Is the Earth a Perfect Sphere?
If you look around you, the earth looks flat. Even before the Middle Ages, people did think it was flat. The shape of the earth has intrigued scientists throughout history.
Pythagoras was the first person to talk about earth being spherical, but people did nto accept his view. The flat earth concept persisted till about 300 years ago. In fact people used to think that if they sailed too far in one direction, they would fall off the edge of the earth
It was Sir Isaac Newton, an English scientist, who first claimed that the earth is not spherical, but 'oval'. Today we know that the earth is not perfect sphere. It is slightly flattened at the poles, and it bulges at the equator. This is caused by the rapid spinning of the earth. In recent years, scientists have learned that there is another small bulge at the North Pole. So the earth is now considered to be somewhat pear-shaped!
What is the Earth's crust and how big is it?
The Earth's Crust
The Earth's outermost surface is called the crust. The thickness of crust varies from 40 kilometres to about 10 kilometres.
The earth's crust is like the skin of an apple. It is very thin in comparison to the other three layers.
The temperature of the crust vary from air temperature on top, to about 870 degrees. Celsius in the deepest parts of the crust. Since you can bake a loaf of bread in the over at 175 degrees Celsius, you will understand why at 870 degrees Celsius, rock begins to melt!
The crust of the earth is now broken into many pieces called plates. The plates 'Float' on the layer below. These plates usually move alone smoothly, but sometimes they stick and build up pressure. The Pressure builds and builds, and the rock bends until it snaps. When this occurs an earthquake is the result!
What is inside the Earth?
If you could make a journey into the heart of the earth, what would you find? Well, you would start from the surface of the earth, which is a hard crust of rock. The rock is not continuous, but has many cracks. This split up into large slowly moving plates called tectonic plates.
As you go deeper and deeper you would notice many changes. To begin with, it would get hotter and hotter, as you enter the next layer. This is called the mantle. The mantle is so hot that the rock here is not solid but molten. The molten rock, know as magma is constantly moving.
About 6400 kms below the surface you will reach the core of the earth. The earth has outer core and inner core. This is made mainly of molten iron, and the temperature in the heart of the earth is fiery 5500 degrees centigrade!
Of Course, no one has ever gone so deep into the Earth but scientists know what it is like from their studies of earthquakes.
Where does the earth's crust end?
The earth's crust is about 40 kilometres thick beneath continents, and about 10 kilometres thick beneath oceans. It occupies only about 1% of the earth's volume and ends at the next layer called the mantle.
The earth's crust meets the mantle along a boundary called the Mohorovic discontinuity. Since the top of the mantle as rigid as the crust, scientists often group the upper portion of the mantle with the crust and call it the lithosphere.
What is the earth's mantle?
The mantle of the earth is 2,900 km thick rocky shell comprising approximately 70% of earth's volume. It lies under the crust. and is made up of two layers
The Earth's mantle is thought to be composed mainly of olivine rich rock. It has different temperatures at different depths. The temperature is lowest immediatly beneath the crust and increases with depth. The highest temperature occurs where the mantle material is in contact with the heat producing core.
The outer part of the mantle is about 600 kms thick. Its temperature is about 2000 degrees celsius and is composed of molten rock called magma, which has the consistency of hot tar. It is under great pressure, and sometimes bursts out of crack on the earth's surface as a red hot volcanic eruption.
The inner layer of the mantle is under even greater pressure. In fact, the pressure is so great that the rock is more sold than the outer layer, but not rigid. It is like plastic, and moves very slowly.
Are there any gaps in the court?
We know that the earth's crust is not one huge shell of solid rock. There are many cracks in the crust and these cracks form gaps
The crust, or rather the lithosphere, is actually broken into 20 or so giant plates called tectonic plates. Since these plates are always shifting, the gaps keeps widening and narrowing. Sometimes molten rock from the layer underneath the crust pushes up through these gaps, and volcanos are formed.
Star Facts
| The earth is the only planet with its crust split into moving plates. |
What is the Core?
The core is the heart of the earth. It consists of a fluid outer core, and a solid inner core.
The outer core has temperature upto 3000 degrees celsius. It is made up of mainly liquid iron that flows in giant corkscrew like currents produce the earth's magnetic field.
The inner core of the earth is even hotter. The temperature can go up to 5500 degrees celcius. Because of the tremendous pressure, the iron crystals in the inner core form a solid ball.
