Download 1-Movement of Crustal Plates - Fellows

Topic 3-Tectonic Impacts
This is topic very similar to ‘Dynamic
Earth’ however you will look at the
tectonic theory in more detail.
Topic Overview
There are 6 parts to this Topic:
1. Movement of Crustal Plates
2. Mountain Building
3. How Australia has changed
4. Natural Disasters-Volcanic Eruptions
5. Natural Disasters-Earthquakes
6. Plate tectonics and Climate
Topic Outcomes
 Describe the lithospheric plates and their motion
 Explain how the movement of plates results in mountain building
 Explain how continents grow and change shape as plate
boundaries move and change
 Describe how natural disasters are often associated with tectonic
activity like earthquakes and volcanoes
 Explain how environmental conditions caused by tectonic activity
may contribute to the problems experienced by people
 Explain the link between plate tectonics and climate, both in the
short term and throughout geological history.
Part 1-Lesson 1
Movement of Crustal Plates
DOT Points:
•Describe the characteristics of lithospheric plates
Introduction
In the last topic you studied: Dynamic Earth, you studied plate
tectonics and the forces which drive them. These plates are
the top part of the lithosphere.
Introduction
 Scientists studying the Earth must have a knowledge and
understanding of crustal plates and their motion. This
understanding helps predict the locations of earthquakes,
volcanoes and valuable minerals.
Introduction
 The movement of crustal
plates also helps us explain
the current distribution of
plants and animals and
allows us to predict
possible future effects of
climate change.
Crustal Plates
 We know the Earth’s crust is not a single layer like the shell
of an egg but broken into at least 23 pieces like a jigsaw
puzzle.
Crustal Plates
 The difference between our crust and a jigsaw puzzle is the
pieces of our crust are moving, so it’s much more complex.
Each of these pieces are called plates and are moving at a rate
of about 5cm per year.
Crustal Plates
 Seven of these plates are very large, some are small. Some
thick and others thin. It’s a very complex system to try and
understand.
Crustal Plates
The West African coastline
seems to fit nicely into the
east coast of South
America.
The fit is even more striking
when the submerged
continental shelves are
compared rather than the
coastlines.
The Theory of Continental Drift
Alfred Wegener (1880-1930)
noticed the same thing and
proposed that the
continents were once
compressed into a singe
protocontinent which he
called Pangea (meaning ‘All
lands’) and over time they
have drifted apart.
The Theory of Continental Drift
Wegener’s Hypothesis lacked
a geological mechanism to
explain how the continents
could drift across the
Earth’s surface as he had
proposed.
In 1929 Arthur Holmes
elaborated one of
Wegener’s many
hypothesis:
The Theory of Continental Drift
Arthur Holmes stated that the mantle undergoes thermal
convection. And that this thermal convection was like a
conveyor belt and that the upwelling pressure could break
apart a continent and then force the broken continent in
opposite directions carried by the convection currents.
The Theory of Continental Drift
Not until the 1960’s did Holmes’ idea receive any attention.
Greater understanding of the ocean floor and the discoveries
of features like mid-ocean ridges, geomagnetic anomalies
parallel to the mid-ocean ridges, and the association of island
arcs and oceanic trenches occurring together and near
continental margins, suggested convection might indeed be at
work.
The Theory of Continental Drift
To understand this theory better we need to look at the
geological processes taking place and the evidence which
supports it.
We’ll start by looking
at the crust.
Earth’s Crust
The crust covers the mantle and is the Earth’s hard outer shell,
the surface we are living on .
Compared to the
other layers the
crust is much
thinner.
Earth’s Crust
The crust is made up of solid material but this material is not
the same everywhere. There is an oceanic crust and a
continental crust.
Crustal Plates
 Oceanic Crust is much
thinner than continental
crust. It ranges from 3-15
km thick. The average is
about 3.7km. Oceanic
crust is very difficult to
study, why do you think
this might be true? How
do you think this is done?
Crustal Plates
 The composition and vertical structure of the ocean floor can
be studied in a number of ways. For example:
 seismic waves generated by earthquakes allow the layered
structure to be determined.
 Boreholes have been drilled through the sedimentary upper
layers to study the basaltic rock beneath. These samples are
recovered and analysed in the lab.
