Rocks
Senior 1 · Geography
Rocks
Lesson 2 of 3
25 min
Near Musanze town there is a cave about 2 km long. It was not dug by people or carved by a river. It is a lava tube: long ago a river of molten rock flowed here, its outside cooled and hardened into a roof, and the liquid inside drained away, leaving a tunnel. The walls are made of the same dark volcanic rock you see in the stone walls around Kinigi. Every rock has a story of how it formed like this, and geologists sort rocks into three families by that story.
The name comes from the Latin ignis, fire. Igneous rocks form when molten rock cools and hardens. How fast it cools decides the size of the crystals.
Figure 1. Extrusive and intrusive igneous rocks.
Definition
Igneous rock: rock formed when magma or lava cools and becomes solid. Intrusive (plutonic) igneous rocks cool slowly underground and have large crystals, e.g. granite. Extrusive (volcanic) igneous rocks cool quickly at the surface and have tiny crystals or glass, e.g. basalt and pumice.
Rocks at the surface are slowly broken up by weather and carried away by rivers, wind and ice. The pieces, called sediments, settle in lakes, seas and valleys, layer on layer. Over thousands of years the weight of the layers above squeezes out the water (compaction) and minerals dissolved in the water glue the grains together (cementation).
Definition
Sedimentary rock: rock formed from layers of sediment (fragments of older rock, remains of living things, or minerals precipitated from water) that have been compacted and cemented. It usually shows layers (beds) and may contain fossils. Examples: sandstone, shale, conglomerate, limestone, and coal (from plant remains).
Worked example 1: how long does a layer take?
Mud settles on the floor of a lake at an average rate of 2 mm per year. How long would it take to build up 10 m of mud?
When the mud is compacted into shale it loses much of its water and may shrink to about half its thickness, so 5,000 years of mud might make only about 5 m of rock. Thick sedimentary rocks record very long periods of time.
Deep in the crust, where mountains are pushed up or magma rises, existing rocks are heated and squeezed but not melted. Their minerals slowly grow, change and line up, making a new, usually harder rock. The name comes from Greek words meaning "change of form".
Definition
Metamorphic rock: rock formed when an existing rock (igneous, sedimentary or older metamorphic rock) is changed by great heat and/or pressure without melting. It is usually harder, with crystals that are often lined up in bands (foliation). Examples: quartzite (from sandstone), marble (from limestone), slate and schist (from shale or mudstone), gneiss (from granite or schist).
Figure 2. Look at the grains, and you can usually tell the family.
| Igneous | Sedimentary | Metamorphic | |
|---|---|---|---|
| How it forms | Molten rock cools and solidifies | Sediments are deposited, compacted and cemented | Existing rock is changed by heat and pressure |
| Texture | Interlocking crystals | Grains stuck together | Crystals often lined up in bands |
| Layers | No layers | Beds, often with ripple marks | Wavy bands (foliation) |
| Fossils | Never | Often | Rare, usually destroyed |
| Hardness | Usually hard | Often soft or crumbly | Usually very hard |
| Examples | Granite, basalt, pumice | Sandstone, shale, limestone, coal | Quartzite, schist, marble, gneiss |
| In Rwanda | Basalt of the Virungas; granite bodies in the old hills | Clay, sand and gravel in valleys and along Lake Kivu; limestone near Bugarama | Quartzite and schist of the Congo–Nile divide and central hills |
No rock lasts for ever. Over millions of years any rock can be broken down, melted or squeezed and become another type. This never-ending series of changes is the rock cycle.
Figure 3. The rock cycle: every rock can become every other kind, given enough time.
Worked example 2: following a grain round the cycle
Describe how a grain of quartz in the granite of a Rwandan hill could end up in a quartzite.
Much of Rwanda's quartzite formed in exactly this way more than a billion years ago.
3 Types of Rocks and the Rock Cycle, by FreeSchool (4.5 min). As you watch, fill in one line of the comparison table for each rock family.
A classmate says: "Metamorphic rocks are just igneous rocks that melted again." Use the rock cycle to explain what is wrong with this statement, and give a Rwandan example of a metamorphic rock and the rock it came from.
Warning
Metamorphism is not melting. If a rock melts, the magma will cool into an igneous rock. A metamorphic rock is changed while it is still solid.
Tip
Three families, three stories: Igneous = Ignite (fire, cooling), Sedimentary = Settle (layers), Metamorphic = Morph (change). Crystal size tells cooling speed: big crystals = slow cooling, deep; tiny crystals = fast cooling, at the surface.
Practice
Remember
Sources and credits
The lesson text, diagrams, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
