The first rocks were formed by volcanoes - as magma cools down it forms rocks. When magma reaches the surface it is called lava. Igneous rocks can be divided into two categories:
Extrusive rocks cool quickly on the surface of the Earth, and form tiny crystals (or sometimes cool so quickly they are like glass). They are also known as volcanic rocks.
Intrusive rocks cool slowly deep underground, and form large, coarse grained crystals often visible to the naked eye. They are also known as plutonic rocks.
High temperature runny lava is produced by shield volcanoes like those in Hawaii, and in deep-sea vents. They are rarely explosive. The most common rocks they produce are basalt.
Basalt Lava Flow
Cooled Lava Flow
Cooler sticky lava produce cinder cone volcanoes. While their eruptions might be spectacular they are rarely dangerous. The most common rocks they produce are andesite.
Cooled Lava Flow
Even cooler, very sticky lava can build up, blocking the volcanic vent and eventually produce explosive eruptions. The most common rocks they produce are rhyolite.
Cooled Lava Flows
In this diagram you can see how volcanoes are associated with tectonic plates, and how hotter magma comes from deep within the Earth.
Sediments and water are drawn down at subduction zones, cooling the magma and producing cooler lava. Volcanoes can also be a mix of different types.
Intrusive rocks form slowly deep underground, and are exposed on the surface when the area is uplifted, and/or exposed by erosion. Granite outcrops are formed when magma chambers cool down, which are then exposed by erosion and sometimes shaped by weathering. See image of granite outcrop in Queensland, and the Devil's Marbles in the Northern Territory.
TASK 22: Create a Venn diagram (see attached file at bottom of page) that compares extrusive and intrusive igneous rocks. How are they similar, and how and they different?
Why are there so many different types of igneous rocks?
There are many different types of igneous rock because there are only eight elements commonly found in magma - the most abundant being oxygen (O) and silicon (Si) which make up nearly 80%. The other minerals in order are aluminium, iron, calcium, potassium, sodium and magnesium. As the magma cools down, silicon and oxygen will combine with other elements to form different silicate minerals, depending on the temperature. These minerals grow together to form rocks, but as different elements are removed from the magma the composition of the magma changes, and different minerals are formed. This is a process called fractional crystallisation. Over 90% of all igneous rocks are formed from silicate minerals.
You can see some different types of igneous rocks here. Igneous rocks are often hard to identify in the field, and some must be brought back to the laboratory and tested before they can be identified.
TASK 23: Activity. Try growing crystals for yourself. Negotiate an activity with your teacher. Here are some suggestions: Crystal Creations, Make a Salt Sculpture, Crystal Growing Activities - you can find others on the internet.
Next we'll have a brief look at minerals.