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Earth and Environmental Science
9.2 Tectonic impacts:
1. Lithospheric plates and their motion
Background: The lithosphere is an outer layer of the Earth that includes the continental crust, the oceanic crust and
the upper, most rigid layer of the mantle. The lithosphere lies above a less rigid layer of the earth called the
asthenosphere.
The lithosphere is not a uniform layer all around the globe. It consists of a series of plates, called lithospheric
plates, which ride and move on the partially molten asthenosphere.
The lithospheric plates move relative to each other. They are created at mid-oceanic ridges and destroyed at
subduction zones.
describe the characteristics of lithospheric plates

The upper layer of each lithospheric plate is composed of crust
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The crust is composed of either continental crust or oceanic crust.

Continental crust is typically made up of relatively less dense rock like granite.

The oceanic crust is typically made up of more dense rock like basalt. Oceanic crust carries sediments that
have been deposited on the oceanic floor.

Some lithospheric plates have just oceanic crust. Others have some oceanic crust and some continental crust.

Oceanic crust is generally thin, usually between 5 and10 kilometres thick. Continental crust is generally
between 25 and 50 kilometres thick. Under large mountain ranges the crust can be over 80 kilometres thick.

The lithospheric plates are up to 70 km thick, where there is oceanic crust, and up to 150 km thick, where
there is continental crust.
outline the motion of plates and distinguish between the three types of plate boundaries (convergent, divergent
and conservative)

Plates move slowly, at speeds of up to 12 centimetres per year.

Plates are created at divergent boundaries, slide past each other at conservative boundaries and are
absorbed back into the earth at convergent boundaries.

At divergent zones, the plates are moving away from each other and new oceanic lithosphere is created to fill
the gap. Divergent boundaries are usually found at the mid-ocean ridges. Some special cases of divergent
boundaries can be found in rift valleys on continental crust where the continent is beginning to divide e.g. the
African Rift Valley, the Dead Sea Rift Valley.

At conservative plate boundaries, crust is neither created nor destroyed. The plates slide past each other
along faults. Near mid ocean ridges these faults are called transform faults. On continental crust, these
boundaries are the cause of many earthquakes e.g. Alpine Fault System in N.Z and the San Andreas Fault in
California, USA.

At convergent zones, the lithosphere is consumed. This occurs when one plate, usually consisting of oceanic
material, is subducted beneath another plate. Deep ocean trenches are usually found along the continent’s
edge when subduction occurs. Quite often, the upper surface of the subducted plate is shaved off, creating
folded sediments at the edge of the overlying plate. As the subducted plate moves deeper into the
asthenosphere, it partly melts and this molten rock rises because it is less dense than the material above it,
creating magma chambers. From these magma chambers, volcanoes are produced in the overlying plate.
identify the relationship between the general composition of igneous rocks and plate boundary type

At divergent boundaries the dominant types of igneous rocks are the mafic igneous rocks like basalt, gabbro
and peridotites. Mafic rocks are dark coloured because they contain minerals richer in magnesium and iron
like olivine, pyroxene, amphibole and biotite. The mafic rocks at divergent boundaries form as a direct
upwelling of dense magmas from the asthenosphere.

At convergent boundaries the dominant types of igneous rocks are felsic rocks like andesite, rhyolite and
granite. Felsic rocks are light coloured because they contain more feldspar and quartz, minerals with
relatively more silica than the dark coloured minerals. Felsic rocks are produced from magmas with higher
water content.

Occasionally at conservative boundaries a variety of igneous rocks occur as molten rock fills cracks to form
intrusions, such as dykes and sills.
describe current hypotheses used to explain how convection currents and subduction drive plate motion
The following are three hypotheses used to explain plate motion.

Idea 1: The plates move because of convection currents in the asthenosphere which transfer heat from the
lower mantle towards the crust. As the currents move, they drag the plates with them. (shear traction).

Idea 2: The higher density of cold rock compared to that of hot rock causes the lithosphere to be dragged
by gravity (ridge-push) from the relatively high mid-ocean ridge to the subduction zone by the sinking denser
lithosphere (slab-pull).

Idea 3: A tensional force is placed on an upper plate caused by subduction of the lower plate. The subducting
zone moves away (called roll-back) from the upper plate to create secondary volcanic arcs (trench suction).
2. Mountain building
Background: Mountains are formed as a result of the interactions between plates. The types of mountains formed
depend on the type of plate interaction.
gather, process and present information from secondary sources which compares formation, general rock type
and structure of mountain belts formed as a result of thermal uplift and rifting with those resulting from
different types of plate convergence
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A good way to compare information is to structure the information in a table.

