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11. EARTH SCIENCE

11.1 INTRODUCTION Men of the Apollo 17 mission to the moon took this beautiful photograph of our planet Earth. This is the world we live on – a huge ball of rock and water, surrounded by an atmosphere of air about 100 km thick. Beyond that is the almost empty region we call space. The Earth has a diameter of about 12,750 km and a mass of almost 6 x 1021 tonnes. It is rushing through space on a roughly circular orbit around the sun at a speed of over 100,000 kph! The radius of the Earth’s orbit is about 150 million km and each orbit takes exactly a year to complete. Our orbit around the sun gives us our year. As the Earth speeds along its orbit, it also spins like a top! At the equator, the surface moves from left to right (West to East) at more than 1600 kph. The Earth makes one complete spin every day. It is the spinning of the Earth that gives us day and night. We will learn more about all this later in the chapter.

Our closest companion in space is the Moon which orbits the Earth every 28 days at a distance of about 384,400 km. The time of the moon’s orbit gives us our month. However, the sun is our most important neighbour. It is our nearest star and it gives us the light and heat that make life on Earth possible. The sun is huge! Its diameter is more than 100 times that of the Earth, and almost 1 million Earths could fit inside it. The temperature at the surface of the sun is about 6000ºC and its light is so bright that we can not usually see any other stars during the day. They are too far away and too faint. But if we look at the sky on a clear night, we can see that space seems to be full of stars. With our eyes alone we can see several thousand stars, and with a telescope we can see many thousands more. Astronomers believe that there are actually many billions of stars! In this chapter, we will start by learning a bit more about the stars. After that we will look at our own ‘solar system’, comprising the sun and its family of planets, moons and other objects. We will focus mainly on the Earth and learn what causes day and night, the seasons of the year, and the changing phases of the moon. We will also explain how eclipses happen, how the moon and the sun cause the tides that move the oceans, and how man-made satellites are changing our world. In the last part of the chapter, we will focus on the Earth itself. Over 70% is covered by water, and the dry land is mostly covered by soil and vegetation. We will discuss how the Earth was formed and how its surface has been shaped by the movements and collisions of gigantic rocky plates, and by earthquakes, volcanos and erosion. We will learn about the making of mountains and valleys, rocks and soil.

1. What is (i) space, (ii) an astronomer, (iii) a satellite, (iv) an orbit, (v) the solar system?

2. How far away are (i) the moon, (ii) the sun?

3. What gives the Earth it’s (i) 24 hour day, (ii) 28 day month, and (iii) 365 day year?

4. Look at the photo above. List all the things you can see in the picture that are mentioned in the last paragraph on this page.

5. Why does life on Earth need light from the sun?

Africa

Antarctica

Indian ocean Atlantic

ocean

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11.2 STARS AND GALAXIES When you look up at the sky on a dark night you can see thousands of stars. They are all very far away but there is one star that is much closer – the sun! A star is a huge ball of very hot gas held together by gravity. Most of the gas is hydrogen and stars change hydrogen into helium by a process called nuclear fusion. Nuclear fusion releases vast amounts of energy as heat and light. The temperature in the middle of a star may be several million degrees. The sun is an average star and nuclear fusion in the sun creates the heat and light that reach us on Earth, even though we are about 150 million kilometres away! The sun was born more than 4.5 billion years ago when enough hydrogen came together for nuclear fusion to start. It is a middle-aged star, but there is still enough hydrogen left for it to continue as it is for about another 5 billion years. In the end, all stars either collapse or explode, but before they do that, they start converting the helium they have made into other elements. In fact, scientists believe that all the elements we know today (except hydrogen) were made in old stars! A constellation is a group of stars that appear to form a pattern in the

sky. Many constellations have been recognised since ancient times and some have been used by travellers to find their way. If you live north of the equator, a constellation called the Plough can be used to find north. Two stars in the plough, point to another fairly bright star called the pole star. The pole star is always in the north. If you live south of the equator, the

Southern Cross can be used to find south. There is no bright south pole star, but the long axis of the cross points to a nearby part of the sky that is always south. The stars in a constellation are not really close together. Some may be much farther away than others – it is only from our point of view on the Earth that they seem to be in a group. However, stars really do occur in huge groups called galaxies. Most of the stars we can see at night are in our own galaxy which is called the Milky Way. On a dark night the Milky Way looks like a fuzzy band of stars right across the sky. Our sun is on the edge of the Milky Way which contains many billions of stars. And the whole universe contains many billions of galaxies, but we can see only a few with our naked eyes. Galaxies look like tiny, fuzzy spots in the sky and we need a good telescope to see them properly. Astronomers believe that the universe started about 13 billion years ago with a ‘big bang’! Since then it has been expanding and now it is more than 13 billion light years from side to side.

1. In astronomy we have to think about huge numbers. How big is one billion?

2. How many km are there (i) in one light year, (ii) to the nearest star, (iii) from one side of the universe to the other?

A light year is the distance light travels in a year. Light from the nearest star (not counting the sun) takes 4.5 years to reach Earth. We say it is 4.5 light years away. Light travels 300,000 km every second, so one light year is nearly 1013 (ten thousand billion) kilometres.