 Ships can dredge the floor to collect samples and bring them to
the surface to be studied.
 Deep sea submersibles can visit the ocean floor allowing
geologists to carefully select and study samples
Crustal Plates
 Another way to study the oceanic crust is to find samples on
the surface of Earth. For example Iceland sits on a midocean-ridge and has allowed scientists to study lava produced
there.
Crustal Plates
 Oceanic crust is produced at mid-ocean ridges where molten
magma rises to the surface and solidifies to form dark Mafic
Rocks on contact with the ocean water. Mafic rocks are
dark coloured igneous rocks that have high concentrations of
ferromagnesian minerals. These rocks are low in silica and
when molten flow smoothly. Volcanic eruptions are classified
as gentle. Rocks include Basalt and Gabbro.
Crustal Plates
 The oldest rocks found on the ocean floor are about 175
million years old (myo). This is the maximum time between
new floor being produced at mid-ocean ridges, its movement
away from the ridges and it being recycled into the mantle at
a subduction zone.
Crustal Plates
 Continental Crust is thicker and more complex than
oceanic crust. It’s mainly composed of Felsic Rocks. Felsic
rocks are light coloured igneous rocks with large amounts of
feldspar and quartz. These rocks are high in silica and when
molten have a low viscosity (do not flow well). Volcanic
eruptions are classified as explosive. Rocks include granites.
Crustal Plates
 Continental crust is about 40km thick but it can be 65km
thick. Where do you think it may be this thick?
 The granitic material is less dense than the gabbro material
below and ‘floats’ on top.
Crustal Plates
 Just as a ship sinks further into the water when carrying
more cargo, the continents sink further into the mantle when
carrying the weight of mountains above. This principle is
called isostasy.
Crustal Plates
 As mentioned before the plates move about 5cm per year.
How do we know this?
 If we know the oldest rocks on a plate are about 175myo we
can measure the distance to an appropriate mid-ocean ridge
and calculate the speed.
Crustal Plates
 Scientists can also study the magnetic pattern left on the
ocean floor and measure speed. As discussed in prelim, we
know the polarity of the Earth’s magnetic poles reverse every
few million years. By studying this it allows geologists to
determine plate movements.
 This method produces results ranging from 2cm at the Mid-
Atlantic Ridge and 12cm in the eastern Pacific
Crustal Plates
 Scientists also try and calculate direction. Crustal plates are
rigid and do not move in one set direction. By locating the
mid-ocean ridge and the subduction zone scientists can infer
about their direction of movement.
Crustal Plates
 The edges of plates (transform zones), also provide
information about the direction of movement. The pattern
of magnetism left on the ocean floor can also indicate if and
how the direction of movement has changed.
Crustal Plates
 Hot spots on the middle of a plate will also show the
direction of movement from the line of volcanic islands they
produce. The Hawaiian Islands are a good example of this.
Crustal Plates
 With advances in technology plate movement data is now
also coming from satellites. GPS like that used in cars, can
be used to measure the movement of continents and show
the direction of the plates.
Crustal Plates
 Satellite results are similar to those produced from magnetic
strip analysis: 1.8cm in the north Atlantic and more than
24cm near Somoa.
 The fastest plates are quite small as most have been
subducted and the slowest have no subduction zones or carry
large continents. Why might this be true?
Activity
 Complete Activity 1.1 pg 3 HSC Spotlight Text Together
Review
 Earths crust is broken into 23 pieces like a jigsaw puzzle




which move at rates of between 1-23cm per year
Oceanic Crust is much thinner than continental crust. It
ranges from 3-15 km thick.
Mafic rocks are dark coloured igneous rocks that have high
concentrations of ferromagnesian minerals.
Continental Crust is thicker and more complex than
oceanic crust; about 40km thick but it can be 65km thick..
Felsic rocks are light coloured igneous rocks with large
amounts of feldspar and quartz.
Homework
 Read Pages 1-3 HSC Spotlight Text
 Start this topics Electronic Vocabulary
 Complete DOT Point 1.1
Part 1-Lesson 2
Movement of Crustal Plates
Review
 Earths crust is broken into 23 pieces like a jigsaw puzzle




which move at rates of between 1-23cm per year
Oceanic Crust is much thinner than continental crust. It
ranges from 3-15 km thick.
