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I.
Mechanism of Plate movement 92+2
1.
The lithosphere is divided into broken segments called plates.
2.
The asthenosphere below is a soft layer where the rock is heated (by the decay of
radioactive elements in the mantle) close to its melting point.
3.
Magma flows and forms convection currents.
4.
So, plates are drifting.
5.
They may converge, diverge or slide past each other.
II. Evidences that supports the plate tectonics theory / Evidences on sea floor spreading
to support plate movements 92+2 & 01#1
1.
Jigsaw puzzle of continents
There is a jigsaw like fit between the east coast of South America and the west coast
of Africa.
2.
Biology
Mesosaurus was a small reptile living in Permian times. Its remains have
been found
only in South Africa and Brazil.
3.
Geology
a)
Rocks of similar type, age, formation and structure occur in south-east Brazil and
South Africa.
b) Appalachian Mountains of the Eastern USA correspond geologically with mountains in
north-west Europe.
4.
5.
Climatology
a)
Coal, formed under warm, wet conditions, is found beneath the Antarctic ice cap.
b)
Evidence of glaciation had been noted in tropical Brazil and central India.
Palaeomagnetism
Magnetic field reverses periodically. Alternations in alignment are almost symmetrical
in rocks either side of the Mid-Atlantic Ridge.
III. Evidences on sea floor to support plate movements 92+2 & 01#1
1.
Age of rocks
a)
After new rocks are formed at the mid-oceanic ridge, they are pushed away by
continuous swelling of magma from the asthenosphere.
b)
So, with the aid of fossils, age of rocks ↑ with distance from the mid-oceanic
ridge.
c)
The newest rocks were at the center and were still being formed. The mid-oceanic
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ridge is a birth place of oceanic crust.
2.
Age + thickness of sediments
a)
age of sediment ↑ from the mid-oceanic ridges outwards.
b)
Sediments were absent at the ridge crest, and they thicken with increasing
distance from the ridge.
3.
Palaeomagnetism
Magnetic field reverses periodically. Alternations in alignment are almost symmetrical
in rocks either side of the Mid-Atlantic Ridge.
4.
Alignment of volcanoes
Alignment of volcanoes of different ages from hot spots indicates the direction of plate
movement. The ages of each progressive volcano and their relative locations show the
speed of the plate movements.
5.
Faults
Displaced blocks indicated the degree and direction of relative movements at transform
faults. All these (1) to (5) are evidences of sea-floor spreading.
IV. Mountain building processes 89#1 ; 93#2
1.
Vulcanicity → volcanoes for pressure release
extrusive vulcanicity : ejection of magma from the mantle onto the earth’s surface
2.
Folding : bending of the earth’s crust ∵ compression → fold mountains
3.
Faulting : breaking of the earth’s crust, displacement of rock strata ∵tension or
compression → block mountains
V.
Landform features associated with mountain building processes 93#2
1.
Volcanism ――
Mid-oceanic ridge
Andesitic volcanoes
Volcanic island arcs
Hot spot volcanoes
2.
Folding ――
Cordilleran +
Himalayan fold mountain
3.
Faulting ――
Block mountains / Horsts
VI. Global distribution of extrusive vulcanicity 89#1
1.
Features : mid-oceanic ridge, volcanic islands, lava flow, volcano, island arcs + hot spot.
2.
Four regions :
a)
Circum-Pacific Ring of Fire ―― The west Pacific includes New Zealand, Indonesia,
the Philippines and Japan, whereas the east Pacific includes the Aleutians, the
Rockies and the Andes.
b)
Mediterranean-East African Belt,
c)
Mid-oceanic Belt, i.e. along the Mid-oceanic ridge, including Iceland and also
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d)
Hot spots near the Hawaiian Islands, S.W. Indian Ocean.
VII.Features associated with extrusive vulcanicity 89#1
A.
Constructive plate margin
1.
mid-oceanic ridge 92+2
a)
diverging convection magma current at the upper mantle drags the plates
b)
2 plate diverge (North American and South American plates move apart from
the Eurasian and African plate)
c)
magma swells up / rises to fill the gap, i.e. fissure eruption of basaltic lava to
release pressure
d)
lava cools down + solidifies → new crust on sea floor → Mid- Atlantic Ridge,
e)
tensional force / tension
→ faulting : vertical displacement of rocks
→ rift
valley / axial rift at its top along the ridge
f)
sea floor spreading in Atlantic Ocean (widening of 5cm/yr.)
2. volcanic islands
a)
In 1783, a fissure of 30 km opened up.
b)
With repeated fissure eruption, Iceland, a volcanic island, is formed.
c)
At the beginning, magma produces submarine volcanoes, e.g. Surtsey, south of
Iceland on the Mid-Atlantic Ridge
d)
With repeated eruption (1963-67) → cone grows above the sea level to form a
volcanic island.
e)
The Heimaey eruption of 1973, on the other hand, began with a fissure 2 km in
length.
