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Chapter 10
Plate tectonics
Unit 10.1
1
a Alfred Wegener (continental drift)
b Harry Hess (seafloor spreading)
2
That there was a Global Rift system and undersea mountains
3
A layer of ‘plastic’ semi-solid rock in the lower mantle—this ‘rock’ can flow very
slowly and is very hot.
4
a The process of the crust sinking down is called subduction.
b Seafloor spreading : new crust is being formed at the ocean ridges and
spreading outwards to form the new sea floor.
5
The first observation was that the continents seemed to fit together a bit like a
jigsaw. The second observation was that fossils of the same species were found in
continents that were a long way apart.
6
Wegener rearranged the continents to join them up so that the pieces of the jigsaw
seemed to fit but also the distribution of the fossils was continuous across the land
masses.
7
Hess proposed that new crust was being formed at the ocean ridges and spreading
outwards. This process was later called seafloor spreading. Hess proposed that the
crust was sinking down into the Earth in other places called ocean trenches. The
process of the crust sinking is called subduction. These processes of forming new
crust and destroying it are part of the theory.
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• Magnetic striping—rocks at a particular distance from the mid ocean ridge on
one side always had the same magnetic direction as rocks the same distance
away on the other side.
• The further the rocks on the seafloor were away from the ridges, the older they
are.
•
Sedimentary rock layers on the ocean floor become thicker as you move away
from the ridges.
9
New seafloor is constantly being added while continents remain at the surface,
moving around on the plates.
10 The coloured areas (green, orange and red) represent rock that was formed when
the Earth’s magnetic field was the same as it is today (i.e. north magnetic pole near
Greenland in the northern hemisphere). Magnetic minerals in these rocks will be
lined up in the rock like a compass and point to the north. The grey areas show
rock that formed when the Earth’s magnetic field was reversed, with the ‘north’
magnetic pole in Antarctica. The magnetic direction (north-seeking poles) of the
magnetic minerals in these rocks will point to the south. The layers are symmetrical
either side of the axis because the magma separates away on both sides of the
axis as it is cooling.
11 The rocks of the asthenosphere (upper mantle) are very hot and plastic, flowing like
very thick mud. The heat creates convection cells in the mantle, with warm rock
being displaced upwards and cooler rock sinking down. As the rock moves upwards
and then flows sideways beneath the crust, the plates may be dragged along by
friction, though there are different opinions on this.
12 They both propose that the continents moved about on the Earth’s surface.
Continental drift had no clear mechanism or evidence explaining its movement.
Wegener thought the continents drifted somehow. Plate tectonics proposed that the
crust was cracked into large pieces called plates—with its mechanism of seafloor
spreading and subduction, this explained the movement of continents.
13 The pattern of stripes of rock with alternating magnetic field direction that are
symmetrical on either side of the ocean ridges is called magnetic striping. Magnetic
field reversal is where the magnetic field of the Earth reverses and north becomes
south. This is preserved in the rocks.
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14 The Red Sea was formed by rifting when two tectonic plates (one on the African
side and one on the Saudi Arabian side) separated by moving in opposite
directions. The sea then entered from the north (Mediterranean) and south (Indian)
and flooded the rift valley.
15 The Great Global Rift system seems to be associated with the lighter blue colours.
These colours indicate that the land around the rifts is below sea level and perhaps
as much as 3000 metres below sea level. The rest of the oceans appear to be
deeper than this—mostly about 5000 metres or deeper. So the rift system is not as
deep as the rest of the oceans—like a series of under-sea mountains.
16 Trenches are where one plate subducts under another and ridges are where new
seafloor (crust) forms. So new crust south of Australia forms and pushes the plate
northwards where it subducts under Indonesia.
17 Tabulated answer (see below). The sketches will depend on the students’ answers.
Figures from the text in simple outline form will probably be what they do.
