Rocks seem to stay the same once they form, but over very long times they are broken, melted and squeezed and turn into other rocks.
Magma that cools and hardens becomes igneous rock, grains broken from rocks that pile up and harden become sedimentary rock, and existing rock changed by great heat and pressure becomes metamorphic rock; this flow from one to another is the rock cycle.
The three are told apart by the marks their formation left: igneous rocks have mineral crystals, sedimentary rocks have layers and fossils, and many metamorphic rocks have bands of minerals lined up (foliation).
Choose each process in the cycle diagram to see which rock passes through what to become which, and compare the insides of four rocks magnified in the picture below.
The Rock Cycle
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Igneous, sedimentary and metamorphic rocks turn into one another in a cycle
weathering and deposition
💡 Routes Between Rocks
①Magma that cools and hardens becomes igneous rock.
②Rock at the surface is weathered and eroded; the grains are carried away, deposited and harden into sedimentary rock.
③Deep underground, great heat and pressure change rock without melting it, making metamorphic rock.
④Hotter still, and the rock melts back into magma.
⑤There are more routes than the arrows show: metamorphic rock exposed at the surface is weathered into material for sedimentary rock, and igneous rock can melt again.
Textures Left by Formation
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Magnified rocks: the texture depends on how the rock formed
Granite (intrusive)
Three kinds of rock
Classifying rocks
Features and Uses of Rocks
Rock comparison
Features of major rocks
Rock
Type
Features
Uses
Granite
Igneous (intrusive)
Large crystals, light-colored, hard
Building facades, gravestones
Basalt
Igneous (extrusive)
Small crystals, dark, holes
Stone walls, millstones, paving
Sandstone
Sedimentary
Sand grains, layers
Building stone
Limestone
Sedimentary
Fizzes in dilute hydrochloric acid (carbon dioxide)
Rocks are told apart by the marks their formation left, though some metamorphic rocks, like marble, have no foliation.
Exam Key Points
Summary
magma → igneous → (weathering, deposition) → sedimentary → (heat, pressure) → metamorphic → (melting) → magma
The three rocks change into one another by many routes in no fixed order, and any rock exposed at the surface is weathered.
🎯 Exam Points
①Igneous rock is magma that has cooled and hardened; intrusive rocks have large crystals, extrusive rocks small ones.
②Sedimentary rocks show layers and may contain fossils (sandstone, limestone).
③Metamorphic rocks have been changed by heat and pressure, and many, like gneiss, show foliation.
④Limestone and marble fizz with carbon dioxide when dilute hydrochloric acid is dropped on them.
⑤The three rocks change into one another through weathering, melting, and heat and pressure.
Examples and Unit Test
Example 1
Which has larger crystals, granite (intrusive) or basalt (extrusive)? Why?
1
Igneous rock forms as magma cools, and the slower it cools, the longer the crystals have to grow, so the larger they get.
2
Granite cooled slowly deep underground and has large crystals; basalt cooled quickly near the surface and has small ones.
▸ Granite (it cooled slowly deep down, so its crystals are large)
Switch between granite and basalt in the lower picture and the difference in grain size is obvious.
Example 2
What kind of rock shows layers, and what kind shows bands (foliation)?
1
Sedimentary rock forms as grains pile up one layer on another, so it shows layers.
2
In metamorphic rock, heat and pressure line the minerals up in one direction and make foliation. So layers mean sedimentary and foliation means metamorphic.
Layers come from piling up, foliation from squeezing; remembering why the lines formed keeps the two apart.
Unit-test style
Which statement about the rock cycle and classification is correct?
①Igneous rock forms from piled-up sand
②Metamorphic rock forms when rock is subjected to heat and pressure
③Basalt cools slowly so it has large crystals
④Sedimentary rock shows foliation (banding)
⑤The three types of rock do not change into one another
▸ ② Metamorphic rock forms when rock is subjected to heat and pressure
1
Metamorphic rock forms when existing rock is changed by great heat and pressure, so ② is correct.
2
Igneous rock forms from cooling magma (①), basalt cooled fast and has small crystals (③), sedimentary rocks show layers (④), and the three rocks change into one another in a cycle (⑤).