Igneous rocks form by crystallisation from a liquid called magma.

Magma is liquid rock that is formed deep within the Earth where pre-existing rocks melt.

The temperature and properties of the magma depend on the composition of the rock that was melted.

Magma may be ‘cold’ (~800 degrees celsius) and stiff if it contains lots of the element silicon (Si) or ‘hot’ (>1300 degrees Celsius) if it has low silicon.

Magma comes to the surface of the Earth through volcanoes where it can flow as lava and then cool and crystallise to form extrusive or volcanic rocks.

Sometimes volcanic eruptions are explosive and these eruptions can produce pyroclastic rocks which are composed of particles. The term pyroclastic comes from two greek words: pyro – fire and klastos – broken into pieces.

Magma can also cool and crystallise deep within the Earth to form intrusive igneous rocks.

The minerals that form in an igneous rock depend on the original chemical composition of the magma.

• High silicon magmas typically crystallise to form lots of quartz and potassium feldspar and only small proportions of dark coloured minerals that contain iron and magnesium.
• Low silicon magmas mainly produce rocks that do not contain quartz or potassium feldspar but which contain lots of dark iron and magnesium-bearing minerals.

The size of the crystals in an igneous rock is mainly controlled by how fast the magma cools:

• Slow cooling deep within the Earth results in large crystals.
• Rapid cooling at the Earth’s surface produces small crystals.

Volcanic rocks occur on land but also erupt under water. When hot magma is erupted into cold water the outer surface cools very rapidly but the interior keeps flowing. This can produce lava flows with lobes or pillow structures. These features tell geologists that the lava was erupted under water.

There are two basic types of igenous rock:

Instrusive (plutonic) igneous rocks - those that form in the Earth and,

Extrusive (volcanic) igneous rocks - those that form on the surface of the Earth.

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Sedimentary rocks are formed from the weathered products of pre-existing rocks or pieces of once-living organisms that accumulate on the Earth’s surface.

The starting material for sedimentary rock formation is sediment which is simply loose fragments of rock or material created by chemical, mechanical or biological processes.

The stages in the formation of a sedimentary rock are:

Weathering – existing rocks are broken down by chemical and mechanical processes to produce both rock fragments and chemicals dissolved in water.

Erosion and Transportation– the weathered rock fragments are removed from where they were formed and transported to a new location. Moving water, wind, ice, gravity and biological processes are the main ways that weathered rock is transported.

Deposition – the weathered rock material eventually stops moving and builds up in a suitable location. This could occur where fast flowing water slows down or where biological processes extract chemicals from the water.

Burial and Compaction – In the right circumstances, thick piles of sediment can accumulate and the lower sediments in the pile are compacted by the weight of the overlying material.

Lithification (cementation) – is the process that turns sediment into rock. Once the sediments have been compacted, new minerals grow to cement the particles together to form a solid rock.

There are two basic types of sedimentary rocks:

Clastic sedimentary rocks – composed of fragments (clasts) of pre-existing rocks that have been compacted and cemented together.

Non-Clastic sedimentary rocks – chemicals dissolved in water that are deposited as solid materials by physical, chemical or biological processes.

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Metamorphic rocks are formed when existing rocks are exposed to high temperatures and/or pressures.

The original rock is called the protolith and could be an igneous rock, a sedimentary rock or an existing metamorphic rock.

In most cases the chemical composition of the rock does not change during metamorphism but new minerals may form and the appearance of the rock may change.

The effects of metamorphism can be complex and depend on the composition of the original rock and the pressure and temperature that the rock has be subjected to.

Metamorphic rocks are usually denser than the original rock, may contain new minerals, and develop structures such as foliations and folds.

Foliations develop when the new minerals that grow in the rock line up in sheets that are oriented at 90 degrees to the maximum force applied to the rock.

Folds are bends that develop when original layers in the rock are compressed in a direction parallel to the layers.

Folds and foliations are used by geologists to interpret the forces that were acting on the rock at the time it was metamorphosed.

Of the three basic rock groups, metamorphic rocks have the most complex textures and mineralogy and they are the most difficult to identify and interpret.

Many different metamorphic styles are recognised but metamorphic rocks can be divided into two basic classes:

Contact metamorphism – occurs where rocks are heated to high temperatures, usually at low pressure. This typically happens to rocks next to a large igneous intrusions. The effects of contact metamorphism usually only extend for a short distance from the cooling igneous rock.

Regional metamorphism – rocks are altered by high temperatures and / or high pressures usually deep within the earth. Regional metamorphism can affect large volumes of the crust and typically happens beneath new mountain range.

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This educational product is designed for Year 7 to 10 secondary students to complement the Earth and Space component of the Australian National Science Curriculum and all Australian State and Territory curricula.

The content and design of this educational product is based upon materials previously published by AusGeol.org