The atmosphere is a layer of gas around the Earth. Air is a mixture of gases. It is most dense at sea-level and thins out as we rise through the layer of atmosphere called the troposphere. The Earth formed about 4600 million years ago. The ozone layer protects us from the Sun's most harmful ultraviolet rays. The atmosphere is maintained by nature recycling substances in living things. All living things depend upon oxygen, nitrogen, carbon dioxide and water. The Earth's surface is formed and forever changing by different processes occurring. Igneous rocks are formed from the hot liquid rock (molten magma) from volcanoes solidifying. Interlocking crystals are often found in these rocks. Sedimentary rock is formed from sediments. Due to weathering and erosion, larger rock may crumble into smaller ones and be transported until they eventually settle on sea or riverbeds. Over the years these fragments of rock compress together. Sedimentary rock is often crumbly due to its formation from small bits of rock. Often they are layered, and on some occasions fossils may be present. Metamorphic rock occurs when heat or pressure or both cause a rock to change its structure.