Rocks - GCSE Chemistry

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ROCKS

The rock cycle and how it works

The rock cycle

is the name given to the process where rocks are formed,

changed, broken and melted as shown in the picture on the left.

It is the movement of

tectonic plates that makes the rock cycle happen. Hot magma can

cool to form igneous rocks. Fast cooling on the

surface makes basalt and slow cooling inside the crust makes

granite. When tectonic plates collide the enormous forces

can produce metamorphic rocks and rocks inside the

Earth's crust can become pushed up onto the

surface by a process called uplift.

Surface rocks are

exposed to weathering and erosion which makes sediments.

Burial of sediments produces sedimentary rock

inside the Earth's crust. Any rock in the crust can be pushed to the

surface by uplift or dragged down towards the mantle

where it can melt and form new magma.

Igneous Rocks

Sedimentary Rocks

Metamorphic Rocks

Igneous Rocks are rocks formed from the

cooling of hot magma. Igneous rocks first

formed when the Earth cooled from its molten state about

4 billion years ago. Igneous rock is still being made today

by the process of tectonic activity.

How can they be identified?

Igneous rocks contain

randomly arranged interlocking crystals. The crystal size depends

on the rate of cooling. When hot magma is

cooled quickly it makes basalt and when it is cooled

slowly it makes granite. Igneous rocks do not

contain fossils because the rock forms at a high temperature

which would destroy any living organisms.

Rocks formed from

sediments are called sedimentary rocks.

Sediments are small

pieces of material that do not dissolve in water. They slowly fall down to the

bottom of a lake or the sea, and gather together over the years forming a large

mass of material.

Imagine stirring very

fine sand into a glass of water. Slowly the sand particles settle on the bottom

of the glass. They are called "sediments" .

Materials which do

dissolve (salts), stay in the water. This is what has made the sea salty over

many millions of years.

Sediments have come

from the weathering and erosion of surface rocks. The

original surface rocks were igneous rocks formed from the

cooling of molten magma. Surface rocks today

are igneous, sedimentary or metamorphic.

Weathering and Erosion

Weathering breaks a

rock down into small pieces. The two main processes

are exfoliation and freeze-thaw weathering. Freeze-thaw weathering

occurs when water gets into cracks in rock and then freezes

(below 0 °C) in cold weather. Water expands when it

freezes and turns to ice, and the expansion can

cause the rock to split and fragment.

In warmer weather

(above 0 °C) the ice melts (called thawing), and new cracks are

exposed allowing the process to repeat itself.

Erosion is a slow

process of wearing away weathered rock by the action of the

wind, rivers and waves. All rocks would have

been worn down flat to sea level long ago if hills and mountains

were not constantly being formed by the movement

of plate tectonics. See the rock cycle.

Transport and compaction

Small pieces of

weathered rock (called sediments) are transported by rivers.

This means that the rock pieces are moved along as the

river flows towards the sea.

During transportation,

the sediments are sorted according to size. The largest pieces

tend to be deposited first. Sand is next to settle out of the

water. The smallest particles are mud or clay, and these are moved

furthest by the river and deposited last.

Compaction - With the passing of

time, the deposits of sediment get thicker and thicker.

The sediment underneath becomes increasingly crushed

by the weight of the new sediment which is being deposited on

it from above. This is called compaction. It is this process of

compaction which eventually makes

layers of sedimentary rock.

The process may take

millions of years. The largest rock

pieces will produce conglomerate. Sand will produce

sandstone. Mud or clay will produce mudstone or shale

https://www.youtube.com/watch?v=Yf4YtDlA1oQ (Embed)

The formation of sedimentary rocks

Compaction eventually

forces the water out of the sediment. Salts in the water

crystallise under pressure and cement the

sediment particles together (like glue).

This is how

sedimentary rock is made.

How can Sedimentary

Rocks be Identified?

Sedimentary rock is

often formed in layers, because the original

sediment is laid down in layers. This gives the rock a

typical layered structure, which can be seen in

mudstone, and more clearly in shale.

Examples of

Sedimentary Rocks are conglomerate, sandstone, mudstone, shale

Sedimentary Rock Formation & Fossils

What is a Metamorphic

Rock?

A metamorphic rock is

a rock which has had its form changed by the action of high

temperatures and pressures (without melting).

The crystal structure

of the rock is changed in the process. Metamorphism can be

low or high grade.

Rocks can become

subject to high temperatures because they come into

contact with an igneous intrusion.

This is called contact

metamorphism or rocks become subject

to high temperatures and pressures because of tectonic

activity in the Earth's crust. This is called regional metamorphism

and it occurs where plates collide.

Examples of

metamorphic rock are Marble Slate, Schist and Gneiss.

Test your knowledge

Quizzes

Flashcards

Rock cycle

The Rock Cycle

The Rock Cycle

The Rock Cycle

Rocks and the Rock Cycle

The Rock Cycle

Products and Uses of Rocks

All raw materials come

from the Earth's crust, the oceans and the atmosphere.

Metals and oil have

already been discussed. Some rocks are used in

construction for building.

These include Granite, Limestone, Marble, Slate. Granite and limestone

are used in road construction.

Limestone has a number

of other important uses.

Limestone is used to

make Lime -

Quicklime - Slaked Lime - Cement -

Concrete - Glass

Powdered limestone can

be used in power station chimneys to remove acidic gases or powdered limestone can

be added to soils or lakes to reduce acidity.

Lakes and soils can become acidic because of acid rain.

Powdered limestone is cheaper than slaked

lime but acts more slowly.