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Module 8.1 (the rock cycle (Igneous rocks form from red hot melted rock…
Module 8.1
the rock cycle
Igneous rocks form from red hot melted rock from inside the Earth. Volcanoes are a good place to see them.
The third type of rock is metamorphic. These rocks have been changed by heat and pressure within the Earth. Common places to see them are around volcanoes and in mountain ranges.
The rock cycle is a model used by scientists to explain the endless cycling of materials that make up rocks by processes within Earth and on its surface. There are three main types of rocks: igneous rocks, sedimentary rocks and metamorphic rocks.
Sedimentary rocks form when small pieces worn off other rocks (sediments) become stuck together, or when minerals in solution solidify. They are commonly seen in river valleys and canyons.
- weathering—rock is broken down into smaller pieces
• erosion—the broken rock material is transported from one place to another
• deposition—the broken rock material settles, forming sediment
• compaction and cementation—the sediment becomes packed together, and then chemically bound
• melting—the solid rock is heated, forming magma
• crystallisation—the magma becomes solid
• metamorphosis—the rock is changed by intense heat and/or pressure.
Deposition
Deposition occurs where moving water, wind or ice that is carrying broken rock material slows down. In rivers, this slowing down occurs at bends, and where rivers leave the mountains and enter plains.
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The layers of material formed are called sediments and so the process is sometimes also called sedimentation.
Prospectors are people who search for precious minerals. They concentrate their efforts on the inside of river bends, where heavy materials such as gold can accumulate
When a river meets the sea, the water slows down, causing the deposition of most of its materials.
Physical Weathering
Anyone who has been on a beach on a windy day will know the feeling of sand blasting against your skin. Fine particles of rock carried by wind can also blast away at a rock surface, wearing pieces away.
Solids expand (get larger) when they are heated, and contract (get smaller) when they are cooled. If the day is very hot but the night is very cold, the temperature change between night and day can cause the rock to expand and contract too quickly, causing it to crack.
Physical weathering can happen because of the action of:
• wind
• temperature change
• water and ice
• crystallisation of salts.
Water can enter cracks in a rock. If the water freezes and becomes ice, the ice expands and makes the crack wider. This is common in colder climates and on the cold tops of mountains.
Water running or washing over rock can also gradually wear away the rock. The rate of weathering depends on the type of rock the water flows over, the speed and power of the water, and what materials the water is carrying in it. Rivers can cut deep through rock, forming gorges. Pounding waves at the beach can wear away cliffs
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Structure of the Earth
Geology is the study of rocks, and the scientists who study them are geologists
The crust is the outermost solid layer of the Earth. The crust varies in thickness from about 7 kilometres under the oceans to about 30–50 kilometres under the continents. The crust is the layer we live on, the layer we build our cities on, and the layer we dig up when we mine for minerals
Within the uppermost portion of the mantle is a layer called the asthenosphere . The rock here behaves plastically. This means the rock can move slowly, like plasticine
Chemical Weathering
Living things and their products can also break down rock. This is known as biological weathering .
plant roots can grow through fine cracks, or even create their own cracks. As the root grows and thickens, it splits the rock. Tree roots can be strong enough to seriously damage homes, paving and driveways.
Water dissolving chemicals from the soil and air will react with rock, changing it or breaking it down. This is known as chemical weathering . Air contains oxygen and carbon dioxide. When these gases are dissolved in water, they can react with minerals in the rock, changing them. This may cause the rock to crumble or change colour.
Erosion
The small rock particles formed by weathering rarely stay near the rock they came from. Instead they are moved elsewhere. This is called erosion . The small rock particles are moved elsewhere by water, wind, gravity or ice, known as agents of erosion .
You can see erosion where rivers have cut away banks, where stormwater pipes have created gullies and where large boulders have rolled downhill.
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