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Introduction

This chapter offers an overview of the formation of the Earth. The Earth is a part of the solar system and it has been influenced by its elements since it was formed. This chapter also discusses the inner structure of the Earth.

Rocks of the Earth

Our physical environment consists of different parts like stars, planets, air and water. Earth is one of the eight planets that form the solar system. A planet is a large celestial body which orbits around a star. The solar system consists of a star called the sun and a group of planets called Mars, Venus, Earth, Mercury, Jupiter, Saturn, Uranus, and Neptune. Satellite photographs of the Earth show that it is a sphere. See image 1.

Calculating the ages of rocks is very important for establishing not only the history of geological events, but also for determining the rates of geological processes. The radiometric method is the most accurate method used for dating rocks. This method is based on calculating the age of rocks according to the radioactive decay of chemical elements that are natural components of all rocks.

Scientists studying the geological history of Earth are called geochronologists. The oldest rocks that have been found on Earth are about 4600 million years old. That is how old our planet is considered to be.

The Earth is born

According to the Big Bang Theory, the solar system was formed from a cloud of space dust and gas that was 'squeezed' by a supernova explosion. Gravity pulled the gas and dust together, forming a solar nebula. This solar nebula grew hotter and denser, forming a disk of gas and dust at its centre. Particles of nebula dust started to stick together, forming clumps that eventually formed planets and their satellites. See image 2.

At first, the Earth looked like a ball of hot bubbling liquid rocks made from different chemical elements and compounds. Some rocks were heavier than others. After a while heavy rocks started to 'sink', moving toward the centre of the Earth. Lighter rocks moved towards the Earth's surface. This separation of rocks took millions of years. As the light-weight surface rocks were cooling down they became harder. The hard surface of the Earth is called the crust.

The Earth became a planet with a hard crust about 4000 million years ago. Even though it was a much cooler place by then, it was still too hot to support any forms of life. Also at that stage, the Earth did not have any water or air. Earthquakes, volcanic eruptions, comets and meteors were common events at that time. They were the main forces shaping the newly-formed surface of the Earth. Plate tectonics and crashing meteors led to the formation of mountains and waterbeds.

Tectonic plates

The Earth's crust is made up of large, slowly-moving blocks of rocks, called tectonic plates. All modern continents sit on tectonic plates. Tectonic plates are made of rocks that were once part of the hot liquid material of the early Earth. Modern tectonic plates move roughly with the same speed as our fingernails grow. Tectonic plates were formed millions of years ago and they have been moving and changing ever since. Ancient tectonic plates were thin and light and moved around more easily. Now they are getting thicker and heavier. This means the Earth is losing heat. The Earth's crust is made of about 29 tectonic plates.

Inner structure of Earth

People have always wanted to know what was beneath the Earth's surface. After studying volcanic lava which comes from underneath the Earth surface, ancient scientists realised that deep rocks were different from surface rocks. Today, various sophisticated drilling machines and measuring devices are used for studying the structure of the inner layers of the Earth.

According to scientific research and calculations, the distance between the surface of the Earth and its centre is about 3900 km. But no-one yet has ever managed to drill down more than 15km. There are ways of finding out what is happening deep down beneath the surface of the Earth without having to go there. Forces generated by earthquakes and volcanic eruptions bring the deeper-lying rocks closer to the surface. These rocks are samples of the material that makes up the 'insides' of the Earth. By studying these rocks, scientists have found out that our planet consists of three layers that are made from different types of rocks and metals. See image 3. These layers are:

The crust

The hard surface of the Earth is called the crust. The Earth's crust is like a skin that covers the Earth. The Earth's crust is covered with soil. Soil is a complex mixture of rotting plants, animals and rocks. Soil is a very important part of the Earth's biosphere. The part of the Earth and its atmosphere that can support life is called the biosphere.

The mantle

The layer that lies directly under the crust is called the mantle. It is the thickest layer and it is made mostly of hot, melted rocks. This hot, bubbling, jelly-like mass is slowly moving up and down from the Earth's core. When volcanoes erupt, magma breaks through the Earth's crust. The whole process is very slow, because this layer is made of very heavy and solid rocks.

The core

The core is the deepest layer of the Earth. Because the deepest layers have the highest temperatures the Earth's core is the hottest part. The Earth's core temperature is about 7000 degrees Celsius. That is 70 times hotter than the temperature of boiling water.

It is not known why the Earth's core is hot,but there are lines of evidence. The changing thickness of the Earth's crust indicates a constant cooling, which indicates that the heat was created at the beginning of the Earth's formation.

The Earth's core is divided into the inner core and the outer core. The inner core is solid and made of pure iron. The outer core is liquid and rich in iron and nickel. There is an irregular boundary between the liquid core and the bottom of the rocky mantle. Scientists have learned that the Earth's core is spinning faster than the Earth itself. Every 400 years, the core is a full turn ahead of the Earth.

See animation 1.
 
All the above-mentioned data come mostly from studying how seismic waves travel through rocks.

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