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

Metamorphic Rocks and Sorting All Three Types

E2.1 E2.2 E2.3 A1.2 A1.4About 50 minutes
Learning goals
  • I can explain how heat and pressure change rocks into metamorphic rocks without melting them.
  • I can match metamorphic rocks to the rocks they came from (shale to slate, limestone to marble, sandstone to quartzite, granite to gneiss).
  • I can describe properties of metamorphic rocks, like bands and lined-up minerals.
  • I can use a key and a vinegar test to classify a rock as igneous, sedimentary or metamorphic, and do it safely.

Words to know

metamorphic rock
Rock that has been changed by heat and pressure deep underground, without melting. "Metamorphic" means "changed form".
pressure
A strong push or squeeze. Deep underground, the weight of rock above and moving rock cause huge pressure.
parent rock
The rock that a metamorphic rock was before it changed. Limestone is the parent rock of marble.
bands
Stripes of light and dark minerals in a rock, made when minerals line up under pressure.
classify
Sort things into groups based on their properties.
key
A set of yes-or-no questions that leads you, step by step, to the name of something.

Rocks that change

A caterpillar changes into a butterfly. That change is called metamorphosis. Rocks can change too!

When rocks are buried very deep, or are squeezed when huge pieces of the Earth's crust push together to make mountains, they get very hot and are under enormous pressure. The rock does not melt (if it melted, it would become magma). Instead, its minerals slowly change and rearrange. The new rock is called metamorphic rock, which means "changed form".

How the change happens

  • Heat comes from deep inside the Earth, or from nearby magma. It "bakes" the rock.
  • Pressure comes from the weight of rock above, and from rock being squeezed when mountains form.
  • Under pressure, flat minerals often line up in the same direction, like books stacked on a shelf. This makes bands (stripes) or flat, shiny layers.
  • Minerals can also grow bigger or change into new minerals.
  • Any kind of rock, igneous, sedimentary, or even another metamorphic rock, can become metamorphic.
Smooth rock on a lakeshore with wavy pink and dark grey bands, beside pine trees and a blue lake.
This banded rock is gneiss, a metamorphic rock common on the Canadian Shield in Ontario. Heat and pressure lined up its minerals into wavy stripes. AI-generated image
Parent rockMetamorphic rockWhat it is like
shale→slatesplits into thin, flat, smooth sheets; used for roof tiles and old chalkboards
limestone→marblesparkly crystals of calcite; can be white or swirled; fizzes in acid; used for statues
sandstone→quartzitevery hard; sand grains fused together; feels smooth, not gritty
granite (or shale, with more heat and pressure)→gneiss (say "nice")light and dark minerals in bands or stripes
Rocks and the metamorphic rocks they can turn into
squeezeminerals pointevery which waysqueezed: mineralsline up in bands
Before (left), the flat mineral grains point in all directions. After being squeezed from above and below (right), the rock is flatter and the grains line up in rows. This is how bands and flat layers form in metamorphic rocks.

Metamorphic rocks in Ontario

Much of the Canadian Shield is made of very old metamorphic rocks, especially gneiss, mixed with granite. If you see a rock in cottage country with wavy stripes of light and dark minerals, it is probably gneiss. Ontario also has marble, for example in parts of eastern Ontario.

Rock detective: Which type is it?

Now you know all three rock types. Here is a key to help you classify a rock. Answer the questions in order. Stop when you get a "yes".

  1. Is it glassy, or full of gas holes? → igneous (obsidian, pumice)
  2. Does it have layers of sand, pebbles or mud grains stuck together, or fossils? → sedimentary (sandstone, conglomerate, shale, limestone)
  3. Does it have bands or stripes of lined-up minerals, or flat, shiny layers that split into sheets? → metamorphic (gneiss, slate, schist)
  4. Does it have interlocking crystals in a speckled pattern, with no layers or bands? → probably igneous (granite)
  5. Is it dark with tiny grains you can't see, and no layers? → probably igneous (basalt)

Extra test: the fizz test. Rocks made of calcite, like limestone (sedimentary) and marble (metamorphic), make tiny bubbles in vinegar. The bubbles are a gas called carbon dioxide. Vinegar is a weak acid, so the fizzing may be gentle. Look closely with a magnifying glass. Real geologists use a stronger acid that only grown-up scientists may handle.

Remember: a key gives you a best guess. Geologists often need more tests to be sure.

1. Glassy or full of gas holes?yesIGNEOUSobsidian, pumiceno2. Layers of sand, pebbles or mud, or fossils?yesSEDIMENTARYsandstone, shaleno3. Bands, stripes or flat shiny sheets?yesMETAMORPHICgneiss, slateno4. Interlocking crystals, or dark and tiny grains?yesprobably IGNEOUSgranite, basalt
The rock key as a flow chart. Start at question 1 and go down until you get a yes. Your answer is a best guess, so test more if you can.
Worked example

Let's classify four mystery rocks

Use the key. Rock A: wavy stripes of white and black minerals. Rock B: gritty, made of sand grains, has layers. Rock C: shiny black glass with sharp edges. Rock D: white and sparkly, fizzes gently in vinegar, no layers or fossils.