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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Unit 10.2
1
Converging (collision), diverging, transform
2
Oceanic and continental
3
a Mid-Atlantic Ocean ridge
b Sunda trench between Australia and Indonesia
c South Island of New Zealand
4
a Diverging boundaries—where the plates are moving apart from each other
b Converging boundaries—where the plates are colliding with each other
c Transform boundaries—where the plates are sliding past each other
5
If oceanic crust is colliding with continental crust, the denser oceanic plate sinks
under the lighter continental plate.
6
a Where a denser plate (oceanic crust) meets a lighter plate (continental crust).
Oceanic plate subducts under the continental plate.
b A trench forms where ocean crust subducts under continental crust under the
ocean.
c Where two continental crustal plates collide, the continental plate becomes
distorted, forming fold mountains and also volcanoes.
d Where oceanic plates are separating (diverging), the land subsides in the centre
and ridges form either side of the ‘crack’.
e When two oceanic plates collide, one always subducts under the other. This
forms a chain of volcanic islands (‘island arc’) as the magma formed by the
subducting plate rises up to the surface.
f When oceanic crust is colliding with continental crust, the continental plate
becomes distorted, forming fold mountains and also volcanoes, and where the
oceanic plate subducts under the continental plate, a deep ocean trench is
formed.
7
Australia had separated from Antarctica completely by about 40 million years ago.
This allowed a cold current to cool Antarctica and lock up a lot of moisture that fell
as rain in Australia. Southern Australia became drier. As Australia drifted north on
the tectonic plate, it became more arid as it entered tropical regions. This led to
flora that was adapted to dry conditions.
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Distance between land masses = 50 000 000 cm /7 cm = 7.1 million years.
9
Plates can move towards each other (converging), apart from each other
(diverging) or past each other (transform). The direction of movement of each plate
in the pair is different for each type of boundary. There can be faults called
transform faults near diverging plate boundaries.
10 When two oceanic plates collide, the faster-moving plate always subducts under
the other. This forms a deep trench. The descending plate melts, and is destroyed.
Lines of volcanic islands (called an ‘island arc’) form. When a continental plate and
an oceanic plate converge, the denser ocean plate subducts under the continental
plate.
11 India is continental crust and Asia is continental crust. The Indian Plate is colliding
with the Eurasian Plate. Because both are light crust, they have been pushed
upwards, forming fold mountains (the highest mountain range on Earth, the
Himalayas).
12 Look for fold mountains—the crust would have been pushed upwards to form a new
continent. There may be volcanic activity as well.
13 Crust is being destroyed elsewhere by subduction as fast as it is being created by
seafloor spreading.
14 Some seafloor sedimentary rocks from the Indian Plate were uplifted as it crashed
into the Eurasian plate to form the Himalayas. Mt Everest is in the Himalayas.
15 The ocean floor is continually being recycled. It is formed at the mid-ocean ridges
and is destroyed where it subducts beneath continental plates or other oceanic
plates. The rate of spread of the plates suggests that no ocean is more than about
200 million years old.
16 The movement of the tectonic plate allowed the formation of a cold circumpolar
current and this locked up a lot of moisture in Antarctica as it became glaciated.
This reduced the rainfall in southern Australia. Rainfall patterns changed elsewhere
as Australia moved northwards, as did other ocean currents, especially the warm
current down the east coast. The temperature gradually rose as Australia moved
into the tropics and rainfall declined.
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17 Tabulated answer (see below)
Unit 10.3
1
From white, yellow, orange, red to black.
2
Gas, ash, solid rock particles, lava
3
Subduction
4
Seismometer
5
A tsunami
6
If it is viscous, high in gas and water, it can become explosive as the gas pressure
builds up within it as it turns to solid.
7
a Pyroclastic flow is a cloud of ash, rock and gas at about 500°C.
b It races down the volcano like an avalanche at over 100 kilometres per hour. It is
so hot and fast that it incinerates living things, and several of the gases are
poisonous.
8
The volcanic ash rises high into the atmosphere, and aircraft can fly into it. The ash
particles melt in the jet engines and can cause them to stop.
