Igneous rocks form by the crystallisation of magma or lava; cooling rate controls crystal size (slow cooling at depth gives coarse-grained intrusive rocks such as granite, fast cooling at the surface gives fine-grained extrusive rocks such as basalt); rocks are classified by crystal size and by silica content (felsic, intermediate, mafic); minerals also crystallise from hydrothermal fluids to form veins.

What this dot point is asking

Eduqas wants you to explain how igneous rocks form by the crystallisation of magma or lava, and to use the central rule that cooling rate controls crystal size: slow cooling at depth gives coarse-grained intrusive rocks (granite), fast cooling at the surface gives fine-grained extrusive rocks (basalt). You also need to classify igneous rocks by crystal size and by silica content (felsic, intermediate, mafic), and to know that minerals also crystallise from hot watery (hydrothermal) fluids to form mineral veins.

The answer

How igneous rocks form

Igneous rocks form when molten rock cools and crystallises. Molten rock below the surface is called magma; at the surface it is lava. As it cools, atoms lock into ordered crystals, so igneous rocks are made of interlocking crystals with no bedding and (usually) no fossils.

Cooling rate controls crystal size

The single most important idea is that how fast the melt cools sets how big the crystals grow:

• Slow cooling at depth. Deep underground the surrounding rock insulates the magma, so it cools slowly over a long time. Crystals have time to grow large, giving a coarse-grained rock (crystals over about 3 mm, visible to the naked eye). These are intrusive (or plutonic) rocks; the classic example is granite.
• Fast cooling at the surface. A lava flow is in contact with cool air or water and loses heat quickly. Crystals have little time to grow, so they stay small, giving a fine-grained rock (crystals too small to see individually). These are extrusive (or volcanic) rocks; the classic example is basalt.

Two rocks can share the same chemistry yet look completely different because one cooled slowly and one quickly. Sometimes a magma cools in two stages, giving large crystals set in a fine matrix (a porphyritic texture), which records a change in cooling rate.

Classifying igneous rocks

Igneous rocks are classified two ways at GCSE:

• By crystal size (which records cooling rate and depth): coarse-grained (intrusive) or fine-grained (extrusive).
• By silica content (which records composition and colour):
  • Felsic (silica-rich, pale, lower density): granite (coarse) and rhyolite (fine).
  • Intermediate: diorite (coarse) and andesite (fine).
  • Mafic (silica-poor, dark, denser): gabbro (coarse) and basalt (fine).

So the pair "coarse versus fine" plus "pale versus dark" places a specimen on the chart: a dark, fine-grained rock is basalt; a pale, coarse-grained rock is granite.

Minerals from hydrothermal fluids

Not all crystallisation happens in a cooling magma. Hot watery hydrothermal fluids, heated by a nearby intrusion or by deep circulation, dissolve metals and other elements and carry them through cracks and faults. As the fluid cools, the dissolved minerals crystallise on the fracture walls, filling the crack to form a vein. This is how economic ore minerals (galena, haematite) and gangue minerals (quartz, calcite) are concentrated, a link to economic geology later.

Examples in context

Example 1. The Giant's Causeway. The basalt columns formed when a thick lava flow cooled and contracted, cracking into hexagonal columns. The fine grain confirms it cooled quickly at the surface.

Example 2. A granite tor. Dartmoor's granite crystallised slowly kilometres underground, then was exposed by erosion. Its large, interlocking quartz, feldspar and mica crystals record that slow, deep cooling.

Try this

Q1. State the difference between magma and lava. [1 mark]

• Cue. Magma is molten rock below the surface; lava is molten rock at the surface.

Q2. Explain why an extrusive rock is fine-grained. [2 marks]

• Cue. It cools quickly at the surface, so crystals have little time to grow and stay small.

Q3. Name one coarse-grained felsic rock and one fine-grained mafic rock. [2 marks]

• Cue. Coarse felsic: granite. Fine mafic: basalt.