A table like the one below is an effective tool to assist you to gather, process and present information. Welldesigned tables assist you to identify useful information and will assist you to notice trends and patterns. Try
using a table to sort out the information from the notes provided after the table.
Mountain belts formed
Mountain belt features
by:
Formation
General rock type
Structure of mountain belts
thermal uplift and
rifting
ocean/ocean boundaries
ocean/continent
boundaries
continent/continent
boundaries
distinguish between mountain belts formed at divergent and convergent plate boundaries in terms of general
rock types and structures, including folding and faulting
Divergent boundaries
Mountain belts formed from the action of thermal uplift and rifting are of two main types:
1.
Mid-ocean ridges form a near-continuous underwater mountain chain that extends for 60 000 kilometres
right around the globe. Mid-ocean ridges rise to over 2.4 kilometres above the floor of the 5 kilometres deep
ocean basins. A mid-ocean ridge can be a wide a 2000 kilometres.
Mid-ocean ridges result from convective upwelling of mantle beneath thin oceanic lithosphere. They are
formed along structurally weak zones created where the ocean floor is being pulled apart lengthwise along
the ridge crest. New magma from deep within the Earth rises easily through these weak zones and eventually
erupts along the crest of the ridges to create new oceanic crust. This process is called seafloor spreading.
At the top of the oceanic crust at mid-ocean ridges are basalt lavas. The lavas often form as pillow lavas.
Beneath are numerous basaltic dykes and deeper down are gabbros. The topography near the ridge axis is
very rough and mountainous. At the centre of each ridge there are steep-sided troughs, several kilometres
wide, which are similar to rift valleys that occur on continents. Mid-ocean ridges are offset by transform
faults that run perpendicular to the ridge axis. The faults are only active between the spreading centres.
2. Young rift zones occur within continental landmasses and are caused by convective upwelling of mantle
beneath weak continental lithosphere. When continental crust stretches beyond its limits, tension cracks
begin to appear on the Earth's surface. Magma rises and squeezes through the widening cracks, sometimes to
erupt and form volcanoes. Rift zones generally have intensive basaltic igneous activity. The rising magma,
whether or not it erupts, puts more pressure on the crust to produce additional fractures and, ultimately,
the rift zone. The uplift produces plateaus adjacent to the rift. These plateaus generally slope upwards
towards the rift valley. Escarpments in the rift valley are formed from normal faulting into the rift. Such
features are seen in Africa along the East African Rift Zone.
Convergent boundaries
The three types of convergent boundaries result in the following mountain types:
Ocean/ocean boundaries:
Mountains formed at ocean/ocean boundaries are of the volcanic island arc type. They form on an oceanic plate that
has another oceanic plate subducting under it. There are two types of mountains that can form at ocean/ocean
boundaries.
1.
Those that comprise elongated mounds of ocean floor sediments that have been tightly folded and chaotically
mixed in the trench by the faulting and folding caused as they are scraped from the down-going oceanic
plate. The southern line of islands of the Indonesian Archipelago is a good example of this type.
2. Those formed of chains of explosive volcanoes. These volcanoes form from andesitic magmas that are
generated as the subducted plate partially melts when it comes in contact with the hot asthenosphere. Steam
and other volatile substances find paths upwards, creating vents for magma to reach the surface to create
the volcanoes. The northern line of islands of the Indonesian Archipelago is a good example of this type.
Island arcs can be deformed by strike-slip faults and folds.
Ocean/continent boundaries:
As an oceanic plate is subducted beneath a continent, the sediments on the upper surface of the lower plate will be
scraped off to produce a wedge of sediment called an accretionary wedge. Where the accretionary wedge is forced
directly against the leading edge of continental crust, the subducting plate will be forced down steeply into the
asthenosphere where the plate will be partially melted. Steam produced in the process also partially melts the upper
mantle. Andesitic magmas are produced from these processes. Mountains will be produced in the continental plate
from the compression and uplift of the low density wedge sediments and the sediments and rocks of the continent,
and from the intrusion of magma produced from the partial melting in the subduction zone. These mountains rise to
very high altitudes and contain highly folded and faulted sedimentary rocks produced from the compressional forces.
The upper sections of sedimentary mountain ranges remain poorly consolidated and quickly erode, producing large
amounts of sediment for the rivers that drain from them. The intrusions of magma are in the form of large granitic
batholiths beneath the volcanic belt. The mountains contain explosive andesitic volcanoes. The explosive volcanoes
produce much pyroclastic sediment that is deposited in the mountain areas. The explosive volcanoes frequently form
calderas where they develop from eruptions from large, shallow magma chambers. The Andes Mountain chain in South
America is a good example of this ocean/continent type of mountain building.
Continent/continent boundaries:
When two continents collide, the ocean between them has been subducted under one of them. The continents will
have been flanked by accreted sediment from the ocean floor that was scraped off from the subduction. This
sediment forms into a huge wedge as it is folded, compressed and uplifted. Rocks from old oceanic plate, called
ophiolites, can also be squeezed between the two continents and be uplifted as part of the mountain range formed.
Ophiolites are very mafic and are composed of rocks like basalt and gabbro. Eventually the two older sections of
colliding continents meet. These older sections of the continents are called cratons. Cratons are made up of
crystalline igneous and high-grade metamorphic rocks. They are old and incompressible. The rocks of the craton
splinter and fault at low angles, stacking on each other as they are compressed to form mountains. The Himalayas are
an example of a mountain range that has been formed from compressed ocean floor sediments and fractured cratons.
Low-angle thrust faults are common.
3. Continents evolve
analyse information from a geological or tectonic map of Australia in terms of age and/or structure of rocks
and the pattern of growth of the continent
The information you collect and analyse about the ages and rock structures from west to east, and from north to
south, will allow you to describe the pattern of growth of the Australian continent.
outline the main stages involved in the growth of the Australian continent over geological time as a result of
plate tectonic processes
It is important to understand that the Australian landmass has existed as an island as we know it since about 55
million years ago (mya). Any outline of how the Australian continent has grown must be set in the broader context of
a smaller Australia linked to other landmasses, particularly to the west and south. Often the eastern border met
ocean with island arcs or with shallow seas.
The oldest rocks of Australia are found in Western Australia and are 3800 million years old. They are found in
cratons, areas that have been through a full cycle of continental crust building processes. An area is cratonised when
it has been through stages of mountain building that includes folding, igneous emplacement and crustal thickening,
and has become stable after continuous erosion and isostatic uplift until it is about 35 kilometres thick.
The general trend across Australia is that the rocks become younger as we move from west to east.

