Rocks are hard substances that are formed together by many different things and ways.  There are three types of main rocks and they are:

  • Sedimentary
  • Igneous
  • Metamorphic


Layers of Rocks

There are three different kinds of rocks that are found in the layers of rocks.  There is no specific order of the layers because the layers are always changing but one thing is for sure that the rocks in the bottom layer are the oldest rocks and the rocks at the top are the youngest layers.

Why Are the Rocks on the Bottom the Oldest?

The reason that the rocks on the bottom of the layers of rocks are the oldest is because rocks will continue to pile on top of each other.  If there is one layer of rock on the bottom and another layer piles on top of it, the layer on the bottom is now the oldest and the layer on top is the youngest.

Rock outcrop along a mountain creek near

The layer on the top of the rock layer is the youngest layer of rocks.  This will stay the youngest layer until another layer piles on top of it and then that will become the youngest layer.


People that study rocks are called geologists.  The study of rocks is geology.  These people look at different kinds of rocks such as:

  • Fossils
  • Mountains
  • Soil
  • Rocks
  • Crystals
  • Minerals

Geologists try to figure out why mountains were formed or how crystals were formed.  They also study the different minerals and figure out how they were made and how long they have been around.

Sample of igneous gabbro

Sedimentary Rocks

Sedimentary rocks are rocks that are made from mud, pebbles and sand.  As time goes by, the layers of these things pile up on top of each other.  As they are piled on top of each other, they get more and more pressure.  When there is enough pressure, sedimentary rocks are made.

Where Are Sedimentary Rocks Made?

Sedimentary rocks are made in places such as:

  • Rivers
  • Lakes
  • Seas
  • Deserts
  • Oceans


Sediment is bits and pieces of debris and dirt that make up sedimentary rocks.  Sediment is formed when bigger rocks get broken down by erosion and other things.  Sometimes rocks break into pieces from wind, rain or even from snow.  When this happens, the rocks become sediment.

Some kinds of sediment include:

  • Pebbles
  • Mud
  • Sand

Different areas that are lower such as oceans and lakes collect the sediment.  When other materials lay on top of them, they get pushed together and they form rocks.

Most Common Rock

The most common rock that we see on the surface of the Earth is sedimentary rock.  This happens because the crust of the Earth has a lot of sediment.  Compared to igneous and metamorphic rocks, the surface of the Earth has more sedimentary rocks.

What Causes Sedimentary Rocks?

When there are mountains, they are usually millions of years old.  Most mountains are tall and are made of different sizes of rocks.  Some of the rocks stick out and are jagged while others are flat and close to the mountain side.

As time goes by, they get older and older and many things happen around the mountain such as wind erosion, water erosion and other things.  Water runs through the mountains, snow falls, wind blows.

Mountains begin to crumble and break a little bit at a time.  When the small rock that breaks off falls, it falls into a river, ocean or stream.

These rocks that fall are called sediment.  Eventually, the water slows down and the layers of rock that have fallen in the water fall to the bottom of the river, lakes or streams.  Over time, more and more rocks stack on top of these and they form layers.

The layers of rocks eventually form sedimentary rocks.

How Can You Tell if a Rock is a Sedimentary Rock?

You can tell a sedimentary rock by the layers that it has.  These rocks can only be formed over time as layers pile on top of layers.  Some layers become hard beds of cliffs or strata.  Strata means layers.

What Can a Sedimentary Rock Tell Scientists?

Scientists study rocks to learn information about an area.  They can tell a lot about an area when there are sedimentary rocks such as:

  • What kind of conditions existed when the rocks were deposited in the area.
  • What kind of organisms or minerals made the sedimentary rock.
  • Thing about the past because of fossils.
  • How the sedimentary rocks go where they are.

Types of Sedimentary Rocks

There are six main types of sedimentary rocks including:

  • Sandstone
  • Limestone
  • Shale
  • Gypsum
  • Fossil Fuels
  • Greywacke
  • Chalk
  • Coal
  • Claystone
  • Flint
  • Mudstone

Clastic Rock

When a rock is under a lot of pressure and when the material dissolves and crystalizes, this is called clastic rock.

When this happens, the rock under the top rock becomes more compacted or stuck together and it makes it to get harder.  This is how sand becomes sandstone.

Consolidated Rock

When rock is under pressure for a long time it becomes consolidated.  This means that it becomes a solid.


