The Rock Cycle
In 2001, geologists from the Australian National University dated zircon crystals from Jack Hills in Western Australia to 4.4 billion years old, the oldest known material on Earth. Those grains have survived dozens of rock cycle journeys. In this lesson you'll learn the three rock types and how the rock cycle has been transforming them ever since.
Printable Worksheets
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Q1 · Pick up any rock. It looks permanent, but it was once something completely different. What processes might have turned it into what it is today?
Q2 · Why do some granite rocks have large visible crystals while some volcanic rocks are completely glassy with no crystals at all?
● Know
- The three rock types: igneous, sedimentary, metamorphic
- How each type forms
- The key processes connecting rocks in the rock cycle
● Understand
- Why intrusive igneous rocks have larger crystals than extrusive ones
- Why only sedimentary rocks contain fossils
- How the same atoms cycle through different rock types over geological time
● Can do
- Classify common rocks as igneous, sedimentary or metamorphic
- Draw and label a rock cycle diagram
- Identify Australian rock examples (Hawkesbury Sandstone, Jack Hills zircons)
- Igneous rock
- Sedimentary rock
- Metamorphic rock
- Rock cycle
- Crystallisation
- Forms from compacted and cemented sediment layers; may contain fossils
- Forms when magma or lava cools and solidifies
- Process by which minerals form organised structures as magma cools
- The continuous process by which rocks transform from one type to another
- Forms when existing rocks are changed by heat and/or pressure without melting
Granite is an igneous rock. It cooled underground, giving crystals time to grow large. Basalt is an igneous rock that cooled at the surface, producing small or glassy crystals.
A lump of granite in your hand is 300 million years old. It formed when magma slowly cooled underground. Before that, the atoms in it were in sediment. Before that, in older rocks. The rock cycle has been running since Earth formed, your rock has probably been through it dozens of times.
Igneous rocks form when magma or lava cools and solidifies. Two types:
- Intrusive (plutonic): magma cools slowly underground. Crystals have time to grow large. You can see them with your eyes. Example: granite, speckled pink/white/grey rock with visible crystals of quartz, feldspar and mica. Common in Snowy Mountains, NSW.
- Extrusive (volcanic): lava cools quickly at the surface. Crystals are tiny or there are no crystals at all (glass). Examples: basalt (dark, fine-grained or glassy, common in volcanic fields like Newer Volcanics Province), obsidian (volcanic glass, jet black, no crystals).
The rule: cooling rate determines crystal size. Slow = large crystals. Fast = small or no crystals.
- Igneous rocks form from cooled magma/lava.
- Intrusive: slow cooling underground → large crystals (e.g. granite).
- Extrusive: fast cooling at surface → small/glassy (e.g. basalt, obsidian).
- Rule: slow cooling → large crystals; fast cooling → small or no crystals.
Sedimentary rocks form at Earth's surface from accumulated sediment:
- Clastic: from rock fragments (sandstone, shale, conglomerate). Formed when sand/mud is deposited, buried, compacted and cemented by minerals.
- Chemical: from dissolved minerals precipitating out of water (limestone, CaCO₃, often containing marine fossils).
- Organic: from accumulated organic material (coal, from compressed plant material).
Sedimentary characteristics: layered (strata), may contain fossils (only sedimentary rocks contain fossils, heat from igneous and pressure from metamorphic processes destroy organic material), formed at Earth's surface. Australian examples: Hawkesbury Sandstone (Blue Mountains), Kimberley limestone, Cooper Basin coal.
Metamorphic rocks form when existing rocks are changed by heat and/or pressure without melting:
- Contact metamorphism: heat only, near a magma intrusion.
- Regional metamorphism: heat AND pressure during deep burial or mountain building.
Examples: marble (limestone → marble), slate (shale → slate), schist, gneiss. Often foliated (layered texture from aligned minerals under pressure). Fossils are usually destroyed in metamorphic rocks.
The rock cycle has no beginning or end, any rock can transform into any other type given enough time and the right conditions:
| Transformation | Process |
|---|---|
| Igneous → sedimentary | Weathering breaks down the rock → erosion transports fragments → deposition → compaction and cementation |
| Sedimentary → metamorphic | Burial under more rock → increasing heat and pressure (without melting) |
| Metamorphic → igneous | If buried deep enough, rocks melt → magma cools → new igneous rock |
| Any rock → magma → igneous | Melting from intense heat |
| Any rock → sedimentary | Surface weathering, erosion, deposition, compaction |
Australian context:
- The Three Sisters, Blue Mountains: Triassic Hawkesbury Sandstone (~240 million years old), sedimentary rock carved by erosion.