Mafic rocks are dark coloured igneous rocks that have high
concentrations of ferromagnesian minerals.
Continental Crust is thicker and more complex than
oceanic crust; about 40km thick but it can be 65km thick..
Felsic rocks are light coloured igneous rocks with large
amounts of feldspar and quartz.
Introduction
 To better understand plate motions we need to understand
Earth’s layers and the areas where the plates interact.
The Lithosphere
The lithosphere is the rigid,
brittle outer layer of the
Earth. Lithos means rocks.
Above the lithosphere is
the hydrosphere and
atmosphere.
Asthenosphere
This is the section below the
lithosphere and consists of
the upper part of the
Mantle. Astheno means
weak or without strength.
It’s made up of partially
molten rock which
scientists call it’s state to be
“plastic”.
Mesosphere
Everything below the
asthenosphere: the rest of
the mantle, outer and inner
core is called the
mesosphere. Meso means
middle.
Earth’s Interior
How do scientists know so
much about Earth’s interior?
After all humans have never
even breached the crust…..
Seismologists provide us with
the information.
Earth’s Interior
Seismologists are scientists who
study earthquakes. When an
earthquake happens an
extreme amount of energy is
released. This energy travels
in waves (seismic waves)
through the Earth. By
studying the behaviour of
these seismic waves scientists
are able to estimate the
thickness and composition of
the Earth’s interior.
Earth’s Interior
As the seismic waves pass through layers of the Earth, the
energy changes speed and direction. This is due to the
materials having different densities.
Earth’s Interior
Understanding the Earth’s interior is vital to our understanding
of crustal plates and their movements. So why do we care?
Earth’s Interior
We care because this information can save lives and property by
helping to predict earthquakes and volcanoes. It also helps us
to explain the current distribution of plants and animals and
predict future outcomes.
Plate Boundaries
The Theory of Plate Tectonics builds on Wegener’s Theory of
Continental Drift. This theory states that the Earth’s crust or
Lithosphere is broken up into “tectonic plates”
Plate Boundaries
If we analyse the distribution of earthquakes around the world,
we are able to identify the locations of the plate boundaries.
Why would we associate earthquakes with plate boundaries?
Plate Boundaries
If we imagine the tectonic plates like blocks of wood, there are
three ways they can interact.
Plate Boundaries
Remember what Holmes said about convection currents? He
stated that the mantle undergoes thermal convection. And that
this thermal convection was like a conveyor belt and that the
upwelling pressure could break apart a continent and then force
the broken continent in opposite directions carried by the
convection currents.
What type of plate boundary would assimilate with Holmes’ idea?
Divergent Plate Boundary
Divergent plate boundaries are located where plates are moving
away from one another. This occurs above rising convection
currents as Holmes suggested. Mid-ocean ridges are found at
this boundary.
Convergent Plate Boundary
Convergent plate boundaries are locations where plates are
moving towards one another. The plate collisions that occur
in these areas can produce earthquakes, volcanoes and crustal
deformation. Subduction zones are found at this boundary.
There are three
different types of
convergent boundaries
which we will discuss
more later.
Transform Plate Boundary
Transform plate boundaries are locations where two plates slide
past one another. The fracture zone that forms a transform
plate boundary is known as a transform fault. Most
transform faults are found in the ocean basin and connect
offsets in the mid-ocean ridges.
Review
 The Mesosphere is everything below the asthenosphere: the rest






of the mantle, outer and inner core.
The asthenosphere is the section below the lithosphere and
consists of the upper part of the Mantle.
The lithosphere is the rigid, brittle outer layer of the Earth.
Seismologists are scientists who study earthquakes.
Divergent plate boundaries are located where plates are
moving away from one another.
Convergent plate boundaries are locations where plates are
moving towards one another.
Transform plate boundaries are locations where two plates
slide past one another.