The Plough

The Milky Way galaxy

The Southern Cross

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11.3 THE SOLAR SYSTEM The solar system consists of the sun and all the planets and other bodies that orbit around it. There are 8 planets and most of them have moons that orbit around them. There are also many smaller bodies including asteroids, dwarf planets and comets. The solar system formed about 4.6 billion years ago from clouds of gas that included the remains of earlier stars. More than 99% of the gas condensed to form the rapidly spinning sun. The remaining gas spread out in a spinning disk but could not escape the pull of the sun’s gravity. Finally, over the next 50 million years or so, the planets and other bodies condensed out of the disk. They are still held in place in their orbits by the gravity of the sun. You will learn more about the origin of the Earth in Module 11.10. The four inner planets, the ones nearest to the sun, are huge balls of rock and metal. Mercury has the smallest orbit, closest to the sun, followed by Venus, Earth and Mars. Mercury and Venus are much hotter than Earth but Mars is colder. Earth is the largest of the inner planets and Mercury is the smallest. As you know, Earth has a moon that orbits around it. Mercury and Venus do not have moons, but Mars has two! The sizes of all the planets, together with their distances from the sun and the time they take to complete their orbits, will be found on the next page with a diagram of the solar system.

Outside the orbit of Mars is the asteroid belt. Like the inner planets, asteroids are made of rocks and metals. But they are much smaller than planets and they are irregular in shape. They range from tiny particles of dust to huge rocks hundreds of kilometres wide. There are hundreds of thousands of big asteroids, more than 1 km wide, but they are so spread out that they are very little danger to space craft. The four outer planets are Jupiter, Saturn, Uranus and Neptune. All the outer planets are very large and very cold compared to the inner planets. Jupiter and Saturn are called the gas giants because they consist mainly of hydrogen

and other gasses. Uranus and Neptune are called the ice giants because they consist mainly of gases and vapours that have frozen into icy solids. Each of the outer planets has many moons and Saturn (pictured on the right) is famous for the rings that surround it. The rings are made of dust and particles of rock and they can easily be seen from Earth with binoculars or a small telescope. Even beyond the orbit of Neptune, there are other bodies orbiting the sun. These include dwarf

planets and comets. There are several dwarf planets but only Pluto is well known. Dwarf planets, like ordinary planets, are roughly spherical, but their orbits are different. Their orbits are elliptical instead of almost circular, and they lie at an angle to the ecliptic. The ecliptic is the flat disk that contains the orbits of the true planets. Comets are irregular lumps, a few kilometres wide, made of icy materials. Their orbits are eccentric ellipses that take them way out into space and back again close to the sun. As they approach the sun, some of the ice is vaporised and streams out forming a ‘tail’ that can sometimes be seen from Earth. The most famous comet is called Halley’s Comet. It takes about 75 years to complete its orbit so we can only

observe it once every 75 years! The picture on the left shows a comet approaching the sun. 1. Name the inner and outer planets and describe

three general differences between them. 2. Estimate the approximate diameter, in AU, of the

orbit of the asteroid.

Astronomical Units (AU) For distances inside the solar system, the kilometre is too small to be a convenient unit but light years are too huge! Astronomers therefore use astronomical units (AU) where I AU is the mean radius of the Earth’s orbit – about 150 million km.

Observing planets: We see stars at night because they shine with their own light. But we can see planets only because they reflect light from the sun. Because it is close to us and close to the sun, Venus looks brighter than any of the stars. It can often be seen in the evening, following the sun down in the west – or in the early morning in the east. Although it is really a planet, people often call it the evening star (or sometimes the morning star). The word planet means ‘wanderer’. Unlike the stars, which always appear in fixed patterns or constellations, the planets appear to move around the sky. Jupiter is also brighter than any star, almost as bright as Venus, but it is not quite so easy to find!

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The Solar System (not to scale)

SUN Mercury

Venus

Earth Mars

Jupiter

Saturn

Uranus

Neptune

Planet Mean diameter

(km)

Mean distance from

sun (AU)

Time of orbit

(years)

Number of moons

Mercury 4,880 0.39 0.24 0

Venus 12,100 0.72 0.62 0

Earth 12,750 1.00 1.00 1

Mars 6,790 1.52 1.88 2

Jupiter 142,800 5.20 11.86 63

Saturn 120,000 9.54 29.46 60

Uranus 51,120 19.18 84.01 27

Neptune 49,530 30.06 164.8 13

direction of motion

asteroids

Dwarf planets and comets orbit beyond Neptune

moon

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11.4 OUR PLANET EARTH Our planet Earth is the largest of the inner planets of the solar system. It is the third planet out from the sun. The photograph on the right was taken from a rocket out in space. The Earth is almost a perfect sphere but it is slightly flattened at the north and south poles. It takes exactly one year (365¼ days) to complete its orbit around the sun and it spins from left to right (west to east) on an axis through the poles. It takes exactly one day (24 hours) to complete each rotation. More than 70% of the Earth’s surface is covered by the oceans and there are large areas of land that we call continents. On this photo, you can see North America and South America, and on the photo in Module 11.1 you can see Africa and Antarctica. The other continents are Europe, Asia and Australia. (Sometimes Europe and Asia together are called Eurasia, and sometimes the name Oceania is used to include Australia, New Zealand and the islands of the Pacific Ocean). The equator is an imaginary line around the Earth, half way between the north and south poles. Some basic facts about the Earth are summarised in the table below.

