Download Where plates meet

Where plates meet
he movement of the Earth’s plates has changed the face of the Earth.
Super-continents have formed and split, and smaller continents have
spread across the globe. Plate movement is also closely linked to other events
common on Earth — such as volcanism and earthquakes.
T
Looking closely at this map of the world,
you’ll discover that the majority of volcanic
activity and earthquakes occur in certain
zones around the world. The zones also
mark the edges of the Earth’s plates. This
is because earthquakes and volcanic
eruptions are often created by movement
at the edges of the plates.
Movement at plate junctions can involve
plates:
• moving apart
• colliding together
• sliding past each other.
It is also possible for a combination of
these movements to occur.
Spreading plates
Where plates diverge, magma from the
mantle spews out onto the Earth’s surface
and forms a ridge of volcanoes. Two very
long volcanic ridges of this type are found
under the Atlantic and Pacific oceans. The
Mid-Atlantic Ridge spans the length of the
world, from Pole to Pole. The East Pacific
Rise runs from Mexico to the South Pole.
We often don’t feel the explosions from
these underwater volcanoes because the
oceans absorb the effect. However,
scientists know that some tsunamis are the
result of underwater volcanic activity.
Mid-ocean
ridge
Plate boundary
Earthquake zone
Volcano
Earthquake
N
0
3000 km
This map shows the location of major volcanic activity and earthquakes
around the world. The large earthquake and volcanic zone around the
Pacific Ocean is known as the ‘Ring of Fire’.
Colliding plates — subduction zones
The edges of some plates move towards each other. Plates that
collide with each other are called converging plates. If the edge of
one plate is made from oceanic crust and the edge of the other plate
is continental crust, the oceanic crust sinks under the continental
crust. The areas where this happens are called subduction zones.
Deep ocean trenches form along subduction zones.
When plates sink under each other, huge amounts of heat
and friction are produced. The heat and friction result in
earthquakes and volcanic activity.
Oceanic crust melts as it sinks under continental crust.
Magma bursts through the crust to form a volcano.
An example of this type of plate
boundary is found along
the western coastline of
South America.
A mid-ocean ridge forms when magma spews to
the surface between plates that are diverging
(moving apart).
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Science Alive for VELS Level 5
A sea floor in the air
Sliding plates
California, in North America, experiences many
earthquakes. The earthquakes there are caused by the
edges of two plates sliding past each other. The plates
do not slide smoothly; they get jammed together and
the pressure builds up. Suddenly, the
plates jolt past each other
and start sliding
again.
Fossils of ancient sea creatures can be found at
the top of the Himalayas, thousands of metres
above sea level. How did they get there?
If you look at a map of the world, you will
notice that India is joined to Asia. But that was
not always the case. India has been moving
towards Asia since it broke away from Gondwana
millions of years ago. At first, seas separated the
two lands. But now, the Indo–Australian Plate
and the Eurasian Plate have collided. The
current edges of these plates are both made of
continental crust, so one plate will not easily
slide under the other. Instead, the two are
crumpling against each other, forming the
Himalaya Mountains. Sediments that once lay at
the bottom of the sea between the two
landmasses have been forced upward and can be
found at the peaks of the mountain range.
REMEMBER
1. Give an example of where diverging plates are
found on Earth.
2. Name a place where two plates are sliding past
each other.
3. On which plate is India found?
4. With what type of plate boundary are ocean
trenches associated?
THINK
5. Why do you think that earthquakes are felt in the
region surrounding the Himalayas?
Asia
DESIGN AND CREATE
6. Make a model of converging continental crust.
Use two piles of paper to represent the two
sections of crust. Push the two piles of paper
together.
(a) What happens at the point where the paper
piles meet?
(b) Describe how this relates to the way the
Himalayas have formed.
(c) What would you expect the rocks in a
mountain range that forms this way to look
like?
India
Himalayas
India
The Himalayas have grown about three kilometres higher
over the past three to four million years.
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14. Geological processes
✓ learning
Che
There is little volcanic activity near the
Himalayas because there is not much rock
melting occurring at the collision zone.
However, the region surrounding this collision
zone does experience many earthquakes.
oint
ckp
I CAN:
list three types of plate boundary:
spreading, sliding and colliding
discuss causes of volcanic
activity and earthquakes in terms
of the movement of the Earth’s
plates.
Stressed rocks
t’s hard to imagine the size of the forces that are responsible for the movement of
the Earth’s plates. These forces are so large they can effortlessly snap rocks or
fold them into intricate patterns. Broken and folded rocks can be found near the
edges of colliding plates, but they are also found in areas far from plate boundaries.