Stage 1: Formation of cornerstone blocks (cratons)
 By 2500 mya, three large cratons were established in Western Australia.

Stage 2: Welding the blocks together
 From 2500mya to 900 mya, the cratons were separated by active, linear mountain chains, known as
mobile belts, that welded the cornerstone blocks together. These belts were highly deformed and
folded and contain metamorphic rocks and granite.
 By 900 mya, the western two thirds of present day Australia had been cratonised. Australia was still
part of Gondwana.

Stage 3: Subduction and accretion in the east.
 From about 500 mya to about 250 mya, the continent was developed further to the east in the
formation of the Tasman Fold Belt. The rocks present in the eastern third of the Australian
continent exhibit evidence of former island arcs and ocean trenches resulting from the subduction of
an oceanic plate. Sediment accumulated between the continental edge and the island arc, filling the
seaway.
 From 320 mya to 280 mya, major mountain building occurred in eastern and central Australia,
including the formation of the Lachlan Fold Belt and the New England Fold Belt. These belts supplied
the sediment for sedimentary basins that developed along the eastern flank of Australia. The active,
or mobile, belt then moved eastward to produce the Lowe Howe Rise. The current mobile belt lies
along the Tonga-Kermadec-New Zealand Line in the Pacific Ocean.
 By 200 mya, the eastern third of Australia was cratonised.

Stage 4: Shallow seas
 160 mya, an area called Argoland rifted away to the northwest. Rift valleys formed down the Western
Australian coast and between Australia and the Indian continent. This was the beginning of the
breaking up of Gondwana. sea levels rose, flooding over the Greta Artesian Basin.
 132 mya, a narrow seaway had developed separating Argoland. South and west of Australia, spreading
began and marked out the continental shapes including India. Faster spreading between India,
Antarctica and Australia continued to 118 mya, opening an ocean up to 600 kilometres wide.
 96 mya, the Lord Howe rise began rifting south of Tasmania and westward, separating Antarctica.
The rift that was moving India away was cut.
 84 mya, the Indian continent moved further north with the same direction as the rift between
Australia and Antarctica.
 64 mya, the Tasman Sea continued spreading, until 49 mya when spreading stopped.
 From 45 mya to the present, the Southern Ocean continued spreading. Resultant downwarping of the
continent allowed shallow seas to cover the Murray Basin.

