The rock cycle document describes the three main types of rocks - igneous, sedimentary, and metamorphic - and how they are formed through different natural processes. Igneous rocks form when magma cools and solidifies. Sedimentary rocks form through the compaction and cementation of sediments. Metamorphic rocks form when existing rocks are changed by heat or pressure deep underground. Rocks are constantly being worn down by weathering and erosion and then reformed through processes like sedimentation, melting, and metamorphism, completing the continuous rock cycle.

1. Rocks
There are three main types of rock: igneous, sedimentary and metamorphic. They are formed
in different ways and have different properties.
Introduction to rocks
Rocks are solid at room temperature. They are made of grains that fit together. Each grain in
a piece of rock is made from a mineral, which is a chemical compound. The grains in a rock
can have different:
 colours
 shapes
 sizes
Granite has interlocking grains Sandstone has rounded grains
Porous rocks
Rocks with rounded grains are more likely to absorb water than rocks with interlocking
grains. This is because the water can get into the gaps between the grains. Rocks that absorb
water are described as being porous.
Rocks with rounded grains are usually softer and more crumbly than rocks with interlocking
grains. So porous rocks tend to be softer than non-porous rocks.
Rocks with interlocking grains are more likely
to be hard and non-porous
Rocks with rounded grains are more likely to
be crumbly and porous

2. Igneous rocks
This means that we get two main types of igneous rock, extrusive and intrusive, as shown in
the table:
Extrusive Intrusive
Where the magma cooled On the surface Underground
How fast the magma cooled Quickly Slowly
Size of crystals Small Large
Examples Obsidian and basalt Granite and gabbro
Igneous rocks are formed from molten
rock that has cooled and solidified.
The inside of the Earth is very hot - hot
enough to melt rocks. Molten (liquid)
rock is called magma. When the
magma cools enough, it solidifies and
igneous rock forms.
What are igneous rocks like?
Igneous rocks contain randomly arranged
interlocking crystals. The size of the crystals
depends on how quickly the molten magma
solidified:
 magma that cools slowly will form an
igneous rock with large crystals
 lava that cools quickly will form an
igneous rock with small crystals
Extrusive igneous rocks form from magma that erupted onto the surface as lava, where it
cooled quickly. On the other hand, intrusive igneous rocks form from magma that cooled
slowly, deep underground.
Obsidian has tiny crystals Granite has large crystals
Igneous rocks do not
contain any fossils. This is
because any fossils in the
original rock will have
melted when the rock
melted to form magma.
Modelling the cooling of magma
You may have done an experiment at school with a
substance called salol. If molten salol cools slowly, you
get big crystals. If it cools quickly, you get small crystals.

3. Sedimentary rocks
Here are the different processes in order:
Sedimentary rocks are formed from the broken remains of other rocks that become joined
together.
A river carries, or transports, pieces of broken rock as it flows along. When the river reaches
a lake or the sea, its load of transported rocks settles to the bottom. We say that the rocks
are deposited. The deposited rocks build up in layers, called sediments. This process is called
sedimentation.
The weight of the sediments on top squashes the sediments at the bottom. This is called
compaction. The water is squeezed out from between the pieces of rock and crystals of
different salts form. The crystals stick the pieces of rock together. This process is called
cementation.
These processes eventually make a type of rock called sedimentary rock. It may take millions
of years for sedimentary rocks to form.
transport → deposition → sedimentation → compaction → cementation
1. Small pieces of rock are transported
by a river
2. Pieces of rock are deposited and layers of
sediment build up
3. Sediments are compacted and water is squeezed out. Crystals form,
which cement the pieces of rock together

4. What are sedimentary rocks like?
Sedimentary rocks contain rounded grains in layers. The oldest layers are at the bottom and the
youngest layers are at the top. Sedimentary rocks may contain fossils of animals and plants
trapped in the sediments as the rock was formed.
Examples of sedimentary rock are:
 chalk
 limestone
 shale
 sandstone
Sedimentary rocks, like this sandstone, form layers

