Download Tectonic Impacts #2

Tectonic Impacts
Glossary
Term
aerosol
andesite
Meaning
A small droplet of liquid is suspended in air
A fine-grained igneous volcanic rock crystalized from magmas from at
subduction zones.
anticline
Folds where the limbs either side of the fold axis are bent downwards
aphanitic
A rock texture in which crystals are too small to be identified without a
microscope.
assimilation
The process in which a rock melts and becomes part of a magma
asthenosphere A region made up of partially molten rock bellow the lithosphere
basalt
A dark, fine-grained volcanic rock that sometimes displays a columnar
structure
Benioff zone
An inclined area of earthquake activity dipping into the mantle away from
a trench
conservative
In relation to plates, it describes a boundary where crust is neither created
boundary
of subducted
constructive
In relation to plates, it describes a boundary where new ocean crust is
boundary
formed
continental
The gradual movement of the continents across the earth's surface
drift
through geological time.
convection
The process in which heat is transferred by the motion of material in a
fluid
convergent
when two crustal plates move towards each other and collide
boundary
destructive
In relation to plates, it describes a boundary where crust is being
boundary
subducted into the mantle
divergent
Where two plates diverge or separate from each other
boundary
earthquake
A sudden and violent shaking of the ground, sometimes causing great
destruction, as a result of movements within the earth's crust
fault
A fracture in a rock where there has been relative movement on either
side of the fracture
felsic
focus
Fold mountain
geosyncline
Gondwana
High in silica, low in iron, light in colour, less dense, found in continental
crust, associated with volcanoes
The point beneath the Earth’s surface where rocks break and shock waves
are produced generating earthquakes
A mountain containing rocks deformed by horizontal forces and intruded
by igneous rocks
The thick accumulation of sediments formed near mountain belts.
A vast continental area believed to have existed in the southern
hemisphere and to have resulted from the breakup of Pangaea in
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granite
granulites
greenhouse
gases
intra-plate
isostatic
adjustment
lahar
Lithosphere
Mesozoic times. It comprised the present Arabia, Africa, South America,
Antarctica, Australia, and the peninsula of India
A very hard, granular, crystalline, igneous rock consisting mainly of quartz,
mica, and feldspar
Rocks formed under high temperature and pressure. Water-containing
minerals, such as mica nad amphibole, are absent.
Gases that increase the heat retained in the atmosphere
Within a plate
Movement that balances weight forces and buoyancy forces
Mud flows
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.
Laurasia
A vast continental area believed to have existed in the northern
hemisphere and to have resulted from the breakup of Pangaea in
Mesozoic times. It comprised the present North America, Greenland,
Europe, and most of Asia north of the Himalayas
Ma
An abbreviation meaning ‘million years’. “M” represents ‘mega’ or
‘million’, and “a” represents annum or year.
mountain
A large natural elevation of the earth's surface rising abruptly from the
surrounding level; a large steep hill
mid-ocean
A giant mountain range that lies under the ocean and extends around the
ridge
world where sea floor spreading occurs
mafic
High in iron, slightly denser, darker in colour, low in silica, found in oceanic
crust
ophiolite
Rocks found on land that have an origin as oceanic crust
orogeny
Episode of mountain building
Pangaea
A supercontinent comprising all the continental crust of the earth,
postulated to have existed in late Paleozoic and Mesozoic times before it
broke into Gondwana and Laurasia
peridotite
Dark coloured, coarse grained ultramafic rock containing olivine and
pyroxene
pillow basalt
Basalt with a pillow-like structure due to be erupted in water
plate
Any of several large pieces of the Earth's lithosphere which participate in
plate tectonics
plate tectonics The lithosphere is being recycled down trenches at subduction zones. As
new sea floor is being produced at the mid oceanic ridge, old sea floor is
being removed at the trenches
platforms
Areas with relatively flat surfaces formed on continents
plutons
Masses of igneous rocks formed below the earth’s surface.
porphyritic
A rock texture in which some crystals are significantly larger than others
P-wave
Type of earthquake wave that can travel through solids and liquids
pyroclastic
Fragmented rock material formed in a volcanic eruption
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reverse faults
rhyolite
sea floor
spreading
seismometer
shield
silica
subduction
syncline
transform
boundary
troposhere
tsunami
ultramafic
viscous
Steep faults caused by compressional forces
A fine grained volcanic igneous rock with the same composition as granite.