Why is it hot inside the earth?
The inside of the earth is unimaginably hot. The deeper you go, the hotter it gets. Every 40 metres, the temperature rises by another degree. About 4 kilometres below the surface of the earth, it is hot enough for water to start boiling. At the very centre, It is a mind blowing 5500 degrees celsius!
Why is so hot at the centre of the earth? we know that the earth was once a hot, whirling mass of gas liquid, and time particles that began to orbit the sun regularly. As millions of years passed, it took a ball like shape and cooled very slowly. A hard crust formed on the surface, but the heart remained as hot as before...and remains as hot to this very day.
Star Facts - 10 LONGEST RIVERS IN THE WORLD
| Name |
Length(km) |
| Nile |
6632 |
| Amazon |
6400 |
| Mississippi - Missouri |
5936 |
| Ob-Irtysh |
5536 |
| Yangtze |
5440 |
| Huang Ho |
4800 |
| Congo |
4350 |
| Amur |
4320 |
| Lena |
4288 |
| Mackenzie - peace |
4216 |
How do we know about the interior of the earth?
Scientist have been able to understand what the interior of the earth is like by studying volcanic eruptions that bring hot gases, and earthquakes.
When an earthquake occurs the vibrations it causes spread out in all directions and are called Seismic waves.
Seismic waves can be detected with a very delicate instruments. It has been notices that they travel at different materials. Their directions also changes as they from one kind of rock to another.
The two main types of seismix waves are body waves and surface waves. Body waves travel through the earth's interior in all directions. Surface waves travel only along the surface of the earth, like ripples on water. It is the behaviour of body waves that gives us clues about the nature of earth's interior
One type of body wave, the 'P' wave slows down and changes direction at the depth of about 550 metres. Another type, the, 'S' wave stops entirely at this depth. This shows that there are significant changes in matter, a scientists have concluded tat this is the boundary between the mantle of the earth and its outer core.
Since 'S' waves cannot travel through liquid, scientists concluded that the outer core is liquid. The 'P' waves continues till a depth of more than 5000 kilometres. When they change direction again, Showing that they have reached a solid innercourse.
As far as surface waves are concerned, the speed at which they travel gives us information about the type of rocks found near the surface of the earth.
So it is the information about the seismic waves gathered by the scientist, and their study of volcanic eruptions that gives us insights into what the interior of the earth is like.
Can we ever visit the centre of the earth?
There is a famous book 'Journey To The centre of the earth' written by the French author Jules Verne. The story is about a professor who leads his nephew and hired guide down a volcano in iceland to the centre of the earth. They encounter many adventures, including prehistoric animals and natural hazards, eventually coming to the surface again in southern Italy.
Jules Verne's classic makes for exciting reading, but it is pure fiction. The centre of the earth is not a nice place to visit, unless you like hanging out in a blast furnace. The outer core of the earth, about two thirds of the way to the centre, is molten iron. Deeper yet at the inner core, the pressure is so great - 3.5 million times surface pressure that iron solidifies even though the temperature is hotter than the surface of the sun. For these very reasons, it can be safely said that it is unlikely that you can visit the centre of the earth anytime. in the near future.
Since the ground is flat, why do we say the earth is round?
We know that the earth is round, yet the ground around us looks flat. This is because, in comparison to the earth, we are very very very tiny. We can only see a small portion of the earth at a time, which is why that portion looks flat.
What are rocks and minerals?
The earth's crust is made up of rocks. A rock is usually defined as a mixture of common minerals. Rocks can be hard or soft, as small as grain, or as large as a building.
Geologists classify rocks based on how the rockes were formed. The three classes are Igneous rocks, metamorphic rocks, and sedimentary rocks.
Igneous Rcoks are formed when molten lava cools and hardens.
Sedimentary Rocks are formed when tiny particles are worn away from other rocks and settle into layers on the sea bed. They slowly get cemented by chemical action into hard rock again.
Metamorphic rocks are formed when existing rocks change to form new rocks because of great changes of pressure or temperature
Minerals are natural compounds through geological processes. Minerals range in composition from pure elements and simple salts, to very complex silicates.