5. Metamorphic rocks
Metamorphic rocks are formed from other rocks that are changed because of heat or
pressure. They are not made from molten rock – rocks that do melt form igneous rocks
instead.
Earth movements can cause rocks to be deeply buried or squeezed. As a result, the rocks are
heated and put under great pressure. They do not melt, but the minerals they contain are
changed chemically, forming metamorphic rocks.
Metamorphic rocks may
form from rocks heated by
nearby magma
What are metamorphic rocks like?
Marble is formed from limestone
and contains tiny interlocking
grains. The layers in slate, formed
from shale, are arranged in layers.
This makes slate useful for making
roof tiles because it can be split into
separate flat sheets.
Slate is a metamorphic rock that can be split into
thin sheets, which makes it ideal for roof tiles
Metamorphic rocks rarely contain fossils. Any that were present in the original sedimentary
rock will not normally survive the heat and pressure.

6. The Rock Cycle
Rocks gradually wear away, a process called weathering. Biological, chemical and physical
weathering are three types of weathering. Weathering and erosion are part of the rock cycle.
People can even cause biological weathering just by walking. Over time, paths in the
countryside become damaged because of all the boots and shoes wearing them away.
Biological weathering
Weathering is the breaking down or wearing away of rocks where they are. It does not
happen because they move or collide with each other.
One type, biological weathering, is caused by animals and plants. For example, rabbits and
other burrowing animals can burrow into a crack in a rock, making it bigger and splitting the
rock.
You may have seen weeds growing through cracks in the pavement. If you have gone for a
walk in the countryside, you may even have seen bushes or trees growing from cracks in rocks
or disused buildings. This is because plant roots can grow in cracks. As they grow bigger, the
roots push open the cracks and make them wider and deeper. Eventually pieces of rock may
fall away.
Trees can grow out of rocks, with their roots finding and
making cracks

7. Chemical weathering
The weathering of rocks by
chemicals is called chemical
weathering.
Rainwater is naturally slightly acidic
because carbon dioxide from the
air dissolves in it. Minerals in rocks
may react with the rainwater,
causing the rock to be weathered.
Some types of rock are easily
weathered by chemicals. For
example, limestone and chalk are
mostly calcium carbonate. When
acidic rainwater falls on limestone
or chalk, a chemical reaction
happens. New, soluble, substances
are formed in the reaction. These
dissolve in the water, and then are
washed away, weathering the rock.
Chemical weathering can produce caves made of
limestone
Some types of rock are not easily weathered by
chemicals. For example, granite and gabbro are
hard rocks that are weathered only slowly.
However, some of their minerals do react with the
acids in rainwater to form new, weaker substances
that crumble and fall away.
Acid rain
Coal, oil and natural gas are fossil
fuels. When they are used, carbon
dioxide and sulphur dioxide escape
into the air. When the sulphur dioxide
dissolves in the water in the clouds, it
makes acid rain – rainwater that is
more acidic than normal.
Acid rain harms fish and trees, but it
also makes chemical weathering
happen more quickly. Buildings and
statues made from rock are damaged
as a result. This is worse when the
rock is limestone rather than granite.
Stone carvings damaged by acid rain

