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

Igneous Rocks: Made from Melted Rock

E2.1 E2.2 A1.2About 45 minutes
Learning goals
  • I can explain how igneous rocks form when melted rock cools and hardens.
  • I can explain why some igneous rocks have big crystals and others have tiny crystals or none.
  • I can describe and identify granite, basalt, obsidian and pumice.
  • I can describe where igneous rocks are found in Ontario.

Words to know

magma
Melted rock deep under the Earth's surface.
lava
Melted rock that has come out onto the Earth's surface, for example from a volcano.
igneous rock
Rock that forms when magma or lava cools and hardens. "Igneous" comes from a Latin word for fire.
cool
Lose heat and get colder. When melted rock cools, it becomes solid.
intrusive
Igneous rock that cooled slowly underground. It has big crystals, like granite.
extrusive
Igneous rock that cooled quickly on or near the surface. It has tiny crystals or none, like basalt and obsidian.

Rocks born from fire

Deep under your feet, the Earth is very, very hot. In some places it is so hot that rock melts into a thick, glowing liquid. Melted rock under the ground is called magma. When it comes out onto the surface, for example from a volcano, we call it lava.

When magma or lava cools, it hardens into solid rock. Rock made this way is called igneous rock. The word igneous comes from a Latin word for fire.

Remember from Grade 2

In Grade 2 you learned that solids can melt into liquids when they are heated, and liquids can freeze into solids when they cool. Igneous rock is the same idea: melted rock is a liquid, and it becomes a solid when it cools down.

Slow cooling or fast cooling?

As melted rock cools, minerals start to grow as crystals. The time it takes to cool makes a big difference:

  • Slow cooling, deep underground: Magma is trapped under thick rock, like a pot under a blanket. It can take thousands of years to cool. The crystals have lots of time to grow, so they are big enough to see. These rocks are called intrusive igneous rocks. Example: granite.
  • Fast cooling, on the surface: Lava meets air or water and cools quickly, in days or even minutes. The crystals have little time to grow, so they are tiny (too small to see). These rocks are called extrusive igneous rocks. Example: basalt.
  • Very, very fast cooling: If lava cools almost instantly, there is no time for crystals at all. It forms natural glass called obsidian.
  • Gas bubbles: Some lava is full of gas bubbles, like a fizzy drink. When it hardens quickly, the bubbles leave holes. Pumice has so many holes that it is very light. Some pumice can even float on water!
A piece of shiny black obsidian next to a speckled piece of granite on a wooden table.
Both are igneous rocks. Obsidian (left) cooled so fast it is glass with no crystals; granite (right) cooled slowly underground, so its crystals are big enough to see. AI-generated image
magmalava cools fast:tiny or no crystalsvolcanounderground magma cools slowly: big crystals
Magma deep underground cools slowly and forms rock with big crystals, like granite. Lava that comes out of a volcano cools fast and forms rock with tiny crystals or none, like basalt or obsidian.
RockHow it formedWhat it looks like
granitemagma cooled slowly undergroundspeckled; big crystals of pink, white, grey and black; hard
basaltlava cooled quicklydark grey to black; crystals too small to see; heavy
obsidianlava cooled almost instantlyshiny black glass; breaks with very sharp, curved edges
pumicebubbly lava cooled quicklylight grey; full of holes; very light; some can float
Four igneous rocks

What do igneous rocks have in common?

  • They are usually hard.
  • Their mineral crystals interlock, fitting together like puzzle pieces (when crystals are big enough to see).
  • They do not have layers made of sand or mud.
  • They do not have fossils. Living things could not survive in melted rock.
  • Some are glassy or have holes from gas bubbles.

Igneous rocks in Ontario

Ontario has no active volcanoes today. But much of northern Ontario is part of the Canadian Shield, which you learned about in Grade 3 Social Studies. The Shield is made of some of the oldest rocks on Earth, and many of them are igneous rocks like granite, along with metamorphic rocks. When you drive north past rocky hills and road cuts in places like Muskoka or near Sudbury, you can often see pink and grey granite, and its striped cousin gneiss (you will meet gneiss in the metamorphic rocks lesson).

The Canadian Shield is very old, and in the far past it had volcanoes. Some of Ontario's rocks are ancient lava that cooled long, long ago.

Worked example

Let's sort igneous rocks by cooling speed

Put these rocks in order from slowest cooling to fastest cooling: obsidian, granite, basalt. Use the crystals as clues.

  1. Big crystals need a long time to grow. Granite has big crystals, so it cooled the slowest, deep underground.
  2. Tiny crystals mean little time. Basalt has crystals too small to see, so it cooled quickly.
  3. No crystals at all means almost no time. Obsidian is glass, so it cooled the fastest.

