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examBoard: Cambridge
examType: IGCSE
lessonTitle: Igneous Rocks Formation and Characteristics
Igneous rocks are one of the three main rock types found on Earth (along with sedimentary and metamorphic rocks). The word "igneous" comes from the Latin word "ignis" meaning "fire", which gives us a clue about how these rocks form. Igneous rocks are created when molten rock (magma or lava) cools and solidifies. They make up about 95% of the Earth's upper crust, even though much of that is covered by other rock types.
Key Definitions:
Igneous rocks form the starting point of the rock cycle. When exposed to weathering and erosion, they break down into sediments that can form sedimentary rocks. Under heat and pressure, igneous rocks can also transform into metamorphic rocks. If any rock type is melted completely, it becomes magma again, which will cool to form new igneous rock.
Igneous rocks provide valuable information about the Earth's interior and past volcanic activity. They contain important minerals and metals used in construction and manufacturing. Understanding igneous rocks helps us predict volcanic hazards and develop strategies for managing these risks.
Igneous rocks form when magma cools and crystallises into a solid rock. The cooling process can happen either below the Earth's surface or after the magma erupts as lava. The speed of cooling greatly affects the final appearance and properties of the rock.
When magma cools, minerals begin to crystallise in a specific order based on their melting points. This sequence is known as Bowen's Reaction Series. Minerals with higher melting points (like olivine) crystallise first, while those with lower melting points (like quartz) form later. The rate of cooling determines crystal size:
When magma cools slowly underground, minerals have time to form large, visible crystals. This creates a coarse-grained texture, as seen in rocks like granite.
Intermediate cooling rates produce medium-sized crystals, creating rocks with a porphyritic texture, where some larger crystals are surrounded by a finer-grained matrix.
When lava cools rapidly at the surface, crystals don't have time to grow. This creates fine-grained or even glassy textures, as seen in rocks like basalt or obsidian.
Igneous rocks are classified based on where they form (their mode of occurrence) and their mineral composition.
These form when magma cools slowly beneath the Earth's surface. The slow cooling allows large crystals to form, giving these rocks a coarse-grained texture. Examples include granite, diorite and gabbro. Intrusive rocks are only exposed at the surface after uplift and erosion remove the overlying rocks.
These form when lava cools rapidly at or near the Earth's surface. The rapid cooling creates small crystals or glass, giving these rocks a fine-grained or glassy texture. Examples include basalt andesite and rhyolite. Extrusive rocks often contain vesicles (bubble holes) from gas that was trapped as the lava solidified.
Igneous rocks are also classified based on their mineral composition, particularly their silica (SiO₂) content. This affects their colour, density and other properties:
High in silica (>65%), low in iron and magnesium. Usually light-coloured (white, pink, or light grey). Examples: granite (intrusive) and rhyolite (extrusive). They're less dense and more viscous as magma.
Moderate silica content (55-65%). Medium-coloured (grey to dark grey). Examples: diorite (intrusive) and andesite (extrusive). Common in volcanic arcs above subduction zones.
Low in silica (45-55%), high in iron and magnesium. Dark-coloured (dark grey to black). Examples: gabbro (intrusive) and basalt (extrusive). They're denser and more fluid as magma.
Let's explore some of the most common igneous rocks you might encounter:
Iceland sits on the Mid-Atlantic Ridge where the North American and Eurasian tectonic plates are moving apart. This makes it one of the most volcanically active places on Earth and a perfect natural laboratory for studying igneous rocks.
The island is almost entirely made of igneous rocks, with basalt being the most common. The famous black sand beaches of Iceland are made of basalt fragments. The country has over 30 active volcanic systems, including Eyjafjallajökull, which erupted in 2010 and disrupted air travel across Europe.
Iceland also showcases unique igneous features like columnar basalt formations (such as at Svartifoss waterfall), where cooling lava forms hexagonal columns. These dramatic formations occur when thick lava flows cool slowly and contract, creating geometric patterns.
Icelanders have adapted to living with active volcanism, using geothermal energy from magma chambers to heat homes and generate electricity, making it one of the world's most sustainable energy systems.
When trying to identify an igneous rock, look at these key features:
Igneous rocks are important for many reasons:
While igneous rocks themselves are natural and non-polluting, mining them can cause environmental damage through habitat destruction, dust and potential acid drainage. Sustainable mining practices and proper site rehabilitation are essential to minimise these impacts. Volcanic eruptions can also release large amounts of carbon dioxide and sulphur dioxide, affecting air quality and potentially contributing to climate effects.
For your iGCSE Environmental Management exam, be prepared to describe the formation process of igneous rocks, distinguish between intrusive and extrusive rocks and explain their importance in the rock cycle. You should also be able to identify common igneous rocks from descriptions or images and understand their environmental significance.
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