Fig. 1 :
3.
Formation of mid-Atlantic Ridge 96+1
lava flow / plateau
a)
2 plates diverge, magma may be ejected through fissures, rather than via
central vent.
b)
Layer upon layer, slow and quiet outflow of basic lava forms an extensive
highland , e.g. Deccan plateau in India.
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B.
Destructive plate margin
1.
andesitic volcano 92+2
a)
convergence of an oceanic and a continental plate → compression
→ the
denser oceanic plate subducts beneath the continental plate.
b)
Upon entering the hot asthenosphere, the oceanic crust melts under great heat
+ pressure → magma.
c)
∵ lighter, magma rises slowly along lines of weakens → volcanic activity.
d)
ejection of ash and eruption of lava → volcano (extrusive vulcanicity)
e)
E.g. Mt. St. Helens along Cascade Range is formed at the edge of North
America due to the convergence of Juan De Fuca plate (oceanic plate) and
North American plate (continental plate)
Fig. 2 :
2.
Formation of volcano 99+3
volcanic islands arc
a)
a chain of offshore volcanic islands formed at the subduction zone
b)
They are parallel to the plate margin along the coast, e.g. The Aleutians and the
Philippines along the Circum-Pacific Ring of Fire
Fig 3 : Formation of volcanic island arcs
92+2
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C.
Plates’ interior / Hot spot (volcanoes)
1.
uneven distribution of heat within the core, where T0 + pressure are high →
2.
magma ejects out of the mantle along the lines of weakness onto the earth’s
surface,
3.
forming magma plumes of hot spot,
4.
e.g. the Hawaiian Islands with Mauna Loa (volcanic eruption in 1975)
VIII.How vulcanicity reflects instability along destructive plate margin 89#1
1.
indicated by subduction, vulcanicity, folding, faulting and earthquakes
2.
Subduction + vulcanicity + earthquake
a)
an oceanic plate + a continental plate converge, the former subducts beneath
the latter ∵ its higher density. → trench, e.g. Japan Trench
b)
oceanic crust melts
c)
→ friction → earth tremours. Earthquake focus are thus located along the
plane of subduction, becoming deeper as sinking material moves further from
the zone of subduction.
d)
Japan is one of the places where earthquake take place frequently, e.g Kobe in
1995 (7.2 on Richter Scale).
3.
Collision + folding + faulting + earthquake
a)
2 plates collide → compression → buckling up of sediments derived from
weathering + erosion of continents nearby
b)
intense folding → cracks / fissures / faulting
c)
∵ friction is created → earthquake
d)
e.g. the Himalayas is formed as a result of the collision of Indo-Australian plate
and the Eurasian plate.
Fig. 4 :
Formation of the Himalayas
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IX. Faulting (Textbook)
1.
Def. : breaking of the earth’s crust / vertical displacement of rock strata ∵
tension or compression
2.
3.
Types :
a)
Normal fault (tension)
b)
Reverse fault (compression)
c)
Tear fault (lateral displacement of rocks)
Landform features
a)
Block mountain / horst, e.g. the Black Forest in Germany
i.
∵ tension or compression
ii.
when parallel faults , either when the central block of land is uplifted (∵
compression) or when the side blocks sink (∵tension)
iii.
b)
appearance : flat top + steep sides
Rift valley/ graben, e.g. the East African Rift Valley
i.
∵ tension or compression
ii.
when parallel faults , either when the central block of land sinks (∵tension)
or when the side blocks are uplifted (∵compression)
iii.
appearance : elongated with steep slopes + flat floor
Case Study : Formation of the East African Rift Valley
i.
a fault line exists between African plate + Arabian plate
ii.
when the plates move apart ∵tension → the Great East African Rift
valley (4000km long) with block mountains on its sides, e.g. Kharas, is
formed
iii.
sometimes it is associated with volcanic activity, e.g. Kilimanjaro + Kenya
iv.
So, it can be said that Red sea is newly formed.
Fig. 5 : formation of block mountain + rift valley
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X.
Associated natural hazards along plate margins 88#2. & 99+3 & 96+1
1.
volcanic activity on surface (C + D)
Constructive plate margin
tension → 2 plates diverge → magma swells up / rises to fill the gap, i.e. fissure
eruption of basaltic lave to release pressure → volcanic islands, e.g. Iceland (1783)
+ Surtsey (1963-67)
Destructive plate margin
compression + subduction → sinking materials are under great heat + pressure →
magma (lighter) → magma rises along fissures in the crust → vulcanicity
2.
