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 aresolid at roomtemperature. 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:
Granite has interlocking grains Sandstone has rounded grains
Rocks with rounded grains are more likely to absorb water than rocks with interlocking
grains. This is becausethe water can get into the gaps between the grains. Rocks thatabsorb
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 withinterlockinggrainsare more likely
to be hard and non-porous
Rocks withroundedgrainsare more likelyto
be crumblyand porous
2. Igneous rocks
This means that we get two main types of igneous rock, extrusive and intrusive, as shown in
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
rock that has cooled and solidified.
The insideof the Earth is veryhot - 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
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 itcools quickly, you get small crystals.
3. Sedimentary rocks
Here are the differentprocesses in order:
Sedimentary rocks are formed from the broken remains of other rocks that become joined
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
aredeposited.Thedeposited rocksbuild up in layers,called sediments.Thisprocessis called
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
These processes eventually makea type of rock called sedimentary rock. It may takemillions
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. Theoldest layers areat 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:
Sedimentary rocks, like this sandstone, formlayers
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
Earth movements can causerocks 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 heatedby
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 becauseit 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 presentin the original sedimentary
rock will not normally survivethe 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 justby walking. Over time, paths in the
countrysidebecome damaged becauseof all the boots and shoes wearing them away.
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
You may have seen weeds growing through cracks in the pavement. If you have gone for a
walkin thecountryside,youmay even haveseen bushesor trees growingfromcracksinrocks
or disused buildings. This is becauseplant 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
Trees can grow out of rocks, with their roots finding and
7. Chemical weathering
The weathering of rocks by
chemicals is called chemical
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
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.
Coal, oil and natural gas are fossil
fuels. When they are used, carbon
dioxide and sulphur dioxide escape
into theair. When the sulphurdioxide
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 aredamaged
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 is caused by physicalprocesses such as changes in temperature,
freezing and thawing, and the effects of wind, rain and waves.
When a rockgets 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 physicalweathering 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 windcan blow tiny grainsof
sand against a rock. These wear the rock
awayand weather it. Rain and waveslashing
against a rock can also wear it away over
long periods of time.
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
brimwith water eventually shatters after it is putinto a freezer.The formation of ice can also
If water gets into a crack in a rock and then freezes, itexpands and pushes the crack further
apart. When theice melts later, water can get furtherinto thecrack.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.
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.
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
Heaps of rocks, causedby weathering thenerosion, lying at the bottomof a
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)
example, slate and
The Earth's rocks do not stay the same forever. They are
continually changing because of processes such as
weathering,erosion and largeearth 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. Thesecan 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
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
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
G Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming
underneath. This is called uplift.
A fossil is the preserved remains or
traces of a dead organism. Fossils
are found in rocks and can be
Hard body parts, such as bones and
shells, which do not decay easily or
are replaced by other materials as
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
Fossils of ammonites - sea creatures that
became extinct about 65 millionyears 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 thatsimple
life formsgradually evolved into more
Fossils containedinlayers of sediment
- Evidence for early forms of life comes from fossils. By studying fossils, scientists can learn
howmuch (or howlittle) organismshavechanged 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. Itshows 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
A species becomes extinct
whenthe lastindividualof 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.
Severalfactors can causea species to become extinct.
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 introductionof newpredators
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 wascolonised by the Dutch in 1638,dodoswerehunted for food.They were
easy to catch because they were not afraid of people. New competitors were broughtonto
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:
geneticvariation- whereeach population has a widerangeof allelesthatcontroltheir
natural selection - wherethealleles which help an organismto surviveareselected in
speciation - where the populations become so different that successfulinterbreeding
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 kmoff
the coast of Ecuador). He noticed that the finches on the different islands were similar to
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 becausethe islandsare sofar fromthe mainland, the finchesthat 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 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 ina coldclimate
18. Adaptations for hot climates
Deserts are hot by day, cold at night, and receive very little rainfall.
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
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
adapted to life in a
Cacti arealso well adapted for survivalin
the desert. Their adaptations include:
stems that can store water
widespreadroot systems thatcan
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
cacti from animals that might eat them.
The stems of cacti are
able to store water