Stage 5: Intra-continental earthquakes and hot spot volcanoes
 As the continent (now the island we recognise) continued its northward drift, it passed over a number
of mantle hot spots, resulting in a series of parallel lines of volcanoes which are younger towards the
south. The largest of these include Mount Warning on the NSW/Queensland border and Mount


Canobolas in the NSW Central West. The most recent volcanic eruption was at Mount Gambier in
South Australia only 4000 years ago
Tensional stresses acting within the continent as the plate boundary to the north pushed against the
Asian and Pacific plates caused some very old faults to move periodically, and blocks to adjust
isostatically. The Great Dividing Range was uplifted to its present height by this process.
Stage 6: Continuing northward
 Interaction between the converging Australian and Pacific plates has produced the current New
Guinea mobile belt.
present information as a sequence of diagrams to describe the plate tectonic super-cycle concept
You could present a summary to show how a super-cycle operates using a cycle diagram like the one below.
summarise the plate tectonic super-cycle
The plate tectonic super-cycle is a theory to explain a sequence of events that have repeated at least three times.
Formation of super-continents Pangea and Rodinia occurred 300 million years ago and 900 million years ago,
suggesting a super-cycle time span for formation and breaking up of super-continents of about 600 million years.
The following is a very general description of possible super-cycles.
During plate tectonic development, a super-continent breaks up and the two new continents become separated by the
new oceanic lithosphere that is produced at a mid ocean ridge between them. As the oceanic lithosphere grows, the
continents drift further apart. If a subduction zone forms near the edge of one of the continents, the oceanic
lithosphere will be consumed in the subduction zone. The continents will be drawn back together, eventually to collide
producing a super-continent again.
If a subduction zone develops on the far side of one of the continents, oceanic lithosphere will be consumed. This
may eventually cause the continent to collide with another continent producing a new super-continent.
The following is another super-cycle scenario, using Pangea as an example:

Begin with a small super-continent, like Pangea, completely surrounded with ocean. (Pangea occupied 30% of
the Earth's surface with the other 70% being ocean.)


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Spreading at a mid ocean ridge some distance from the super-continent will cause the oceanic lithosphere
near the super-continent to begin to subduct beneath it.
This subduction produces the characteristic andesitic volcanoes. The volcanism at the edges of the supercontinent causes some weakness in the crust there.
Subduction continues until the subduction zone becomes choked and ceases, causing a new subduction zone to
develop a few hundred kilometres offshore. This new subduction zone will result in a chain of new andesitic
volcanoes, and thus new continental material developing offshore. The weakness in the continental margin
between the new island chain and the original super-continent allows spreading to occur creating a trough
called a back-arc basin. The area west of the islands of Japan is an example of this.
Now, marginal seas and island arcs surround the super-continent. Back-arc basins eventually fill with
sediment, thus extending the size of the super-continent.
Eventually, due to the presence of weaknesses in the zones that were once marginal seas, the super-continent
is able to split up, allowing the formation of separate continents, like we see today.
The cycles continue for each continent. If subduction of the ocean plates continues, it may bring continents
together once again creating a supercontinent and thus the cycle can continue.
4. Natural disasters
Background: Earthquakes are vibrations or tremors that occur in the Earth as a result of rocks suddenly moving
against each other. The epicentre of an earthquake is the point on the Earth’s surface that is directly above where
the movement occurred. The focus of an earthquake is the exact position of the earthquake. The precise location of
an earthquake can be described by stating the map coordinates for the epicentre and a depth for the focus.
identify where earthquakes and volcanoes are currently likely to occur based on the plate tectonic model

Because we know that earthquakes are caused by sudden movement of rocks that are under stress, the
location of earthquakes can be predicted by using the plate tectonic model to identify places where sections
of rocks are being forced to move against each other. These places would correspond to known active fault
lines and plate boundaries.

Rocks that are under stress can frequently adjust to the stress by folding or sliding. However, if sections
lock up, stress may be released by the rocks fracturing, creating a sudden large movement. Earthqaukes are
likely to occur in places such as these.