- Jack Hills, WA: 4.4-billion-year-old zircon crystals, oldest known geological material on Earth. These tiny crystals survived multiple rock-cycle transformations.
You find a rock with visible layers and what appears to be a shell embedded in it. Before identifying it, predict: what type of rock is it? What does the presence of a fossil shell tell you about how and where this rock formed?
How close was your prediction?
Exactly right, layers + fossil = sedimentary, and the shell means it formed in water.
Key rule: only sedimentary rocks contain fossils. Layers are also a sedimentary hallmark. Fossils require burial in sediment, not volcanic or high-pressure processes.
In your workbook, draw a rock cycle diagram. You must include:
- All three rock types in boxes or circles
- At least four labelled arrows showing processes connecting them (e.g. weathering/erosion, compaction/cementation, heat/pressure, melting, cooling)
- One example rock for each type
Complete this paragraph about granite formation by filling in the correct words from the box below:
Words: magma, cools, intrusive, slowly, large, granite, crystals, underground
Granite is an example of an igneous rock. It forms when cools beneath Earth's surface. Because the cooling happens , minerals have time to form crystals visible to the naked eye. The resulting rock, is a hard, speckled rock commonly found in mountain ranges.
At the start of the lesson, you read that the Three Sisters in the Blue Mountains are made of sandstone that was once sand on a riverbed 240 million years ago.
Now that you understand the rock cycle, trace the full journey: how did ancient river sand become the solid sandstone cliffs we can touch today? Use the words sediment, compaction and cementation.
Q1. Explain why intrusive igneous rocks have larger crystals than extrusive igneous rocks. (3 marks)
Q2. Describe how a sandstone rock could eventually become a metamorphic rock. Include the processes involved. (3 marks)
Q3. Draw and label a rock cycle diagram showing all three rock types and at least four processes. (4 marks)
Answers
▾MCQ 1
C Only sedimentary rocks contain fossils. Fossils form when organisms are buried in sediment that slowly compacts around them. The heat that forms igneous rock and the heat/pressure that forms metamorphic rock destroy organic material.
MCQ 2
B Granite is an intrusive igneous rock. It forms when magma cools slowly underground over millions of years. This slow crystallisation gives mineral grains time to grow large enough to see with the naked eye.
MCQ 3
C Marble is a metamorphic rock. Limestone (sedimentary) is changed by heat and pressure, without melting, into marble. The calcium carbonate recrystallises into larger interlocking crystals.
MCQ 4
C Heat and pressure without melting is the defining process for metamorphism. If the rock melted, it would become magma and eventually igneous rock, not metamorphic rock.
MCQ 5
C Coal is an organic sedimentary rock. It forms when plant material accumulates in swampy environments, is buried under sediment, and compressed over millions of years. It is sedimentary, not igneous or metamorphic.
Short Answer 1
Model answer: Intrusive igneous rocks (like granite) form when magma cools very slowly deep underground, sometimes over millions of years. This slow cooling rate gives mineral atoms time to arrange themselves into large, well-formed crystal structures. Extrusive igneous rocks (like basalt) form when lava cools rapidly at or near Earth's surface, sometimes in seconds or minutes. This fast cooling doesn't allow crystals time to grow, resulting in tiny crystals or even volcanic glass (no crystals at all). The slower the cooling, the larger the crystals.
Short Answer 2
Model answer: Sandstone is a sedimentary rock. For it to become metamorphic, it would need to be buried deep underground by the accumulation of more sediment and rock layers above it. As burial depth increases, the temperature and pressure increase. These conditions, heat and pressure without melting, cause the minerals in the sandstone to recrystallise and change. The resulting metamorphic rock formed from sandstone is called quartzite. If the pressure is high enough for minerals to align, the rock may become foliated.
Short Answer 3
Model answer: Three boxes or circles labelled: Igneous (e.g. granite), Sedimentary (e.g. sandstone), Metamorphic (e.g. marble). Four arrows with labels: (1) Weathering and erosion, from igneous to sediment; (2) Compaction and cementation, from sediment to sedimentary rock; (3) Heat and pressure, from sedimentary to metamorphic; (4) Melting and cooling, from any rock back to igneous. Full marks require all three rock types, at least four correctly labelled processes, and arrows showing direction of transformation.