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Earth and Space · Chapter 2: Structure of the Earth
Rocks and the Rock Cycle
Igneous, sedimentary and metamorphic rocks, how they form, examples from Pakistan, and the rock cycle, with a rock-cycle virtual lab.
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01Concept
A rock is a naturally formed solid made of one or more minerals. Geologists put rocks into three groups according to how they were formed.
1. Igneous rocks: formed from melted rock
Igneous rocks form when hot molten rock cools and becomes solid. Molten rock underground is magma; when it comes out of a volcano it is lava.
| Type | How it forms | Crystals | Example |
|---|---|---|---|
| Intrusive (plutonic) | Magma cools slowly, deep underground | Large, easy to see | Granite |
| Extrusive (volcanic) | Lava cools quickly, at the surface | Tiny or none | Basalt, pumice |
2. Sedimentary rocks: formed from layers
Sedimentary rocks form when small pieces called sediment (sand, mud, pebbles, shells) are carried by rivers, wind or ice and settle in layers. Over time, the layers are pressed together (compaction) and glued by minerals (cementation).
- Sandstone: made from sand grains.
- Shale: made from fine mud; splits into thin layers.
- Limestone: made mostly of shells and lime mud from ancient seas. The Margalla Hills near Islamabad are mostly limestone.
- Rock salt: left behind when ancient seawater evaporated. Khewra's salt formed this way.
Sedimentary rocks often show layers and are the only rocks that usually contain fossils.
3. Metamorphic rocks: changed rocks
Metamorphic rocks form when an existing rock is changed by great heat and pressure, without melting, usually deep underground or where plates collide.
| Original rock | Becomes |
|---|---|
| Limestone | Marble: quarried in Khyber Pakhtunkhwa and Balochistan |
| Shale | Slate: used for roofs and old school writing boards |
| Sandstone | Quartzite: very hard |
| Granite | Gneiss: has light and dark bands |
The rock cycle
Rocks are not permanent. Over millions of years, any type of rock can be changed into any other type. This never-ending process is called the rock cycle.
02Virtual Lab
Try this topic for yourself in the virtual lab. It opens on its own page with steps, tasks and questions.
03Real World Examples
Marble floors
Many floors in Pakistani homes and mosques are marble, a metamorphic rock made from limestone.
Cement
Cement factories near Karachi, D.G. Khan and the Margalla foothills use limestone, a sedimentary rock.
Kitchen worktops
Polished granite worktops show the large crystals of an igneous rock that cooled slowly underground.
Thar coal
The coal under the Thar Desert is a sedimentary rock formed from ancient plants.
04Practice Questions
Try to answer in your own words first, then open the model answer.
Short questions
Q1.Name the three main types of rock with one example each.Show answer
Answer
- Igneous: granite
- Sedimentary: limestone
- Metamorphic: marble
Q2.Why does granite have large crystals but basalt has tiny ones?Show answer
Answer
Granite forms from magma that cools slowly deep underground, so crystals have time to grow large. Basalt forms from lava that cools quickly at the surface, so the crystals stay tiny.
Q3.How are sedimentary rocks formed?Show answer
Answer
- Weathering breaks rocks into small pieces (sediment).
- Rivers, wind or ice carry the sediment away.
- It settles in layers (deposition).
- Layers are pressed together (compaction) and glued by minerals (cementation).
Q4.What are metamorphic rocks? Give two examples with their original rocks.Show answer
Answer
Rocks changed by great heat and pressure without melting. Limestone → marble; shale → slate.
Long questions
Q5.Describe the rock cycle with the help of a labelled diagram.Show answer
Answer
- Magma cools → igneous rock (granite, basalt).
- Weathering and erosion break any rock into sediment.
- Compaction and cementation turn sediment into sedimentary rock.
- Heat and pressure change igneous or sedimentary rock into metamorphic rock.
- Melting turns any rock back into magma, and the cycle continues.
Diagram: draw five boxes (magma, igneous, sediment, sedimentary, metamorphic) joined by arrows labelled cooling, weathering and erosion, compaction and cementation, heat and pressure, and melting.