I
Faulting
Strike–slip fault
Strike–slip faults are those where
rocks move sideways along a fracture.
A large-scale, strike–slip fault is the
San Andreas Fault, near California.
This fault is the result of two of the Earth’s
plates sliding past each other. You can
recognise smaller strike–slip faults by
looking at surface rocks from above. The
rocks on either side of the crack will be
mismatched.
Spencer
Gulf
A
B
nc
e nt
Yorke
Peninsula
Adelaide
Vi
A fault is a fracture in a rock. The
sections of rock on either side of the
fracture move upward, downward
or sideways. Large faults can extend
as far as several kilometres along
the crust. However, some faults are
much smaller. Major faults appear
near plate boundaries, but smaller
ones are found throughout the
Earth’s plates. Cracks can appear in
rocks that experience sudden and
strong movements, such as during
an earthquake. Rocks tend to fault
in three ways: strike–slip, reverse
and normal.
The Gulf region of South
Australia was formed by a
series of faults.
Gulf S
t
Kangaroo Island
B
Gulf St Vincent
Yorke
Adelaide
Peninsula
A Spencer Gulf
Normal fault
The rocks on one side of a
normal fault slip downwards.
Normal faults appear in rocks
that have been stretched.
Large-scale normal faults
create long cliff faces. Look
for small-scale normal faults
along road cuttings. They
appear as a crack in the rock,
with mismatched layers on
either side of the crack.
Reverse fault
The rocks on one side of a reverse fault have been forced upwards.
Reverse faults appear in rocks that have been squeezed together. Large-scale
reverse faults create mountains with one steep side. Subducting plates can
cause this type of fault. An example is the Japan Alps.
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Science Alive for VELS Level 5
Folding
Not all movements that occur on Earth are sudden, like earthquakes.
The slow movement of the Earth’s plates causes rocks to fold rather
than fault. When rocks are squeezed, they may soften and bend without
breaking. Folded and buckled rocks form hills and mountains. Near
the edges of converging plates, folds and faults create massive mountain
ranges. Examples include the Himalayas and the European Alps.
Small-scale folding is more likely to form hills.
Anticline
that occur in rocks.
2. What type of fault is created
when a rock is pulled apart?
3. Give an example of a
Overturned rock
landform that has been
created by a fault.
4. Give an example of a
landform that has been
created by folded rocks.
5. Draw a diagram that
explains the difference
between an anticline and a
syncline.
Rocks that have been
squeezed under extreme
pressure can fold so much
that they overturn. This
type of fold is common
where two plates collide,
as the pushing force is
very strong.
Syncline
A downward bend in a rock
is called a syncline.
REMEMBER
1. Describe three types of fault
THINK
6. Is an earthquake more likely
An upward fold in a rock
is called an anticline.
to create folds or faults in
rocks? Explain your answer.
DESIGN AND CREATE
7. Make three separate cakes,
Modelling folds
each a different colour. Cut
slabs of each cake and use
cream to ‘cement’ the
different coloured layers
together. Use the layered
cake to model the
three types of
fault. Draw a
diagram of how
you used the cake
to model the
faults. On the
diagram, use
arrows to
represent the direction the
slabs of cake moved to
create the fault. (Note:
Layered plasticine or pieces
of foam work equally well.)
Rocks are usually folded well below the Earth’s surface. The
anticlines and synclines can be seen only along road cuttings or
where erosion has exposed the layers of rock. A model is a useful way to
describe how folded rocks would appear under the surface.
You will need:
3 or 4 pieces of differently coloured plasticine
knife or blade
board.
• Roll the pieces of plasticine into 1 cm thick layers.
• Place the layers of coloured plasticine on top of
each other. Press down lightly on the layers, so
that they stick together.
• With the palms of your hands, very gently
compress the layers from the side.
1. Describe the appearance of the plasticine when the layers are
compressed.
2. Draw a diagram of the plasticine after compression, labelling anticlines
and synclines.
INVESTIGATE
8. Find out what horst and
3. Imagine that the rock layers are eroded at the Earth’s surface.
How could this process be represented on your plasticine model?
graben are. (Hint: They are
related to faults.)
4. Draw diagrams of the eroded layers when viewed from above and
when viewed from the side. Label the oldest and youngest layers.
(Remember that the oldest layers are deposited before the younger
ones.)
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14. Geological processes
✓ learning
• Model the processes of weathering and erosion on your plasticine
layers.
I CAN:
explain the geological terms ‘fault’
and ‘fold’
describe the types of fault and fold
give examples of landforms
created by folds and faults.