We know that volcanoes result from the upwelling of magma generated at hot locations in the mantle, and
from partial melting of crust. This means that the location of volcanoes can be predicted by using the plate
tectonic model to identify typical geological settings for magma generation.
Plate boundaries were discovered by plotting past earthquakes and volcanoes on a map of the world. The following
predictions can be made from the plate tectonic model.

Most volcanoes and earthquakes will occur on plate boundaries.

Shallow focused earthquakes, down to depths of seven kilometres, will occur along those sections of
transform faults that are between the spreading rift axes.

Earthquakes and explosive volcanoes will be produced in subduction zones. Shallow focused earthquakes will
occur near the ocean trench; deep focused earthquakes will occur further away from the trench.

Earthquakes will frequently occur at conservative boundaries, down to depths of 30 kilometres.

Volcanic eruptions occur progressively along the rifts of the mid ocean ridges. More activity will occur away
from the hinge of rotation for the two plates.

Relatively passive eruptions can be expected from volcanoes located at divergent boundaries.

Mid plate volcanoes are usually the result of a hot spot under the plate. Observation of the direction of plate
movement over the hot spot can assist in predicting where new volcanos will occur.
The plate tectonic model does not currently provide reliable predictions related to earthquakes in continental
lithosphere.
distinguish between plate margin and intra-plate earthquakes with reference to the origins of specific
earthquakes recorded on the Australian continent

The Australian continent lies entirely within the Australia-India plate, and so it does not experience plate
boundary processes.

Plate margin earthquakes account for ninety percent of all earthquakes and are the result of the constant
movement of the rocks at plate boundaries against each other.

Intra-plate earthquakes are those that occasionally occur in the crust of plates and away from the more
active plate boundaries. The causes of intra plate earthquakes are not well understood, but they are usually
caused by compressive stress in rocks.

Australia has three distinct regions of earthquake activity. These are:
o the Eastern region, covering the eastern highlands and coastal areas
o the Central region, extending from near Adelaide to the Simpson Desert
o the Western region, encompassing several distinct zones.
o
The most disastrous Australian earthquake in the last 200 years was the Newcastle earthquake of 28
December 1989. It was a magnitude 5.6 earthquake that caused $1.2 billion damage. The most likely
cause was by readjustments along the Hunter-Mooki Thrust, a curved fault running from Newcastle
and through Maitland, Murrurundi, Quirindi. Narrabri and Mackay, The fault is sporadically active due
to strong easterly compression from the expanding Pacific Ocean floor.

In the central seismic region of Australia, earthquakes have been associated with a 120 kilometre long fault
as a result of north-south compressive forces.

The stress causing intra-plate earthquakes may be associated with isostasy, which is the tendency for rock
masses to rise or sink to achieve a balance between downward weight forces and upward buoyancy forces.
Erosion and deposition change historically balanced isostatic forces, causing new regions of stress and strain.

Some intra-plate earthquakes may be related to the stress at plate boundaries and to temperature changes
in the lithosphere caused by processes in the mantle. The Australian plate has many north-south trending
concentrations of earthquakes, so it may be that the Australian plate is adjusting to the twisting motion of
the plate as it moves north. The forces that drive the continent may not be uniform and adjustment to the
different stresses created by this may cause the earthquakes.

In Western Australia, a linear zone of seismic activity extends from near Moora, southeast to Albany. This is
known as the Southwest Seismic Zone. It is the most seismically active area in Australia. The town of
Meckering, that experienced a magnitude 6.9 earthquake on 14 October 1968, lies within this zone. Though
the reason for the concentration of seismic activity in the Southwest Seismic Zone remains unknown, it could
be caused by a major structure/discontinuity of crustal or lithospheric scale that has been reactivated.
gather information from secondary sources to identify the technology used to measure crustal movements at
collision boundaries and describe how this is used
The following table is an example of one way to gather data. It can be used as a starting point for the collection of
more detailed information. The table describes some of the developed or experimental technologies currently used
to measure crustal movements. Many of these exhibit potential as technologies which might contribute to more
accurate prediction of volcanic eruptions and earthquakes.
Technology used to measure
crustal movements
How this is used
Laser geodimeter
Measures changes in the distance between stable
units on either side of the fault
Wire strain meter (10 m long)
Measures the deformation of the ground surface
around a fault
Tilt meter
Monitors ground tilting
Data gathered by satellite global
positioning systems (GPS) is being
used to analyse deformations in
the Earth's crust
Monitoring the relative and absolute motion of
stations set up across plate boundaries enables the
determination of regional-scale deformation and
associated stress fields.
Two-colour geodimeter
Measures crustal deformation along faults and near
volcanoes. It is an ultra-precise, distance-measuring
instrument that employs light pulses. It has a
precision of 0.5 to 1.0 mm for ranges between 1 and
12 km.
describe methods used for the prediction of volcanic eruptions and earthquakes
The following are descriptions of some of the developed or experimental technologies currently used to predict
volcanic eruptions and earthquakes. No reliable method of predicting volcanic eruptions and earthquakes has yet been
developed.

