All rocks are made from minerals. There are around 2000 different types of minerals but not all of them are found in rocks. There are actually only about ten of these minerals that are common in rock formations. These include silicon, aluminium, iron, magnesium, calcium, sodium, potassium and hydrogen. Most differences between rocks are found in the way in which they were formed. This chapter examines igneous rocks, which are formed from magma and lava.
What are igneous rocks?
Igneous rocks are known as 'fire rocks'. Igneous rocks got their name from the Latin word ignis, which means 'fire', because they are formed from hot molten rocks inside the Earth. When they are inside the Earth, the hot molten rocks are known as magma. When the hot molten rocks reach the surface of the Earth, they are called lava.
Most igneous rocks contain large quantities of silica. This is because of the high quantity of silicate minerals occurring in magma. See video 'What are igneous rocks?'
How are igneous rocks formed?
Igneous rocks are formed from magma and lava. When the magma or lava cools, it hardens, and igneous rocks are formed. See Animation
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Intrusive rocks are formed when magma cools and hardens within the Earth's crust. Intrusive rocks are formed underneath the surface of the Earth. Magma finds fissures, or cracks, within the crust and fills in these areas. Sometimes these areas are small cracks and at other times they are more open and cavernous. The lower temperature of the surrounding rocks cools the magma so it hardens slowly. Slower cooling rocks produce larger crystals and are considered coarse grained.
Layers of soil and other rocks above intrusive igneous rocks are often worn away due to erosion and weathering. This exposes the igneous rock underneath. There are many different formations that can occur. Batholiths are large granite rocks that are composed of various sections of cooled magma, known as plutons. Dykes are vertical or near vertical intrusions and sills are horizontal intrusions. These intrusions into other rocks can extend for large distances and are always younger than the other rocks.
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Extrusive rocks are formed when magma reaches the surface of the Earth, then cools and hardens. Extrusive rocks are formed on the surface of the Earth from volcanic activity. Lava on the surface cools more rapidly than magma still within the crust. For this reason, most extrusive igneous rocks have smaller crystals and are considered fine grained. Sometimes extrusive rocks can cool so quickly that very little or no crystals form. These rocks produce a glassy appearance, such as seen in obsidian.
Identification of igneous rocks
Igneous rocks do not have a specific classification system. One of the most common methods is to classify igneous rocks based on texture and mineral composition. The texture, or crystal size, tells us how long the rock took to cool and harden. The mineral composition tells us where the rock was formed.
Most igneous rocks are dominated by the mineral silicate.
Ultra-basic rocks are less than 45 percent silica.
Basic rocks are between 45 and 52 percent silica.
Intermediate rocks are between 52 and 66 percent silica.
Acid rocks are above 66 percent silica.
Igneous rocks can be described as mafic or felsic. Mafic rocks are generally darker in colour and have a lower silica content than felsic rocks. Felsic rocks are generally lighter coloured and have a higher silica content.
The origin of an igneous rock will determine whether it is intrusive or extrusive. The origin will also play a part in the occurrence. The occurrence describes the form of the rock once the magma or lava has cooled and hardened. Mineral content and composition are important factors in determining which type of igneous rock is formed. Colour will help determine whether the rock is felsic or mafic. Crystal shape determines how quickly the rock cooled. Grain size indicates if the rock is extrusive or intrusive.
Types of igneous rocks
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Basalt is a dark coloured rock with small crystals. The small crystals form because this rock is extrusive and cools quickly. It is found on both continental and oceanic crusts. Basalt has a low silica percentage and is considered basic. See Image 4
Granite is generally a lighter coloured rock that has black and glassy flecks. Granite comes in a variety of colours including white, grey, black or pink. Granite is an intrusive rock and so is slow cooling with generally large crystals and a medium grain. Because granite is very durable, it is often used in the construction of buildings. See Image 5
Obsidian is an extrusive rock formed from the rapid cooling of magma. This glassy black rock has a smooth texture. The magma cools and hardens so rapidly that there is no time for crystal formation. Historically, this rock was used for tools and arrowheads.
Pumice is a pale coloured, light weight, porous stone. The texture is hard and it is often used for scrubbing. Pumice is formed when lava erupts violently from volcanoes and meets the cooler atmosphere.