Igneous Rocks: Texture, Classification and Bodies

Crystals remember how they cooled

An igneous rock begins as molten rock and freezes solid as it cools, growing crystals as it goes. The single most useful thing about it is that the SIZE of those crystals is a record of HOW FAST it cooled. Read the grain size and you read the history of the rock. ⭐⭐ Slow cooling grows big crystals. Molten rock trapped deep underground, wrapped in warm rock, loses its heat very slowly, so the crystals have plenty of time to grow large. A rock full of large crystals you can see with the naked eye cooled slowly, deep down. ⭐ Fast cooling grows small crystals. Molten rock that reaches the surface as lava loses its heat quickly, so the crystals have no time to grow and stay tiny. And if it is chilled almost instantly, it sets as a glass, with no crystals at all. ⭐⭐ Composition classifies it further. A rock rich in silica tends to be pale and light-coloured; one rich in iron tends to be dark. So naming an igneous rock uses two things together: the crystal size, and the colour that hints at what it is made of. Where the rock cooled splits it into two families: intrusive rock, cooled underground, and extrusive rock, cooled from lava at the surface. This module is about igneous rocks; the rock cycle and mineral tests have their own modules. ⚠️ Carry one question the whole way through: how big are the crystals, and what does that tell me about where it cooled?

Slow, fast, and how to name it

The first column is rock cooled slowly, the second cooled fast, and the third how you classify what you find.

Tap the two that cooled slowly

Tap the TWO descriptions of an igneous rock that cooled SLOWLY.

• A rock full of large crystals you can pick out by eye.
• A rock that formed from molten rock trapped deep underground.
• A smooth, glassy rock with no crystals at all.
• A rock whose crystals are far too small to see.

The rock with no crystals

A shiny, glassy igneous rock has no crystals at all. What does that tell you about how it cooled?

• It cooled almost instantly, giving crystals no time to grow
• It cooled very slowly, deep underground
• It cooled at a medium rate over many years
• Cooling rate has no effect on the crystals

The language of igneous rocks

Six terms this topic turns on. The first two are the raw material and the clue; the rest are the families.

Match each setting to its result

Two true of an intrusive rock

Select the TWO statements that are TRUE of an intrusive igneous rock.

• It cooled underground, from molten rock that never reached the surface
• It has large crystals, because it cooled slowly
• It formed from lava flowing across the ground
• It is always a crystal-free glass

Answering an igneous rock question

Igneous questions reward reading the cooling story from the rock and naming it in two parts. ⭐⭐ START FROM THE GRAIN SIZE. Large crystals mean slow cooling deep underground, so intrusive; small crystals mean fast cooling at the surface, so extrusive; no crystals, a glass, means the fastest cooling of all. State the crystal size and what it tells you before anything else. ⭐ NAME IT IN TWO PARTS. A full classification uses the crystal size AND the composition. Say whether the crystals are large, small or glassy, and whether the rock is pale (silica-rich) or dark (iron-rich). Two clues together pin the rock down. ⭐⭐ KNOW THE BODIES. Intrusive rock forms underground: large masses deep down, sheets that cut across the layers, and sheets that run along the layers. Extrusive rock forms from lava at the surface. Match the body to whether it cooled below ground or above it. EXPLAIN, DO NOT JUST LABEL. Naming a rock intrusive earns little; saying it is intrusive BECAUSE its large crystals show slow, deep cooling earns the mark. Always give the reason from the evidence. ⚠️ A wording habit. Say magma for molten rock below ground and lava for molten rock at the surface, describe cooling as slow or fast rather than reaching for a temperature, and give silica and iron as words, not formulae. Keep the one question to hand: how big are the crystals, and what does that tell me about where it cooled?

Order how a deep rock forms

Put the stages by which a coarse-grained igneous rock forms deep underground into a sensible order.

• Molten rock rises but is trapped deep underground
• Surrounded by warm rock, it loses heat very slowly
• Crystals begin to form and, with time, grow large
• The whole mass sets solid, full of large crystals
• Much later, uplift and erosion expose it at the surface

Read the rock from its crystals

Assemble one sentence that reads an igneous rock history from what you can see.

The igneous run

Five quick decisions about igneous rocks. Three lives.

Making sweets from hot syrup

Leave geology aside for a screen. Think about a pan of hot sugar syrup and the two very different sweets you can make from it, just by changing how fast it cools. ⭐ Cool the syrup SLOWLY, left in a warm place for hours, and big sugar crystals have time to grow. You get something grainy and crystalline, like fudge or rock candy, where you can see and feel the crystals. ⭐⭐ Cool the same syrup FAST, poured out thin or plunged into cold water, and the sugar has no time to organise into crystals. It sets into a smooth, hard, see-through sweet, a boiled sweet, with no grains in it at all. Same syrup, opposite results, and the only thing that changed was the speed of cooling. Slow makes big grains; fast makes a smooth glass. ⭐ A cook can even read a finished sweet backwards: crunchy grains tell you it was cooled slowly, a smooth clear sweet tells you it was cooled in a hurry. ⚠️ Hold that picture. Molten rock is the hot syrup, the crystals are the sugar grains, and how fast it cooled decides everything: slow and deep grows big crystals, fast at the surface makes tiny ones or a glass. Read the grains and you read the cooling.

Finish each igneous line

Molten rock below the ground is called _____. The size of the crystals in an igneous rock is its _____. A rock that cooled slowly deep underground has _____ crystals. An igneous rock that formed underground, from molten rock that never reached the surface, is _____. An igneous rock that formed from lava at the surface is _____.

Three rocks on the bench

Three igneous rock samples are handed to you. For each, choose the reading the evidence supports.

• A sample is packed with large crystals you can pick out by eye.
• A sample is a smooth, shiny glass with no crystals at all.
• Two samples have the same small crystal size, but one is pale and one is dark.

Explain how to read an igneous rock

Explain how the crystal size of an igneous rock records how fast it cooled, and how you would use crystal size and colour to classify a sample as intrusive or extrusive and to say something about its composition.

• Explain that crystal size is set by how fast the molten rock cooled
• Explain what large crystals tell you about where the rock formed
• Explain what small crystals or a glass tell you
• Explain how to classify a rock as intrusive or extrusive
• Explain how colour hints at whether it is silica-rich or iron-rich