  1. Rock A: not glassy, no sand or pebble layers, but it has bands → metamorphic, probably gneiss.
  2. Rock B: layers of sand grains → sedimentary, sandstone.
  3. Rock C: glassy → igneous, obsidian.
  4. Rock D: fizzes, so it contains calcite. Limestone or marble? It is sparkly with crystals and has no layers or fossils. That fits marble (metamorphic). Limestone is usually dull and may show fossils.

A: gneiss (metamorphic). B: sandstone (sedimentary). C: obsidian (igneous). D: marble (metamorphic).

Just for fun

A caterpillar can turn into a butterfly in a few weeks. A rock changing into a metamorphic rock can take millions of years. Talk about being patient!

Safety: hammers and vinegar

  • Vinegar can sting your eyes. Wear safety glasses or sunglasses if you have them. Use only a few drops. Never taste it after it touches rocks. Wash your hands after.
  • Breaking rocks is a job for a grown-up. Put the rock inside an old cloth bag or fold it inside a thick towel, and everyone nearby wears safety glasses. Children stand well back. Rock chips fly out fast and can hurt eyes. Never hit two rocks together near your face.
  • Rice and small pieces in the model below can be a choking hazard for little children. Clean up afterwards.
Hands-on activity

Squeeze a rock and do the fizz test

Part 1: Model metamorphism. You need: modelling clay or play dough, a handful of uncooked rice (or small strips of paper), a heavy book, waxed paper, a plastic knife.

  1. Mix the rice into a ball of clay so the grains point in all directions. Cut the ball in half with a plastic knife and look. Draw it.
  2. Put the other half between two sheets of waxed paper. Put the heavy book on top and press down hard for 30 seconds.
  3. Remove the book. Cut the flattened clay and look at the edge. Which way do most of the rice grains point now? Draw it.
  4. Write: What part of the model was the pressure? What part stood for the minerals? What part of real metamorphism did our model not show? (Answer: heat.)

Part 2: The fizz test. You need: vinegar, a dropper or spoon, a plate, a magnifying glass, safety glasses, your rock collection, and a piece of eggshell or seashell.

  1. Test the eggshell first. Eggshells are made mostly of calcite, so they should bubble. This is your control: it shows the test works.
  2. Put a few drops of vinegar on each rock. Watch closely for tiny bubbles.
  3. Record fizz / no fizz in your chart. A rock that fizzes probably contains calcite (limestone or marble).
  4. Rinse each rock with water and dry it.
  5. Use the key to classify each rock in your collection as igneous, sedimentary or metamorphic. Write your evidence for each one.

Your turn

1

What two things change a rock into a metamorphic rock?

2

Does the rock melt when it becomes metamorphic?

3

What metamorphic rock does limestone become?

4

What metamorphic rock does shale become?

5

What is special about the way gneiss looks?

6

Which rocks fizz in vinegar, and why?

7

Use the key: a rock is full of gas holes. What type is it?

8

Who should break rocks with a hammer, and how?

More practice: warm-up

1

What does "metamorphic" mean?

2

What does sandstone become?

3

Which metamorphic rock is used for statues?

4

Which metamorphic rock splits into thin sheets for roofs?

5

What is a classification key?

More practice: on your own

1

In the clay model, what happened to the rice grains when you pressed the clay? What real rock feature does that model?

2

Why do we test an eggshell before testing the rocks with vinegar?

3

Sandstone feels gritty, but quartzite feels smooth. Why?

4

A rock has sparkly crystals but no bands. It does not fizz. Is it more likely granite or marble? Explain.

5

Why are fossils rarely found in metamorphic rocks?

Challenge

1

Can an igneous rock become a metamorphic rock? Give an example.

2

Slate and shale look similar. How could you tell them apart? Give two clues.

3

Leila's key says "has layers" means sedimentary. But her rock has stripes and is gneiss. How should she improve her key?

Check your understanding

Quiz

1.

Metamorphic rocks form from...

2.

Limestone can change into...

3.

A rock with bands of light and dark minerals is probably...

4.

Which rock would fizz in vinegar?

5.

Using the key, a glassy black rock with sharp edges is...

6.

What is the safest way to break a rock?

7.

Shale becomes...

For parents and teachers

Big idea (E2.1, E2.2, E2.3): Metamorphic rocks form when existing rocks are changed by heat and/or pressure without melting. Key examples: shale → slate, limestone → marble, sandstone → quartzite, granite (or shale) → gneiss. Typical properties: banding or foliation, aligned minerals, harder and denser than the parent, fossils usually destroyed. Classification uses properties and simple tests (fizz test for calcite).

How to help: Marble and slate are common in homes (tiles, counters, old chalkboards, flooring). Gneiss is everywhere in the Canadian Shield. Let your child use the key on your rock collection and explain their evidence. Accept a reasoned "best guess": even geologists often need lab tests.

Common difficulties:

  • Thinking metamorphic rocks melt. They change while solid.
  • Confusing layers (sedimentary) with bands (metamorphic). Sedimentary layers are made of grains settled from water; metamorphic bands are lined-up crystals.
  • Expecting a big fizz. Household vinegar gives a gentle fizz, clearer on powdered or scratched rock. The eggshell control helps.

Safety (A1.4): Safety glasses for vinegar and rock breaking; only adults use the hammer, with the rock wrapped in a towel or bag; keep rice and small pieces away from young children.