9. Erosion and transport
Physical weathering
Physical weathering is caused by physical processes such as changes in temperature,
freezing and thawing, and the effects of wind, rain and waves.
Temperature changes
When a rock gets hot it expands a little, and
when it gets cold the rock contracts a little.
If a rock is heated and cooled many times,
cracks form and pieces of rock fall away.
This type of physical weathering happens a
lot in deserts, because it is very hot during
the day but very cold at night.
Wind, rain and waves
Wind, rain and waves can all cause
weathering. The wind can blow tiny grains of
sand against a rock. These wear the rock
away and weather it. Rain and waves lashing
against a rock can also wear it away over
long periods of time.
Freeze-thaw
Water expands slightly when it freezes to form ice. This is why water pipes sometimes burst
in the winter. You might have seen a demonstration of this sort of thing - a jar filled to the
brim with water eventually shatters after it is put into a freezer. The formation of ice can also
break rocks.
If water gets into a crack in a rock and then freezes, it expands and pushes the crack further
apart. When the ice melts later, water can get further into the crack. When the water freezes,
it expands and makes the crack even bigger.
This process of freezing and thawing can continue until the crack becomes so big that a piece
of rock falls off.
Erosion
Weathering and erosion are often confused:
 weathering is the wearing away of rocks
 erosion is the movement of the broken pieces away from the site of weathering
For example, a limestone cliff may be weathered by freeze-thaw, a type of physical
weathering. This means that rock in the cliff becomes broken into smaller pieces.
Erosion happens when these pieces of rock fall away down the cliff.

10. Transport
Rivers and streams can move pieces of rock. This is called transport. Fast-flowing rivers can
transport large rocks, but slow-moving rivers can only transport tiny pieces of rock.
As the pieces of rock are carried along by the water, they bash against each other and the
river bed. They gradually wear away because of this. They become smaller and more
rounded.
Heaps of rocks, caused by weathering then erosion, lying at the bottom of a
limestone cliff

11. Rock cycle summary
The processes in the rock cycle are summarised in this diagram:
Remember that there are
three main types of rock:
 igneous (for example,
basalt and granite)
 sedimentary (for
example, limestone,
sandstone and shale)
 metamorphic (for
example, slate and
marble)
Continual change
The Earth's rocks do not stay the same forever. They are
continually changing because of processes such as
weathering, erosion and large earth movements. The rocks
are gradually recycled over millions of years. This is called
the rock cycle.
For example, sedimentary rocks can be changed into
metamorphic rocks. These can be weathered, eroded, and
the pieces transported away. The pieces of rock could be
deposited in a lake or sea, eventually forming new
sedimentary rock. Many routes through the rock cycle are
possible.

12. Letter Description
A Weathering breaks down rocks on the surface of the Earth. There are three types of
weathering (biological physical and chemical). Wind and water move the broken rock
particles away. This is called erosion.
B Rivers and streams transport rock particles to other places. Rock particles are deposited in
lakes and seas.
C Rock particles form layers.
D Compaction and cementation presses the layers and sticks the particles together. This
creates sedimentary rock.
E Rocks underground get heated and put under pressure, and are changed into metamorphic
rock.
F Rocks underground that get heated so much they melt turn into magma. Magma also comes
from deeper inside the Earth, from a region called the mantle. Pressure can force magma
out of the ground, creating a volcano. When the magma (lava) cools quickly, it turns into
solid extrusive igneous rock. Magma that cools slowly underground forms solid intrusive
igneous rock.
G Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming
underneath. This is called uplift.

13. Fossils
A fossil is the preserved remains or
traces of a dead organism. Fossils
are found in rocks and can be
formed from:
Hard body parts, such as bones and
shells, which do not decay easily or
are replaced by other materials as
they decay.
Parts of organisms that have not
decayed. For example, dead
animals and plants can be
preserved in amber, peat bogs, tar
pits, or in ice.
Casts or impressions, such as
footprints or burrows. These
become covered by layers of
sediment, which eventually become
rock.
Fossils of ammonites - sea creatures that
became extinct about 65 million years ago
The fossil record
Fossil remains have been found in
rocks of all ages. Fossils of the
simplest organisms are found in the
oldest rocks, and fossils of more
complex organisms in the newest
rocks. This supports Darwin’s theory
of evolution, which states that simple
life forms gradually evolved into more
complex ones.
Fossils contained in layers of sediment
- Evidence for early forms of life comes from fossils. By studying fossils, scientists can learn
how much (or how little) organisms have changed as life developed on Earth. However, many
early forms of life were soft-bodied so they have left few traces behind.

