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Earth

Earth is a planet in the Solar System that revolves around the Sun. Most theories about the origin of the Universe suggest that the great galaxies, or clusters of stars, which abound in space are formed by the coming together of a great deal of dust and gas (mostly hydrogen), which slowly condenses into a vast, rotating system of stars. Within each galaxy there are millions of stars, and our own Sun is just an average star. The Sun has acquired a family of planets in orbit around it. Our Earth is one of these planets.

Some scientists explain the origin of these planetary systems, which must also exist for other stars, by suggesting that both the Sun and its planets condensed at the same time from part of the material forming the galaxy. Other scientists believe that the Sun was formed first, and that a close approach by another star drew from it a large thread, or filament, of material. This then separated into a number of pieces revolving around the Sun and condensed to form the individual planets.

The Earth is, roughly speaking, a ball, or sphere about 8,000 miles in diameter. It is rocky on the outside, and is about 5% times as heavy as a quantity of water the same size would be. Parts of its surface are very rugged, and almost three-quarters of the total surface area is covered by water. Scientists cannot learn much about the inside of the Earth by studying the surface of it. They learn more by studying earthquake waves. When there is an earthquake, the shock may travel a long way through the Earth, and the scientists can tell from the type of shock wave and the speed at which it travels and what type of substance it is traveling through.

The Earth's crust consists of two main types of rock sima and sial. Sima is made mainly from silicon and magnesium and sial is made mainly from silicon and aluminum. Sima is a dense rock similar to that which comes out of volcanoes, and may have been formed when most of the Earth was in a molten (liquid) state. Sial occurs as great 'rafts' floating in the heavier sima. Each of the 'rafts' is a continent.

Beneath the crust there is a layer, called the mantle, of very heavy rocks about 1,800 miles deep. The mantle surrounds the core, which is in two parts. The outer core, about 1,350 miles deep, is liquid, and probably made from molten iron and nickel. The inner core is also believed to be composed of iron and nickel, but is solid. It is about 1,700 miles across. The crust is, of course, the part of the Earth we know most about. It has been bent and folded by great pressures at various times in the distant past. In some areas this earth movement has resulted in the formation of extensive mountain ranges.

Volcanic activity and earthquakes have also played a part in the formation of the Earth's crust. Parts of the crust have been covered by igneous (crystallized) and sedimentary (deposited) rocks. Some rocks have later been metamorphosed (changed) by heat or pressure. These so-called metamorphic rocks may in turn be covered by sedimentary rocks deposited in more recent times.

The form of the Earth's surface has developed during the past few million years. The landscapes of today show the results of great rock movement during several periods of the Earth's past. The oldest mountain ranges, such as those of Scotland and Wales, are about 400 million years old. They have been much reduced and worn away (eroded) during that time. The most striking mountains are those of the youngest ranges, such as the Alps, which are 20 million years old, and the Himalayas, which are 70 million years old. These ranges have not been in existence long enough to be greatly altered by erosion.

An important factor in forming the landscape of the Earth is glaciation. In the past, the Earth has had long periods during which large areas of its surface were covered by vast sheets of ice. These periods are known as Ice Ages. During the most recent Ice Age, which began nearly a million years ago, most of Europe, Asia and North America was under ice. As the ice advanced, it scraped the land clear of soil, leveling the surface, smoothing the mountain peaks, and carrying large quantities of rock for hundreds of miles. Glaciers fed by the ice sheet flowed down river-beds towards the sea, scooping them out into deep valleys and fjords.

The landscape is also shaped by the sea, wind, rain, rivers and weather which wear away the land. Rivers play an important part in the creation of a landscape. When they are young, they may be fast moving streams, carrying away large boulders as well as smaller bits of rock, ratting steep-sided gorges for themselves, and forming waterfalls and rapids. In times, rivers become gentler, smoothing out the rugged scenery created by their own earlier activity.

The study of the Earth's surface features is called geomorphology.