The formation of new areas of oceanic crust, which occurs through the
upwelling of magma at mid-ocean ridges and its subsequent outward
movement on either side
An instrument that records earthquake or other seismic waves
Old parts of continents composed of rocks formed deep in the Earth
Silicon dioxide
The sliding of the sea floor beneath a continent or island arc. Lithosphere
is consumed
Folds where limbs either side of the fold axis are bent upwards
A fault which runs along the boundary of a tectonic plate
Layer of the atmosphere in which clouds form
A water wave with a very long wavelength
An igneous rock composed only of minerals rich in iron and magnesium
The property of a fluid that describes it resistance to flow. A very viscous
fluid does not flow easily.
Volcanic island An arc of volcanic activity marked by a chain of volcanic islands. It lies
arc
parallel to an ocean trench.
analyse
Identify components and the relationships among them, draw out and
relate implications
assess
Make a judgment about the value of something
compare
Show how things are similar or different
deduce
Draw conclusions
define
State the meaning of and identify essential qualities
discuss
Identify issues and provide point for and against
distinguish
Recognize or note/indicate as being distinct or different from, note
differences between things
evaluate
Make a judgment based on criteria
examine
Inquire into
predict
Say what might happen based on available data
summarize
Express concisely the relevant details
Focus Area 1-Lithospheric plates and their motion
-Describe the characteristics of lithospheric plates:
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Upper layer of each lithospheric plate is composed of crust
Crust is composed of either continental crust or oceanic crust
Continental crust is typically made up of relatively less dense rock like granite
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 general thin, usually between 5 and 10km thick
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Continental crust is generally between 25 and 50 km thick
Under large mountain ranges the crust can be over 80 km thick
The lithospheric plates are up to 70km thick, where there is oceanic crust, and up to
150km thick, where there is continental crust
Under large mountain ranges the crust can be over 80 kilometres thick
Feature
Average Density
Average Thickness
Composition
Oceanic crust
Continental
2.7g/cm³
35km
Felsic rocks, sedimentary, igneous,
andesite and granite
Similarities and
differences
Oceanic
3.0g/cm³
7km
Mafic rocks, basalt and gabbro
Continental crust
Thinner
Thicker
More Dense
Less dense
Mafic
Felsic
High in iron
Both contain iron
Low in iron
Low in silica
Both contain silica High in silica
Darker in colour
Higher in colour
Creates sedimentary rocks
Creates igneous and metamorphic rocks
-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 and amphibole and biotite
 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
 Compare samples of andesite, basalt, granite, gabbro, diorite and rhyolite:
1. Create a table to tabulate their physical characteristics, mineral composition and
typical boundary location to illustrate trends
Granite
Diorite
Gabbro
Rhyolite Basalt
Andesite
Characteristics Pale
White grey
Dark grey Grey
Fine
Grey,
colour,
and black
with black with red crystals, small
specks of
specks,
specks,
streaks,
dark
white
pink black
mostly dark small
small
grey
speckles
and grey,
in colour,
crystals
crystals
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Volume
large
crystals
4 cm3
Mass
10.7 g
Density
2.67
=mass/volume
large
crystals
4 cm3
7 cm3
6 cm³
3 cm³
17 cm³
9.4 g
2.35
18.8 g
2.7
14 g
2.3
7g
2.9
43.1 g
2.5
2. Write a procedure for measuring the density of an irregular shaped object.
Title: Measuring densities of irregular shaped objects.
Aim: To measure the densities of an object with an irregular shape.
Hypothesis: We will find out the density by finding out the mass and the volume of the
object.
Equipment and risk assessment:
Equipment
Potential Hazard
Prevention or control
Water
Spilling the water on the Handle water carefully and wipe up all spills
floor then slipping on it
immediately
Measuring
Dropping it and it
Keep measuring cylinder in the centre of the
cylinder
smashes
table at all times and clean up immediately if
you do drop it. Handle with caution
String
Getting a rope burn
Do not let the string slide on your skin
rapidly, hold with a tight grip at all times
Various different Getting a cut from a
Be gentle with all rocks and avoid rocks with
shaped rocks
sharp edge on the rock
sharp edges
Measuring scales Dropping them and they Keep on the table at all times, do not
land on someone's feet
remove
Method:
1. Fill a measuring cylinder with 40mL of water
2. Tie a knot using the string around one rock and make sure it is firmly tied
3. Gently lower the rock using the string into the measuring cylinder until it reaches
the bottom. Keep hold of the string.
