Types of Rocks

There are three different types of rocks: igneous, metamorphic, and sedimentary. Each of these types is related through the rock cycle. That said, they’re all unique. Rock types are actually classified by their separate formation processes. Just as well, how they form gives them their distinct properties.

An image of a sea cave formed out of marble. This is a metamorphic rock, one of the three types of rocks.

Marble, forming a cave. Marble is a type of metamorphic rock.

Igneous Rocks

Of the three types, we’ll start with igneous rocks. There are two different kinds of igneous rocks, intrusive and extrusive. This depends on how the rocks form.
 

Characteristics of Igneous Rocks

Igneous rocks are special in that they’re made by a process known as crystallization. They’re actually made out of a bunch of crystals! Intrusive igneous rocks tend to have large crystals. Oppositely, extrusive igneous rocks can have crystals so small, you can’t even see them. This gives the rock a glassy appearance.
 

How are Igneous Rocks Formed?

Igneous rocks form as hot magma cools down. This causes it to harden into solid rock. Yet, how fast it cools matters a lot. Cooling speed is what determines whether an igneous rock is intrusive or extrusive.
 
Anytime that magma breaks through the earth’s surface (becoming lava), it will form extrusive rock. The relatively cold air on the surface rapidly cools the magma. Additionally, large crystals don’t have enough time to form in extrusive rock. It cools too fast!
 
Oppositely, there are times when the magma in a plume never even reaches the earth’s surface. It pushes into the crust without breaking it. This creates volcanic landforms. Inside of them, you find intrusive igneous rock.
 

Igneous Rock Examples

An image of Basalt Columns. These are Igneous rocks, one of the types of rocks.

Columns of basalt. Basalt is the rock that makes up oceanic crust.

An image of obsidian, a rock made by lava.

Obsidian, an extrusive igneous rock. It almost looks like black glass.

Metamorphic Rocks

Characteristics of Metamorphic Rocks

Metamorphic rocks are any rock that has been changed significantly by heat or pressure. They’re extremely common. Along with igneous rocks, they make up about 95% of the volume of the upper crust. Moreover, metamorphic rocks are classified into two types: foliated and non-foliated.
 

Foliated Metamorphic Rocks

Foliated metamorphic rocks are those which are ‘banded.’ This means they have flat bands of crystal in them. We call this effect ‘foliation.’ It gives the rock a distinct appearance, as you can see below.

An image of Gneiss, a type of metamorphic rock.

Gneiss. Gneiss is a foliated metamorphic rock. If you look closely, you can see its color bands.

How are Metamorphic Rocks Formed?

Remember, metamorphic rocks are just changed sedimentary and igneous rocks. When either of these two types is heated or put under pressure, they can become metamorphic rock. We call this process metamorphism.
 
Usually, metamorphism takes a long time. Over thousands of years, pressure can deform rock layers deep within the earth. This converts them into metamorphic rock. Another way this can happen is through contact metamorphism. This is when magma rises through the earth and ‘bakes’ the rock it touches.
 

Sedimentary Rocks

Characteristics of Sedimentary Rocks

Sedimentary rocks are a bit different from the two other kinds. For one, they’re only found on earth’s crust, where all of its sediment is. That’s because they’re made out of sediment. What’s more, this makeup gives them a grainy appearance. Some are literally made out of grains of sand.

A close-up image of sandstone, which lets you see the texture of the individual grains of sand.

Sandstone. You can see that this sedimentary rock has lots of rough grains.

 

Sediment

As shown above, the sediment which makes up a sedimentary rock determines its type. Sand makes up sandstone, and mud makes up mudstone. This seems obvious. But, it’s not as clear for some kinds of sedimentary rocks. As a result, scientists have invented a classification system for them. You can read more about it here.
 

How are Sedimentary Rocks Formed

Finally, whatever sediment we’re looking at, we still have to ask: how do we turn small grains into hard rock? The answer: time and pressure.
 
Over long periods of time, sediment builds up in certain areas. For example, rocks might erode and have their sediment deposited at the bottom of the lake. As more and more sediment builds up, the grains at the bottom compact. If there’s enough pressure, they might turn into sedimentary rock. This process is cementation. It tends to produce rocks in large layers, called strata.
 
To clarify, cementation sounds a lot like metamorphism. But, it’s different in that it’s creating a rock, not changing one.

Other Great Resources:

Rock Cycle Facts by Kiddle: https://kids.kiddle.co/Rock_cycle

Rocks and the Rock Cycle by Ducksters: https://www.ducksters.com/science/rocks.php

Short Animation on Rock Types: https://www.youtube.com/watch?v=CeuYx-AbZdo

Types of Sedimentary Rock: https://geology.com/rocks/sedimentary-rocks.shtml

Written by: Noah Louis-Ferdinand