Grade by Grade
E. Earth and Space Systems · Rocks, Minerals, and Geological Processes

The Rock Cycle

E2.1 A2.1 A1.5About 45 minutes
Learning goals
  • I can explain the rock cycle and how any rock can change into another type over time.
  • I can name the processes in the rock cycle: weathering, erosion, deposition, compaction, cementation, heat and pressure, melting and cooling.
  • I can trace the journey of a rock through the rock cycle.
  • I can read, follow and fix code that models the rock cycle.

Words to know

rock cycle
The never-ending, very slow way rocks change from one type to another.
cycle
A set of events that repeats over and over, with no real beginning or end.
uplift
When rock deep underground is slowly pushed up toward the surface, for example when mountains form.
melting
When rock gets so hot that it turns into liquid magma.
code
Instructions written for a computer to follow.
debug
Find and fix mistakes in code.

Recycled rocks

You know about recycling: an old plastic bottle can be melted and made into something new. The Earth recycles too, but it recycles rocks! It just does it very, very slowly, over millions of years.

This never-ending recycling is called the rock cycle. You have already met all its parts. Now let's put them together.

Igneous rockSedimentsSedimentary rockMetamorphic rockMagmaweathering, erosioncompaction,cementationheat and pressuremeltingcooling
The main path of the rock cycle. Magma cools into igneous rock. Weathering and erosion break rock into sediments. Compaction and cementation turn sediments into sedimentary rock. Heat and pressure change it into metamorphic rock. Melting turns it back into magma. There are many shortcuts too.

Following the main path

  1. Magma cools and hardens into igneous rock.
  2. Uplift brings the rock to the surface. Weathering breaks it into pieces, and erosion carries the pieces away as sediment.
  3. The sediment is deposited in layers. Compaction and cementation turn it into sedimentary rock.
  4. If the sedimentary rock is buried deep, heat and pressure change it into metamorphic rock.
  5. If it gets even hotter, it melts into magma. And the cycle goes around again!
Clear river water flowing over rounded, colourful pebbles, with a gravel bar and forest along the banks.
These pebbles were broken off bigger rocks by weathering, then rolled and rounded by the river (erosion). Where the water slows, they are deposited as sediment, the start of new sedimentary rock. AI-generated image

Shortcuts!

The rock cycle is not a one-way street. Rocks can take many paths:

  • Any rock at the surface can be weathered and eroded into sediment, even metamorphic rock.
  • Igneous rock can go straight to metamorphic with heat and pressure (granite → gneiss).
  • Metamorphic rock can be changed again by more heat and pressure (slate → schist).
  • Any rock can melt if it gets hot enough.

The cycle has no beginning and no end. The rocks under Ontario today have been through many of these steps. Some Canadian Shield rocks are billions of years old!

Worked example

Let's trace a grain of sand

A tiny quartz grain starts inside a piece of granite in the Canadian Shield. Tell one possible journey for the grain through the rock cycle.

  1. Igneous: The grain is part of granite, which formed when magma cooled slowly underground.
  2. Weathering: Over many winters, water freezes in cracks and breaks the granite apart. The quartz grain comes loose.
  3. Erosion and deposition: A river carries the grain to a lake. It settles on the bottom with other sand.
  4. Compaction and cementation: More layers pile on top. The sand is squeezed and glued into sandstone (sedimentary).
  5. Heat and pressure: The sandstone is buried deep and becomes quartzite (metamorphic).

One possible journey: granite → sediment → sandstone → quartzite. Other paths are possible too, as long as each step uses the right process.

graniteigneousweathering +erosionloose sandsedimentcompaction +cementationsandstonesedimentaryheat +pressurequartzitemetamorphic
One possible journey for a quartz grain through the rock cycle. Each arrow is a process that changes the rock into the next type.

Coding the rock cycle

Computer scientists use code to model how things change. Code uses if to make decisions: if this happens, then do that. Here is code for a rock that changes depending on what happens to it.

Just for fun

If a grain of sand could send postcards, it might write: "Spent 500 million years inside some granite. Then I took a river trip, settled down in a lake, and got squeezed into sandstone. Wish you were here!"

rock = "sedimentary"
event = choose_random("heat and pressure", "melting", "weathering")
if event == "heat and pressure":
rock = "metamorphic"
if event == "melting":
rock = "magma"
# (later, when the magma cools, it will become igneous rock)
if event == "weathering":
rock = "sediment"
say("I am now " + rock)

A rock cycle code. The computer picks a random event, then the "if" lines decide what the rock becomes.