Local seismic recording stations: used with artificially generated micro-earthquakes to produce precise
estimates of the P-wave and S-wave velocities in a test region. An earthquake may be expected when the
ratio of these velocities changes slightly.

Modern seismic monitoring networks including the use of seismic instruments placed down boreholes (to
depths of 500 m): Some large earthquakes are preceded by foreshocks. Knowledge of past earthquake
patterns allows scientists to estimate the odds that an earthquake striking is a foreshock to a larger
mainshock in the same area. Additionally, characteristic levels of background seismicity may drop
substantially in the months or years prior to a large earthquake. Tiny changes in seismic velocity in the
stressed region may be due to cracks forming just before failure.

The VAN Technique measures changes in the earth’s electric field prior to an earthquake: Three Greek
scientists, P. Varotsos, K. Alexopoulos, and K. Nomicos, (VAN) have pioneered methods of detecting,
recording, and interpreting signals from the earth that precede an earthquake. These electromagnetic signals
are apparently generated through piezoelectric processes, induced by tectonic stress. Other similar research
is investigating if changes in the Earth’s background noise in the low (LF), very low (VLF), and extremely low
(ELF) frequency bands may indicate a pending earthquake.

Geochemical samplers: detect increases in the radioactive gas radon in the ground water prior to an
earthquake. It is believed that changes in stress in the crust enables radon trapped in cracks to move toward
the surface.

Because increased earthquake activity is an indicator of an imminent volcanic eruption, the methods above can
all be used to predict eruptions. Additionally, systematic seismic monitoring of activity before, during and
after an eruption is used in some situations. Volcano monitoring consists of keeping a detailed record of the
changes in a volcano over time. Scientists look for:
o increase or decrease in steaming of vents
o emergence of new steaming areas
o development of new ground cracks or widening of old ones
o unusual or inexplicable withering of plant life
o changes in the colour of minerals encrusting fumaroles
o increase in volume of the volcano (swelling)
o precise location of earthquakes associated with magma movement
o localised changes in the Earth’s magnetic field
o any other obvious and recordable change.
gather information from secondary sources to present a case study of a natural disaster associated with
tectonic activity that includes:




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an analysis of the tectonic movement or process involved
its distance from the area of disaster
predictions on the likely recurrence of the tectonic movement or process
technology available to assist prediction of future events
an investigation of possible solutions to minimise the disastrous effects of future events

Select a recent natural disaster associated with tectonic activity to research as your case study. You will
find it easier to find information you need for your case study than for earlier natural disasters.
describe hazards associated with earthquakes, including ground motion, tsunamis and collapse of
structures
o
Ground motion can cause built structures to collapse, can damage and displace vehicles, can cause
water in harbours to be displaced, and can trigger other devastating events such as landslides and
mudslides. People and other animals can be buried in crevasses.
Major earthquakes in the lithosphere below oceans can trigger tsunamis. Such earthquakes can change the
level of the ocean floor by several metres and displace an enormous volume of water. The waves produced
contain the energy of the earthquake as it lifts up to 14 kilometre of ocean above it. A wave generated has
twice the wavelength of the diameter of the affected area and it travels very fast (800 km per hour). Upon
reaching shallow water, the front of a tsunamis wave-set slows down while the back catches up to produce a
massive wall of water. Tsunamis devastate low lying coastal areas. Houses and other structures are usually hit
by a wall of water from the ocean and again as the water rushes back out to sea. Floating debris increases
the impact on life and property.
describe hazards associated with volcanoes, including poisonous gas emissions, ash flows, lahars and lava
flows and examine the impact of these hazards on the environment, on people and other living things
o
One of the greatest hazards of volcanoes is the explosive eruption. At least 200 000 people have lost
their lives as a result of explosive volcanic eruptions in the past 500 years. Well known examples of
explosive eruptions are:
 Mt Pelée, Martinique, in which erupted in 1902, killing 30 000 people

