The rock cycle: sedimentary, igneous & metamorphic

Rocks are constantly being formed, broken down, and reformed over vast timescales in a continuous process called the rock cycle. Igneous rocks form when molten rock (magma or lava) cools and solidifies — quickly at the surface (like basalt) or slowly underground (like granite). Sedimentary rocks form when eroded fragments of other rocks (sediment) are deposited in layers and compressed over time, often preserving fossils (like sandstone or limestone). Metamorphic rocks form when existing rock is transformed by intense heat and pressure, without fully melting (like marble from limestone, or slate from mudstone). Each rock type's properties — hardness, layering, crystal size — reflect exactly how it formed, which is why geologists use them to read Earth's history.

Key terms

Igneous rock:

Rock formed when molten rock cools and solidifies.

Sedimentary rock:

Rock formed from compressed layers of sediment.

Metamorphic rock:

Rock transformed by intense heat and pressure without fully melting.

Questions

1. 1. Igneous rocks form when:

   • Molten rock cools and solidifies
   • Sediment is compressed
   • Rock is heated but never melts
   • Rocks dissolve completely

2. 2. Sedimentary rocks form from:

   • Compressed layers of sediment
   • Cooling magma
   • Intense heat alone with no sediment
   • Nothing physical at all

3. 3. Metamorphic rocks form when existing rock is:

   • Transformed by intense heat and pressure
   • Fully melted into lava
   • Left completely unchanged
   • Dissolved in water

4. 4. Granite is an example of a/an:

   • Igneous rock
   • Sedimentary rock
   • Metamorphic rock
   • None of these

5. 5. Sandstone is an example of a/an:

   • Sedimentary rock
   • Igneous rock
   • Metamorphic rock
   • None of these

6. 6. Marble is an example of a/an:

   • Metamorphic rock
   • Igneous rock
   • Sedimentary rock
   • None of these

7. 7. The rock cycle describes rocks as:

   • Constantly forming, breaking down and reforming
   • Permanently fixed and never changing
   • Only ever forming once and staying that way
   • Unrelated to geological processes

8. 8. Why does granite have large, visible crystals while lava-formed rock has tiny ones?

   • Granite cools slowly underground, giving crystals more time to grow; surface lava cools quickly
   • Crystal size has no connection to cooling speed
   • All igneous rocks have identical crystal sizes
   • Granite never actually contains any crystals

9. 9. Why do sedimentary rocks often contain fossils?

   • Sediment layers can bury and preserve remains over long periods as they compress into rock
   • Fossils only ever appear in igneous rocks
   • Sedimentary rocks form too quickly to preserve anything
   • Fossils and sedimentary rock have no connection

10. 10. Why does marble typically show no fossils, even though it originally formed from limestone (which often does)?

    • The intense heat and pressure of metamorphism destroys or erases original fossil structures
    • Marble and limestone are completely unrelated rock types
    • Metamorphism has no effect on a rock's internal structure
    • Limestone never contains fossils in the first place

11. 11. Visible layering in a rock sample is most likely a sign of:

    • A sedimentary rock
    • An igneous rock
    • A metamorphic rock only
    • No particular rock type

12. 12. Why is the rock cycle described as "continuous" rather than a one-way process?

    • Any rock type can eventually be broken down, transformed, or melted and re-formed into another type
    • Rocks are formed once and can never change afterward
    • Only sedimentary rocks can ever transform into other types
    • The rock cycle only moves in a single fixed direction

13. 13. A geologist finds a rock with fine layers and embedded shell fragments. It is most likely:

    • Sedimentary
    • Igneous
    • Metamorphic
    • Impossible to classify

14. 14. Why might metamorphic rock be described as forming "without fully melting"?

    • If it fully melted, it would become magma and eventually form igneous rock instead, not metamorphic rock
    • Metamorphic rocks always fully melt during formation
    • Melting and metamorphism are the exact same process
    • Metamorphic rocks form with no heat or pressure involved at all

15. 15. Why can examining a rock's properties (crystal size, layering, hardness) help geologists determine its formation history, even without having witnessed it form?

    • Each formation process leaves distinct, predictable physical evidence in the rock's structure
    • Rock properties are entirely random and reveal nothing about formation
    • Geologists can only determine a rock's history by directly observing its formation
    • All rock types have identical physical properties regardless of formation

16. 16. Why might a mountain range contain all three rock types (igneous, sedimentary and metamorphic) in close proximity?

    • Mountain-building involves intense heat, pressure and volcanic activity, all of which can produce different rock types in the same region
    • Only a single rock type can ever exist within any given mountain range
    • Rock types are always evenly distributed across the entire Earth
    • Mountain ranges have no connection to varied rock formation processes

17. 17. Why is understanding the rock cycle important for interpreting the age and history of a landscape?

    • Different rock types and their arrangement reveal a sequence of geological events over time
    • Rocks provide no information about a landscape's geological history
    • All landscapes have an identical, universal rock formation history
    • The rock cycle has no connection to interpreting a landscape's past

18. 18. Why might sedimentary rock layers sometimes reveal ancient climate conditions (e.g. desert sand layers versus ocean sediment layers)?

    • The type of sediment deposited reflects the environment present at the time of formation
    • Sediment type has no connection to the environment it formed in
    • All sedimentary layers form under identical environmental conditions
    • Climate history cannot be studied through rock layers

19. 19. Why might extracting resources like marble or granite require understanding which rock cycle process formed a particular deposit?

    • Knowing the formation process helps predict where similar deposits are likely to be found and how they can be safely extracted
    • Rock formation processes have no relevance to resource extraction
    • All rock deposits are found and extracted using an identical, universal method
    • Understanding rock formation provides no practical benefit at all

20. 20. A geologist finds a metamorphic rock that clearly shows the compressed, wavy layering typical of a sedimentary rock, but is now much harder and denser. What does this most likely reveal about its history?

    • It began as a sedimentary rock and was later transformed by intense heat and pressure, retaining some original layering while changing in hardness
    • It must have formed directly from cooling magma with no prior rock stage
    • Layering and hardness have no connection to a rock's formation history
    • Metamorphic rocks never retain any features from their original rock type

21. 21. Understanding the rock cycle mainly helps you to:

    • Explain how different rock types form and relate to each other over geological time
    • Assume all rocks are identical and unchanging
    • Ignore the connection between heat, pressure and rock formation
    • Treat rock formation as a completely random, unexplainable process