Slowest to fastest: granite, basalt, obsidian.

granitebig crystalscooling: slow (underground)basalttiny crystalscooling: fast (surface)obsidianno crystals: glasscooling: almost instantfaster cooling = smaller crystals
Crystal size tells how fast an igneous rock cooled. Slow cooling gives big crystals (granite), fast cooling gives tiny crystals (basalt), and almost instant cooling gives no crystals at all (obsidian).
Worked example

Let's identify a mystery rock

A light grey rock is full of tiny holes. When Ava puts it in a bowl of water, it floats. What rock is it, and how did it form?

  1. Holes in an igneous rock come from gas bubbles in lava.
  2. Lots of holes make a rock very light. A rock that floats has so many air spaces that it is lighter than the same amount of water.
  3. A light, holey rock that floats matches pumice.

It is pumice. It formed when bubbly lava cooled quickly, trapping gas holes.

Just for fun

Funny but true: some pumice is so full of gas holes that it floats. It may be the only rock that can go for a swim without a life jacket!

Safety

  • Obsidian breaks into very sharp edges. Handle it gently and never try to break it.
  • Hot water is used in the activity below. A grown-up should pour it.
  • Do not eat or drink Epsom salt. Wash your hands after the activity.
Hands-on activity

Fast cooling vs slow cooling crystals

Do crystals grow bigger when they form slowly? Let's test a model.

You need: Epsom salt (from a pharmacy), hot tap water, a measuring cup, a spoon, 2 small clear dishes or lids, the fridge, a magnifying glass.

  1. A grown-up mixes 125 mL (½ cup) of hot tap water with Epsom salt, a spoonful at a time, stirring until no more will dissolve.
  2. Pour a thin layer of the liquid into each dish.
  3. Put Dish A in the fridge (fast cooling). Leave Dish B on the counter (slow cooling and drying).
  4. Predict: Which dish will grow bigger crystals?
  5. Check Dish A after 1 to 3 hours and Dish B after 1 to 3 days. Look with the magnifying glass. Draw what you see.
  6. Compare: Which dish had bigger crystals? Did your results match how granite and basalt form? If not, think about what might have been different (amount of liquid, temperature, how long you waited).

Fair test: Both dishes get the same liquid, the same amount, and the same kind of dish. Only the cooling place changes.

Extra: If your family has a pumice stone for feet in the bathroom, see if it floats. (Some are real pumice; some are made in a factory.)

Your turn

1

What is the difference between magma and lava?

2

How does igneous rock form?

3

Why does granite have big crystals?

4

Why does obsidian have no crystals?

5

Why does pumice have holes?

6

Name one place in Ontario where you can see a lot of granite.

7

Why are there no fossils in igneous rock?

More practice: warm-up

1

What does "igneous" come from?

2

Is basalt usually light or dark?

3

Which igneous rock can float?

4

True or false: Ontario has active volcanoes today.

5

Which igneous rock is natural glass?

More practice: on your own

1

A rock has big pink, white and black crystals fitted together like a puzzle. Is it intrusive or extrusive? Name it.

2

A dark rock has crystals too small to see and no holes. What is it probably, and where did it cool?

3

In the Epsom salt model, what does the fridge dish stand for, and what does the counter dish stand for?

4

Give two properties most igneous rocks share.

5

Why does a pot of soup under a lid stay hot longer than soup in an open bowl? How is that like magma deep underground?

Challenge

1

Pumice and obsidian can form from the same kind of lava. Why do they look so different?

2

Granite forms underground. So why can we see it at the surface in the Canadian Shield?

3

Kai says, "A rock with crystals I can't see must not be igneous." Do you agree? Use an example.

Check your understanding

Quiz

1.

Melted rock under the ground is called...

2.

Igneous rock forms when...

3.

Which rock cooled slowly, deep underground?

4.

Why does pumice have holes?

5.

Obsidian has no crystals because it...

6.

Much of the granite in Ontario is found in the...

For parents and teachers

Big idea (E2.1, E2.2): Igneous rocks form from cooled magma (intrusive, slow cooling, large crystals, e.g., granite) or lava (extrusive, fast cooling, fine crystals or glass, e.g., basalt, obsidian, pumice). Typical properties: hard, interlocking crystals, no sedimentary layers, no fossils.

How to help: Granite is easy to find: countertops, gravestones, landscaping stones and Canadian Shield road cuts. Ask your child to point out the crystals and guess how fast the rock cooled. In the Epsom salt model, results can vary with temperature and amount of liquid; that is fine. Ask your child to explain what happened and what they might change (A1.2).

Common difficulties:

  • Mixing up magma and lava. Magma is underground; lava is at the surface.
  • Thinking slow cooling makes small crystals. It is the opposite: more time, bigger crystals.
  • Thinking a rock must be from a volcano to be igneous. Granite cooled underground and never erupted.

Safety: An adult handles the hot water. Obsidian (sometimes sold in rock kits) is very sharp when broken.