A Very Brief History of the Earth
Earth's Age
The Earth's age is probably at least 4.5 billion years old. The oldest rocks ever discovered are about 4.3 billion years old. Scientists learn the age of rocks by measuring the amount of radioactive isotopes in the rocks. A radioactive isotope gives off invisible rays and changes into a different element. For example, uranium gives off rays and slowly changes into lead.
Earth's Early Developments.
Scientists theorize that the earth began as a waterless mass of rock surrounded by a cloud of gas. Radioactive materials in the rock and increasing pressure in the earth's interior gradually produced enough heat to melt the interior of the earth.
The heavy metals such as iron then sank, whilst the light silicates ( rocks made of silicon and oxygen ) rose to the earth's surface and formed the earliest crust.
The heating of the earth's interior also caused other chemicals inside the earth to rise to the surface. Some of these chemicals caused water, and others formed the gasses of the atmosphere.
The water took up 70% of the surface of the earth, and as today, the land masses take up 30% of the earth's surface.
The Earth's Crust
The earth's crust is about 8 kilometres thick under the oceans, to about 40 kilometres thick under the continents.
The crust is made up of three kinds of rock, igneous, sedimentary and metamorphic. All are formed from the same basic materials, but they are different because each was formed by a separate process.
Igneous rocks are formed when melted rock deep inside the crust cools and hardens, or erupts at the surface as lava.
Sedimentary rocks develop from materials that were once part of older rocks or of plants or animals. These materials were worn away from the land. They then collected in low places, layer upon layer, and hardened into rock.
Many sedimentary rocks contain shells, bones, and other remains of living things. Such remains, or the impressions of the remains in sedimentary rocks, are called fossils.
Metamorphic rocks are formed deep in the crust when igneous and sedimentary rocks are changed by heat and the weight of the crust pressing on them.
All rocks on the earth's surface are made of minerals, the most common solid materials found on earth. Minerals are themselves made up of basic chemical substances called elements.
Rocks in the earth's crust consist mainly of two elements, silicon and oxygen. The next most common elements in the crust are aluminum, iron, calcium, sodium, and magnesium, in that order.
The Shaping of the Continents
The earth's crust and upper mantle consist of about 15 plates. These plates move about 1.3 to 20 centimetres each year, in the direction shown by the arrows, The plates carry the continents and ocean floor with them.
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Scientists theorize that the continents once formed part of a single land mass.
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Little is known about the position of the earth's land before 200 million years ago. But some scientists theorize that a number of land masses came together to form Pangaea. It is thought that this single land mass started breaking apart 200 million years ago.
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Scientists theorize that the single land mass broke into two major land masses, Laurasia in the northern hemisphere, and Gondwanaland in the southern hemisphere. The above map depicts how the continents may have looked 150 million years ago.
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The above map shows the positions of the continents about 60 million years ago. At that time the continents were approaching their present positions, which are shown in black outlines.
Outline of the Earth's History
From the table above, you can see how important the Yea Fossil Site is, being of the Silurian Period which was over 400 million years ago.
Again the Yea Fossil site is very important because the fossils found there are the most ancient leafy foliage so far found on earth.
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For more information on Yea's Fossil Site, drop in to the Information Centre at the Old Railway Station, or phone them on 0357 972 663 |
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