Some basic facts about planet Earth

Diameter at equator

Circumference at the equator

Mass Mean distance from sun

Time for 1 rotation

Time for 1 orbit

12,756 km 40,075 km 5.97 x 1024 kg 149.6 million km 24 hours 365¼ days

Day and night: The axis of rotation of the Earth is an imaginary line passing through the north and south poles. The Earth rotates around this axis once every day (24 hours) and it is this rotation that causes day and night. In the photo above, the sun is shining from the left. The side of the Earth facing the sun is having daylight. As the Earth rotates from left to right (west to east),

you can see that South America will soon pass from daylight into night. Of course, to people on the Earth, it looks as if the sun rises in the east, moves across the sky, and sets in the west. We are not usually aware that the cycle of day and night is actually caused by the earth spinning, not by the sun moving! Try the ‘thought experiment’ in the box using the simple model below.

1. How do you think we can define an hour? 2. How many times does the Earth spin on its

axis in a year? 3. On Earth, how long are the longest day and

the longest night?

South Pole

North Pole

South Pole

Equator

S America

N America

A thought experiment: Push a pencil or a sharpened stick through an orange. You will also need a torch. Go into a dark room and switch on the torch. Hold the pencil upright about a metre away from the torch so that light shines on the orange. The Orange represents the Earth and the torch represents the sun. The half of the orange facing the torch is in ‘daylight’ and the other half is in ‘night’. Mark a cross near the middle of the orange on the ‘night’ side.

Now imagine that you are as small as a tiny ant and that you are standing on the cross. Slowly rotate the orange so that the cross moves from left to right, from the dark into the light, and then right around and back to its starting point. Imagine what you will see as you move from the dark into the light. You will see the torch (sun) slowly appearing over the edge of the orange (earth). The sun is rising in the east! As the orange turns, the ‘sun’ seems to climb in the ‘sky’ until it is overhead. Now it is mid day! As the ‘earth’ keeps turning, turn round and keep looking at the ‘sun’. It seems to get lower and lower in the ‘sky’ until it disappears behind the edge of the ‘earth’. Now the sun has set in the west!

torch

orange and pencil

Atlantic ocean

Pacific ocean

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11.5 EARTH’S CLIMATES AND SEASONS The Earth’s axis of rotation is not at right angles to its orbit. It is tilted to the right by about 23½º. This tilt makes a huge difference to the Earth’s climate. Latitude and climate. Lines of latitude are imaginary lines parallel to the equator. Latitude is measured in degrees north or south. The equator is 0º and the two poles are 90º north or south. The most important lines of latitude are shown in the diagram on the right. Near the equator, between the tropics of Cancer and Capricorn, the sun is almost

straight overhead every day. This region has a hot, tropical climate. There are no summer and winter seasons, and day and night are roughly the same length all year. The regions north of the Arctic Circle and south of the Antarctic Circle have polar climates with long, very cold winters and short, mild summers. In summer there are days when the sun is still visible at midnight, although it gives only a little heat. And in winter there are long nights when the sun never rises! Finally, the regions between the tropical and polar regions have temperate climates with warm summers and cool winters. In summer, the days are longer than the nights and in winter, nights are longer than days. The seasons. Study the diagram below. With your

finger, follow the Earth as it orbits around the sun. The orbit is almost circular so the Earth’s distance from the sun does not change much. The seasons have nothing to do with the small changes of distance from the sun. Notice particularly that the tilt of the Earth’s axis does not change. Now focus on what happens on the 21st of March every year. On this date, the Earth’s axis is tilted neither towards the sun nor away from it and the sun shines straight down on the equator. Now move on three months. Every June, the Earth’s axis is tilted towards the sun so it shines straight down on the Tropic of Cancer. Now it is mid-summer in the northern hemisphere. Notice that, as the Earth spins, the North Pole remains exposed to the sun so that daylight there lasts for 24 hours. At the same time, the southern hemisphere is tilted away from the sun so it is mid-winter there. As the Earth spins, the South Pole remains hidden from the sun so the night there lasts for 24 hours. Moving on around the orbit again, in September the situation is the same as in March. The sun is over the equator again. And finally, in December every year, the Earth’s axis is tilted away from the sun. The sun is straight over the Tropic of Capricorn. Now it is mid-summer in the southern hemisphere and mid-winter in the northern hemisphere. 1. Draw a diagram of the Earth showing the axial tilt.

Mark the poles and the tropical, temperate and polar regions. Where is the land of the midnight sun?

2. Explain in your own words why, in December, it is summer in the southern hemisphere and winter in the northern hemisphere.

