Rock Formation

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Igneous rocks
The second type of rock we'll look at is igneous rock.
Formation
The inside of the Earth is very hot - hot enough to melt rocks. Molten (liquid) rock forms when rocks melt. The molten rock is called magma. When the magma cools and solidifies, a type of rock called igneous rock forms. What are they like?

Igneous rocks contain randomly arranged interlocking crystals. The size of the crystals depends on how quickly the molten magma solidified. The more slowlythe magma cools, the bigger the crystals. You may have done an experiment at school with a substance called salol. If molten salol cools slowly, you get big crystals. If it cools quickly, you get small crystals. Obsidian and basalt

If the magma cools quickly, small crystals form in the rock. This can happen if the magma erupts from a volcano. Obsidian and basalt are examples of this type of rock. They are called extrusive igneous rocks because they form from eruptions of magma. Granite and gabbro

Granite has large crystals.
If the magma cools slowly, large crystals form in the rock. This can happen if the magma cools deep underground. Granite and gabbro are examples of this type of rock. They are intrusive igneous rocks because they form from magma underground. Unlike sedimentary rocks, igneous rocks do not contain any fossils. This is because any fossils in the original rock will have melted when the magma formed.

Metamorphic rocks

Metamorphic rocks are the third type we'll look at. They formed from other rocks that are changed because of heat or pressure.

Formation
Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are heated and put under great pressure. They do not melt, but the minerals they contain are changed chemically, forming metamorphic rocks.

Sometimes, metamorphic rocks are formed when rocks are close to some molten magma, and so get heated up.

Metamorphic rocks may form from rocks heated by magma

Remember that metamorphic rocks are not made from melting rock. (Rocks that do melt form igneous rocks instead.)

What are metamorphic rocks like?
When a metamorphic rock is formed under pressure, its crystals become arranged in layers. Slate, which is formed from shale, is like this. Slate is useful for making roof tiles because its layers can be split into separate flat sheets.

Marble bedrock on the coast.

Marble is another example of a metamorphic rock. It is formed from limestone.

Metamorphic rocks sometimes contain fossils if they were formed from a sedimentary rock, but the fossils are usually squashed out of shape.

Metamorphic rocks can be formed from any other type of rock - sedimentary or igneous. Remember these two examples of common metamorhpic rocks and where they come from:

slate is formed from shale

marble is formed from limestone

Weathering
Rocks gradually wear away. This is called weathering. There are three types of weathering: •physical weathering
chemical weathering
biological weathering
Remember, when you answer questions about weathering, mention what is causing the weathering and what it does to the rock. Physical weathering
Physical weathering is caused by physical changes such as changes in temperature, freezing and thawing, and the effects of wind, rain and waves. Temperature changes
When a rock gets hot it expands a little, and when a rock gets cold itcontracts a little. If a rock is heated and cooled many times, cracks form and pieces of rock fall away. This type of physical weathering happens a lot in deserts, because it is very hot during the day but very cold at night. Wind, rain and waves

Wind, rain and waves can all cause weathering. The wind can blow tiny grains of sand against a rock. These wear the rock away and weather it. Rain and waves can also wear away rock over long periods of time. Freeze-thaw

Water expands slightly when it freezes into ice. This is why water pipes sometimes burst in the winter. You might...
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