Rock types

 

Types of rocks

The Earth’s outer layer is called the crust, and it is made of rocks. Rocks are hard things that have different kinds of minerals in them. Minerals are natural substances that have different kinds of atoms in them.

 

There are three kinds of rocks on Earth: igneous, metamorphic, and sedimentary. They are different because they are made in different ways.

 

• Igneous rocks are made from melted rock that cools down and becomes hard.
• Metamorphic rocks are made from other rocks that change because of heat and pressure deep in the Earth.
• Sedimentary rocks are made from small pieces of rock that stick together in layers over time.

 

 

Let’s look at these in more details:

 

 

Igneous Rocks

Igneous rocks are made from melted rock that becomes hard when it cools down.

 

The Earth is very hot inside, so some rocks are liquid there. We call this liquid rock magma. Sometimes, magma comes out of the Earth through volcanoes. We call this lava. When lava cools down, it makes igneous rocks.

 

Igneous rocks have crystals that fit together like a puzzle. The crystals can be big or small, depending on how fast the magma cools down. There are two kinds of igneous rocks: extrusive and intrusive.

 

 

Extrusive igneous rocks

Extrusive igneous rocks are made from lava that cools down quickly on the surface. They have very small crystals that you can’t see without a microscope. Obsidian is an example of an extrusive igneous rock. It is black and shiny.

 

 

Intrusive igneous rocks

Intrusive igneous rocks are made from magma that cools down slowly underground. They have big crystals that you can see with your eyes. Granite is an example of an intrusive igneous rock. It has different colours and is good for making kitchen counters.

 

 

Sedimentary rocks

Sedimentary rocks are made of layers of different things. The bottom layers are older than the top layers.

Some examples of sedimentary rocks are chalk, limestone, shale, and sandstone. They are made from pieces of other rocks that stick together.

Sedimentary rocks have layers that you can see in this picture:

Sedimentary rock showing different layers within the rock There are five steps to make a sedimentary rock:

 

• The pieces of rock move with water or wind. This is called transportation.
• The pieces of rock stop moving and settle down. This is called deposition.
• More pieces of rock pile up on top of each other. This is called sedimentation.
• The weight of the top layers squeezes the bottom layers. This is called compaction.
• Some minerals in the rock harden and glue the pieces together. This is called cementation.

 

 

Formation of Sedimentary Rocks

 

 

Metamorphic rocks

Metamorphic rocks are made from other rocks that change because of heat and pressure.

The rocks that change can be sedimentary rocks or igneous rocks. Sometimes, metamorphic rocks can also change into other metamorphic rocks.

There are three steps to make a metamorphic rock:

 

• The rocks get buried deep in the Earth or squeezed by the Earth’s movements.
• The rocks get hot and have a lot of pressure on them.
• The rocks do not melt, but the things in them change and make new metamorphic rocks.

 

🪨 Knowledge Check: Rock Types

Test your knowledge on the three main types of rocks and how they are formed.

1. Which type of rock is formed from the cooling of molten rock (magma or lava)?

• Sedimentary
• Igneous
• Metamorphic

2. Why do intrusive igneous rocks, like granite, have large crystals?

• Because they cool very quickly on the Earth's surface.
• Because they form from layers of sediment.
• Because the magma cools slowly deep underground.

3. Sedimentary rocks are the only rock type that might contain which of the following?

• Fossils
• Interlocking crystals
• Magma

4. Which rock is a metamorphic rock formed from limestone?

• Slate
• Basalt
• Marble

5. What two factors change existing rocks into metamorphic rocks?

• Heat and pressure
• Erosion and weathering
• Cooling and melting

Check Your Answers

1. Igneous (Formed from molten rock cooling).
2. Slow cooling underground (Slow cooling allows larger crystals to grow).
3. Fossils (Sedimentary rocks are the only type where fossils may be found).
4. Marble (Limestone changes into marble under heat and pressure).
5. Heat and pressure (These factors change existing rocks without melting them).

 

Summary:

 

Earth has three kinds of rocks: igneous, metamorphic, and sedimentary.

They are made differently:

 

• Igneous rocks are made from melted rock that cools down.
• Metamorphic rocks are made from other rocks that change because of heat and pressure.
• Sedimentary rocks are made from small pieces of rock that stick together in layers.