21st March

23rd September

21st December

21st June

Arctic Circle, 66.6ºN

Tropic of Cancer, 23.5ºN

Equator, 0º

Tropic of Capricorn, 23.5ºS

Antarctic Circle, 66.6ºS

Why is the sun hotter at the equator? The diagrams show the sun’s rays hitting the Earth at the equator (A) and in the temperate region (B). Near the equator (i) the sun’s rays pass through less air, so less heat is absorbed, and (ii) the heat is concentrated on a much smaller area of the ground.

A B

North Pole

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11.6 THE MOON The moon is our closest neighbour in space and the brightest object in the night sky. It shines by reflected sunlight. Many ancient cultures regarded the sun as a god and the moon as a goddess and they have given their names to Sunday and Monday (moon day). The moon was probably formed at about the same time as the Earth. It has a diameter of about 3,474 km so it is less than a third the size of the Earth. It orbits the Earth at an average distance of about 384,400 km, completing one orbit in 28 days. The moon’s orbit is at an angle to the Earth’s equator and also to the ecliptic. As it orbits, the moon turns slowly on its axis so that the same face is always towards the Earth. The moon has no atmosphere. Its surface is bare and rocky with some flat, dark areas and many circular craters. Astronomers believe that the craters have been caused by the impact of meteorites – irregular chunks of rock, like the asteroids but generally much smaller. The phases of the moon: The shape of the moon, as seen from the Earth, changes every day on a one month cycle. Study the diagram of the moon orbiting the Earth. Sunlight comes from the

right and shines on the half of the moon that faces it. We see the full moon when it is on the opposite side of the Earth from the sun. In the diagram, the arrow from the Earth, through the moon at A, points to the view that we see. As the Earth spins on its axis, the full moon appears to rise in the east at about 6 pm, just as the sun is setting in the west. A week later, the moon has moved on a quarter of the way around its orbit to B. Now we can see only the left half of it. And a week later, it has moved on to C. Now the sun shines on the back of the moon! The moon rises and sets at about the same time as the sun but we can not see it because its face is not illuminated. A week later the moon has moved on to D, so now we can see the right half of it. And finally, a week after that, the moon is back at A and we can see the full moon again! 1. (i) What makes the moon rise and set? At about

what times does the moon rise and set when (ii) it is full, (iii) you can see its left half, (iv) there is ‘no moon’ and (v) you can see its right half?

2. Sometimes we can see a crescent moon in the west, early in the evening. Use the diagram above to explain exactly what is happening and why the moon appears to be this crescent shape.

Earth rotating

NO MOON FULL MOON

rises 6pm – sets 6am

rises 9pm – sets 9am rises 3am – sets 3pm

rises 6am – sets 6pm

rises 9am – sets 9pm rises 3pm – sets 3am

Moon orbiting

A

B

C

D

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11.7 ECLIPSES Eclipse of the sun. The pictures on the right show a partial eclipse of the sun and a total eclipse of the sun. An eclipse of the sun is called a solar eclipse. A solar eclipse happens somewhere in the world every year, but at any one place a total eclipse is a rare and dramatic event. The light of the sun is cut off by the moon and it becomes quite dark. At first, it looks as if a bite has been taken out of the sun, and in a total eclipse the whole of the sun gradually disappears behind the dark moon, except for a faint halo called the corona. Fortunately, eclipses only last for a few minutes. Study the diagram. An eclipse of the sun happens when the moon comes between the sun and a particular place on Earth. This can only occur at a time of ‘no moon’, and when the moon lines up exactly between the Earth and the sun. Solar eclipses are rare because the moon’s orbit is at an angle to the ecliptic. Because of this, the moon does not line up exactly with the sun and the Earth very often. An eclipse can only be seen from the area of the Earth that falls under the moon’s shadow.

Eclipse of the moon. The picture on the left below shows a partial eclipse of the moon. An eclipse of the moon is called a lunar eclipse. A partial lunar eclipse happens two or three times a

year, but a total eclipse is quite rare. An eclipse of the moon can occur only at a time when the moon is full, and when the Earth lines up exactly between the moon and the sun. The diagram shows the full moon in its orbit as it approaches, and passes through, the Earth’s shadow. A partial lunar eclipse occurs when only part of the moon passes through the shadow. But a total eclipse, as shown in the diagram, occurs when the whole of the moon passes through the shadow. You can see a solar eclipse only from one particular place on the Earth’s surface, and it lasts only a few minutes. However, you can see a lunar eclipse from all over the night side of Earth at the same time, and a total eclipse may last for two or three hours.

1. Why do total lunar eclipses last longer than total solar eclipses?

2. We can often see the moon during the day, but a lunar eclipse can only happen at night. Why is this?

3. If the moon’s orbit lay in the ecliptic, how would that affect solar and lunar eclipses?

Partial solar eclipse Total solar eclipse

moon

Safety note: Never look straight at the sun, even for a second – this can cause permanent damage to your eyes. If you are watching an eclipse, always make sure you only look at the sun through very dark glass.

Partial lunar eclipse

Solar panels and lunatics! The word solar comes from ‘solaris’ which was the Latin word for the sun. Now, more and more houses have solar panels which collect the sun’s energy to make electricity, or heat water. The word lunar comes from ‘lunaris’ which was the Latin word for the moon. In the past, people were very frightened by eclipses; some even went mad! People used to think that madness was caused by the moon. That is why mad people are sometimes called lunatics.