Sandstone is a sedimentary rock that is made when sand gets stuck together.  This type of rock is usually soft and usually has different layers of different colored sand.


Limestone is rock that is made of calcium carbonate.  Most limestone has small shells in it.  This type of rock forms when there are layers and heat and pressure are causing it to fuse together.  Marble is a type of limestone but is also a metamorphic rock.

Limestone is where most fossils are found.


Shale is a sedimentary rock that is made of clay that became hard.  Most shale can break apart in sections that are flat and not round.


Gypsum is a sedimentary rock that is made of minerals.  This is usually white and is sometimes used to make plaster and cement.  Gypsum usually has round rocks with pebbles stuck together in it.  It has different patterns.

Breccia is a type of gypsum that has little rocks that are stuck together.

Fossil Fuels

Fossil Fuels are natural gases, oil and coal that is formed from organisms that have died and decayed.

Coal is a type of fossil fuel that is more than 300 million years ago.  Coal can turn into diamonds if it has enough pressure for a long time.  Coal helps to make electricity.

Fossil Fuels are not good for the environment because they are non-renewable, which means once they are gone, they can never come back and because they release greenhouse gases when they are burned.

Greenhouse gases are not good for the environment and cause the ozone layer to get smaller.

Igneous Rocks

Igneous rocks are rocks that are formed by magma.  When magma cools down, it becomes a solid.  This can happen below the surface of the Earth or on the surface of the Earth.

Where Does the Word Igneous Come From?

The word igneous comes from a word in Latin that means fire.  This word is “ignis.”  The reason that the word ignis means fire is because igneous rocks are formed by lava that gets hard and cools down and turns into magma.


Magma is lava that comes out of a volcano.  When the lava is in a volcano, it is called lava, but as soon as it leaves the opening of the volcano, it is called magma.

Magma can go into rocks and can cover the surface of an area.  Magma has temperatures that get as hot as 700 degrees Celsius to 1300 degrees Celsius.

What is Magma Made Of?

Magma is made up of different molecules and different atoms that of minerals that have been melted.  When the magma cools down, the atoms and molecules change from a liquid state to a mineral and the mineral grows together forming the magma rock.

The Earth’s Crust

The Earth’s crust is made up of a lot of igneous rock.  Some scientists believe that the crust is 95% igneous rock.  Since lava can form above or below the surface of the Earth, the crust is made of igneous rock from lava forming below the surface.

Intrusive Rocks

When rocks start below the surface, these are called intrusive rocks.

Extrusive Rocks

When a rock becomes cooled on the surface of the Earth, it is called an extrusive rock.

How Do Igneous Rocks Form?

When lava is a liquid and it cools into a solid, it is also known as crystallization.  The minerals of these rocks get bigger and the crystals grow until they hit into other crystals.  When there is enough crystals that are formed together, they cannot be removed from one another and they form intrusive igneous rocks.

These rocks are coarse grained crystals which means that they have mineral crystals that help them to form.  Sometimes, when people look at intrusive igneous rocks, they can see crystals that have formed.  Some of these are different colors such as:

  • Pink
  • White
  • Black
  • Grey

If you look at a rock and you see crystals on it, you are studying a rock that is an intrusive igneous rock.

How Do Extrusive Igneous Rocks Form?

When lava is hot and it cools down, we know it turns into a solid.  When an intrusive igneous rock is formed, the lava cools down very slowly, but with an extrusive igneous rock, the cooling happens very fast.

When you look at an extrusive igneous rock, you do not see any crystals because they are very small.  The type of igneous rock that is formed depends on how fast or how slow the lava cooled down.

Minerals of Igneous Rocks

There are many minerals that help to make up igneous rocks.  Some of them include:

  • Oxides
  • Quartz
  • Feldspars
  • Micas
  • Amphiboles
  • Olivine’s
  • Pyroxenes

How Many Different Igneous Rocks Are There?

There are more than 700 different igneous rocks.  Some of these include:

  • Gabbro
  • Andesite
  • Pumice
  • Obsidian
  • Tuff
  • Diorite
  • Granite
  • Basalt
  • Kimberlite
  • Olivine Basalt
  • Rhyolite
  • Gabbro


Basalt is a type of igneous rock.  When basalt has a lot of heat and a lot of pressure on it, it changes from an igneous rock to a metamorphic rock.  When this happens, it is called metamorphism.


