Unit 1 - People and
the Physical World
Plate Tectonics
Plate Tectonics
Plate Tectonics ‘ABC’
A. Where do earthquakes and
volcanoes occur?
B. What are the causes and effects
of earthquakes and volcanic
eruptions?
C. Why people live in areas of crustal
instability?
A. Where do earthquakes and
volcanoes occur?
a. What is the structure of the Earth?
b. What are tectonic plates?
c. What is tectonic plate theory?
d. How do tectonic plates move?
e. What are plate boundaries?
f. Where do earthquakes and volcanoes
occur?
a. What is the structure of the
Earth?
Structure of the Earth
The earth is divided into 4 layers:
1. Inner core
A solid ball of nickel and iron. The core is very
hot due to radioactive decay.
2. Outer core
A layer of molten (liquid) rock.
3. Mantle
A layer of semi-molten rock called ‘magma’.
4. Crust
A layer of solid rocks at the Earth’s surface.
Structure of the earth
b. What are tectonic plates?
The Earth’s crust is not a continuous
layer of rock.
It is broken into slabs of solid rock
called tectonic plates.
There are about 15 of these plates
that fit together like pieces of a
jigsaw.
The 8 large tectonic plates are:
1. Eurasian Plate
2. Indo-Australian Plate
3. Pacific Plate
4. North American Plate
5. Nazca Plate
6. Antarctic Plate
7. South American Plate
8. African Plate
1
4
3 8
5
2 7
6
The 7 smaller tectonic plates are:
9. Philippine Plate
10. Juan de Fuca Plate
11. Cocos Plate
12. Caribbean Plate
13. Scotia Plate
14. Arabian Plate
15. Indian Plate
10
12
9 14
15
11
13
Solid rocks on the Earth’s surface
are less dense (lighter) than the
liquid rock beneath.
This means that tectonic plates are
able to ‘float’ like rafts on the
molten rock in the mantle.
There are actually two types of
crust.
1
Continental crust is very old and was
formed over 1,500 million years ago.
2
Oceanic crust is much younger and
was formed less than 200 million
years ago.
1 Thicker but lighter (less dense)
2 Thinner but heavier (more dense)
Continental crust is thicker than oceanic
crust but it is less dense.
If continental crust moves towards
oceanic crust, the oceanic crust will
be forced downwards (subducted)
into the mantle because it is more
dense (heavier).
When oceanic crust sinks into the
mantle it melts and is destroyed.
If continental crust moves towards
cont continental crust, neither plate
will be subducted because they are
the same density.
Instead the plates crumple or buckle
to make ‘fold mountains’ (e.g. the
Himalayas, the Alps).
Oceanic crust does sink back into the
mantle. It is constantly being destroyed
and replaced.
Continental crust never sinks back into
the mantle so it is never destroyed.
c. What is plate tectonic
theory?
The theory of plate tectonics is based
upon the idea of ‘continental drift’. What
evidence is there that the plates were
once joined but continue to move?
The continents ‘fit’ together like pieces of
a jigsaw.
Volcanic rocks found on the east of South
America and the west of Africa are the
same.
Fossils of animals and plants that lived in
the same place can be found on both
continents.
Animals and plants with the same
ancestors live on different continents
today.
d. How do tectonic plates
move?
The Earth’s core is very, very hot
(over 5,000°C).
Ridge
Plate
Trench
Trench
Mantle
Convection currents
1. Heat rises upwards and creates convection
currents in the mantle.
2. Convection currents cause the mantle to
move very slowly.
3. Tectonic plates float on top of the mantle.
4. As the mantle moves it drags tectonic
plates along with it.
e. What are plate boundaries?
The place where two tectonic plates meet
is called a plate boundary (or plate margin).
These places are also called ‘active zones’
because this is where most tectonic activity
occurs.
There are 4 different types of plate
boundary.
Oceanic crust
Continental crust
1
2
3
4
‘Diverging’ means to move apart.