4. Measure the volume of water now. Be cautious of parallax error
5. Subtract the original volume of water from the new volume, this will give you the
volume of the rock
6. Take the rock out of the water and untie the knot
7. Measure the rocks mass using the measuring scales.
8. Divide the mass by the volume to find out the density.
9. Repeat steps 2 to 8 three times to make sure your result is accurate.
10. Repeat these steps with all of the other rocks.
Summary:
Rocks with a higher density had a higher crystal content and contained more minerals
-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 12cm 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
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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 Africa 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 NZ 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 usually found along a 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 motel rock rises because it is less dense than
the material above it
Motion
Plate Type
Major geological features
Examples
Divergence
-Ocean –
Volcanoes (non-explosive) e.g. shield
Mid-Atlantic Ridge
Ocean
volcanoes, fissure volcanoes, cinder cones
(Eurasian and
divergent
Mountains
North American
zone
New ocean crust
Plates)
-Mid-ocean
Basalt rock from basaltic lava (low viscosity, Iceland
ridge
low in silica, mostly dark, mafic minerals
-Ocean
high in iron and magnesium)
spreading
Pillow basalt
zone
Normal faults
Shallow earthquakes
-Continent – Volcanoes (non-explosive) shield volcanoes, Great East African
Continent
cinder cones
Rift Valley (African,
-Continental Mountains
Arabian and Indian
Rift Zone
Rift valleys
plates)
New ocean crust
Basalt rock cools from from basaltic lava
(low viscosity, low in silica, low gas content,
mostly dark, mafic minerals high in iron and
magnesium)
Normal faults
Shallow earthquakes
Convergence
-OceanMore dense ocean plate subducts less
Japan Islands
Ocean
dense continental plate
(Eurasian plate and
-Subduction Reverse faults
Indo-Australian
Volcanoes form an island arc
Plate)
Strato-volcanoes
Marinana Trench
Explosive volcanism as lava is viscous being (11 km)
high in silica, high gas content
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-Continent –
Continent
-OceanContinent
-Subduction
Mostly felsic minerals which are light
coloured
Lava cools to form andesite and rhyolite
rocks
Strong earthquakes (Benioff Zone)
Ocean trench
No subduction as both plates low density
Massive fold mountains
Crustal thickening and shortening
Shallow to deep earthquakes
Very little volcanism
Reverse faults
Folding
Regional metamorphism of rocks (gneiss,
schist)
Granite plutons from deep melting of crust
beneath mountains
Dense ocean plate subducts less dense
continental plate
Some fold mountains as crust pushed up
Reverse faults
Strato-volcanoes
Explosive volcanism as lava is viscous being
high in silica, high gas content
Mostly felsic minerals which are light
coloured
Himalayas (IndoAustralian Plate
and Eurasian Plate)
-Gather and analyse information from secondary sources about the forces driving plate
motion:
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The main forces behind plate motion are convection currents, gravity and heat
With plate tectonics subducting through the crust how they begin to move is by motion
of convection currents beneath the earth's surface in the mantle
The convection currents are stimulated and powered by heat and from this heat,
convection cells begin to rotate causing crust movement above in the lithosphere
As the plates begin to subduct ridge pull and slab push are factors that contribute to the
gravitational pull that drives the plates down into the mantle
With mid-ocean ridges spreading the sea floor and pushing away from the ridge the slab
push and then as the crust descends ridge pull carries the plate
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Convection currents in the asthenosphere contribute most to the movement of
lithospheric plates
-Describe current hypotheses used to explain how convection currents and subduction
drive 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)
 Convection Currents
3 forces: convection currents, slab pull, ridge push
Materials: -Water (hot and cold)
-Food colour x2 (red and blue)
-Clear plastic tray (8" or greater), radial ridges not concentric ridges
-Eyedropper
-4 Styrofoam cups
Procedure:
1.
Fill tray with cool water. Place tray on top of 3 evenly spaced, inverted Styrofoam
cups. (This should well support your display)
2.
Fill the 4th cup with hot water and slide under the centre of the tray.
3.
Using the eyedropper, gently place a blob of food colour inside the tray, on the
bottom directly over the heat source.
