Min menu



How are Rocks Formed?

Where do the rocks come from?

If you imagine the Earth as a giant house with foundations made of all kinds of different rocks, what would those foundations look like? Think about it… some rocks form the outline of hills and mountains, others are at the bottom of rivers and oceans, or under fields, in plains and valleys. However you think about it, you'll see that rocks are everywhere!

The rock cycle

Rocks are all around us. Did you know the Earth has a floor? This floor is made of rocks, and rocks change in a cycle, commonly referred to as the rock cycle (see Figure 1 below). Rocks are subdivided into three broad categories, depending on how they were formed:

Igneous, sedimentary and metamorphic rocks.

Plate tectonics explains how rocks are recycled and change from igneous rocks to sedimentary rocks, then to metamorphic rocks and then back to igneous rocks.

  1. Igneous rocks:  Deep in the earth's crust, there are places where rocks have melted and turned into magma, a mixture of crystals and liquid rock. Sometimes magma comes to the Earth's surface during a volcanic eruption, and it is then called lava. When lava cools and hardens on the Earth's surface, it forms volcanic igneous rocks. If magma cools and hardens underground, it forms plutonic igneous rocks.
  2. Sedimentary rocks:  Wind and water cause rocks to erode. They break off pieces of rocks and carry them from place to place. Gradually, these pieces of rock mix with sand, gravel, shells and plant matter and form layers of sediment. Sediments gradually turn into rocks, over long periods of time. This is how sedimentary rocks are formed. Most of the rocks found on the Earth's surface are sedimentary rocks.
  3. Metamorphic Rocks:  Just as cookie dough changes when baked, rocks change when heated or crushed, or both. Rocks that are buried deep in the Earth are subjected to intense heat and enormous pressure. Over time, these forces change sedimentary or igneous rocks into another kind of rock called metamorphic rock.

Rocks are everywhere

Minerals mined from rocks are in hundreds of products we use. Many common products * that we use every day would not exist without rocks and minerals. Do you need proof? Look at the image below and you will see that rocks are in almost everything around you. Remember that if you can't grow it, you have to go and extract it!

Do you need more examples? If you brushed your teeth this morning, you did it with rocks! Study the Birth of a Bicycle poster to find out why rocks are important and help you get around. Even cyberspace and rocks have things in common. Find out why famous Canadian activities like ice skating and hockey wouldn't be possible without rocks and minerals.


Are you planning to go somewhere? You'll find out that we wouldn't get far on the roads we take...  without the rocks!  As you can guess, rocks, geology and mining together play an important role in our society. Find out more  about the mining industry in Manitoba, or  take a journey through the fascinating history of mining in Manitoba .


If you examine a rock trying to find out how it was formed, you might feel like a detective looking for clues. That's what geologists do. They study the Earth to understand the origins of rock. They examine the rocks carefully to find clues that will allow them to determine the age and composition of the rock.

Geologists will also look at the shape of the grains or crystals inside the rock to see how they are arranged in relation to each other. All of this can give them valuable clues and help them understand the hidden history of the rock.


As a geodetective, you'll also explore time the way geologists do, that is, you won't use seconds, minutes, and hours, but eons, eras, and periods. Look at the geochronological table below. Can you find the geological period during which the human being appeared?