earthquakes / seismic activities (C + D + C)
Constructive plate margin
Heimaey eruption (1973)
Destructive plate margin
a)
crustal material sinks under the overlying plate at the subduction zone
b)
→ friction ∵ sinking material rules against overlying rock → earth tremours
c)
earthquake foci are thus located along the plane of subduction,
d)
becoming deeper as the sinking material moves further from the subduction
zone
Conservative plate margin
a)
plates slide past each other
b)
e.g. N American plate moves NWwards by 1 cm /yr. whereas Pacific plate moves
NWwards by 6 cm /yr.→ San Andreas Fault in California
c)
pressure builds up → friction → San Francisco earthquake : 1906(8.2 on the
Richter scale) & 1989 (6.9 on the Richer scale)
3.
tidal waves /tsunamis (C + D + C)
if the earthquake focus is under-sea
4.
landslips / landslides / mudflow / avalanches (C + D + C)
if the earthquake focus is on land
XI How volcanic eruption causes mudflow 99+3
1.
volcanic eruption → snow-melting → melt-water looses the pyroclastic materials
→ mudflow
2.
avalanche enters a river channel → mudflow
3.
mudflow = lahar (a technical term)
4.
volcanic debris avalanche (including mixture of mud, block of pyroclastic materials
and trees) may occur.
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XII Damages brought by mudflow 99+3
1.
mudflow my spread over a long distance or
2.
even extend and floods down-valley
3.
→ burial of land + settlements + inudation by water
XIIIDamages caused by vulcanicity
88#2. & 99+3 & 96+1 & 99+1 &
Case study : Kobe earthquake, Japan (January 1995)―― 7.2 on Richter scale
1.
2.
Causes
a)
located on Pacific Ring of Fire
b)
convergence of Pacific plate + Eurasian plate ∵ compression
c)
Pacific plate is subducted ∵ its higher density (by 10 cm /yr.)
d)
→ Japan trench (8 km deep)
Associated natural hazards
a)
volcanic activity on surface (C + D)
b)
earthquakes / seismic activities (C + D + C)
c)
tidal waves /tsunamis (C + D + C)
d)
landslips / landslides / mudflow / avalanches (C + D + C)
3.
Damages to man
A.
Immediate / Primary effects
a)
loss of lives (＞5,000 people)
b)
injury (＞30,000 people)
c)
destruction of properties (about 500,000 houses before the earthquake,
68,000 were fully destroyed, 55,000 were partially destroyed + 7,0000 were
completely burnt)
d)
ruin old habitat + crops
e)
destroy cities, towns + community services,(public facilities, e.g. the City Hall +
hospitals are collapsed)
B.
Secondary effects, e.g. fires, ruptured gas- and water supplies
a)
displace population (some 300,000 homeless)
b)
fires ( 175 fires → 80 ha. were burnt)
c)
cut off of power supply for 8 days + destruction of 80% gas supplies
d)
disruption of water supplies for 2.5 months
e)
destruction + interruption of transport network, e.g. ＞130 km of bullet train
network was closed, Hanshin Expressway (collapse of 1 km of the elevated
highway)
f)
interruption of communication network, e.g. 25% phone lines were damaged
g)
interrupt community facilities (80% of the schools, museums + sport facilities
were severely damages)
h)
disrupt commerce ( ∵ collapse of buildings + long period of residence in
evacuation shelters + unemployment)
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XIV Why damages varies in severity 96+1
Severity of damages depends on :
1.
the intensity and magnitude of crustal movements
2.
population density of the areas involved
3.
preventive measures adopted
4.
knowledge + perception
XV Measures to minimize damages by earthquake 99+3
1.
Avoid / minimize construction on earthquake prone areas
2.
Forecasting volcanic eruptions over the short-term can ↓ the loss of lives. This
can be achieved by :
a)
seismic monitoring : deploying a network of portable seismometers around the
volcano ∵ seismometers are capable of detecting rock movement in the
earth’s crust as some rock movements may be associated with the rise of
magma beneath an awakening volcano.
b)
gas monitoring
c)
deformation studies
3.
earthquake prediction by radar → early warning for evacuation
4.
construction should be done by earthquake proof materials / upgrade aseismatic
standards for buildings
5.
Construct single-storey buildings instead of high-rise buildings ∵ the latter
responds slowly and shock waves are amplified as they move up the structure
6.
education or training of emergency exit to ↑ awareness of disaster evacuation
7.
dam construction to minimize strong wave attack along the coast / on the volcanoes’
slope to block the down-slope flowing floods
XVI.Benefical impacts of vulcanicity 96+1 & 99+3 & Textbook
1.
basic lava and volcanic ashes provide fertile soils for farming
2.
create valuable mineral deposits / ores, e.g. gold, silver, copper
3.
create beautiful scenery, e.g. geysers + volcanoes are tourist attractions, e.g.
Yellowstone National Park → promote tourism → ↑ revenue for the local
community
4.
geothermal power can be generated for electricity, e.g. Iceland + New Zealand
5.
igneous rocks can be used as building materials (ignimbrites, pumice, etc) in Naples +
Aberdeen
XVII.Conclusion
1.
Many tectonic processes are found along plate margins.
2.
Plate boundaries = zones of instability which associate with vulcanicity and earthquake.