INTRODUCTION – PLANET EARTH

Planet Earth is our home. It is also home to millions of different plants and animals. Like the human race, the planet Earth has a history – only many millions of years longer.



 





 



 



 



The Utah desert in the United States. Deserts cover a fifth of the Earth’s surface.



 



 



 



 



Have you ever wondered what makes the light and dark of the day and night? Or what causes leaves to fall from trees, the sun to rise, or the tides to change? Find out how the movement of our planet around the Sun causes different patterns for life on Earth, and how the Earth’s climate has evolved throughout the planet’s history.



 





 



 



 



The Earth’s oceans are home to many different species of wildlife.


PLANET EARTH



 



When you look out to sea, the horizon (or skyline) seems curved. This is because our planet is shaped like a ball; it measures about 13,000 km across. The land and water that you see are only a small part of the Earth’s surface. Seen from space, Earth is almost perfectly round and appears to have a smooth surface. The highest mountain is 9 km high, and the deepest ocean is about 11 km deep. These distances are very small compared to the size of the Earth.



 



The Earth is one of many planets in our solar system. But it is the only planet which has the right conditions for human life. Other planets are too cold or too hot for us to live there, or their atmospheres are too poisonous.



Our planet Earth is made up of the atmosphere the land and the oceans.



 




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.



 



 


Continue reading "THE EARTH’S ORIGIN"

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.


SPINNING EARTH



 



 



 



The Sun appears to us as if moves around the Earth. In fact, the Earth spins around on its axis like a top, one turn every day. The place where you live, points towards the Sun in the day, and away from it at night.



The Earth spins on its axis at a great speed: the surface of the Earth moves at more than 1,500 km/h. Gravity provides the force which stops us being thrown off the Earth. Gravity extends a long way from the Earth, and pulls anything within its range towards the Earth. The Moon and artificial satellites are held in orbit round the Earth by the pull of its gravity.



 



 





 



 



The seasons



The seasons are caused by the Earth’s orbit around the Sun. The diagram shows how the Earth’s axis is slanted. During summer in the northern world, the midday Sun is over a region north of the equator, so its rays are more concentrated and feel hotter. During winter, the Sun is over a region south of the equator, and the rays reaching the north spread out over a larger area. They are less concentrated, so they feel less hot. Places south of the equator have their summer when places north of the equator have winter.



 



 



 



 



 


Continue reading "SPINNING EARTH"

CLIMATE



Different parts of the globe have very different climates. It gets hotter as you travel towards the equator because the Sun’s rays are more concentrated there. Places near the sea are usually cooler in summer and warmer in winter, than places inland. This is because the sea heats up and cools down less easily than the land, and so keeps the land near the sea warm in winter and cool in summer.



These differences in temperature cause movements of air across the Earth’s surface, called winds. Air movements, in turn, cause clouds and rain to form, as warm, moist air is cooled by rising up over hills.



 





 



 



The Sun’s rays travel about 150 million km to reach the Earth, providing our planet with light and heat. By rotating once every 24 hours, the Earth keeps the distribution of heat and light balanced with regular alternating periods of daylight and darkness.



The Sun’s rays fall at both the equator and the poles. At the equator, the Sun’s rays are spread over a smaller area than rays reaching the Earth at the poles. This means that sunlight reaching Polar Regions is less intense than sunlight reaching the equator. It also means that places near the poles are colder than places near the equator.



 



 





 



 



 



Temperatures drop considerably at the two poles, which are mainly covered in snow and ice.



 



 



 



 



 





 



 



The hottest and driest climates on Earth are found in the deserts.



 



 


THE ATMOSPHERE



 



 



 



 



The air you breathe forms a thin layer – the ‘atmosphere’ – around the Earth. This stretches up for a few hundred kilometres. As you go higher, there is less air. At the height where planes fly, there is too little air to breathe.



Most plants and animals need atmospheric gases in order to live. Animals rely on ‘oxygen’ to breathe and plants rely on ‘carbon dioxide’ to make their food. The air also contains nitrogen, small amounts of other gases, and some water vapour and dust. The ‘stratosphere’ and ‘ionosphere’ contain small amounts of ‘ozone’, which is a form of oxygen. A layer of ozone prevents harmful ultraviolet rays from the Sun reaching the Earth’s surface.



 



 



 



 





 



 



 



The aurora is caused by charged particles from the Sun hitting the atmosphere.



 



 



 



 



 



 





 



Exosphere



The highest, outer region of the Earth’s atmosphere is called the ‘exosphere’. Here, conditions are not very different from outer space as there is only very little air indeed.



Ionosphere



The ‘ionosphere’ is made up of electrically charged particles produced when radiation from the Sun hits the upper atmosphere. Near the poles, this causes a brilliant display of lights, the ‘aurora’.



Man-made pollution is now causing a hole to appear in the ozone layer.



Stratosphere



The ‘stratosphere’ extends up to about 80 km above Earth. As you travel upwards in this layer, the temperature rises slightly; nevertheless temperatures are below freezing point.



Troposphere



The ‘troposphere’ contains the air we breathe. Clouds rain and snow all form in this layer. As you travel up through the troposphere, it becomes colder.


EARTH’S BALANCE


The parts of the world affecting our lives are called the ‘environment’. This includes the Earth’s crust, oceans, atmosphere, plants and animals. If the balance between things added and things taken away from the environment is upset, the environment will change.



Many of the things we take from the Earth, like metals and fossil fuels, take millions of years to be replaced naturally. Other things, like wood from trees, are replaced more quickly, but if we use them too fast, they will disappear. Some things we produce, like poisons and waste, are not removed as quickly as we add them to the environment, so they build up as pollution.



Household waste is a serious pollution threat – the UK produces 27 million tonnes each year.



 





 



 



 



Cutting down huge forests can cause changes in the Earth’s atmosphere.



 



 



 



 





 



 



Pollution from industry can also disturb the Earth’s balance.


MORE ABOUT PLANET EARTH


Moving Crust



As the plates of the Earth’s surface move towards each other, one plate may rise over the other one. The one which goes up forms mountains and the one which goes down is slowly melted below the Earth’s surface. Where two plates move away from each other, magma is free to escape. It solidifies to form rock, and a ‘mid ocean ridge’ or a ‘volcanic island’ is formed.



 



 



 





Folding



Movements in the Earth’s plates can also cause the Earth’s surface to buckle and split. Where the surface splits and slips, a ‘fault’ is produced. Where the surface buckles, a ‘fold’ is formed. Many familiar features of the landscape are caused by folds and faults, and these can often be seen in layers of rocks at seaside cliffs. The diagram shows some of the features that can be produced in these ways.