What is an igneous rock?
Igneous means 'made by fire'. An igneous rock forms when melted rock cools and becomes solid.
Melted rock under the ground is called magma. When it comes out on the surface it is called lava. It is the same material in a different place.
When magma cools, its atoms join into ordered patterns called crystals of minerals such as quartz, feldspar, mica, pyroxene and olivine. Most igneous rocks are a tight mix of these crystals. Igneous rocks have no layers and no fossils, because they were once fully melted.
Igneous rocks make most of the ocean floor and much of the deep continental crust. They are also the starting point of the rock cycle: weathering breaks them into sediment, and heat and pressure can change them into metamorphic rock.
How does magma form? (Magma at plate boundaries)
The mantle is hot but mostly solid, because high pressure keeps it from melting. Rock melts in three main ways:
- Pressure drops (decompression melting): at divergent boundaries such as mid-ocean ridges, and at hot spots like Hawaii, hot mantle rises. Pressure falls, so it melts. This makes low-silica basaltic magma.
- Water is added (flux melting): at subduction zones (convergent boundaries), a sinking ocean plate releases water into the mantle above it. Water lowers the melting point, so rock melts. This gives medium-silica andesitic magma, as in the Andes and Japan.
- Heat is added: hot magma rising into continental crust can melt the crust itself. This gives high-silica rhyolitic (granitic) magma.
Magma evolution: as magma cools, minerals crystallise in a set order. Dark, iron-rich minerals (olivine, pyroxene) form first at high temperature and can sink out. The liquid left behind becomes richer in silica. So one basaltic magma can slowly change into andesitic and then rhyolitic magma. This is called fractional crystallisation. Magma can also change by melting and mixing in surrounding rock.
Intrusive and extrusive rocks: cooling speed and crystal size
The size of the crystals tells you how fast the magma cooled.
- Intrusive (plutonic) rocks cool slowly inside the crust, over thousands to millions of years. Crystals grow large enough to see by eye (coarse-grained). Examples: granite (light, high silica), diorite (medium), gabbro (dark, low silica). Large bodies are called batholiths; sheets are dykes (cut across layers) and sills (lie between layers).
- Extrusive (volcanic) rocks cool quickly on the surface in hours to years. Crystals are tiny (fine-grained). Examples: basalt, andesite, rhyolite. If cooling is almost instant, no crystals form and you get glass: obsidian. If the lava is full of gas bubbles, you get pumice, light enough to float.
- A rock with a few big crystals in a fine background is porphyritic: it started cooling slowly underground, then erupted and finished cooling fast.
| Silica | Intrusive (slow) | Extrusive (fast) |
|---|---|---|
| Low (mafic, dark) | Gabbro | Basalt |
| Medium | Diorite | Andesite |
| High (felsic, light) | Granite | Rhyolite, obsidian, pumice |
Magma types, eruption types and volcanic landforms
Viscosity means how sticky or thick a liquid is (honey is more viscous than water). Silica chains make magma sticky.
- Basaltic magma (about 45–52% silica, about 1000–1200 °C): runny. Gas escapes easily, so eruptions are gentle (effusive): lava fountains and long lava flows. It builds broad, gently sloping shield volcanoes (Mauna Loa, Hawaii) and wide lava plateaus (Deccan Traps, India).
- Andesitic magma (about 52–63% silica): medium. Eruptions switch between lava and ash, building steep, layered stratovolcanoes (composite cones) such as Mount Fuji (Japan).
- Rhyolitic magma (above about 63% silica, about 700–850 °C): very sticky. Gas is trapped until it bursts out in an explosive eruption with ash, pumice and pyroclastic flows (fast, hot clouds of gas and rock). It can form lava domes or a huge collapse crater called a caldera (Yellowstone, USA).
Other landforms: small cinder cones of loose fragments, volcanic necks left after the cone erodes, and pillow lavas formed when lava erupts under water.
Try it: model crystal growth at home
Dissolve as much sugar or salt as you can in a cup of hot water (ask an adult). Pour half into a saucer and put it in a warm, still place to dry slowly over days. Put the other half in a shallow dish in the fridge or a breezy spot to dry fast. Compare with a magnifying glass: slow drying gives bigger crystals, just like intrusive rock. In the 3D, set the slider to 'deep' and then to 'surface' and watch the crystal cube change.
Key formulas and definitions
- Igneous rock = magma or lava that has cooled and become solid
- Slow cooling (deep) → big crystals → intrusive / plutonic
- Fast cooling (surface) → tiny crystals or glass → extrusive / volcanic
- More silica → lighter colour, stickier magma, lower temperature, more explosive
- Gabbro ↔ basalt (low silica); diorite ↔ andesite; granite ↔ rhyolite (high silica)
- Decompression (ridges, hot spots) → basaltic; water from subduction → andesitic; melted crust → rhyolitic
Worked examples
1. A rock has light-coloured crystals of quartz and feldspar, each about 5 mm across. Name it and explain how it formed.
Large crystals mean slow cooling deep underground, so it is intrusive. Light colour and quartz mean high silica. The rock is granite, formed from felsic magma that cooled slowly inside the crust.
2. Why is the ocean floor made mostly of basalt?
At mid-ocean ridges plates move apart, mantle rock rises and melts as pressure drops. This gives low-silica basaltic magma. It erupts on the sea floor and cools quickly in cold water, so it forms fine-grained basalt (often as pillow lava).
3. Two volcanoes: A has gentle slopes and long lava flows; B is steep and has had violent ash eruptions. Compare their magmas.
A has low-silica, hot, runny basaltic magma; gas escapes easily, so it erupts gently and builds a shield volcano. B has higher-silica, cooler, sticky andesitic or rhyolitic magma; gas is trapped, pressure builds and it explodes, building a steep stratovolcano.
4. A dark rock has a few large green olivine crystals in a very fine-grained background. What does this tell you?
It is porphyritic. The large crystals grew first while the magma cooled slowly underground; then the magma erupted and the rest cooled quickly into tiny crystals. Dark colour and olivine show low silica, so it is a porphyritic basalt.
Common mistakes
- Thinking magma and lava are different materials. They are the same melted rock: magma underground, lava on the surface.
- Mixing up crystal size: slow cooling gives BIG crystals, fast cooling gives small crystals or glass.
- Believing the whole mantle is liquid. It is mostly solid; only small parts melt where pressure drops, water is added or heat increases.
- Thinking runny lava is the most dangerous. Sticky, high-silica magma is usually more explosive because it traps gas.