4.
A second colour can be added to the outside edge of the tray, away from the heat
source, (again on the bottom).
5.
Observe 5 minutes
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Discussion questions:
Which way does the warmed water move? Towards the centre of the heat source
What happens when it reaches the surface? It spreads away from the heat source and
then sinks again.
Which way does the cooler water on the edges move? Down
Focus Area 2-Mountain building
-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:
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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
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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.
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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.
-Gather, process and present information from secondary sources which compares
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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
Mountain
belts
formed by: Formation
Mountain belt features
General rock type
OceanOccurs when two plates
ocean
converge-one edge of the ocean
boundaries crust is subducted beneath the
other at an ocean trench
OceanOccurs when oceanic crust is
continent
subducted under continental
boundaries crust which forms an active
continental margin between the
subduction zone and the edge of
the continent
Continent- Results when two continents
continent
collide. The continents were
boundaries serparated at one time by
oceanic crust that was
progressively subducted under
one of the continents. No
subduction as both plates have
low density
Felsic mineralsAndesite and
rhyolite
Felsic mineralsandesite and
rhyolite
Structure of mountain
belts
Island arcs and chain
Intensely folded
mountain ranges
Metamorphism of Intensely folded
rocks: gneiss and mountain ranges
schist
Focus Area 3- Continents evolve as plate boundaries move and change
-Outline the main stages involved in the growth of the Australian continent over
geological time as a result of plate tectonic processes
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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.
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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 Great 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.
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Stage 6: Continuing northward
-Interaction between the converging Australian and Pacific plates has produced the current
New Guinea mobile belt.
-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 Pangaea 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.)
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 super-continent 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.
14
-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 picture below shows that the different parts of Australia are of different
ages. The oldest parts are the Pilbara and Yilgarn Cratons in Western Australia, which were
cratonised before 2500Ma ago. Note that most of Eastern Australian is less than 500 Ma old.
In general terms Australia has grown from west to east as we view it now and it continues to
grow to our north in Papua New Guinea.
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Name: Archaean
Eon: Archaean
Time: Before 2500MA
Facts:
 Tectonic processes at this time, were very different to those today
 There was much more heat in the mantle
 Convection was more rapid and chaotic
 There was a thinner lithosphere and more molten material in the mesosphere
 Two types of material formed:
1. Belts of heavily metamorphosed granulite-gneisses
2. Greenstone belts
 During the late Archaen the small cratons came together to form a single but not very
rigid supercontinent
 Diverse microbial life flourished in the primordial oceans
 Continental shields developed from volcanic activity
 Reducing (anaerobic) atmosphere enabled anaerobic microbes to develop
 Plate tectonics followed a different regime of continental drift
 One type of organism the Cyanobacteria (blue-green algae) produced oxygen as a
metabolic by-product
Name: Proterozoic
Eon: Proterozoic
Time: Between 2500MA and 545 MA
Facts:
 Atmosphere changes from reducing to oxygenated
 Original anaerobic inhabitants of the Earth restricted into a few anoxic refuges
 Rise of aerobic life
 Stromatolites were common
 During the period 2500 to 900 MA the development of the continent was complex
and its interpretation remains open to debate
 Then Australia consisted of Archaean and early Proterozoic cratonic nuclei (Yilgarn,
Pilbara and Gawler cratons) separated by linear fold belts such as the Central
Australian mobile belt
 Mobile belts were progressively cratonised, finally welded and consolidated into the
Australian Precambrian Craton 900 MA ago
 Supercontinent Rodinia formed (1120-850 MA)
 The supercontinent Sturtia formed (720-560 MA)
Name: Phanerozoic
Eon: Phanerozoic
Time: 545MA to present
Facts:
 Devonian, Mesozoic
 Palaeozoic:
-The further development of eastern Australia is marked by the eastward movement
of subduction zones
-A volcanic island arc ran through what is now Western Victoria and was accreted to
the continental coast
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-During the Ordovician there was a subduction zone in what is now central NSW and
this was accreted to the continental coast
 Oogenesis:
-The process of mountain building
-Can take tens of millions of years and build mountains from plains or even the ocean
floor
-Can occur due to continental collision or volcanic activity
-Frequently rock formations that undergo orogeny are severely deformed and undergo
metamorphism
-Usually produces long linear structures, known as Orogenic belts associated with
subduction zones
 Palaeozoic continued
-During the Silurian and Devonian the subduction zone moved east again to the New
England extending northwards in Queensland
 During the late Carboniferous to the Triassic the supercontinent Pangaea was forming
 By the end of the Triassic all of Eastern
 Early Cretaceous, sea floor spreading began in north western Australia and moved
down the western edge of the continent as India moved away from Aus and the Indian
ocean grew
 Rifting occurred on the southern margin of the continent resulting in the formation of
the Tasman sea
 Papua New Guinea began to form in the Early Cretaceous as the pacific plate was
subducted under the Australia-Indian plate
 In the last 5 MA New guinea has risen some 6 KM
-Present information as a sequence of diagrams to describe the plate tectonic super-cycle
concept
17
Focus Area 4- Natural disasters
-Identify where earthquakes and volcanoes are currently likely to occur based on 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.