Different samples of the same mineral react in the same way. There are over 3000 different minerals. All minerals except mecury are solid, Though they do melt under tremendous heat and pressure. Some minerals make up the common rocks on the earth's surface. Some like gold and diamonds, are extremely rare and valuable.
GOLD & DIAMOND
What is the difference between Metamorphic rocks and Sedimentary Rocks?
sedimentary rocks are made from pieces of other rocks that have been worn away by the elements. These pieces have been pushed down underneath the surface of the earth or seabed, and have been cemented together by extreme pressure. Some sedimentary rocks are formed from particles of once living things, such as ancient forests. Others are formed from minerals that crystallize from sea water.
Limestone, Sandstone and shale are sedimentary rocks.
Metamorphic rocks are formed much deeper beneath the earth's crust. Here, Igneous and sedimentary rocks are altered by extreme heat and pressure to form new rocks.
Marble and slate are metamorphic rocks.
Why do rocks change?
There is a phase in English that says 'as unchanging as a rock' - but the fact is that rocks are changing all the time. They are constantly being worn away by the elements.
Wind, rain, sleet and snow wear away igneous and sedimentary rocks on the earth's surface. These fragments are washed away by rain or carried by rivers into the sea. Here, they accumulate in layers and gradually turn into new sedimentary rocks.
Some of the sedimentary rock is pushed so deep beneath the surface that it is transformed by heat and pressure into metamorphic rock. The movement. of the earth's tectonic plates pushes up the metamorphic rock to form new mountains. Thus a cycle is completed and this process is known as the rock cycle.
The cycle continues. Mountains made of metamorphic rocks can be broken up and washed away by streams. New sediments from these mountains can make new sedimentary rock. So that is why we can say that rocks are ever changing!
Which rocks come from fire?
Deep inside the earth, Igneous rocks are formed when the fiery melted rocks called magma is trapped in small pockets. As these pockets of magma cool slowly underground, the magma solidifies to form igneous rocks. Such rocks formed underground are called intrusive igneous rocks.
Igneous rocks are also formed when volcanoes erupt. When this happens, magma appears lava. Igneous rocks are formed as lava cools and solidifies above the ground. These igneous rocks formed above the surface are called extrusive igneous rocks.
Igneous rocks are also called 'Fire rocks' because they are born from fire.
What are Precious stones?
A precious or semi precious stone, also called a gem or gemstone, is a highly attractive and valuable piece of mineral. They are greatly prized, because they are rare beautiful and hard-wearing.
There are over 3000 types of minerals, but only abouve 50 are valued as precious stones. Most of them are crystals formed when magma cooled slowly deep underground. So it is very difficult to reach them. They can be found by mining, quarrying or shifting the mud and sand from river and stream beds.
The most precious of stones are diamond, rubies emerald and sapphires. Of these, the diamond is the most prized gem.
Ruby (LEFT) Emerald (CENTRE) Sapphire (RIGHT)
Why is the desert Dry
Have you ever wondered why the desert is so dry and hot? The main reason for this is the deserts very little rainfall, and the amount of rain determines the amount of plants that grow there.
Deserts, near the equator are dry, even though the oceans is not far away. This is because in these places, instead of hot air rising and are being replaced by cool moist air, air settles downwards and becomes warmer and drier as it does so. The phenomenon is responsible for the lack of moisture in deserts. The Sahara Desert and deserts of Western Australia are caused in this way.
Deserts that are far away from the equator are generally dry because of their distance from the sea. This prevents moisture laden winds from reaching them. Sometimes, there are mountains which act as barriers to the winds. These mountains will get rainfall on the side facing the sea, but the side facing land doesnt get any rain. The desert of Central Asia and the Western United states are good examples of this.
There are sandy deserts, where the wind builds sand hills or dunes. Rocky deserts consists of bare rocks. Other desert contains barren mountains and dry plains of soil and gravel. Cold deserts are located near the Arctic part of the world.
Can Land bend and crack?
Indeed it can, Actually, only very few parts of the land we see around us are actually flat. Even if the surface looks flat, the layers of rock beneath may be bent. Hills and valleys, cliffs and canyons can be caused by rocks being worn away by the weather. They can also be caused by rocks being bent into folds by the movements of the tectonic plates beneath the surface of the earth.
These plate movements can also cause layers of rock to crack and form faults or rifts. The Great Rift Valley in East Africa is actually a series of cracks that are upto 4000 kms deep!
