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Rocks
They are made in a variety
of ways, but …
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… most of them start out in the mantle
And this is how
they get out
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These rocks are Igneous rocks
Igneous rocks
are formed
when lava or
magma cools
down and
solidifies
If the lava or magma cools
QUICKLY it has SMALL crystals
If the lava or magma cools
SLOWLY it has BIG crystals
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Igneous rocks
Obsidian
Granite
Which one has
crystals you can
see? How did it
cool?
Pumice
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What are igneous rocks like?
When they were formed they were a liquid.
They were very hot rock that originally came
from the magma at the centre of the earth.
Once they escaped from the mantle they began
to cool
On the surface (where the liquid rock is called
lava) they cool quickly and like toffee do not
have crystals in it but they are very hard
If the magma pushed in between layers of other
rocks, away from the surface, they cool slowly.
They contain crystals, very hard, never contain
fossils (because there were no living things in
the mantle)
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Metamorphic rocks
Metamorphic rocks are formed by the combined effect of
heat and pressure on other rocks:
Pressure from rocks above…
…and heat from magma nearby
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Metamorphic rocks
Quartzite
I was
sandstone
I was
limestone
Marble
Slate
Did you
know I was
once clay?
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What are metamorphic rocks like?
They were made from other rocks were
acted on by heat and pressure over a long
time
Sometimes have tiny crystals, no fossils
because any remaining animal and plant
bits would have been cooked in the
process!
They always hard and sometimes arranged
in layers
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Sedimentary rocks
How sedimentary
rocks are formed:
1) Weathering and erosion
2) Transportation
3) Deposition
4) Burial
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What is the difference between
weathering and erosion?
Erosion is the wearing away of rock by
wind, water or ice (which means also that
is taken away too). We will come back to
that another day!
Weathering is the breaking down of rock
by weather, plants and animals (which
means it tend to stay where it is) – a bit
more about this in a minute.
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Sedimentary rocks
Sandstone
Limestone
Conglomerate
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Sedimentary rocks are the only ones …
… that may have plant or animal remains
trapped in them
Metamorphic and igneous rocks were too
hot or too squashed to allow them to
survive
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More about weathering …
Physical weathering
(a) Freeze-thaw weathering
Water may enter cracks in rocks.
In areas where the temperature falls
below the freezing point of water, the
water will turn into ice when it is cold
enough.
Water expands as it freezes.
The ice now exerts pressure on the
walls of the crack, pushing them
further apart and widening the crack.
Over time, the crack becomes wider and
the rock may break apart.
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More about weathering …
Physical weathering
(b) Onion-skin weathering
Large changes in temperature:
Changes in temperature cause the surface of
the rock to expand and contract more than the
interior of the rock.
This weakens the rock and causes cracks to
form.
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More about weathering …
Biological weathering
Plant roots and animal
burrows can
destabilise rocks.
Lichen that grows on
rock surfaces
gradually wears it
away.
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More about weathering …
Chemical weathering occurs when rocks are
exposed to water.
Rainfall is naturally acidic due to the carbon
dioxide dissolved in it.
Chemical reactions taking place between the
water and the rock cause the rock to break
down
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Three types of rocks
Igneous:
Metamorphic:
Sedimentary:
65%
27%
8%
Igneous:
Sedimentary:
Metamorphic:
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Summary
Igneous or
metamorphic or
sedimentary?
(e.g. sandstone,
limestone, chalk)
(e.g. basalt, granite)
(e.g. marble, slate)
How they were
formed
Small pieces of
sediment were stuck
together by salt and
pressure from rocks
above
Appearance
Usually soft, can
contain fossils, easily
eroded
Liquid rock (magma or Contain crystals, very
lava) cooled down and hard, never contain
turned back into a
fossils
solid
Other rocks were
acted on by heat and
pressure over a long
time
Sometimes have tiny
crystals, no fossils,
always hard and
sometimes arranged
in layers
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The rock cycle
The rock cycle describes the perpetual
recycling of minerals by
weathering and erosion processes on the
surface
transport and deposition processes involving
water, ice, waves or wind
rock creation and transformation involving
crust and tectonic processes
Landscape creation
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The Rock Cycle
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This is a bit easier to understand
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Rocks revision quiz
1.
What are the 3 types of rock?
2. What type of rock are the following:
Sandstone, limestone, granite, basalt, slate
3. What type of rock would a mountain probably be made out of?
4. What are the two conditions needed for a metamorphic rock to form?
5. Which metamorphic rock is formed from limestone?
6. Why wouldn’t you find any fossils in igneous or metamorphic rocks?
7. You find a rock containing crystals. What type of rock would it most
probably be?
8. What is molten rock that is inside the Earth called?
9. What is molten rock that is outside the Earth (i.e. erupted) called?
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Rocks revision quiz
10. Name one example of physical weathering.
11. Name one example of biological weathering.
12. Acid rain is an example of chemical weathering. What poisonous gas
(given off by car exhausts and power stations) causes acid rain?
13. Granite is an igneous rock and contains large crystals. Did it cool down
slowly or quickly?
14. In the rock cycle what causes igneous rock to become sedimentary
rock?
15. In the rock cycle again what causes the sedimentary rock to become
metamorphic rock?
16. Gravestones are usually made out of marble or granite instead of
limestone or sandstone. Why?
17. A piece of rock contains layers. What type of rock would it probably
be?
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