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Plate Tectonics 1
The Structure of the Earth
The structure of the Earth
The Earth is divided into three layers—the crust, the
mantle, and the core—based on the compounds that
make up each layer.
These are
compositional
layers !
What is the internal structure of the earth?
The Crust
Outermost layer of the Earth.
The crust is 5 to 100 km thick, and is the thinnest layer of the
Earth.
There are two
types of crust—
continental and
oceanic.
Continental crust
Continental crust is
thicker, older and lighter,
and is composed mainly
of granite.
It is 35 km thick on
average, but this increases
under mountain chains,
and is less dense than
oceanic crust.
Continental crust
Continental crust is more complex than
oceanic crust in its structure and origin and
is formed primarily at subduction zones at
destructive plate margins.
It is also very old, much of it
being over 1,500 million
years. The rocks contain
mainly silica and aluminium
(SiAl) and granites are the
most common.
Oceanic crust
Oceanic crust is younger
and heavier, and is
mainly composed of
basalt and Gabbro.
This material
contains Silica and
Magnesium (SiMa)
and is very young,
most of it being
under 200 million
years old).
Oceanic crust
It is mainly formed at constructive
margins or spreading mid ocean
ridges and tends to be between 6
and 10km thick.
It is constantly destroyed
and recreated because
of the processes taking
place along ocean ridges
and destructive plate
margins.
Ocean and Continental Crust
• Oceanic Crust
– primarily basalt
– 6-10 km thickness (thin
relative to continental
crust)
– denser (heavier) than
continental crust
• Continental Crust
– primarily granite
– 30-70 km thickness
– less dense (will not
undergo subduction)
The physical structure of the Earth
• The Earth is divided into five physical layers:
1. The lithosphere
2. The asthenosphere
These are
structural /
3. The mesosphere
mechanical
4. The outer core
layers !
5. The inner core
• Each layer has its own set of physical
properties.
The Lithosphere
Lithosphere
= Crust + Uppermost (rigid) part of the mantle
• Makes up the Earth’s rigid outer shell
• When the rocks in the lithosphere melt, hot molten
rock called magma is formed.
The Asthenosphere
Asthenosphere
= Weak, plastic
layer of the upper
mantle on which
the tectonic plates
move.
The Mesosphere
The strong, lower
part of the mantle
between the
asthenosphere and
the outer core.
It has a greater
density and is
more rigid than the
asthenosphere.
The chemical
composition is the
same (since it is
part of the mantle)
The Core
The outer core is
the liquid layer of
the Earth’s core that
lies beneath the
mantle.
The inner core is the
solid, dense centre of
our planet that extends
from the bottom of the
outer core to the
center of the Earth
What is a tectonic plate?
Plate tectonic theory says that the Earth’s
lithosphere is broken into several pieces called
tectonic or lithospheric plates.
A tectonic plate is made up
of the lithosphere (i.e.
crust + uppermost part of
the mantle).
These plates
move in
relation to one
another.
The surface of the Earth is divided into 7
major and 7 minor plates. The largest plates
are the Pacific, Eurasian, North American
and African plates.
What is a tectonic plate?
Tectonic plates contain:
oceanic crust
continental crust or
a combination of both
Those
tectonic
plates that
contain only
oceanic
crust are
called
oceanic
plates.
Tectonic plates
that contain
only / mainly
continental
crust are called
continental
plates.
What is a tectonic plate?
Some plates are large enough to
consist of both continental and
oceanic crustal portions (e.g. the
African or South American plates)
whilst the Pacific Plate is almost
entirely oceanic
Plates are on average 125 km
thick, reaching maximum
thickness below mountain
ranges.
Oceanic plates (50-100km)
are thinner than the
continental plates (up to
200km) and even thinner at
the mid-ocean ridges where
the temperatures are higher.
Why do tectonic plates move?
1. Convection currents
• Convection currents are movements of heat within
the mantle.
• Material in the mantle is heated by the decay of
radioactive isotopes in the core.
• This causes convection currents in the molten mantle
material.
• Mantle expands, rises and spreads out beneath the
plates.
• Plates are dragged along and move away from each
other.
Why do tectonic plates move?
1. Convection currents
•Subsequently, the hot molten mantle cools slightly
and sinks, pulling the plates along
•Hence plates move towards each other.
•The sinking mantle material heats up again as it
nears the core and the whole process repeats.
Why do tectonic plates move?
Plates moving away from each other
Ocean floor
Plates
Tectonic plates float on molten mantle, driven by heat
energy/convection currents
Why do tectonic plates move?
2. Slab-pull force
• This occurs when an oceanic plate (denser) subducts
under a less dense plate and pulls the rest of the
plate along.
• The subducting plate drives the downward-moving
portion of convection currents.
• While mantle material away from the subduction
zone drives the rising portion of convection
currents.
Effects of Plate Movement
• Plate movements occur very slowly, averaging
about 5 cm a year.
• However, over several hundred million years,
such movements have altered the distribution of
the earth’s land masses. (see Wegener’s theory
on Continental Drift)
Effects of Plate Movement
• Plate movement also causes plates to interact
with each other along their margins (edges).
• These interactions have resulted in many
phenomena (including earthquakes, volcanoes
and mountain building) being found along
these plate margins or boundaries.
• Differences in:
– the directions of movement and
– because some plates are continental and some are
oceanic,
• result in different types of volcanoes and different
types of earthquakes at the edges of the plates.