Tuff is another form of igneous rock.  Tuff is formed when volcanic ash turns to rock.


Granite is an igneous rock that has quartz in it.  This kind of rock is very strong, and people use it to help build things.


Pumice is a type of igneous rock that is not very strong.  It is formed when molten rock is blown quickly out of a volcano and bubbles are formed.  These bubbles cool down quickly and have hardly any pressure.  When this happens, it forms pumice.  When you look at pumice, it looks like it has bubbles in it, and it is a rock that is very easy to break.


Obsidian is an igneous rock that is made from volcanic rock that forms quickly and crystals grow on it making it very sharp.  This is also called volcanic glass because it is shiny and looks like glass.  This type of rock can be used to make weapons because of how sharp it is.

Chemical Types of Igneous Rocks

There are two classifications for igneous rocks that are based on what the rocks are made up of:

  • Felsic-this is igneous rocks that are lighter.  Some examples of felsic igneous rocks are silicon, sodium and potassium.
  • Mafic-these types of igneous rocks contain a lot of iron and magnesium.

Why Are Igneous Rocks Important?

Igneous rocks are important because they help to tell the age of the world.  Also, many of the tectonic plates that move are made up of igneous rocks.

Another reason that igneous rocks are important is because many of them have important mineral ores.  Some of the mineral ores that are found in igneous rocks include:

  • Platinum
  • Chromium
  • Tungsten
  • Tin
  • Uranium

Metamorphic Rocks

Metamorphic Rocks are rocks that are exposed to a lot of heat and a lot of pressure and this causes them to change.  Most of the changes happen many, many years later.  Many of the metamorphic rocks are found on the surface of the Earth where there is a lot of pressure and heat that comes from the magma that is under the surface.

Even though rocks can be made from the same type of minerals, they become classified by how they are made and not by what kinds of minerals they have.

Kinds of Metamorphic Rocks

There are many different kinds of metamorphic rocks such as:

  • Anthracite
  • Schist
  • Gneiss
  • Marble
  • Slate
  • Quartzite
  • Granulite


Anthracite is a metamorphic rock that looks very shiny.  This is a type of coal and it is high in carbon.  When a rock looks shiny, it is also called a rock’s luster.


Marble is an interesting rock because it is a metamorphic rock that was once a sedimentary rock.  This rock followed the rock cycle and because of the sedimentary rock was under so much pressure and heat, it turned the rock into a metamorphic rock.

Limestone is the sedimentary rock that marble is formed from.  Marble is very durable, but it can be dissolved if someone puts different kinds of acid or lemon juice on it.


Slate is another metamorphic rock that was formed from a sedimentary rock.  Slate was made from mudstone that was under a lot of heat and pressure and it formed slate.

When slate is formed, clay is replaced by mica.


Quartzite is once again a metamorphic rock that changed from the sedimentary rock, sandstone.


Granulite is a metamorphic rock that changed from an igneous rock, basalt to a metamorphic rock due to heat and pressure.


Schist is a rock that comes from slate.

What Does Metamorphic Mean?

The word “metamorphic” comes from the world “morph” which means “to be changed or transformed.”  The reason that metamorphic means changed, is because metamorphic rocks are made when other rocks change because of heat or pressure.

There has to be a lot of heat and a lot of pressure for this to happen.


When rocks start to change, this is called metamorphosis.  When metamorphosis happens, the rocks that are close to the surface of the Earth are changed by magma.

Why Are Metamorphic Rocks Different?

Metamorphic rocks are different because not only can they be formed by igneous and sedimentary rocks; they can also be formed by other metamorphic rocks.

When a metamorphic rock is formed, it has to have a lot of pressure and a lot of heat for a very, very long time.

Contact or Thermal Metamorphism

When rocks are changing, this is called contact metamorphism.  This happens when bits of rocks are broken down or when they are changed by the heat from the magma.

When magma goes up into the Earth’s crust, it is changing all of the rocks around it.  When this happens, there is so much heat that new rocks are being formed.

Regional Metamorphism

When rocks are buried beneath the surface of the Earth, deep into the Earth, the rocks begin to change.  These changes happen from the chambers of magma that reach these rocks.

Even though the rocks are really hot and there is a lot of pressure, the rock will not melt because of the pressure.  If the rock did melt instead of staying solid, it would form into one of the other two types of rocks either a sedimentary rock or an igneous rock.

