The Earth Rocks, Minerals and Soils The surface of the Earth is covered by a layer of rock. This layer is called the Earth’s crust. Scientists who study rocks are called geologists. Rocks are made up of grains of different materials. These different materials are called minerals. Each mineral is made up of one chemical substance. In some rocks the minerals form small crystals. In other rocks the crystals are much larger. Granite is a rock made from quite large crystals of three different materials: quartz, feldspar and mica. The particles in the soil may be of different sizes. Sand particles are large. Clay particles are small. Soils also contains different amounts of humus. These difference give soils different properties. Igneous Rocks Magma: The molten rock is called magma. The crust of the earth is made of solid rock. But deep inside the earth it’s very hot. When rock is very hot it melts to form liquid. Beneath the crust the rock is molten (hot and liquid). Igneous rocks When magma cools, it solidifies and forms rocks. Rocks that have been made in this way are called igneous rocks. Magma is a mixture of different minerals. Different samples of magma may contain different minerals in different quantities. Cooling of magma and its effect on rock formation When magma cools deep underground, it cools very slowly. The slow cooling gives plenty of time for large crystals to grow. When magma cools near the surface, it cools more quickly. There is only enough time for small crystals to form. When it comes out of a hole as a liquid, it cools very quickly and may not have enough time to form crystals. Granite forms when magma cools deep underground. Basalt forms when magma cools near the surface. Obsidian forms when magma cools very quickly. Volcanoes • When magma reaches the surface of the earth, it is called lava. • The lava erupts from volcanoes. Sedimentary rocks Sediment: Made up of little fragments (pieces) of rocks. River often carry lots of sediment. How are sedimentary rocks formed? Layers of fragments of rocks or mud collect on the sea bed. As more layers build up on top of them, the weight of the new layers presses the particles in the deeper layers together. The solid rock formed, is called sedimentary rock. Sometimes, the remains of dead plants and animals fall into the sediment. They become part of the rock. They may form fossils. Important clues Sedimentary rock has layers. Sometimes these layers contain fossils. Sedimentary rock is made of grains or particles that are stuck together. There are often tiny gaps in between the grains, so the rock is porous. Water can soak into the rock into the little gaps between the grains. Example Limestone is a sedimentary rock formed from little fragments of shells from animals, such as corals. The grains are made of calcium carbonate. Limestone is often almost white, because it is made of calcium carbonate. The limestone is full of fossils of animals. Metamorphic Rocks The high temperatures and pressures change the rock buried very deep under the ground. They often squash the grains closer which makes the rock harder. Rocks that have been changed like this are called metamorphic rocks. As we go deeper into the earth, the temperature and the pressure increase. In the gold mine, the miners can only work for a few hours at a time. This Photo by Unknown Author is licensed under CC BY Grains do not fit perfectly together in the limestone. Marble is a metamorphic rock made from limestone. It is hard without any pores. This Photo by Unknown Author is licensed under CC BY This Photo by Unknown Author is licensed under CC BY Sandstone is made of sand grains with tiny gaps between them. Quartzite is a metamorphic rock formed from sandstone. Some more information about Metamorphic rocks Metamorphic rocks don’t only form deep underground. They can also form when lava flows close to the rocks near the surface of the earth. Rocks can be changed when the Earth’s surface moves. Example: In an earthquake, rocks may get pushed against each other. If this keeps on happening, they may get squashed and folded. Weathering Rocks do not stay the same forever. They get worn away slowly. This Photo by Unknown Author is licensed under CC BY-SA-NC Rain, wind, frost and temperature changes can all wear away rocks. When rocks are worn away by rain, wind, temperature, and frost, it is called weathering. Types of weathering Chemical weathering Physical weathering Biotic weathering This Photo by Unknown Author is licensed under CC BY This Photo by Unknown Author is licensed under CC BY-NC-ND Chemical Weathering Acidic rainwater attacks the limestone due to reaction between acid and salt. This is called chemical weathering. Limestone is