Rocks and Minerals
Senior 6 · Geography
Rocks and Minerals
Lesson 2 of 4
25 min
Rwanda's rocks tell two very different stories. Most of the country is built of quartzites, sandstones and shales laid down as sediment more than a billion years ago and later altered by heat and pressure, with bodies of granite pushed up into them. In the north-west, by contrast, the Virunga volcanoes have poured out young lavas within the last few million years. Granite and lava both began as molten rock, yet they look completely different. This lesson explains the three classes of rock, the rock cycle that links them, and how igneous rocks are classified.
Definition
Igneous rock: rock formed when molten rock cools and solidifies. Magma is molten rock below the surface; lava is molten rock that has reached the surface.
Sedimentary rock: rock formed from sediment (fragments of older rock, chemical precipitates or remains of organisms) that has been compacted and cemented.
Metamorphic rock: rock changed in its minerals or texture by heat, pressure or both, without melting.
Rock cycle: the continuous set of processes by which rocks of each class are changed into the others.
Figure 2: the rock cycle.
Internal energy (the Earth's heat) drives melting, metamorphism and uplift; external energy (the Sun, through weather, water and gravity) drives weathering, erosion and deposition. The cycle has shortcuts: an igneous rock can be metamorphosed without first being weathered, and a sedimentary rock can be uplifted and weathered straight back into sediment.
Definition
Intrusive (plutonic) rock: igneous rock that solidified underground, slowly, so it has large crystals (coarse grained). Example: granite.
Extrusive (volcanic) rock: igneous rock that solidified at the surface, quickly, so it has small crystals (fine grained) or glass. Example: basalt.
Texture: the size, shape and arrangement of the grains in a rock.
Igneous rocks are classified by two things: their grain size, which records how fast they cooled, and their composition, especially the proportion of silica ().
| Composition (silica content) | Typical minerals | Coarse grained (intrusive) | Fine grained (extrusive) |
|---|---|---|---|
| Felsic (over about 65%) | Quartz, orthoclase feldspar, mica: pale | Granite | Rhyolite |
| Intermediate (about 55–65%) | Plagioclase, amphibole | Diorite | Andesite |
| Mafic (about 45–55%) | Plagioclase, pyroxene, olivine: dark | Gabbro | Basalt |
| Ultramafic (under about 45%) | Olivine, pyroxene | Peridotite | (rare) |
Felsic magma is viscous and traps gas, so felsic volcanoes tend to erupt explosively; mafic magma is runny, so basalt flows spread widely, building plateaus and gentle shield volcanoes. Fast-chilled lava can form obsidian (volcanic glass), and gas bubbles frozen into lava make pumice or scoria.
Worked example 1: cooling rate and crystal size
Problem. To a first approximation, the time a sheet of magma takes to solidify grows with the square of its thickness. A dyke 2 m thick solidifies in about 10 days (illustrative). How long would a sill 200 m thick take, and what does this predict about their crystal sizes?
Solution.
The sill cools for centuries, so its crystals have time to grow large (coarse grained); the dyke is fine to medium grained, and its edges, chilled against cold rock, are finest of all.
Worked example 2: naming an unknown rock
Problem. A dark rock has crystals too small to see without a lens, contains small gas holes, and a chemical analysis gives 49% silica. Name it and say where it formed.
Solution. Fine grained: it cooled quickly, at the surface (extrusive). Gas holes: gas escaped from a lava. 49% silica: mafic. A fine-grained mafic rock is basalt, formed from a lava flow.
| Form | Description |
|---|---|
| Batholith | A huge, dome-shaped mass of granite, often hundreds of square kilometres, exposed when the rocks above are eroded |
| Dyke | A vertical or steep sheet that cuts across the layers of the rock it intrudes |
| Sill | A horizontal sheet that runs along the layers |
| Laccolith | A sill-like body that domed up the layers above it |
Because igneous rock is usually harder than the rock around it, erosion often leaves dykes as walls, sills as ledges and steps (and waterfalls where rivers cross them), and batholiths as uplands with domes and tors.
Warning
Common mistakes
Tip
Exam technique. To name an igneous rock, decide two things in order: grain size (where it cooled) and colour or silica content (its composition). Then read the name off the table.
Professor Dave Explains: the rock cycle and the three classes of rock. Match each process he names to an arrow in Figure 2.
Practice
Summary
Sources and credits
Photo: Lilesville Granite close-up (Scarlett016, CC BY-SA 4.0), via Wikimedia Commons.
Figure 2, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "The Rock Cycle", Professor Dave Explains, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
