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.

 

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.

Three Types of Rocks

There are three major types of rocks that form rock layers.  These include metamorphic rock, igneous rock and sedimentary rock.  These different types of rocks help to classify rocks by how they were formed but each of these different types of rocks have rocks that are made from them and so the cycle is always continuing.

Metamorphic Rocks

Metamorphic rocks are rocks that are formed when there is a lot of pressure and a lot of heat.  These rocks are usually found inside of the Earth’s crust because this is the area of the Earth where there is a lot of heat and pressure.

Metamorphic rocks are made up of different kinds of rocks such as:

• Shale
• Slate
• Gneiss
• Marble
• Anthracite
• Soapstone
• Schist

Igneous Rocks

Igneous rocks are rocks that are made when lava turns into magma.  Igneous rocks are formed from magma.  When the lava leaves the volcano, it forms magma and when magma cools, igneous rocks are made.

When magma cools down, it can be a fast cooling or a slow cooling.  Depending on how fast or how slow magma cools depends on what kind of igneous rocks are formed.

Some igneous rocks include:

• Basalt
• Gabbro
• Granite
• Pumice
• Marble
• Quartzite
• Slate

Sedimentary Rocks

Sedimentary rocks are rocks that form when different debris and sediment are pushed together for a long, long time and then rocks form.

Most sedimentary rocks are carried to different places by water such as rivers or streams and they are taken to bigger bodies of water.  These rocks are then left where they are and they sit there for years and years, sometimes even millions of years.  After they sit for all of that time, they become sedimentary rocks.

Some sedimentary rocks include:

• Shale
• Limestone
• Chalk
• Flint
• Sandstone
• Greywacke

When there are many layers of sedimentary rocks, this is called a strata.

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

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.

More Facts About Layers of Rocks:

• The Earth’s Crust is made up of layers of rocks.
• Rocks have been used by people for millions of years for weapons and to build things.
• When force shoots lava out of the volcano, it then becomes magma.
• Water helps to break down rocks into different pieces.
• The name of igneous rocks comes from the Latin word, “ignis.” “Ignis” means “of fire.”  Igneous rocks are called “of fire” because they are formed from lava.
• Many lakes and oceans are formed from sedimentary rock layers, that is why you have sand.
• Marble is a rock that follows the rock cycle.  It is made up of limestone which is an igneous rock.  Then when the limestone is under extreme heat and pressure, it becomes marble, a metamorphic rock.
• The Earth is over 4 billion years old.
• We live in what is called the Quaternary Period.
• Scientists are able to study rock in order to know more about the Earth and the history of the Earth.

What Did You Learn?

• What are rock layers?
  Rock layers are layers upon layers of rocks that have formed over time.
• What layer of rock is the oldest?
  The layer of rock that is the oldest is the bottom layer.
• What layer of rock is the youngest?
  The layer of rock that is the youngest is the top layer.
• What are the three main types of rocks?
  The three main types of rocks are metamorphic rocks, igneous rocks and sedimentary rocks.
• What is the rock cycle?
  The rock cycle is when rocks begin as one type of rock and then turn into a different type of rock based on pressure, heat and the environment.

• Back to –
  Earth Science