SUN

Earth

partial solar eclipse

total solar eclipse

Eclipse of the sun (not to scale)

SUN

Eclipse of the moon (not to scale)

total lunar eclipse

Earth

moon

moon

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11.8 GRAVITY AND TIDES Gravity. We met gravity in Module 10.4 as the force with which massive bodies like the Earth and the sun pull other bodies towards them. The strength of the force increases with the masses of the bodies, and decreases with the distance between them. (In fact, gravity decreases with the distance squared. If we move twice as far away from a body, the force of its gravity will go down to a quarter of what it was!). For everyday purposes, we only need to worry about the gravity of the Earth because it is the only massive body that is close to us! Earth’s gravity is the force that gives everything weight, and that makes anything that is not supported fall towards the ground. In the solar system, it is gravity that holds the planets and other bodies in their orbits around the sun. It is also gravity that holds the moon in orbit around the Earth. Look at the picture on the right. The athlete is ‘throwing the hammer’. She has a heavy weight on the end of a long wire. She whirls it round and round in a circle as fast as she can. When she lets go, the weight flies away in a straight line. It will go in whatever direction it is moving at the time. It was only the wire, pulling in towards the centre, that was keeping the weight moving in a circle. In the same way, it is only the force of gravity, pulling in towards the sun, that keeps the planets moving around their orbits instead of flying off into space!

Tides. Anyone who lives close to the sea knows that the sea moves up and down the beach every day. This regular movement is called the tide. When the sea reaches its highest point on the beach we call this high tide, and when it reaches its lowest point we call this low tide. The photographs show low tide and high tide at the same place. There are two high tides and two low

tides every 24 hours. The diagram below (not to scale) shows a view looking down on the Earth from above the North Pole. The moon’s gravity pulls on the oceans and causes a ‘bulge’ at the point closest to the moon. This bulge is high tide. Because of the way the water flows around the globe, there is also a high tide on the opposite side of the Earth and low tides half way between these. As the Earth turns on its axis every 24 hours, every place gets two high tides and two low tides. The gravity of the sun effects the tides too, but the sun’s effect is much smaller than the moon’s because it is so far away. At the time of full moon and no moon, the sun is lined up with the moon so its gravity adds to that of the moon. The high tides are a bit higher than usual, and the low tides are a bit lower. These bigger than average tides are called spring tides. Smaller than average tides occur at the times when we can see a half moon and these are called neap tides.

1. What are the two factors that determine the strength of the force of gravity between two bodies.

2. What is a neap tide? Use a diagram to show how the sun and moon combine to cause neap tides.

high tide

high tide

low tide

low tide

moon’s gravity

the oceans Earth

moon

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11.9 SPACE EXPLORATION AND TECHNOLOGY On October 4th 1957, a Russian rocket carried the first man-made satellite into orbit around the Earth. The satellite was called Sputnik 1. For three months it circled the Earth every 96 minutes at a height of about 7000 km, sending back information about conditions in space to scientists on the ground. Since then, tens of thousands of man-made satellites have been launched. The satellites have a useful life of 5 to 20 years and there are now about 3000 working satellites in orbit. They are used mainly for communications, global positioning, weather monitoring and research. Two of the most exciting are the Hubble telescope that sends back images of the distant universe to astronomers on Earth, and the International Space Station that is an orbiting laboratory where scientists carry out research that is impossible on Earth. In addition to launching satellites, space rockets have carried men to the moon and back, and have

flown past all the planets collecting scientific data and taking pictures. Remote controlled vehicles have been landed on Mars as well as the moon, and plans are being made for astronauts to travel to Mars. Some highlights of space exploration are listed in the table.

Global positioning system. Towards the end of the last century, a global positioning system (GPS) was set up by the government of the USA. 24 to 32 satellites are maintained in orbits which are chosen so that, for any place on Earth, at least six will be in the sky there all the time. A GPS receiver uses signals from these satellites to calculate the receiver’s position on the Earth’s surface. Even a small, hand-held GPS receiver is accurate to within 10 to 20 metres. GPS receivers are now used for navigation by aircraft, ships and even cars. Other uses include map making, surveying, locating mobile phones and tracking migrating animals. Communications satellites. The majority of man-made satellites are communications satellites. These are often placed in geosynchronous orbits which means that they remain over the same

place on the Earth’s surface all the time. The satellite is a relay station which receives microwave radio signals from one or more transmitter stations and passes the signals on to many receivers below. One of the receivers may be a new transmitter station on the ground which then passes the signals on to another satellite and so on. Important uses of communications satellites include telephone and video links, radio and television broadcasts, and high speed internet connections.

1. What is (i) an astronaut, (ii) a relay station? 2. List 4 uses of man-made satellites. 3. Why is ‘space debris’ a problem? Find out

and report on what is being done about it.