Homework
Complete DOT Point 1.1, 1.3
Read pages 1-5 HCS Spotlight Text
Start electronic vocab list of all bold terms pg1-5 Spotlight Text
Eg:
Tectonic Impacts Vocab List
1. Plate: Large pieces of the Earth’s crust
Part 1-Lesson-3
Movement of Crustal Plates
Review
 The Mesosphere is everything below the asthenosphere: the rest






of the mantle, outer and inner core.
The asthenosphere is the section below the lithosphere and
consists of the upper part of the Mantle.
The lithosphere is the rigid, brittle outer layer of the Earth.
Seismologists are scientists who study earthquakes.
Divergent plate boundaries are located where plates are
moving away from one another.
Convergent plate boundaries are locations where plates are
moving towards one another.
Transform plate boundaries are locations where two plates
slide past one another.
Divergent Plate Boundary
This is where two plates separate. These spreading zones are where
new crust is being generated. Mid-ocean ridges mark divergent
plate boundaries.
Divergent Plate Boundary
These boundaries are some of the most active on Earth.
Volcanoes at these boundaries produce basalts low in silica.
This means the lava has a low viscosity and releases gasses
easy. Eruptions are therefore less explosive.
Divergent Plate Boundary
Lava usually erupts in long cracks in the Earth’s surface called
fissures rather than mountains. This is because the crust is
being pulled apart rather than forced together.
Divergent Plate Boundary
Rocks at divergent boundaries do not usually undergo
metamorphism. Earthquakes are shallow as the forces cause
rocks to crack and sink along fault lines.
Divergent Plate Boundary
General Composition of Igneous Rocks:
Oceanic Divergence
 Basaltic
 Low silica content
 High concentration of dark coloured mafic minerals
Continental Divergence
 Rhyolitic
 Rich in silica and felsic minerals
 Light in colour
Convergent Plate Boundary
This is where two plates come together ‘collide’. There are three
types of convergent zones:
oceanic-oceanic
oceanic-continental
Continental-continental
Oceanic-Oceanic Convergence
This type of boundary occurs between two pieces of oceanic crust.
Because no buoyant continental crust is involved we do not get large
mountain ranges.
Oceanic-Oceanic Convergence
As one plate is forced beneath the other, it is melted and a line
of volcanoes form in the same way as described in a oceaniccontinental convergent boundary.
Oceanic-Oceanic Convergence
The volcanoes that are created at this boundary form a curved
line out of the sea and scientists call these island arcs.
Oceanic-Oceanic Convergence
Examples of island arcs
include the Philippine
Islands, Japan and
Indonesian Islands.
Oceanic-Oceanic Convergence
Metamorphism occurs at
these boundaries and the
trenches formed are the
deepest on the planet. The
Mariana Trench is 11
kilometres deep. These
boundaries also have deep
and shallow earthquakes.
Ocean-ocean Convergence
Plate Boundary
General Composition of Igneous Rocks:
 Mainly Basalts
 Increase in Andesites as island arcs mature
 High silica granite and less silica gabbro intrusions can be
found within the crust
Oceanic-Continental Convergence
Because oceanic crust is more dense than continental crust, when
these two types of crust collide the oceanic crust is always forced
into the mantle (subducted).
Oceanic-Continental Convergence
Where the oceanic crust is
bent downward, trenches
form. These are the
deepest parts of the ocean
and are used to help
indicate the edge of a plate.
Oceanic-Continental Convergence
As the subducting oceanic plate is forced beneath the overriding
continental plate, the continental plate is lifted and folded
upwards producing mountains.
Oceanic-Continental Convergence
An example of a oceaniccontinental convergent
plate boundary is along the
west coast of South
America. Here we find the
Andes Mountains.
Oceanic-Continental Convergence
The Oceanic Nazac plate is slamming into the Continental
South American plate
Oceanic-Continental Convergence
If we visit this plate boundary we can see the evidence which
supports this theory.
Oceanic-Continental Convergence
Something else happens at this boundary which produces
mountains. As the oceanic crust is forced deeper into the
Earth, intense heat and pressure cause it to melt. And what
happens to materials when we heat them?
Oceanic-Continental Convergence
They rise! The oceanic plate turns into magma and rises into
the overlying continental crust. This is called a magma
plume. If this plume reaches the surface we get volcanoes!
