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The Earth’s Layers 4 Journey to the Center of the Earth The deepest places on the earth are in South Africa. Mining companies trying to extract gold have mined into the eart.. about 3.5 km (about 6 miles). No one has gone deeper into the earth than the South African miners because of the heat and pressure at these depths prevents humans from going much deeper. So if we can’t go very deep into the earth, how do we know what is below the surface when we cannot see it? Isdac Newton (the scientist that discovered gravity) was one of the first scientists to develop theories about the structure of the earth. Based on his studies about gravity, Newton calculated the average density of the earth and found it to be more than twice the density of rocks on the surface. Newton realized that the inside of the earth had to be much denser than the surface. Most of our knowledge about the structure of the earth comes from studying earthquakes. Every earthquake sends out waves in all directions like the way that dropping a rock into a lake sends out waves through the water. These earthquake waves are called seismic waves. Observing these seismic waves as they travel through the earth gives scientists an idea of the different materials that the waves move through. There are two types of seismic waves, S-waves and P-waves. These waves behave differently when they pass through different types of materials. Just like a sound wave behaves different when it passes through water instead of air seismic waves behave differently when they pass through different phases of matter. Scientists know that P-waves will travel through all types of materials but S-waves will not travel through liquid. By calculating these seismic waves scientists have learned that there are four major layers that make up the earth. Three of these layers are solid, one of these layers is liquid, and one of these layers is gas. The Atmosphere ‘The earth is surrounded by all kinds of gases called the atmosphere. Without the atmosphere life on earth would not be possible. The atmosphere gives us air, water, and warmth. The atmosphere protects us from the harmful rays of the sun and from meteorites. This layer around the earth is a colortess, odorless, tasteless sea of gases. The atmosphere is made up of gases such as nitrogen, oxygen, argon, and carbon dioxide. It is about 800 kilometers (500 miles) thick and is less dense than the crust that is floats above. The density of the atmosphere is (0.0013 g/cm’). The atmosphere itself is made up of many different layers. If you ever climb a high mountain yourself, you will notice that as you climb upward the air gets colder. The temperature of the air decreases as the altitude (height) increases because the air becomes less dense. That is, there are fewer and fewer molecules of air ina given amount of space. The thin, less dense air cannot hold as much heat. The temperature of the atmosphere can range from -90°C — 2000 °C (-130 °F — 3632 °F) depending on the layer of the atmosphere, how close it is to earth, how much ultraviolet radiation it absorbs and how dense the particles are in the layer. The Crust The crust lies above the mantle and is the earth’s hard outer shelt. The crust is the surface on which we are living. The crust is O—32 KM (0 -19.8 miles). In relation to the other layers the crust is the thinnest and the least dense layer, It floats on the softer, denser mantle. The crust is made up of z solid rock but these rocks are not the same all over the world. There are two major types of crust: crust that makes up the Bee pe ocean floors and crust that makes up the continents. Oceanic crust is a E epee be lall thin layer found under the oceans. Even though it is relatively thin it is 2 7 DUE Te feste| 