Year Event

1957 First man-made satellite orbits Earth; Sputnik 1

1957 First animal to orbit Earth; the dog Laika

1959 First photos of the back of the moon

1961 First man to orbit Earth; Yuri Gagarin

1963 First woman to orbit Earth; Valentina Tereshkova

1969 First man to land on the moon; Lance Armstrong

1990 Launching of Hubble Space Telescope into orbit

1998 Establishment in orbit of International Space Station

Astronaut John Young on the moon

Escape velocity is the speed which a body such as a rocket has to reach in order to escape from the gravity of a planet, moon or star. For our planet Earth, the escape velocity is 11.2 km/sec.

receiver

receiver

transmitter station

receiver

communications satellite

receiver

Space debris. Space around the Earth is now littered with parts of old rockets, satellites that no longer work, and similar rubbish. All this ‘space debris’ eventually falls back towards the Earth. It is all moving very fast and most of it burns up high above the Earth because of friction with the air. Very occasionally, something big reaches the ground, but so far no one has been hurt!

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11.10 THE ORIGIN AND STRUCTURE OF THE EARTH The Earth was formed with the rest of the solar system about 4,600,000,000 years ago. A cloud of gas and dust started to collapse under the force of its own gravity. Some of the dust was the remains of a star that exploded. Most of the material condensed to form the sun, while the rest became spread out in a thin rotating disk. Gradually, because of gravity, the gas and dust in the disk started to clump together. First it formed the gas-giants Jupiter and Saturn, and then the rest of the planets including Earth. As the Earth grew more and more massive, the force of its own gravity created enormous pressures, crushing and heating the new planet into a ball of molten rock and metal. Finally the outer layer started to cool, forming a solid crust, and water started to accumulate in the atmosphere. At about this time, still more than 4.5 billion years ago, the moon was probably formed when a very large meteoroid skimmed past the Earth and knocked off a huge chunk.

The internal structure of Earth is shown in the diagram. The atmosphere is a mixture of gases including nitrogen (79%), oxygen (20%), inert gases (1%) with a small amount of carbon dioxide and a variable amount of water vapour. It has no clear outer limit, but a line 100 km above the Earth’s surface is often taken as the boundary between the atmosphere and space. The crust is composed of rocks and minerals. More than 70% of it is covered by the oceans and most of the rest is covered by soil and vegetation. Compared to the size of the Earth, the crust is very thin, varying from about 5 to 50 km in thickness. It is thinnest under the oceans and much thicker on the continents. If the Earth was shrunk to the size of a tomato, the thickness of the tomato skin would be about right for the crust!

The mantle is composed of denser rocks than the crust and is about 2,890 km thick. The mantle is hot varying from 500ºC or more under the crust to 4000ºC where it meets the core. Although it is mainly solid, the mantle behaves like a very sticky liquid. Over millions of years, slow convection

currents (see Module 7.3) carry the hottest rocks near the core towards the crust where they spread out and cool before sinking back towards the core again. The core has a radius of about 3,470 km and consists mainly of iron with some nickel. The temperature in the core varies from about 4000ºC to 6000ºC. The outer part of the core is liquid but, because of the huge pressure, the central part is solid. 1. What is nickel, and what do you know about it? 2. Copy the diagram of the internal structure of the Earth and add arrows to

show a convection current in the mantle.

The origin of the solar system

1. Cloud of gas and dust

2. Sun and gas-giants condense first, remaining dust spreads out in rotating disk

3. Other planets condense

core

mantle

crust

atmosphere

The internal structure of the Earth

The biosphere is the name given to the layer of the Earth that supports life. It includes the lower atmosphere, the oceans, and the upper parts of the crust.

How do scientists know? Earthquake vibrations travel right through the Earth. Like light rays, these vibrations are refracted and reflected by different materials. Study of how these vibrations move through the Earth has helped scientists to build up their knowledge about its internal structure.

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11.11 THE EARTH’S TECTONIC PLATES The Earth’s crust includes ocean floors, islands, and continents with high mountains, plains and deep valleys. In some places, earthquakes shake the ground and volcanos spew out ash, smoke and red hot larva. In the rest of this chapter, we will consider how these things come to be. The Earth’s crust is not all one piece! It is made of a number of huge ‘plates’ which float on the upper part of the restless mantle. These tectonic plates are jammed tightly together and jostle one another as they move with the convection currents at the top of the mantle. The next page shows where the plates are located on the world map. Look at the map and find out which plate you live on. The plates jostle each other in slow motion, moving only a few centimetres a year, but huge forces are involved. If you live near the edge of a plate you are probably familiar with the earthquakes and volcanos that these movements can cause! If you live nearer to the middle of your plate, life is probably more peaceful. There are three main kinds of boundaries where plates meet: divergent boundaries, convergent boundaries and transform boundaries. Divergent boundaries (shown on the map by ) occur when two plates are moving apart. This happens when hot, semi-molten rock in the mantle rises by convection, and spreads out under the plate. The plate is forced upwards, then cracks and divides, forming a ‘rift valley’ with parallel ridges on each side. As the plates move apart, magma (molten rock) fills the gap creating new crust. Some magma may reach the surface and create volcanos. A divergent plate boundary runs down the middle of the Atlantic ocean. This gives rise to the mid-Atlantic ridge – a long range of underwater mountains with a rift valley between them. There are volcanos at several places along the ridge including Iceland and the islands of the Azores. Another divergent plate boundary is between the African and Arabian plates. This has created the Red Sea between Africa and Asia. Running south from this is the well-known rift valley of Africa where the African plate is probably starting to break into two plates. Convergent boundaries (shown by ) occur when two plates are colliding. One plate often gets pushed under another. The plate on top buckles forming mountains. If the plates meet under