Mt St Helens, USA, which erupted in 1980 resulting in 57 dead or missing and $1.2 billion
damage.
o
Poisonous gas emissions from volcanoes include carbon monoxide (CO), sulfur dioxide (SO 2), sulfur
trioxide (SO3), hydrogen sulfide (H2S) hydrochloric acid (HCl), hydrofluoric acid (HF), sulfurous acid
(H2SO3) and boric acid (H3BO3). Carbon dioxide (CO2), although not poisonous, can asphyxiate by
displacing air that contains oxygen. Most of these emissions are associated with eruptions. One
specific example recently occurred in Lake Nyos, in Africa, where the crater-lake became saturated
with carbon monoxide gas. A minor disturbance in the lake caused about one cubic kilometre of gas to
be released, killing 1700 people in a nearby village and all livestock in surrounding areas.
o
Ash flows can kill because of heat and poisonous gas. In March and April 1982, El Chichon in Mexico
erupted three times producing high velocity incandescent ash flows that levelled villages up to eight
kilometres away. The number of deaths exceeded 500. In 79 AD, Mt Vesuvius buried the cities of
Pompeii and Herculaneum so completely that they weren’t discovered again until 1700 years later.
o
Different types of lava flow at different speeds. Highly viscous lava tends to block volcanic vents and
lead to explosive eruptions. High temperature, low viscosity lava flows freely and is often associated
with hot spot volcanoes and sea floor rifts. These lavas do not usually endanger human life because
there is time for evacuation. However all property in their path is destroyed by the lava. Lava flows
regularly from Mt Etna in Italy and Kilauea in Hawaii. Villages are buried but people have enough time
to escape the flow.
o
Lahars are mud and ash flows generated from the melting of an ice cap on a volcano or associated
with release of water from a crater lake. Flows of volcanic debris can have the consistency of wet
cement. They can sweep down the sides of a composite volcano burying everything in their path.
Nevada del Ruiz Volcano, in Colombia, buried the city of Armero with a lahar, killing 25 000 people.
o
A nuée ardente is a highly mobile, turbulent gaseous cloud erupted from a volcano. It can be
incandescent. The most infamous nuée ardente occurred when Mt Pelée erupted in 1902, killing 30
000 people.
describe and explain the impacts of shock waves (earthquakes) on natural and built environments
o
Shockwaves from earthquakes are of three main types:
1. P-waves are compression waves.
2. S-waves are transverse or shear waves.
3. L-waves are surface waves and can be transverse or elliptical. The elliptical waves are the
slowest, but often the largest and most destructive, of the wave types caused by an
earthquake.
o
The impact of shockwaves is related to their intensity.
o
Factors affecting intensity include the location of the focus, the triggering mechanism, the quantity
of energy released and the nature of the local geology.
Earthquake intensity is measured using a relative scale, such as the modified Mercalli scale.
The magnitude of an earthquake is an absolute value and is related to the amount of strain energy released,
as recorded by seismographs. Magnitude is measured on the Richter scale, a numerical scale that describes
an earthquake independently of its effects on people or objects such as landforms or buildings.
The following table relates some of the Modified Mercalli scale of earthquake intensities to some well-known
examples. The Richter scale values are provided for comparison.
Intensity
(Mercalli)
Title
Effects on natural and built
environments
Example
(Richter magnitude)
II
Feeble
Suspended objects sway
IV
Moderate
Windows and dishes rattle
Port Jackson, 1788
Rather
strong
Dishes and windows broken
Lithgow, 1985 (4)
VI
Strong
Chimneys topple
VIII
Destructive
Weak structures severely damaged;
strong structures slightly damaged
IX
Ruinous
Total destruction of weak
structures. Foundations damaged.
Underground pipes broken
X
Disastrous
Only best buildings survive. Ground
badly cracked
XI
Very
disastrous
Few masonry structures remain
standing. Broad cracks in ground
Kobe, 1995 (7.2)
Western India, 2001 (7.9)
XII
Catastrophic
Total destruction. Waves seen on
the ground
Chile, 1960 (9.5)
Meckering, 1968 (5)
Newcastle, 1989 (5.6)
describe the general physical, chemical and biotic characteristics of a volcanic region and explain why people
would inhabit such regions of risk
o
Volcanic regions have extremely fertile soils. Volcanic rocks break down physically and chemically very
quickly. Volcanic rocks weather readily producing soils rich in iron and magnesium . Soil formation can
occur in as little as a few hundred years, but there are instances recorded of seeds germinating on
erupted rock soon after cooling. Volcanic mountains often have very high altitudes resulting in
favourable conditions for plentiful rainfall.
o
There is generally a great diversity of biota in volcanic regions. If adequate rainfall is available,
natural vegetation and crops grow quickly and these can support a great variety of animal species.
o
Volcanic landscapes have aesthetic attraction for people. Mountains create beautiful scenery and
symmetrical volcanic cones have been important to many cultural beliefs.
o
People are often willing to take the risk that eruptions will not occur in their lifetime. Many people
who live in volcano and earthquake prone regions accept earthquake activity, like climate, as a
condition of life.
justify continued research into reliable prediction of volcanic activity and earthquakes
Some possible arguments for continued research are:
o
There are large populations in many areas prone to volcanic activity and earthquakes. Given that
prediction of impending volcanic eruptions and earthquakes are currently unreliable, people will not
move until it is too late. Thus reliable early warning would save many lives and reduce losses due to
poor preparation for a disaster.
o
Although research and the use of new technologies are expensive, the cost is small compared to the
possible savings in lives, the provision of emergency services and loss of work, after a devastating
event.
o
The use of new technologies, such as modern microcomputers, and remote sensing technologies, offer
great potential for reliable methods of prediction to be developed in the near future.
Some alternative arguments:
o
Earthquakes are difficult or impossible to predict because of their inherent random behaviour.
Efforts should be channelled into hazard mitigation.
o
Providing warnings can cause panic in a population, potentially causing more problems than if an
earthquake or a volcanic eruption was not predicted.
o
The geological hazards of most regions are now known and the choice to live in a potentially
hazardous area is an individual one. Education about ways to survive and cope with the effects of a
natural disaster is more appropriate than continued research into prediction.
5. Plate tectonics and climate
describe and explain the potential and observed impacts of volcanic eruptions on global temperature and
agricture
The potential impacts of volcanic eruptions on global temperature:

