Category Social Sciences

THE EARTH’S ORIGIN

 

The Earth began over 4,600 million years ago. We believe that the Earth and other planets were formed from a flat gas cloud around the Sun. This cloud formed into small, cold particles which attracted one another, collided, and formed larger particles. This took place over a few million years. As the larger particles collided, they became hot, and melted. Iron from these formed the central core of the Earth, and other substances surrounded it.

The molten outer layer of the Earth cooled to form a thin shell. Sometimes molten rock escaped from under the surface in volcanic eruptions, as it still does today. Gases escaped from inside the Earth to form an ‘atmosphere’.

 

 

 

 

Structure of the Earth

The outer layer of the Earth is a thin, solid skin, called the ‘crust’. Below it is a region called the ‘mantle’? The outer layer of the mantle is made of molten rock, called ‘magma’. Below the mantle is a region of molten rock under great pressure. The central region of the Earth is a solid core.

Scientists predict that the temperature in the Earth’s core is about  6,000 degree C. They have studied temperature changes at different depths beneath the Earth’s surface and also believe that the melting point of iron – found near the Earth’s central core – is a good indication.

 

 

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CHANGING EARTH

The remains of ancient animals, or ‘fossils’, tell us that our planet and the life on it were once very different from today. Throughout the ages, the Earth and its climate have changed, sometimes dramatically – like the coming of the ice ages. The most recent ice age began 70,000 years ago, and lasted for 60,000 years.

 

 

 

Plants and animals living on Earth have adapted to changes in the planet, to improve their chances of survival. Sometimes plants and animals have also caused changes to the Earth. For example, the soil in your garden was partly formed from the rotting remains of dead plants and animals, called ‘humus’.

 

 

 

 

 

 

 

 

During the ice age, the Earth’s land was mainly covered with snow and ice.

 

 

 

 

 

The Earth in a day

Imagine the changes of the Earth’s lifetime squashed to fit into a 24-hour day. On this scale, there would be no life on Earth until about 6.30am, when the first microscopic plants and animals would appear. Large animals with backbones, like fish, would not appear until about 9.15pm. The first plants would appear about 9.45pm and some fish would take to the land at 10.00pm. Reptiles would appear at about 10.30pm. Dinosaurs and early mammals would be living on Earth about 11 pm, but mammals would only become common about 11.40pm. People would appear on Earth 40 seconds before midnight and the first written history would be documented about ten seconds before midnight.

MOVING CONTINENTS

The Earth’s crust is relatively thin, and is formed of large, flat pieces, called ‘plates’. Each crustal plate may be thousands of kilometres across. These plates are moved very slowly by movements of the magma underneath. Where two plates push against each other, the crust crumples to form mountains. Where they move apart, magma can escape to form new rocks. Originally the Earth’s land was close together, but over time the crustal plates have moved the land apart.

 

 

 

 

Changing times

200 million years ago the main land masses that we know today were all grouped close together (1). Over many millions of years, crustal plates carrying the continents have moved away from one another (2). 65 million years ago the continents had moved even further apart (3). Today the continents are still moving a few centimeters each year (4). The coastlines are also being slowly eroded by the sea. How will the continents look in 100 million years time?

 

 

 

 

When crustal plates press against each other, they do not slide smoothly. Instead, they press and press until suddenly they break at a weak point. As they break, there is a sudden movement of the crust, which we feel as an earthquake.

Occasionally, some of the mantle can push through weak points in the crust, and form a volcano. The crust is usually weaker along or near the lines where plates meet.

Earthquakes can cause a great deal of destruction.

 

 

 

 

 

The Sierra Nevada Mountains, USA, were formed by two crustal plates pushing against each other.

ROCKS AND EROSION

 

Rocks are not as permanent as they seem. Rain, wind and freezing weather can cause them to break up over a period of time, into tiny pieces which are washed away by rain and rivers. This process is called ‘erosion’. Where rivers slow down or meet the sea, the rock fragments are left behind forming ‘sediment’, such as sand or mud. Layers of sediment gradually thicken and the tiny pieces of rock become stuck together to form larger rocks. These are called ‘sedimentary’ rocks.

Erosion and sedimentation are slow processes. They may take thousands of years to have a noticeable effect on the landscape about us.

 

 

 

Sedimentary rocks

‘Sedimentary’ rocks are made of layers of small particles. For example, chalk cliffs are made from layers of very small shells and skeletons of sea animals.

 

 

 

 

Metamorphic rocks

Sedimentary rocks are changed when they are subjected to heat and pressure near a volcano. This is how ‘metamorphic’ rocks, like marble are formed.

 

 

 

 

Igneous rocks

‘Igneous’ rocks are formed from magma which has cooled, either slowly inside the Earth, or quickly on the surface. Pumice or pummy stone and granite are formed like this.

 

 

 

 

The Delicate Arch, Utah, USA. Rock shapes like these are formed by erosion.

RICHES OF THE EARTH

All the metals we use are taken from the Earth. Most of them are found in rocks, combined with other substances, while some, like gold and copper are found as pure metal. Many of the ‘fossil fuels’ we use, like coal, gas and oil, are also taken from the Earth. Coal is usually dug out of the ground while gas and oil are extracted by drilling deep holes in the Earth.

Rocks in the Earth’s crust can often be used for specialized jobs. For example, one rock called ‘mica’ is found in thin transparent sheets. It can be used to make windows for high-temperatures ovens, where glass would melt.

 

 

How coal was formed

Coal has formed over millions of years from the remains of ancient forests growing on swamps.

As forest trees and plants died, they fell into the swamps forming layers of dead vegetation. Later, sedimentary rocks formed on top of these layers.

The pressure caused by the rocks made the layers of vegetation denser and harder and formed them into a solid rock – coal.

 

 

 

 

 

Coal is a fossil fuel often mined from underground.

EARTH’S OCEANS

The oceans cover about two-thirds of the Earth’s surface and have existed for more than 3,000 million years!

The top layer of the ocean has a rich plant and animal life, the ‘plankton’. Millions of these minute organisms produce food for all the other sea animals. Plankton is one of the oldest forms of life on Earth. Deeper down at about a hundred metres, it is too dark for plants to grow, so the ocean bottom is mostly bare.

Winds blowing across the ocean cause waves. When winds blow over a great distance across the sea, they build waves which may be many metres tall.

 

 

 

The tides

Tides are caused by the pull of the Moon and the Sun on the waters of the Earth. When both the Sun and the Moon are in line with the Earth, their pulls add up, and the tides are very large. These are called ‘spring tides’. When the Sun and Moon are out of line with the Earth, their pulls oppose each other, and the tides are not so large. These are called ‘neap tides’.

 

 

 

 

Cannon Beach, Oregon, USA. The action of the waves can cause erosion of the coastline.

 

 

 

 

 

Papua New Guinea islands, Pacific Ocean – the seas make up two thirds of the Earth’s surface.