Rocks - GCSE Chemistry

ROCKS

The rock cycle and how it worksThe rock cycle is the name given to the process where rocks are formed, changed, broken and melted as shown in the picture on the left. It is the movement of tectonic plates that makes the rock cycle happen. Hot magma can cool to form igneous rocks. Fast cooling on the surface makes basalt and slow cooling inside the crust makes granite. When tectonic plates collide the enormous forces can produce metamorphic rocks and rocks inside the Earth's crust can become pushed up onto the surface by a process called uplift. Surface rocks are exposed to weathering and erosion which makes sediments. Burial of sediments produces sedimentary rock inside the Earth's crust. Any rock in the crust can be pushed to the surface by uplift or dragged down towards the mantle where it can melt and form new magma.

Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Igneous Rocks are rocks formed from the cooling of hot magma. Igneous rocks first formed when the Earth cooled from its molten state about 4 billion years ago. Igneous rock is still being made today by the process of tectonic activity. How can they be identified?Igneous rocks contain randomly arranged interlocking crystals. The crystal size depends on the rate of cooling. When hot magma is cooled quickly it makes basalt and when it is cooled slowly it makes granite. Igneous rocks do not contain fossils because the rock forms at a high temperature which would destroy any living organisms.

Rocks formed from sediments are called sedimentary rocks. Sediments are small pieces of material that do not dissolve in water. They slowly fall down to the bottom of a lake or the sea, and gather together over the years forming a large mass of material. Imagine stirring very fine sand into a glass of water. Slowly the sand particles settle on the bottom of the glass. They are called "sediments" . Materials which do dissolve (salts), stay in the water. This is what has made the sea salty over many millions of years. Sediments have come from the weathering and erosion of surface rocks. The original surface rocks were igneous rocks formed from the cooling of molten magma. Surface rocks today are igneous, sedimentary or metamorphic.

Weathering and ErosionWeathering breaks a rock down into small pieces. The two main processes are exfoliation and freeze-thaw weathering. Freeze-thaw weathering occurs when water gets into cracks in rock and then freezes (below 0 °C) in cold weather. Water expands when it freezes and turns to ice, and the expansion can cause the rock to split and fragment. In warmer weather (above 0 °C) the ice melts (called thawing), and new cracks are exposed allowing the process to repeat itself. Erosion is a slow process of wearing away weathered rock by the action of the wind, rivers and waves. All rocks would have been worn down flat to sea level long ago if hills and mountains were not constantly being formed by the movement of plate tectonics. See the rock cycle.Transport and compaction Small pieces of weathered rock (called sediments) are transported by rivers. This means that the rock pieces are moved along as the river flows towards the sea. During transportation, the sediments are sorted according to size. The largest pieces tend to be deposited first. Sand is next to settle out of the water. The smallest particles are mud or clay, and these are moved furthest by the river and deposited last. Compaction - With the passing of time, the deposits of sediment get thicker and thicker. The sediment underneath becomes increasingly crushed by the weight of the new sediment which is being deposited on it from above. This is called compaction. It is this process of compaction which eventually makes layers of sedimentary rock. The process may take millions of years. The largest rock pieces will produce conglomerate. Sand will produce sandstone. Mud or clay will produce mudstone or shale

The formation of sedimentary rocksCompaction eventually forces the water out of the sediment. Salts in the water crystallise under pressure and cement the sediment particles together (like glue). This is how sedimentary rock is made. How can Sedimentary Rocks be Identified? Sedimentary rock is often formed in layers, because the original sediment is laid down in layers. This gives the rock a typical layered structure, which can be seen in mudstone, and more clearly in shale. Examples of Sedimentary Rocks are conglomerate, sandstone, mudstone, shale

What is a Metamorphic Rock? A metamorphic rock is a rock which has had its form changed by the action of high temperatures and pressures (without melting). The crystal structure of the rock is changed in the process. Metamorphism can be low or high grade. Rocks can become subject to high temperatures because they come into contact with an igneous intrusion. This is called contact metamorphism or rocks become subject to high temperatures and pressures because of tectonic activity in the Earth's crust. This is called regional metamorphism and it occurs where plates collide. Examples of metamorphic rock are Marble Slate, Schist and Gneiss.

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Products and Uses of Rocks

All raw materials come from the Earth's crust, the oceans and the atmosphere. Metals and oil have already been discussed. Some rocks are used in construction for building. These include Granite, Limestone, Marble, Slate. Granite and limestone are used in road construction. Limestone has a number of other important uses.Limestone is used to make Lime - Quicklime - Slaked Lime - Cement - Concrete - Glass Powdered limestone can be used in power station chimneys to remove acidic gases or powdered limestone can be added to soils or lakes to reduce acidity. Lakes and soils can become acidic because of acid rain. Powdered limestone is cheaper than slaked lime but acts more slowly.