9
Volcanoes form near the edges of tectonic plates because the collision of the plates
and especially the subduction of one creates weaknesses in the crust and also
generates a lot of heat, which can melt rock. The magma rises up through the lines
of weakness to reach the surface.
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10 Each island forms as it sits over the hot spot. As the plate moves, the island goes
with it and so no longer sits over the hot spot. A new part of the plate is now above
the hot spot and this gradually forms a new volcano. Gradually a chain of islands
(called an island chain) forms.
11 a An earthquake is the rapid movement of the ground, usually back and forth and
up and down in a wave motion.
b Seismic waves are waves in the Earth caused by earthquakes.
12 Earthquakes are caused by the rapid release of energy as the tectonic plates move.
This sudden movement sends out waves of energy through the rock and water.
13 The Australian plate is subducting under the Eurasian plate along the Sunda
trench. This creates a great deal of magma that rises to the surface along lines of
weakness and reaches the surface in the islands of Indonesia, especially Java.
14 Most earthquakes occur at plate boundaries. Because earthquakes are rapid waves
moving through the Earth, places where the Earth moves the most would be
expected to have the most severe earthquakes. Where the plates move the most is
where the most energy is generated for earth movements. Volcanoes occur at plate
boundaries because magma can reach the surface through areas of the crust
weakened by the plate movements. Subduction also generates a great deal of heat
by friction.
15 • Dampers—structures that move in opposition to the seismic waves and oppose
their effect
• Base isolation—pads, springs and bearings that allow the building to suppress
the waves by moving rather than vibrating with the Earth.
16 (45 km × 100 000 cm) divided by 5 000 000 years = 0.9 cm/year
17 Island chains form over a hot spot as the plate keeps moving and new volcanoes
form one after the other. There is only one volcano active at a time in the chain. An
island arc is a string of volcanoes, all of which are usually active at once, formed
where one plate subducts under another, often over hundreds of kilometres.
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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18 There can be explosive eruptions where oceanic plates subduct under continental
plates. Continental plates tend to have more silica in them than oceanic plates, and
this often makes the magma formed from them quite viscous. Rocks with a lot of
water in them also create a lot of steam when they are heated during subduction.
Where two oceanic plates collide, the magma is generally not as viscous due to the
lack of silica in the magma.
19 • Primary waves (P-waves) are longitudinal waves that travel fast through the
Earth
• Secondary waves (S-waves) are transverse waves that are slightly slower than
P-waves through the Earth
20 The place where the quake starts is called the focus. This may be many hundreds
of kilometres deep in the Earth. The point on the Earth’s surface directly above the
focus is called the epicentre.
Chapter review
1
Alfred Wegener was probably the first (though several earlier scientists had a
hunch that they may move).
2
Any ten such as: Australian, Pacific, Eurasian, African, South American, Antarctic,
North American, Nazca, Philippine, Arabian, Indian
3
Rocks at a particular distance from the ridge on one side always have the same
magnetic direction as rocks the same distance away on the other side.
4
a The rocky material of the crust and upper mantle
b The lower mantle, which is semi-solid and on which the plates move
5
New crust (seafloor) is being formed at the ocean ridges and spreading outwards.
The spreading eventually results in the ocean floor colliding with nearby plates and
subducting. These processes are being driven by convection currents in the mantle.
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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• Diverging boundaries—the plates are moving apart from each other. These allow
new crust to form.
• Converging boundaries—the plates are colliding with each other. These can
result in plate destruction and volcanic activity if one subducts, or in mountain
building up when both plates are pushed upwards.
• Transform boundaries—the plates are sliding past each other. These generally
do not create or destroy crust but often build mountains.
7
Earthquakes occur when tectonic plates move. The boundaries are where the
greatest movement and energy will be generated, and therefore the earthquakes
should be strongest there. Volcanoes also result from tectonic movements (apart
from the hot spot volcanoes). Most volcanoes occur at convergent and divergent
boundaries because these allow magma to reach the surface through weaknesses
created in the crust. Some extra heat is generated in subduction and this creates
extra magma. Diverging boundaries thin the crust and crack it allowing magma to
rise up to the surface.