-Describe methods used for the prediction of volcanic eruptions and earthquakes
Technology
How it is used
Radar images
Images taken from satellites before earthquakes, show horizontal
and vertical shifts in the ground surface. This can be used to predict
what could happen in the future. Radar images from satellites show
the topography of the land and can help to detect the history of past
earthquakes. When radar images are combined with other
information including seismometers and GPS, it is easier to detect
when an Earthquake may occur
Magnetometers:
Magnetometers measure the magnitude and direction of a magnetic
field. The magnetic field of the Earth changes as strain in rocks vary,
so changes in magnetism may warn that plates are moving.
Magnetism is measured by using magnetometers. They can
distinguish between general changes and changes caused by tectonic
plate movements
GPS Stations:
Global Positioning Systems receive satellite signals which are
transmitted to an observatory. Signals record the exact location of
the GPS. Changes in positions indicate the crust has moved. Scientists
can monitor changes in direction, speed and altitude of plates.
18
Creep meters
Used to measure ground movement, or creep. A wire is stretched
between two rods on either side of the fault. A weight is placed at
one end of the wire which lines up with a scale. Measurement
changes if the fault moves. Blocks of crust along faults either move in
a slow and steady motion known as a creep, or they jolt in a violent
and sudden movement. Creeping movements cause small to
moderate earthquakes. Sometimes the creeping areas along a fault
become stuck. Seismic activity drops and a seismic gap results
Seismometers
Seismometers are used to record vibrations in the ground. They are
incredibly sensitive and can pick up even the slightest tremors. Many
are powered by solar energy. All of the earth’s seismic activity can be
recorded from vibrations in the Earth’s crust and drawn onto a
seismogram. Signs of tiny earthquakes recorded may mean that a
volcanic eruption can occur
-Describe the general physical, chemical and biotic characteristics of a volcanic region and
explain why people would inhabit such regions of risk
-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
-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
-Volcanic landscapes have aesthetic attraction for people. Mountains create beautiful
scenery and symmetrical volcanic cones have been important to many cultural beliefs
-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.
-Describe hazards associated with earthquakes, including ground motion, tsunamis and
collapse of structures
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
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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
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.
Poisonous gas emissions from volcanoes include carbon monoxide (CO), sulfur dioxide (SO2),
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.
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.
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.
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.
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.
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-Justify continued research into reliable prediction of volcanic activity and earthquakes
Some possible arguments for continued research are:
-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.
-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.
-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:
-Earthquakes are difficult or impossible to predict because of their inherent random
behaviour. Efforts should be channelled into hazard mitigation.
-Providing warnings can cause panic in a population, potentially causing more problems
than if an earthquake or a volcanic eruption was not predicted.
-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.
-Describe and explain the impacts of shock waves (earthquakes) on natural and built
environments
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.
The impact of shockwaves is related to their intensity.
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.
21
Intensity
(Mercalli)
II
IV
Title
VI
VIII
Feeble
Moderate
Rather
strong
Strong
Destructive
IX
Ruinous
X
Disastrous
XI
Very
disastrous
Catastrophi
c
XII
Effects on natural and built
environments
Suspended objects sway
Windows and dishes rattle
Dishes and windows broken
Chimneys topple
Weak structures severely damaged;
strong structures slightly damaged
Total destruction of weak
structures. Foundations damaged.