1
‘Converging’ means to move towards.
2
3
1 Constructive plate boundary
Two oceanic plates are being forced apart or
are ‘diverging’.
New oceanic crust is being constructed.
2 Destructive plate boundary
An oceanic and a continental plate move towards
each other (they are converging).
Oceanic crust is being forced down (subducted)
by continental crust and is destroyed.
3 Collision plate boundary
Two continental plates collide.
Neither plate is forced downwards (subducted).
4 Conservative plate boundary
fast
slow
Two continental plates move in opposite
directions or at different speeds.
Crust is neither constructed or destroyed.
1. Constructive plate boundary
Mid-ocean ridge
New crust is formed
E.g. North American E.g. African
1. Constructive plate boundaries
Oceanic plates diverge (move apart)
Lava erupts to fill the gap.
New crust is formed on the ocean
floor.
Underwater volcanoes form mid-ocean
ridges and islands.
Gentle volcanic eruptions and
earthquakes
2. Destructive plate boundaries
Oceanic and continental plates
converge (move together)
Denser oceanic plate is subducted
(forced downwards) by the continental
plate.
Oceanic plate is melted in the mantle.
Lava erupts to form explosive
volcanoes.
Powerful earthquakes occur as the
plates converge.
3. Collision plate boundaries
Continental plates converge (move
together)
Neither plate sinks into the mantle.
Plates buckle to form fold mountains.
There are powerful earthquakes but
no volcanoes.
4. Transform plate boundaries
Tectonic plates move in opposite
directions or at different speeds.
Friction locks the plates together.
Pressure builds up until the rock
‘snaps’ along a fault.
Violent earthquakes occur when the
plates move suddenly.
f. Where do earthquakes and
volcanoes occur?
Most of the world’s earthquakes and volcanoes
occur along plate boundaries.
Most volcanoes occur in narrow belts or are
grouped together in small clusters. One belt runs
all the way round the edge of the Pacific Ocean
and is called the ‘Ring of Fire’.
Earthquakes also occur in narrow belts and are
most common at the edge of plates.
One earthquake belt goes down the middle of the
Atlantic Ocean. Another follows the west coast
of the North and South America and goes all the
way round the edge of the Pacific Ocean to New
Zealand.
A. Where do earthquakes and
volcanoes occur?
a. What is the structure of the Earth?
b. What are tectonic plates?
c. What is tectonic plate theory?
d. How do tectonic plates move?
e. What are plate boundaries?
f. Where do earthquakes and volcanoes
occur?
Where do earthquakes and
volcanoes occur?
ENDS
1.1.2 a Causes and effects of
earthquakes
Learning objectives / Revision:
1. What causes earthquakes to happen
2. The effects of earthquakes
3. How people try to protect themselves from
these effects
4. One earthquake case study
1.1.2 b Causes and effects of
volcanic eruptions
Learning objectives / Revision:
1. What causes volcanic eruptions to happen
2. The effects of volcanic eruptions
3. How people try to protect themselves from
these effects
4. One volcano case study
1.1.3 Why people live in areas
of crustal instability
Learning objectives / Revision:
1. Why many people still live in areas which may
have earthquakes or volcanic eruptions
(To be covered as part of case studies)
4
1
10
12
9 14
15
11
8
3 5
2 7
13
6
PLATE BOUNDARIES
DIVERGENT CONVERGENT Conservative
(away) (towards) (sideways)
1. CONSTRUCTIVE 2. DESTRUCTIVE 3. COLLISION 4. PASSIVE
Volcanoes +
Volcanoes Earthquakes Earthquakes
earthquakes
Mid-ocean ‘Fold’ mountains,
ridges + e.g. Andes, ‘Fold’ mountains, Fault lines, e.g.
Islands, e.g. + island arcs, e.g. e.g. Himalayas San Andreas
Iceland Japan
Mt. St. Helens; Gujarat California
Heimay
Kobe