the oceans, then there will be islands instead of mountains, and there will be a deep ocean trench where one plate slides under the other. The lower plate is forced down into the mantle where it is slowly absorbed. Materials in the lower plate react with the mantle generating heat and forming magma. This may force its way to the surface creating volcanos. For example, the Nazca plate is colliding with, and going under, the South American plate. This has forced up the mountain range of the Andes which runs along the west side of South America and includes a number of volcanos. It

has also created the Peru-Chile trench in the ocean off the west coast. Other examples of convergent boundaries include the mountains and volcanic islands of Indonesia where the Australian plate is colliding with, and going under, the Eurasian plate. The highest mountains in the world are the Himalayas to the north of India. They have been created by the collision of the Indian and Eurasian plates. Both plates are covered with thick, continental crusts so neither is going under and both plates are buckling. The mantle is not disturbed so there are no volcanos. Transverse boundaries (shown by ) occur when plates are sliding past one another. The example shown on the map is where the Pacific plate is sliding past the North American plate. This is the famous San Andreas Fault that causes earthquakes in California.

1. Explain how tectonic plates are created at divergent boundaries and destroyed at convergent boundaries.

2. Look at the map. Africa and South America were once parts of the same continent. What happened?

PLATE PLATE

MANTLE MANTLE

magma

trench mountains or islands

PLATE PLATE magma

MANTLE

mountains mountains

rift valley

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The Earth’s Main Tectonic Plates

mid-Atlantic ridge

mid-Atlantic ridge

rift valley

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11.12 EARTHQUAKES AND VOLCANOS Most earthquakes and volcanos occur near tectonic plate boundaries in the regions listed in the table. The Ring of Fire refers to the volcanic regions around the Pacific Ocean in the Americas, Asia and Oceania. Earthquakes are sudden movements of the Earth’s surface. Hundreds of minor earthquakes occur somewhere every day and we don’t even notice them. However, a major earthquake can be very destructive as you can see in this picture from

Haiti. Most earthquakes originate deep in the crust when huge pressures at tectonic plate boundaries are suddenly released as rocks break or move. They can also be caused by other events, including volcanic eruptions. The epicentre of an earthquake is the point on the Earth’s surface immediately above the event that causes it. Most of the damage in major earthquakes is due to surface waves that spread out from the epicentre like ripples on a pond. The ground shakes, causing landslides, cracking roads, and destroying bridges and buildings. In the Haiti earthquake of 2010, about 230,000 people died, mostly in collapsing buildings. More than a million were

made homeless, but many well-constructed modern buildings survived. Major earthquakes under the sea may cause tsunamis. These are huge waves that can cross oceans causing destruction and death when they arrive at coasts that may be thousands of miles away. The tsunami of 2004 killed over 200,000 people, mainly in Thailand, Indonesia and Sri Lanka. Volcanos are mountains that periodically erupt, spewing out molten rock called larva, and clouds of ash and poisonous gas. Many volcanos are cone shaped, the cones being made of the ash and lava from earlier eruptions. The lava emerges from a vent at the bottom of a crater. A volcano may have several craters – the main crater at the summit, and one or more secondary craters lower down. The lava is magma from

the mantle below that has been forced up through cracks in the Earth’s crust. This usually happens near tectonic plate boundaries, but a few volcanos (for example those in Hawaii) are caused by hot spots where magma has broken through in the middle of a plate. Lava may be as hot as 1,000ºC or more. Larva, hot ash and hot rocks can flow down the sides of a volcano like rivers, burning, burying and suffocating everything in their paths. The most violent volcano in recent times was Krakatoa, an island volcano between Sumatra and

Java in Indonesia. On 27th August 1883, the whole mountain exploded causing earthquakes and tsunamis. It threw so much ash into the atmosphere that it affected the Earth’s climate for years.

1. What is the Pacific Ring of Fire? Describe exactly where it is, naming as many countries as you can.

2. What can help to save lives in earthquakes?

3. (i) What are conical volcanos made of? (ii) Find out what active, dormant and extinct volcanos are. (iii)

What would it be like to live close to an active volcano?

Type of plate boundary

Place E-quakes/

V-canos

Convergent

- Oceanic

- Continental

Pacific Ring of Fire

South Italy, Greece, Turkey

Himalayan region

E and V

E and V

E only

Divergent Mid-Atlantic ridge

Red Sea, African Rift Valley

E and V

E and V

Transverse California E only

Tsunami arriving in Thailand, 2004

Volcano in Alaska

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11.13 MOUNTAIN BUILDING, WEATHERING AND EROSION

Earth’s natural landscapes are shaped by the ongoing processes of mountain building, weathering and erosion. Apart from volcanos, mountain building occurs mainly when tectonic plates collide and the edges of one or both are crumpled and thrust upwards over tens of millions of years. The photo shows Kanchenjunga in the Himalaya where the Indian plate is colliding with the Eurasian plate. But at the same time that mountains are being thrust up, they are also being worn away by weathering and erosion.