made of calcium carbonate (a salt). Acid + Calcium Carbonate -> Salt + Carbon dioxide + water This Photo by Unknown Author is licensed under CC BY Physical Weathering Water from rain gets into the spaces and cracks in rocks. When water freezes, it expands. This makes the cracks larger. Slowly due to repeating of this process, rocks are broken up. This Photo by Unknown Author is licensed under CC BY-NC-ND Another way The heat of the sun can make rocks expand. This Photo by Unknown Author is licensed under CC BY-SA-NC At night, the temperature falls and the rock contracts. Over time, due to repetition of the process, the rocks crack. Another way Wind and running water can also wear way rocks. Rocks are made smooth by water running over them (pebbles). This Photo by Unknown Author is licensed under CC BY-NC Biotic Weathering Living organisms can cause rocks to break apart. Plants can grow in cracks in the rocks. This Photo by Unknown Author is licensed under CC BY-NC The growth of the plant’s root causes cracks and damage to the rocks. Fossils The remains of living organisms that have changed to rock are called fossils. How is it formed? When animals and plants die, their bodies may fall into sediments. They usually just decay. But sometimes they can become part of sedimentary rocks. As the rock layers build up, the rock becomes solid. The minerals in the rock may replace minerals in parts of the dead bodies. This takes place over millions of years. Another way of making fossils Fossils can also be made when animals leave an imprint of its footprint or burrow in wed sand or mud. When more sediment is deposited on top of the imprint and the rock hardens, they may create a mark on the rock. Clues from fossils Fossils can help us to work out how a rock formed. Example: Limestone contains fossils from sea animals and plants. This is how we know that limestone was formed under the sea. Coal sometimes contains fossils of plants that look like ferns. So we know that coal was not formed under the sea. It was formed when trees and other plants fell into swaps millions of years ago. What do fossils tell us? Fossils tell us about the plants and animals that lived millions of years ago. Some of them were very similar to those found today. This tells us that those plants and animals have been on Earth for millions of years. But other fossils show us strange organisms that do not live on Earth today. The fossil record There are many different types of animal and plant fossils. These fossils were formed at different times over many millions of years. Fossils in different rocks made up the fossil record. What can we learn from a fossil record? We can learn about organisms that lived on Earth long ago. We can understand when the species first appeared and disappeared. We can also understand how they changed over time. The oldest fossils that have been found of simple bacteria lived 3.5 million years ago. Where do they form? Fossils form in sedimentary rocks. New sedimentary rocks form on top of old rocks. So, deeper a rock is, the older it is. Therefore, the deeper the rock in which a fossil is found, the older the fossil. Information about some fossils Sometimes fossils that are found in older rocks are not found in younger rocks. This tells us that this type of organism has died out. Example: flying reptiles called pterosaurs lived between 220 and 65 million years ago. Fossils show that there were many different kinds of pterosaurs. Some of them has wing spans of 10 meters. Create a fossil record of pterosaurs The structure and age of the Earth According to Geologists, the Earth is about 4500 million years old. The Earth has a crust of solid rock. Beneath the crust is mantle, which is molten. The centre of the Earth is core, made up nickel and iron. This Photo by Unknown Author is licensed under CC BY Mantle: Molten Outer Core: Molten Inner Core: Solid Changing Ideas In 1912, a German scientist Alfred Wegner suggested that millions of years ago the whole land was one large continent. Over millions of years, the land broke up and drifted (moved) apart. This idea is called continental drift. Evidences (proof) behind the idea The shapes of continents fit together Types of the rock on the different continents match up where they fit together. The fossils on the different continents match up where they fit together. He couldn’t explain how the continents drifted. That’s a lot of people didn’t believe in his ideas. Idea of Tectonic Plates In the 1960s, the theory of tectonic plates was developed. According to this theory, the Earth is made up of large plates. These plates move slowly on the molten magma underneath (below) them. The plates move only a few centimeters every year.
