3. The earth's surface is constantly being eroded. This means that rocks are broken up into smaller pieces by
weathering agents such as wind, water, and ice. These small pieces of rock turn into
pebbles, gravel, sand, and clay. They tumble down rivers and streams. These pieces settle in a new place
and begin to pile up and the sediments form flat layers. Over a long period of time, the pieces become
pressed together and form solid rock called sedimentary rock. Most sedimentary rocks form under water.
Most of the earth has been covered by water some time in the past. 70% of the earth is covered by water
now. So sedimentary rocks are common all over the world. Sedimentary rocks are often rich in fossils.
Sediments can harden into sedimentary rock in two ways.
pressure-As layer after layer of sediments are deposited, the lower layers are pressed together tightly
under the weight of the layers above.
cementing-Some sediments are glued together by minerals dissolved in water.
Some examples of sedimentary rocks are sandstone, limestone, conglomerate, and shale. Sandstone is
formed from grains of sand pressed tightly together. Sandstones are very common rocks. They are formed
from the sand on beaches, in riverbeds, and sand dunes. Sandstones are usually made of the mineral
quartz. Limestone is formed from tiny pieces of shells of dead sea animals that have been cemented
together. Conglomerate contains sand and rounded pebbles that have also been cemented together. Shale is
formed from mud or clay that has been pressed together. Shale forms in quiet waters such as swamps and
bogs. Sedimentary rocks are easy to identify because you can actually see the layers.

4. Below is a summary of the major characteristics of sedimentary rocks.
Classified by texture and composition
Often contains fossils
May react with acid
Often has layers, flat or curved
Usually composed of pieces cemented or pressed together
Has great colour variety
Particle size may be the same or vary
Usually has pores between pieces
May have cross-bedding, mud cracks, worm burrows, raindrop impressions
Here's a checklist of some common sedimentary rocks and their characteristics. Look for these same
characteristics in the rocks you find, and you'll be well on your way to becoming a rockhound.
Some Common Sedimentary Rocks
Name Colour Composition
Sandstone Red or Grey Sand grains cemented together
Limestone White to Grey Calcite and sometimes fossils
Shale Dark Grey Compacted mud
Conglomerate Different Colours Rounded cobbles and pebbles cemented together

5. Formation
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 form a sort of glue that sticks or cements 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.
These are the different processes in order:
Sedimentation → compaction → cementation

9. The oldest type of all rocks is the igneous rock (IG nee us). The word "igneous" comes from a Greek word
for fire. Deep inside the earth, the temperature is very high and the minerals there are in liquid form called
magma. As the magma pushes towards the earth's surface, it starts to cool and turns into solid igneous
rock.
All igneous rocks do not cool the same way. That is why they do not look all the same. Some cool
slowly, deep under the earth's surface. These are called intrusive igneous rocks. The slow cooling formed
rocks with large crystals. Granite is an example of a rock that cooled slowly and has large crystals.
Other rocks formed when the magma erupted from a volcano or reached the earth's surface through long
cracks. Magma is called lava when it reaches the earth's surface. Lava cools quickly and forms rocks with
small crystals. They are called extrusive igneous rocks. Basalt is an example of this type of rock. Obsidian
is an example of another extrusive igneous rock that cooled so fast that it has no crystals and looks like
shiny, black glass.

10. Below is a summary of the major characteristics of igneous rocks.
Classified by texture and composition , Normally contains no fossils , Rarely reacts with acid , Usually
has no layering , Usually made of two or more minerals, May be light or dark coloured , Usually made of
mineral crystals of different sizes , Sometimes has openings or glass fibres , May be fine-grained or glassy
(extrusive) ,
Here's a checklist of some common igneous rocks and their characteristics. Look for these same
characteristics in the rocks you find, and you'll be well on your way to becoming a rockhound.
Some Common Igneous Rocks
Name Colour Texture
Granite Pink/Grey Intrusive
Gabbro Dark Grey to Black Intrusive
Rhyolite Light Pink or Grey Extrusive
Basalt Dark Grey to Black Extrusive
Obsidian Usually Dark Extrusive
Scoria Dark Extrusive