Underground pipes broken
Only best buildings survive. Ground
badly cracked
Few masonry structures remain
standing. Broad cracks in ground
Total destruction. Waves seen on
the ground
Example
(Richter magnitude)
Port Jackson, 1788
Lithgow, 1985 (4)
Meckering, 1968 (5)
Newcastle, 1989 (5.6)
Kobe, 1995 (7.2)
Western India, 2001 (7.9)
Chile, 1960 (9.5)
-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:



the Eastern region, covering the eastern highlands and coastal areas
the Central region, extending from near Adelaide to the Simpson Desert
the Western region, encompassing several distinct zones.
-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
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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, process and present information from secondary sources to chart the location of
natural disasters worldwide associated with tectonic activity and use available evidence
to assess the patterns in terms of plate tectonics
ONE NOTE
-Gather information from secondary sources to identify the technology used to measure
crustal movements at collision boundaries and describe how this is used
ONE NOTE
Gather information from secondary sources to present a case study (2011 Christchurch
Earthquake) of a natural disaster associated with tectonic activity that includes:
– An analysis of the tectonic movement or process involved
February 22nd 2011-Earthquake in Christchurch magnitude 6.3 hit 20km South East of
Christchurch 5km in depth. Aftershock of Canterbury earthquake 5 months ago situated 20
miles (32km) west of the city with a magnitude of 7.1. New Zealand situated on the Pacific
and Australian plate boundary causing major seismic activity. Cause of this earthquake: a
previously unrecognized fault at a conservative boundary, an offshoot from the Alpine fault
– Its distance from the area of disaster
Most earthquakes near Christchurch occur due to the Alpine fault, a conservative plate
boundary 100km from Christchurch. This earthquake was caused due to a new fault, or a
fault which has previously gone unrecognized
– Predictions on the likely recurrence of the tectonic movement or process
The high magnitude of this earthquake lessens the chance of another major earthquake in
this region for a large period of time
– Technology available to assist prediction of future events
Radar images: Images taken from satellites before earthquakes, show horizontal and vertical
shifts in the ground surface. This can be used to predict what could happen in the future.
Radar images from satellites show the topography of the land and can help to detect the
history of past earthquakes. When radar images are combined with other information
including seismometers and GPS, it is easier to detect when an Earthquake may occur
Magnetometers: Magnetometers measure the magnitude and direction of a magnetic field.
The magnetic field of the Earth changes as strain in rocks vary, so changes in magnetism
23
may warn that plates are moving. Magnetism is measured by using magnetometers. They
can distinguish between general changes and changes caused by tectonic plate movements
GPS Stations: Global Positioning Systems receive satellite signals which are transmitted to
an observatory. Signals record the exact location of the GPS. Changes in positions indicate
the crust has moved. Scientists can monitor changes in direction, speed and altitude of
plates.
Seismometers: Seismometers are used to record vibrations in the ground. They are
incredibly sensitive and can pick up even the slightest tremors. Many are powered by solar
energy. All of the earth’s seismic activity can be recorded from vibrations in the Earth’s crust
and drawn onto a seismogram. Signs of tiny earthquakes recorded may mean that a volcanic
eruption can occur
Creepmeters: Used to measure ground movement, or creep. A wire is stretched between
two rods on either side of the fault. A weight is placed at one end of the wire which lines up
with a scale. Measurement changes if the fault moves. Blocks of crust along faults either
move in a slow and steady motion known as a creep, or they jolt in a violent and sudden
movement. Creeping movements cause small to moderate earthquakes. Sometimes the
creeping areas along a fault become stuck. Seismic activity drops and a seismic gap results
– An investigation of possible solutions to minimise the disastrous effects of future events
Earthquakes cannot be predicted, however, the effects of earthquakes can be minimised.
Buildings in earthquake prone areas are built stronger so they can survive earthquakes.
Earthquake-proof buildings are built on concrete rafts and reinforced. This is so that when
the ground shakes, the whole building sways and stays in one piece
Focus Area 5- Plate tectonics and climate
-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 sulfur 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.
-Describe and explain the potential and observed impacts of volcanic eruptions on global
temperature and agriculture
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
24
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 agriculture:
Volcanic eruptions have the potential to devastate agricultural 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 SO2 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:
-Crop loss was estimated at $100 million, or seven percent of the national crop value for
that region.
-Fifty percent of the alfalfa hay crop was ruined
-Timber to the value of $100 million dollars was destroyed
25