Worked example

Let's follow and debug the code

Read the code above. (a) If the event is "heat and pressure", what does the computer say? (b) Mo wrote one more line: if event == "cooling": rock = "sedimentary". Find the bug.

  1. (a) Start: rock = "sedimentary". The event is "heat and pressure", so the first if is true. rock becomes "metamorphic". The other if lines are false, so they are skipped.
  2. The computer says: I am now metamorphic.
  3. (b) Think about the science: cooling turns magma into igneous rock, not sedimentary rock.
  4. Fix: if event == "cooling": rock = "igneous". (It also only makes sense if the rock was magma first.)

(a) I am now metamorphic. (b) Cooling makes igneous rock, so the line should set rock to "igneous".

Hands-on activity

Rock cycle journey game

You need: one die, paper, a pencil. (Optional: a block-coding app like Scratch, with a grown-up, to make your game into code.)

You are a rock! Start as igneous. Each turn, roll the die and follow the chart for the type of rock you are now. Write each step on your paper.

If you are igneous: 1–2 stay buried (no change); 3–4 weathering and erosion → become sediment; 5 heat and pressure → become metamorphic; 6 melting → become magma.

If you are sediment: 1–2 still being carried by a river (no change); 3–6 deposition, compaction and cementation → become sedimentary.

If you are sedimentary: 1–2 stay buried (no change); 3–4 heat and pressure → become metamorphic; 5 uplift and weathering → become sediment; 6 melting → become magma.

If you are metamorphic: 1–2 stay buried (no change); 3–4 uplift and weathering → become sediment; 5–6 melting → become magma.

If you are magma: 1–3 cooling slowly underground → become igneous (big crystals); 4–6 erupting and cooling fast → become igneous (tiny crystals).

  1. Play 10 turns. Record each roll, the process and your new rock type.
  2. Draw your journey as a cycle diagram with arrows.
  3. Compare with a family member. Did you take the same path? (Probably not! Real rocks take different paths too.)

Coding challenge: With a grown-up, write your game in Scratch using a random number from 1 to 6 and "if" blocks.

Your turn

1

What is the rock cycle?

2

What process turns magma into igneous rock?

3

What processes turn sediment into sedimentary rock?

4

What turns sedimentary rock into metamorphic rock?

5

What turns any rock into magma?

6

Can metamorphic rock be weathered into sediment?

7

Does the rock cycle happen fast or slow?

More practice: warm-up

1

Does the rock cycle have a beginning and an end?

2

What brings deep rocks up to the surface?

3

Name the three types of rock.

4

In code, what word is used to make a decision?

5

What does "debug" mean?

More practice: on your own

1

Fill in the blank: granite → (weathering and erosion) → ____ → (compaction and cementation) → sandstone.

2

Name one shortcut in the rock cycle that skips sedimentary rock.

3

In the code, if the event is "weathering", what does the computer say?

4

Why is it called a cycle and not a line?

5

Marble is at the surface. List the processes that could turn it into limestone.

Challenge

1

A piece of slate (metamorphic) melts and then cools quickly on the surface. What kind of rock forms? What would its crystals be like?

2

Write one more "if" line for the code to model uplift and weathering of metamorphic rock.

3

Why can't we watch a rock go all the way around the rock cycle?

Check your understanding

Quiz

1.

The rock cycle happens...

2.

Magma cools to form...

3.

Which processes turn sediment into rock?

4.

Heat and pressure turn sedimentary rock into...

5.

Which statement is true?

6.

In the code, if event == "melting": rock = "magma". If the event is melting, the rock becomes...

For parents and teachers

Big idea (E2.1): The rock cycle links igneous, sedimentary and metamorphic rocks through cooling, weathering, erosion, deposition, compaction, cementation, heat and pressure, uplift and melting. There are many paths, not one fixed loop, and it operates over very long time scales.

How to help: Ask your child to retell the rock cycle using the diagram, then without it. The dice game shows that rocks take different paths. Coding (A2.1): reading code, predicting output, and fixing a bug are all part of the Grade 4 coding expectations; Scratch (free, scratch.mit.edu) is a good next step with adult supervision.

Common difficulties:

  • Thinking the cycle always goes in one order. Stress shortcuts.
  • Mixing up which process makes which rock. Cooling → igneous; compaction + cementation → sedimentary; heat + pressure → metamorphic.
  • Underestimating time. A simple comparison: a human life is about 80 years; many rock cycle steps take a million years or more.