8
The land mass of Australia has slowly been moving north since it split from
Antarctica. It has not collided with any land mass in that time. Australia is well away
from its plate boundaries and therefore has few earthquakes and no active
volcanoes.
9
Recent volcanic explosions have shown that the amount of sulfur (and therefore
sulfuric acid) generated has a large impact on global temperatures, lowering them
by reflecting sunlight. The Deccan Traps in India erupted 65 million years ago and
may have had an impact on the falling global temperatures experienced at the time.
10 The movement of the tectonic plate allowed the formation of a cold circumpolar
current and this locked up a lot of moisture in Antarctica. This reduced the rainfall in
southern Australia. Rainfall patterns changed elsewhere around Australia as it
moved northwards. Other ocean currents, especially the warm current down the
east coast, were blocked from reaching Tasmania. The temperature gradually rose
as Australia moved into the tropics and rainfall declined. This led to flora that was
adapted to dry conditions. Many species developed leaves coated with thick
waterproof wax, stomata in sunken pits, leaves reduced in size or hanging
vertically. These are adaptations to avoid losing too much water in the heat of the
day. Many of the mammals, reptiles and frogs became nocturnal and burrowed
underground to avoid the heat, thus saving on precious water that was scarce.
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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11 Ocean currents can be affected by plate movement, such as the formation of the
cold Antarctic Circumpolar Current when Australia separated from Antarctica. This
cooled Antarctica greatly and led to climatic change there where temperatures fell
and ice sheets formed. Warm currents down the east coast of Australia were
blocked in reaching Antarctica. The global overturning circulation shifts heat around
the Earth, and would have been different in the past because it would have been
unable to reach the Pacific Ocean between Australia and Antarctica.
12 Oceanic crust is dark coloured, dense and low in silica and lighter elements such as
aluminium, whereas continental crust is a lighter colour and less dense due to high
silica content.
13 In both cases one plate subducts under another, forming an ocean trench, and
volcanic activity occurs. An island arc often occurs in the ocean when two oceanic
plates converge, while on land volcanoes form in the fold mountains created by the
collision between an oceanic and continental plate. The oceanic plate always
subducts under the continental plate.
14 Converging plates create fold mountains, either where oceanic and continental
crust collide, or where continental plates meet each other. However, mountains
also form along some transform boundaries. Diverging boundaries can have
mountains formed by volcanic activity. Island arcs are mountains in the sea formed
by volcanic activity. Hotspots can result in mountains as well—shield volcanoes can
be very high.
15 Rifting on land lowers the land. As seen in the Dead Sea, this can fall well below
sea level. If the rift reaches the ocean, it will flood and the ocean area becomes
larger.
16 Wegener’s two crucial observations were that the same fossils were found on
widely separated land masses, and that some continents fitted like a jigsaw. He
could not explain a mechanism by which the continents could move.
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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17 Depends on student answers. Should cover similar points to those made in their
summaries prepared for each of units 10.1 to 10.3.
•
Mid-ocean ridges exist.
•
Magnetic striping is symmetrical either side of the ridges.
•
The seafloor is older further from the ridges.
•
Sediment thickness increase going away from the ridges.
•
The same fossils are found in land masses separated by oceans.
•
Land masses seem to fit together like a jigsaw.
•
Ocean trenches occur where one plate subducts.
•
Mountain ranges occur where plates collide.
•
Volcanic islands occur where plates collide.
•
Plates move in three different directions: converging, diverging and transform.
•
Iceland is widening by a measurable amount each year.
•
Most volcanoes occur along the boundaries of tectonic plates.
•
Hot spot volcanoes leave an island chain behind as the plate moves.
•
GPS and satellites show the plates move.
18 Diagrammatic answer (see below)
Copyright © Pearson Australia 2012 (a division of Pearson Australia Group Pty Ltd) ISBN 978 1 4425 2361 6
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Thinking scientifically
Q1 C
Q2 D
Q3 B
Q4 D
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