITUDANAL and TRANSVERSE.
LONGITUDANAL WAVE – Each particle moves backwards and forwards in the same plane as the direction of wave movement. Each particle simply vibrates to and from about its normal position.
TRANSVERSE WAVE– Each particle moves up and down at right angles (90˚C) to the direction of wave movement. Each particle simply vibrates up and down about its normal position.
The distance travelled by a wave can be worked out using the formula:
Distance (metres) = Wave speed (metres per second m/s) x Time (seconds)
All waves have several important features:
- Amplitude – The maximum disturbance caused by a wave. It is measured by the distance from a crest or trough of the wave to the undisturbed position.
- Wavelength – The distance between corresponding points on two adjacent cycles.
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- Frequency – the number of waves produced in one second. Frequency is measured in hertz (Hz).
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Wave speed, frequency and wavelength are related by the following formula:
Wave speed (metres per second, m/s) = Frequency (hertz, Hz) x Wavelength (metres, m)
For a constant wave speed, the wavelength is INVERSELY PROPORTIONAL to the frequency