14. Evolution of the horse
Scientists have a fairly complete fossil record for the evolution of the horse. It shows that over
50 million years, the horse evolved from a dog-sized creature that lived in rainforests into an
animal standing up to 2 metres high and adapted to living on the plains.
Its multi-toed feet, adapted for walking across the forest floor, evolved into single-
toed hooves more suited for running over open country.
Horses through the ages

15. Extinction
A species becomes extinct
when the last individual of that
species dies. Animals that
have not adapted well to their
environment are less likely to
survive and reproduce than
those that are well adapted.
This can lead to extinction.
Several factors can cause a species to become extinct.
They include:
 new diseases
 new predators
 new, more successful competitors
 changes to the environment over geological
time, such as climate change
 a single catastrophic event, such as a massive
volcanic eruption or a collision between an
asteroid and the Earth
A species may also become extinct through speciation.
The fossil record shows that many species have become extinct since life on Earth began.
Extinction is still happening and often it is due to human activities. We compete with other
living organisms for space, food and water, and we are very successful predators.
The dodo became extinct following the introduction of new predators
The dodo was a heavily-built flightless bird, roughly the size of a swan.
Dodos lived on Mauritius, an island in the Indian Ocean. The island was uninhabited and the
birds had no natural predators.
When Mauritius was colonised by the Dutch in 1638, dodos were hunted for food. They were
easy to catch because they were not afraid of people. New competitors were brought onto
the island, including pigs, cats and rats. They ate the dodos' eggs and their young. Within 80
years, the dodo was extinct.

16. New species
New species can arise as a result of:
 genetic variation - where each population has a wide range of alleles that control their
characteristics
 natural selection - where the alleles which help an organism to survive are selected in
each population
 speciation - where the populations become so different that successful interbreeding
cannot happen anymore
New species can also arise as a result of isolation. This is where two populations of a species
become geographically separated. For example, Charles Darwin described speciation of
finches in this way.
Darwin studied the wildlife on the Galápagos Islands (a group of islands almost 1,000 km off
the coast of Ecuador). He noticed that the finches on the different islands were similar to
each other.
Darwin's drawings of the different heads and beaks of finches
However, the finches showed wide variations in their size, beaks and claws from island to
island. For example, their beaks were different depending on the local food source. Darwin
concluded that because the islands are so far from the mainland, the finches that had arrived
there had changed over time.

17. Adaptations for cold climates
The Arctic is cold and windy with very little rainfall. Plants in the Arctic often grow very close
to the ground and have small leaves. This helps to conserve water and to avoid damage by
the wind.
The polar bear
Polar bears are well adapted for survival in the Arctic. Their adaptations include:
 a white appearance - as camouflage from prey on the snow and ice
 thick layers of fat and fur - for insulation against the cold
 a small surface area to volume ratio - to minimise heat loss
 a greasy coat that sheds water after swimming - to help reduce heat loss
 large feet - to distribute their load and increase grip on the ice
The polar bear is adapted to life in a cold climate

18. Adaptations for hot climates
Deserts are hot by day, cold at night, and receive very little rainfall.
The camel
Camels are well adapted for survival in the desert. Their adaptations include:
 large, flat feet - to spread their weight on the sand
 thick fur on the top of the body for shade, and thin fur elsewhere to allow easy heat
loss
 a large surface area to volume ratio - to maximise heat loss
 the ability to go for a long time without water - they lose very little water through
urination and perspiration
 the ability to tolerate body temperatures up to 42°C
 slit-like nostrils and two rows of eyelashes - to help keep out sand
Camels are
adapted to life in a
hot climate
Desert plants
Cacti are also well adapted for survival in
the desert. Their adaptations include:
 stems that can store water
 widespread root systems that can
collect water from a large area
In addition, cacti have spines instead of
leaves. These minimise the surface area
and so reduce water loss by
transpiration. The spines also protect the
cacti from animals that might eat them.
The stems of cacti are
able to store water