Weathering is the physical and chemical break-down of rocks into smaller rocks and soil, by natural processes including the heat and light of the sun, corrosion by gases in the atmosphere, and the action of wind, rain, snow, frost and ice. Erosion refers to the carrying away of rocks and soil, mostly by water and gravity through the action of rain, streams and rivers. Wind may also cause erosion, and in cold climates and high mountains, glaciers (slow moving rivers of ice) grind down the rocks beneath them. Weathering

and erosion can wear away even the highest mountains in time, leaving only flat plains. The photo on the left shows the famous grand canyon in the USA. A flat plain, left behind after the erosion of earlier mountains, was uplifted millions of years ago. Now the plain is being cut down again by weathering and erosion. The canyon is 440 km long, 29 km wide and almost 2 km deep. It has taken about 17 million years for the Colorado river to carve out the grand canyon.

In a natural landscape, erosion is slowed by vegetation. Roots may help to crack rocks, but they also hold the soil together and prevent it being washed or blown away. Grass, trees and other plants also shield the soil from the force of heavy rain and strong winds. Some animals contribute to erosion, for example by digging burrows, but humans are the most destructive of all. When we clear land to plant, crops, or allow our animals to overgraze, the soil can be washed or blown away. Deforestation (cutting down trees without replacing them) can be very harmful. On steep slopes, millions of tonnes of good soil may be washed away very quickly, impoverishing the ground and spoiling nearby rivers. Villagers living near the river in the first photo used to eat fish, but now the river is brown with eroded soil and fish no longer live in it. The second photo shows a traditional

way of preventing erosion. Terraces slow down run off, trap the soil, and help to retain water. 1. What are (i) weathering, (ii) erosion, (iii) soil, (iv)

overgrazing, (v) deforestation, (vi) run off, and (vii) terracing? 2. What is the approximate time scale for building mountains,

and for wearing them down again?

Soil is essential for the growth of most plants. It is a mixture of sand and clay (from the weathering of rocks) and humus. Humus is organic matter from rotting leaves and similar materials. Have look at soil with a hand lens and see what you can find.

Deforestation and erosion Terracing helps to prevent erosion

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11.14 ROCKS AND MINERALS

Rocks are made of minerals, often mixtures of many minerals. They are classified as igneous, sedimentary or metamorphic according to their origins.

Igneous rocks are formed when molten rock, in the form of magma or lava, cools and solidifies. More than 90% of the Earth’s crust is composed of igneous rocks, with only a thin covering of sedimentary and metamorphic rocks. Granite is an example of an igneous rock that cooled slowly, well below the Earth’s surface. It consists of large crystals of several different minerals. Gabbro is an igneous rock that formed closer to the surface. It cooled more quickly so the crystals are smaller. It also contains less quartz and more dark minerals than granite. Basalt is solidified lava. It cooled very quickly on the surface of the Earth and the crystals are too small to be seen. All igneous rocks contain quartz (translucent), feldspars (white or pink), and various darker minerals. When igneous rocks are weathered, the quartz becomes sand and most of the other minerals become clay.

Sedimentary rocks are formed from layers of sediment that accumulated on the floors of ancient lakes or seas. Many of the particles in these sediments were carried there by rivers as a result of weathering and erosion. The sediments are consolidated by their own weight and by chemical changes over geological periods of time. They may later be uplifted by tectonic events. The layers or strata can be seen in the sandstone and limestone cliffs in the photos below. The strata may be crumpled during uplift. Sandstone is made of sand particles (mostly SiO2), and limestone is made of crushed shells and the remains of other sea creatures (mostly CaCO3). Coal is a sedimentary rock made of the remains of trees and other plants. It contains a lot of carbon and is a useful industrial fuel. Sedimentary rocks may contain fossils. Fossils are the remains of animals or plants, preserved in the sediments and turned into stone.

Metamorphic rocks began as igneous or sedimentary rocks, but they have been changed by heat and pressure in the Earth’s crust. These changes are the result of plate tectonic movements or of magma moving up into the crust. Examples of metamorphic rocks are slate and marble. Slate began as a sedimentary rock called siltstone and marble began as limestone. Marble is a beautiful rock used for carving statues and for public buildings. 1. Name three minerals. For each one, name some of the rocks in which it is found. 2. What is the main factor that determines the size of crystals in an igneous rock? 3. Explain how each of the following are formed: (i) crumpled strata, (ii) fossils, (iii) marble.

GRANITE GABBRO BASALT

feldspar

quartz

LIMESTONE CLIFFS SANDSTONE STRATA CRUMPLED STRATA

Minerals are solid chemical substances that occur naturally in the Earth’s crust. The most common mineral is silica (quartz or SiO2). Many rocks also contain complex aluminosilicates called feldspars. Various forms of calcium carbonate (CaCO3) are also common.

LIMESTONE

fossil of shell

MARBLE STATUE

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