When there is more pressure, the rocks will change more.  If the rock is really deep and it is around more heat, it can change more too.

A great example of this is that mud can turn into shale if it is under just a little bit of pressure.  But when the shale is under even more pressure, it turns into slate.

When a rock is close to the surface of the Earth, and it is exposed only to a little bit of pressure, it causes the rock to sometimes have different layers and to split easy.  This is called foliation.

Foliated Rocks

When a rock has different layers of grain in them, these rocks are called foliated rocks.

Non-foliated Rocks

Rocks that have no grain in them are called non-foliated rocks.

How Do Metamorphic Rocks Get to the Surface?

There are different ways that metamorphic rocks get to the surface of the Earth such as volcanic activity and erosion.  When erosion happens, it can lift rocks out of the ground and bring them to the surface.

What Kind of Rocks Are There?

It is impossible to really name all of the rocks, so rocks are classified by how they are made.  If a rock is made by pressure it is a metamorphic rock and if a rock is made by magma, it is an igneous rock and so on.  The rocks are not made with exact formulas because they are made up of different things and some rocks have the same minerals.

Rock Cycle

The rock cycle is the way that rocks change over time.  A rock can be a sedimentary rock and then change to a metamorphic rock.  This happens because rocks are always changing.  Even though it takes millions of years for rocks to change, they are changing all the time.

Here is how the rock cycle works:

  • Lava flows out of the volcano and turns into magma.  When the magma cools down it becomes igneous rocks.
  • Water in a river or a lake will break down the igneous rock into smaller pieces and the igneous rock will become just pieces of sediment.
  • Sediment piles on top of each other and gets hard and after many years, it becomes sedimentary rocks.
  • Sedimentary rock is normally found deep into the surface because it is always getting covered up.
  • When sedimentary rock is covered for a long time, the pressure builds up and the heat and it can cause the sedimentary rocks to become metamorphic rocks.

The interesting thing about rocks is that they do not have to follow the rock cycle and often times, they do their own thing.

Rocks and the Environment

When the environment changes, it can cause rocks to change.  Fore example, when there is a volcano that erupts, it shoots out magma and all kinds of ash.  The ash hardens and the magma gets cooler.

Even though the ash is on top, it is still the newest layer and because of the change in the environment, there is now an older layer of rock underneath it.

Tectonic Plates

We talk a lot about tectonic plates because they play such a huge role in the world changes.  When there are rock layers that are moved or tilted, this can be because of the tectonic plates.  As the plates change and move, it can cause the layers of rocks to move also.  When this happens, the rocks will stay in a tilted formation until something happens that moves them back, and this is sometimes never.

Radiometric Dating

Scientists use radiometric dating in order to tell how old rocks are.  When a scientist uses radiometric dating, it is looking at the elements of the rocks that are radioactive.  These rocks are able to give scientists a specific date that they were formed.

Radiometric dating is a process of reading how much radioactive material is in a rock.


Strata is when layers of rocks are formed.  A stratum is many different layers.  When a strata forms, it is usually a specific time or a specific place.  When this happens, geologists are able to figure out where the rocks are from and when they formed.

When there is a normal rock stratum, the strata form horizontal or sideways layers.  The rocks will sometimes become deformed because of pressure as there are more and more layers.

When there is heavy rain or other strong weather, there can be spaces that happen between the different strata.  When this happens, these spaces are called unconformities.

Some strata formations repeat themselves such as a sandstone layer followed by a carbonate layer then a sandstone layer followed by a carbonate layer and this will repeat over and over again.  When this happens, it is called a rhythmites.

Space Rocks

A space rock is rocks that come from space.  These rocks are called meteorites.  Most meteorites are made up of iron and not the same elements of rocks on the Earth.

What Did You Learn?

  • What are the three types of rocks?
    The three types of rocks are metamorphic, sedimentary and igneous rocks.
  • Which rocks are formed by magma?
    Igneous rocks are formed by magma.
  • Which rocks are formed by a lot of heat and pressure?
    Metamorphic rocks are formed by a lot of heat and pressure.
  • Which rocks are formed by sediments of sand, dirt and pebbles?
    Sedimentary rocks are formed by sediments of sand, dirt and pebbles.
  • What is the rock cycle?
    The rock cycle is where rocks change from one type of rock to another based on the elements that it is in.  Such as if a sedimentary rock is under a lot of heat and pressure, it can become a metamorphic rock.