Our Planet Earth



 



What are metals?



Metals are one of the major groups of elements. Most of them are shiny and hard. Mercury is the only metal that is a liquid at normal temperatures. Most metals can be bent and stretched, and they can be mixed to make alloys.



Metals are usually found with other elements in the form of compounds. Bauxite is an ore, or mixture of substances, that contains aluminium, the commonest metal. It makes up about eight percent of the Earth’s crust. Osmium is the heaviest metal, and is twice as heavy as lead. Lithium is the lightest metal. It is half the weight of the sane volume of water, so it floats in water.



 



 



 





 



 



How much of the Earth is covered by water?



More than two-thirds of the Earth’s surface is covered by seas and oceans. About 72 percent of the Earth’s surface is water. This water is either in the oceans, locked away as ice at the poles, or held as water vapour in the atmosphere. All of the Earth’s water is known as the hydrosphere. 



 



 



 



 



 





 



 



 



Why is the Earth round?



Gravity pulled the Earth and the other planets into a sphere when they were being formed. The Earth is not truly round, but is slightly flattened making it bulge out at the Equator. This effect is due to the speed with which the Earth spins, causing the Equator to try to fly out from the axis of the spinning Earth. It is rather like a heavy object whirling round on the end of a piece of string. The shape of some of the other planets is distorted in the same way.


Our Planet Earth


 



 



What is the Earth made of?



The Earth is made up of elements. These are the simplest possible substances, which are composed of one kind of atom. Elements cannot normally be broken down into other substances, except under the special conditions that you might find inside the hot core of a star. A total of 92 different elements are found in nature. Other elements can be made in laboratories, but they have only a very short life.



Minerals, which make up the solid surface of the Earth, consist mostly of combinations of elements. The minerals gold, diamond and graphite are examples of single elements, while most others contain several elements. 



 



 



 



 





 



What are crystals?



Crystals are solid substances that are naturally formed into regular geometric, angular shapes. All crystals can be classified into seven systems. Sometimes a crystal is formed when water evaporates, and substances dissolved in the water gradually grow into a crystal. Other crystals are formed under great pressure. Diamond crystals are made when carbon dissolves in molten rock deep down below the Earth’s surface. The diamond crystallizes out and is later uncovered on the surface either by volcanic action or by erosion of the Earth’s crust. Other valuable gems are formed in a similar way, and most of them contain compounds of aluminium.



 



 



 





 



 



What is the Equator?



The Equator is an imaginary line drawn around the outside of the Earth. It lies midway between the North Pole and the South Pole, at the Earth’s broadest point.



The Equator was invented by map-makers because it makes a convenient point from which to measure distances, together with the geographic North Pole and South Pole. On a map, the Equator is positioned at 0 degrees of latitude. It divides the Earth into two halves, which we call the north and south hemispheres. 


Our Planet Earth



What is inside the Earth?



The Earth is not solid rock all the way through. It has an inner core of solid rock, which is mostly iron. The temperature here is probably around 4,500°C. Beyond the inner core is a layer of liquid rock, called the outer core, which extends about halfway to the surface. Beyond this is a thick layer of rock called the mantle, which is partly molten and runny. The thinnest layer, or crust, is on the Earth’s surface, floating on the red-hot liquid mantle. The crust is about 6 km thick under the oceans, but 30 to 40 km thick beneath the land. 



 



 





 



 



Why is the Earth like a magnet?



The Earth acts as if it is a huge magnet. When the solid rocky core moves inside the liquid rock above it, it creates a magnetic field with a north pole and a south pole. This field surrounds the Earth and extends right out into space. The Earth’s magnetic field is changing constantly. This means that the magnetic north pole is not always the same as the geographic, or true, North Pole that you find on a map, although it wanders about in the same region. Millions of years ago, the north magnetic pole lay in what is now the Sahara desert. 



 



 



 





 



How old is our Earth?



The Earth is thought to be about 4,600 million years old. The oldest rocks so far discovered are up to 3,800 million years old. The Universe is much older, and probably began about 15,000 million years ago. We can calculate the ages of the Earth and the Universe by examining meteorites, and also by looking at changes in the atomic structure of some of the elements found on the Earth. Radioactive elements decay at a steady rate, and these changes can indicate the age of the Earth and other planets, as well as the age of the stars.