Oceanic-Continental Convergence
A key feature to this magma is that it is rich in silica which
comes from the sediments in the oceanic crust or the melted
part of the continental crust.
Oceanic-Continental Convergence
This high silica magma makes
it more viscous (thicker)
which allows it to trap
gasses within it. Volcanic
eruptions at such
boundaries tend to be very
explosive because of the
intense heat and pressure
build up.
Oceanic-Continental Convergence
This type of silica rich and explosive eruption is called andesitic
volcanism (after the Andes Mountains).
Oceanic-Continental Convergence
But what happens if the magma never makes it to the surface?
That’s a great question! The magma that has collected within
the continental crust can slowly cool to form granite or
similar intrusive igneous rocks.
Oceanic-Continental Convergence
Eventually the surrounding material is eroded away and we can
see these frozen granite plumes today on the surface of the
earth.
Oceanic-Continental Convergence
Stone Mountain in Georgia (USA) is an example of just how big
these frozen granite magma plumes can get.
Oceanic-Continental Convergence
So what happens to the rocks on the continental crust that
come into contact with these magma plumes?
Oceanic-Continental Convergence
They are changed ‘metamorphosed’ into a different rock. This
is how we get metamorphic rocks.
Oceanic-Continental Convergence
When these rocks are uncovered due to weathering and
erosion, we can see where this contact metamorphism has
happened.
Oceanic-Continental Convergence
Oceanic-continental convergence boundaries also experience a
lot of seismic activity. Earthquakes are quite common as the
two plates are crushing into each other.
Oceanic-Continental Convergence
An earthquake’s focus is the exact point where the rocks in the
crust break or move. At a subduction zone, scientist can
measure both deep and shallow
earthquakes. This
provides further
evidence that the
oceanic plate is being
forced into the mantle.
Ocean-continental Convergence
Plate Boundary
General Composition of Igneous Rocks:
 Andesites are most common
 Rhyolites and basalts can be found
 High silica content
 Intrusions of high silica granite and low silica gabbro can be
found within the crust
Continental-Continental
Convergence
This type of boundary is where two continental pieces of crust slam
into each other. This boundary is different than the last because there
is not complete subduction.
Continental-Continental Convergence
This is because the continental crust is so buoyant. Instead the
plates become intensely folded and uplifted. An example of
this is the Himalayas.
Continental-Continental Convergence
Earthquake foci are shallow at this boundary and there is no
contact metamorphism. Instead there is regional
metamorphism. Low heat, high pressure.
Continent-continent Convergence
Plate Boundary
General Composition of Igneous Rocks:
 No volcanic rocks
 Granite intrusions common (high silica content)
 Remnants of oceanic crust (basalts) found here
Review
 Divergent plate boundaries are located where plates are
moving away from one another.
 Convergent plate boundaries are locations where plates
are moving towards one another.
 Transform plate boundaries are locations where two
plates slide past one another.
Homework
Complete DOT Point 1.2, 2.1.1, 2.2
Part 1-Lesson 4
Movement of Crustal Plates
Two Theories
There are two hypothesis to explain the driving force behind plate
tectonics.
1.
Plates are carried and moved on top of the mantle by convection
currents within this plastic molten material.
2.
Plates are carried and moved by a combination of forces; ridgepush and slab-pull acting under the force of gravity. The push
results from the weight of crust high on mid-ocean ridges. The
pull from the weight of denser crust as it sinks into the mantle.
Video
Convection Currents
Convection currents within
the mantle generated by
radioactive decay in the
core, cause molten material
to rise towards the crust.
Convection Currents
As this molten material reaches the crust it cools and moves
horizontally dragging the crust with it.
Convection Currents
As the molten material drags the lithosphere horizontally and
cools, it begins to sink back into the mantle. The lithospheric
plate is carried with it.
Convection Currents
The plate is recycled into the molten mantle and the convection
cycle starts again.
Activity
 Discuss questions in Activity 1.2 pg 5 HSC Spotlight Text
 Complete ‘To Think About’ pg 7 HSC Spotlight Text
 Students split into groups, each group gets a question and 3
minutes to discuss. Groups then present their answer to the
class. Complete all questions this way.
Homework
Read pages 5-6 HSC Spotlight Text
Complete DOT Point 1.4-1.5