The injection of sulfur dioxide (SO2 ) into the stratosphere causes the greatest impact on the atmosphere
and global temperatures. The SO2 converts to sulfuric acid aerosols that block incoming solar radiation and
contribute to ozone destruction. The reduction in solar radiation can cause global cooling. The plume of ash
from an eruption causes an increase in the amount of sunlight reflected by the Earth's atmosphere back to
space causing the surface of the planet to cool.
The potential impacts of volcanic eruption on agricture:

Volcanic eruptions have the potential to devastate agrictural activity. Areas close to the erupting cone can be
destroyed by lava and mud flows. Poisonous gases can kill herds of stock. Areas further from the cone can be
covered in thick layers of pyroclastic debris.
The observed impacts of volcanic eruption on global temperature:

El Chichon and Mount Pinatubo emitted the greatest amounts of SO 2 into the stratosphere. El Chichon
produced about 7 million tonnes of SO2 and Mount Pinatubo produced about 20 million tonnes. Both of these
volcanoes are at low latitudes but they both had high eruption rates. The impact of eruptions may not last
very long. For a large eruption like Mount Pinatubo, the impact may last for up to three years.
The observed impacts of volcanic eruption on agricture:

Mt St Helens produced a layer of debris six-tenths of an inch (about 15mm) thick, five hundred miles (about
800km) away. In the regions affected by Mt St Helens:
o crop loss was estimated at $100 million, or seven percent of the national crop value for that region.
o fifty percent of the alfalfa hay crop was ruined
o timber to the value of $100 million dollars was destroyed
o the wheat, potato, and apple crops were above normal, through a decrease in popations of destructive
insects.
predict the possible effects of explosive volcanic activity on global and local climates
Global effects:

Explosive volcanism will produce large amounts of ash and aerosols that can reach into the stratosphere. The
high levels of material in the atmosphere at this height will rest in a reduced amount of radiation from the
sun reaching the Earth’s surface. Less radiation reaching the surface reduces the surface temperature and
the heating of air in contact with the surface. If widespread enough, there will be a reduction in the global
temperature.
Local effects:



Fine ash will increase precipitation in the area around a volcano.
The precipitation will be acidic because of the reaction of sfur dioxide with water in clouds developing around
the volcano.
There may be reduced local temperatures because of reduced radiation if a volcanic plume persists for a
prolonged time.