Chapter 8 plate tectonics

Internal structure of the Earth
• The Earth has three concentric layers
Chapter 8: Plate Tectonics

1. Core
• Innermost layer
• About 3,500km thick in radius
• Divided into inner layer and
outer layer
• Has the highest temperature
• Consists mainly of iron and
nickel
2. Mantle
• About 2,900km thick
• Rich in iron and magnesium
• A mixture of solid rock and
molten rock
• The molten portion is known as
magma
• Convection currents occur here
due to the heat from the core

3. Crust
• Outermost layer
• Ranges from 5 to 65 km thick
• Two types of crust – continental
and oceanic
• Continental crusts are:
– less dense
– composed mainly of granite
– rocks are rich in silicates and
aluminium (SIAL)
• Oceanic crusts are:
– denser
– rocks are rich in silicates and
magnesium (SIMA)
• According to the generally accepted
Plate Tectonics Theory, the crust is
broken into several crustal or
tectonic plates which are shifting
due to convection currents in the
mantle

Tectonic plates and their boundaries

Before the Plate Tectonics Theory
• Idea that all the continents once formed a supercontinent
now called Pangaea (meaning ‘all land’)
• The continents eventually drifted apart and this
supercontinent was split up
• In 1912, German geographer Alfred Wegener proposed
the Theory of Continental Drift
• According to his theory, the continents floated over
denser rock

250 million years ago 180 million years ago

Evidence for Continental Drift Theory
• The west coast of Africa fitted with the east coast of
South America
• Matching reptile fossils in Africa and South America
• Presence of coal, formed in warm wet conditions, under
the Antarctic ice cap
• Rocks of the same age and geological structure formed
in southwest Africa and southeast Brazil
• Presence of the same fossil fern in all the southern
continents

Evidence for Plate Tectonics Theory
1. Mid-oceanic ridges
• Presence underwater chains of mountains and volcanoes
running along the central part of major oceans
• Hot magma constantly rises through a gap in the crust called
a rift and forms new crust upon cooling
• Submarine volcanoes form occasionally
• Rocks nearer the rift are newer while those further away are
older
• The sea floor is composed of rocks younger than the earth,
an indication of sea floor renewal
• The Atlantic Ocean is growing wider at a rate of 2cm per year

2. Ocean trenches
• Long, narrow and deep depressions
along the sea floor
• Mark the boundaries where the sea
floor suddenly plunges to great
depths
• Marianas Trench off the Philippines
is the world’s deepest ocean trench
with a depth of 11,033m
3. Island arcs
• Chains of islands of volcanic origin
• Develop along the margins of
continents
• Usually parallel to the coast
Peru-Chile
Trench

4. Volcanoes
• Found where the sea floor is spreading or near subduction zones (i.e. where
one plate moves under another one)
• They can be in the oceans or on continental surfaces or at “hot spots”
• Concentrated in the Pacific Ring of Fire which stretches from the Andes to
California, Japan and New Zealand

5. Earthquakes
• Result from sudden movements of the Earth’s crust
• Often occur where there is frequent volcanic activity

Mechanism of plate tectonics
• Intense heat in the mantle leads to convection
currents which drive the movement of the plates
above
Continent Continent
Mid-oceanic
ridge

• The plates move very slowly with an average speed of
about 7cm per year

Types of Plate Movement
1. Divergent plate movement
• Plates move away from each other
• Heated magma rises at the rift and cools to form new crust
• Boundary where the new crust is created is called a
constructive plate boundary

• When only oceanic plates are involved, results in:
– Mid-oceanic ridges (e.g. Mid-Atlantic Ridge)
– Earthquakes
• When only continental plates are involved, results in:
– Rift valleys (e.g. Great Rift Valley)
– Earthquakes
A view of the
Great African
Rift Valley

2. Convergent plate movement
• Plates move towards each other
• Boundary is known as a destructive plate boundary
• The denser crust dips under the lighter crust and is
destroyed in the magma
• Magma rises through breaks in the plates to form volcanoes
• Folding of plates results in the formation of mountains

• When only oceanic plates are
involved, results in:
– Ocean trenches (e.g. Marianas
Trench)
– Island arcs
– Earthquakes
– Volcanoes
The Himalayas
• When only continental
plates are involved,
results in:
– Fold mountains (e.g.
Himalayas)
– Earthquakes

• When the collision is between an oceanic plate and a
continental plate, it results in:
– Ocean trenches (e.g. Peru-Chile Trench, Java Trench)
– Fold mountains (e.g. Andes)
– Earthquakes
– Volcanoes
World distribution of fold mountains

3. Transform plate movement
• Plates slide past each other
• Boundary is known as a transform or conservative plate
boundary

Case study: Puerto Rico Trench and the Lesser
Antilles
• The Puerto Rico Trench lies to the north of Puerto Rico
• It is located at a boundary where there is sliding as well
as convergence of plates
• The rising of the eastern border of the Caribbean Plate
forms the Lesser Antilles
• The Caribbean Plate slides eastwards at its northern
boundary with the North American Plate
• At its eastern border, the Caribbean Plate overrides parts
of the North American and South American Plates
• This subduction zone explains the formation of the island
arc of the Lesser Antilles with its active volcanoes
• The trench is the deepest part of the Atlantic Ocean
(9,219m) and is poorly studied because of its depth

• A transform fault is formed at the boundary, e.g. the San
Andreas Fault in California, USA
• Earthquakes are a common occurrence along transform
boundaries
San Andreas Fault

Chapter 8 plate tectonics

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Chapter 8 plate tectonics