The Earth began as dust and planetismals which was drawn together by gravity.
About 4–3.5 billion years ago, the earth was covered oceans and volcanoes. The surface was originally molten which cooled down, creating a surface layer. The volcanoes ejected massive amounts of carbon dioxide, stead, ammonia and methane. At this point, there was no oxygen. The steam condensed to form water which then rained down and produced seas where there were gaps in the molten–layer surface.
We have evidence of bacteria from 3.8 billion years ago, which means that life were in process about 700 million years after the Earth was created. These were found in shallow oceans close to thermal vents, as these were a source of heat and minerals.
These then became small plants and started to photosynthesise. This is the process in which plants uses sunlight to convert carbon dioxide and water to food energy and oxygen.
Most of the carbon from the carbon dioxide in the air became locked up in sedimentary rocks as carbonates and fossil fuels. Carbon also dissolved into the oceans. This, along with the photosynthesis, slowly reduced the levels of carbon dioxide.
The ammonia and methane (from the eruptions of the volcanoes) were now in the atmosphere and reacted with the oxygen to create nitrogen.
The oxygen from the photosynthesis was taken out again as soon as it had been produced. This was because of the reactions with other elements, such as iron. This continued until about 2.1 billion years ago when the concentration of oxygen increased markedly.
The oxygen levels built up until it created the ozone layer. The ozone layer filtered out the harmful ultraviolet rays from the sun which allowed the evolution of new living organisms to develop.
The Earth’s atmosphere now is:
- 21% Oxygen
- 78% Nitrogen
- 0.04% Carbon Dioxide
- 0.9% Argon
Underneath the Earth
As the Earth condensed, it heated up and began to melt. The impacts from meteorites added to the increasing heat. Molten rocks separated to form a crust, mantle, outer core and inner core.
The crust is the surface layer. It varies in depth – in the sea, is can be as little as 5km but it is normally, on continents, 70km. The crust is made up of tectonic plates, which I will explain more about later.
The mantle is below the crust. The mantle is over 1000°C and is molten rock nearer the crust of the Earth. Most of the rocks found in the mantle, however, are solid. Most of the Earth’s mass is in the mantle, which is composed of iron, magnesium, aluminum, and oxygen.
The outer core is the thickest section of the earth, after the mantle. This is extremely hot (3700°C – 4300°C) and contains about 10% sulphur.
The inner core is the centre of the Earth. It is solid, due to the high pressure from the other layers. It mostly contains iron, which makes it magnetic.
But the deepest a drill has ever tunnelled down is 12km. So how do we know about this? There is a lot of evidence to support this.
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The overall density of the Earth is much higher than the density of the rocks we find in the crust. This tells us that the inside must be made of something much denser than rock.
- Meteorites (created at the same time as the Earth) have been analysed. These meteorites have roughly the same density as the whole earth. A meteorite minus its iron has a density roughly the same as mantle rock.
- Scientists can follow the path of seismic waves from earthquakes as they travel through the Earth. The inner core of the Earth appears to be solid whilst the outer core is liquid (seismic waves cannot travel through liquids)
Tectonic Plates
200 million years ago, the continents were not shaped the way it is now. It was once all grouped together. This arrangement was called the Pangea. The continents are slowly moving away from each other. This was because the crust of the Earth is broken into seven large and many small moving plates. These plates, each about 50 miles thick, move away from each other at an average of a few inches a year.
There are three types of movement recognised at the boundaries between plates: convergent, divergent and transform–fault.
At convergent boundaries, the plates move towards each other and collide. This
causes the plates to fold up and often creates mountains.
At divergent boundaries, plates move away from each other. This causes gaps in the spaces where they used to be, and so molten rock would often have to rise up and fill the space.
At transform–fault boundaries, the plates move horizontally past each other. This causes
the plates to scrape across each other which then creates earthquakes.
This is the reason why earthquakes and volcanic eruptions are more likely to be found at the boundaries of the plates.
This picture shows the boundaries of the plates. England is on the Eurasian plate.
Evidence for this theory is the shape of the world itself. Some parts (such as Africa and South America) look like pieces of a jigsaw puzzle – they look like they were once joined. We have also found the same fossils on the coast of West Africa and East South–America.
Conclusion
In this piece of writing, I have learnt how the earth was created 4.6 billion years ago and how it was formed. I have also learnt the evolution of the Earth’s atmosphere, from being toxic to habitable. I now know the structure of the Earth in more detail and that the movements of the Tectonic plates. 250 million years later, The Earth will be a very different place from how it is now. Because the tectonic plates are moving away from each other, they will eventually collide once again and cause another pangea. This is an image of how it may look like.