11. Heat and pressure can change many things. They can even change rocks. The name for rocks that has been
changed is metamorphic (met uh MOR fik) rocks. Metamorphic comes from Greek words meaning "change"
and "form".
Metamorphic rocks form deep in the earth where high temperature, great pressure, and chemical
reactions cause one type of rock to change into another type of rock. Metamorphic rocks begin to form at
12-16 kilometers beneath the earth's surface. They begin changing at temperatures of 100 degrees Celsius
to 800 degrees Celsius. If you squeeze and heat a rock for a few million years, it can turn into a new kind
of rock.
Where does the heat come from? The heat comes from magma. Where does the pressure come from? The
pressure comes from layers of rock piled on top of layers and layers of rock. The layers on the bottom get
squeezed. The thicker the layers, the more pressure there is.

12. Some examples of how metamorphic rocks were changed: sandstone becomes quartzite, shale becomes
slate and limestone becomes marble.
Below is a summary of the major characteristics of metamorphic rocks.
Classified by texture and composition ,rarely has fossils, may react with acid , may have alternate bands
of light and dark minerals , may be composed of only one mineral, ex. marble & quartzite , may have
layers of visible crystals , usually made of mineral crystals of different sizes, rarely has pores or openings
, may have bent or curved foliation
Here's a checklist of some common metamorphic rocks and their characteristics. Look for these same
characteristics in the rocks you find, and you'll be well on your way to becoming a rockhound.
Some Common Metamorphic Rocks
Name Colour Texture
Gneiss Pink/Grey Foliated
Marble Light Coloured Unfoliated
Quartzite Light Coloured Unfoliated
Slate Dark Grey to Black Foliated

14. Physical weathering is caused by physical changes 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 a rock gets cold it 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 can also wear away rock over long periods of time. Freeze-
thaw Water expands slightly when it freezes into ice. This is why water pipes sometimes
burst in the winter. You might have seen a demonstration of this sort of thing at school -
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 rock freezes again, 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.

15. 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 made of a mineral called calcium carbonate. When acidic rainwater falls on
limestone or chalk, a chemical reaction happens. New soluble substances are formed in the reaction. These
are washed away and the rock is weathered.
Chemical weathering can hollow out caves form and make cliffs fall away.
Some types of rock are not easily weathered by chemicals. For example, granite and gabbro are hard rocks
that are weathered only slowly. Still some of their minerals do react with the acids in rainwater to form
new, weaker substances that crumble and fall away.
Acid rain
When fossil fuels such as coal, oil and natural gas are burned, carbon dioxide and sulphur dioxide escape
into the air. These dissolve in the water in the clouds and make the rainwater more acidic than normal.
When this happens, we call the rain 'acid rain'.
Acid rain 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. Acid rain also kills trees
and fish.

16. Plants can cause biological weathering. Animals and plants can wear
away rocks. This is called biological weathering. For example,
burrowing animals such as rabbits 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. 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.

18. Sedimentation creates layers or rock particles
Compaction and cementation presses the layers and sticks the particles together. This creates
sedimentary rock.
Rocks underground that get heated and put under pressure are changed into metamorphic rock.
Rocks underground that get heated so much they melt turn into magma. Magma is liquid rock.
Magma also comes from deeper inside the Earth, from an region called the mantle.
Pressure can force magma out of the ground. This creates a volcano. When the magma cools it turns
into solid rock.
Magma that cools underground forms solid rock called intrusive igneous rock.
Areas of rock can move slowly upwards, pushed up by pressure of the rocks forming underneath.
This is called uplift.
Weathering breaks down rocks on the surface of the Earth. There are three types of weathering -
physical, chemical and biological.
Wind and water move the broken rock particles away. This is called erosion.
Rivers and streams transport rock particles to other places.
Rock particles are deposited in lakes and seas, where they build up to form layers. This starts the
process of sedimentation which will create sedimentary rock.