Plate Tectonics and Associated Hazards

Notes from the bluey silver book by Philip Allan Updates for AQA A2 Geography.

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  • Created on: 24-04-12 18:01
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Plate Movement
The theory of plate tectonics
1912, Alfred Wegener published theory of single continent (Pangaea) that existed 300 million
years ago
o Later split into 2 continents
Laurasia in north
Gondwanaland in south
Different evidences to prove single continent
o Geological evidence
Bulge of south America fitting into indent below west Africa
Evidence of glaciation of late Carboniferous period ­ deposits found in south
America, Antarctica and India
Rock sequences in northern Scotland and Eastern Canada
o Biological evidence
Fossil brachiopods found in Indian limestones ­ comparable with fossils in
Australia
Fossil remains in South America and southern Africa
Fossil remains in coal in India and Antarctica
Earth's layers
o Core ­ made up of dense rocks containing iron and nickel alloys
Divided into solid inner core / molten outer core
o Mantle ­ made up of molten / semi-molten rocks containing lighter elements such as
silicon and oxygen
o Crust ­ even lighter because of elements
Most abundant = silicon, oxygen, aluminium, potassium, sodium
Varies in thickness
Oceanic crust ­ 6-10 km thick
Continental crust ­ 30-40km thick
Under highest mountain ranges ­ 70km thick
o Lithosphere ­ consists of crust and the rigid upper section of the mantle
Approximately 80-90 km thick
o Asthenosphere ­ below lithosphere
Semi-molten
Continental Crust Oceanic Crust
Thickness 30-70 km 6-10 km
Age Over 1,500 million years Less than 200 million years
Density 2.6 (lighter) 3.0 (heavier)
Composition Mainly granite; silicon, Mainly basalt; silicon,
aluminium, oxygen (SIAL) magnesium, oxygen (SIMA)
o Hot spots ­ generate thermal convection currents within asthenosphere
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Seen in Hawaii (see picture below)
Features of plate margins
Constructive (divergent) margins
o Plates move apart in oceanic areas
o RIDGE VALLEYS
Longest continuous uplifted features
Precise form influenced by rate at which plates move apart
Slow rate ­ 10-15mm/year, produces wide ridge axis (30-50km) and
deep central rift valley (3,000m)
Intermediate ­ 50-90mm/year, produces well-marked rifts (50-200m
deep) with smoother outline
Rapid rate ­ >90mm/year, produces smooth crest and no rift
Destructive (convergent) margins
OCEANIC / CONTINENTAL CONVERGENCE
Where oceanic and continental plates…read more

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OCEANIC/OCEANIC CONVERGENCE
Ocean trenches and island arcs are the features associated ­ takes place
offshore
CONTINENTAL/CONTINENTAL CONVERGENCE
Plates forming continental crust have much lower density than underlying
layers, not much subduction where they meet
Due to there being no subduction ­ no volcanic activity.…read more

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Vulcanicity
Distribution
Most volcanic activity associated with plate tectonic processes, mainly located along plate
margins
Such activity therefore found:
o Along ocean ridges
o Associated with rift valleys
o On or near a subduction zones
o Over hot spots
Volcanic Eruptions
Vary in form, frequency and type of volcanic eruption
Intrusive volcanic landforms
o When magma forced the surface, only small amount of lava reaches that level
o Most magma is intruded into crust where it solidifies
o Often exposed after erosion
o Batholith ­…read more

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Main types of extrusive volcanic landforms
o Basic/shield volcanoes ­ formed from free flowing lava. Have gentle sides and cover a
large area
o Fissure/lava plateaux ­ extensive lava flows are basaltic in nature, flow great distances
o Acid/dome volcanoes ­ steep sided convex cones, viscous lava that is rhyoltic
o Ash and cinder cones ­ formed from ash, cinders and volcanic bombs ejected from
crater
o Composite cones ­ classic pyramid shaped volcanoes.…read more

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Volcanic gases ­ include carbon dioxide, carbon monoxide, hydrogen sulphide, sulphur
dioxide and chlorine
Secondary effects :
o Lahars ­ volcanic mud flows
o Flooding ­ melting of glaciers and ice caps
o Tsunamis ­ giant sea waves generated after violent caldera-forming events
o Volcanic landslides
o Climatic change ­ ejection of vast amounts of volcanic debris into atmosphere can
reduce global temperatures and is believed to have been an agent in past climatic
change
Volcanic effects become a hazard when they have an impact…read more

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Seismicity
Causes of earthquakes
As crust of Earth is mobile, tends to be a build up of stress within the rocks
When pressure is suddenly released part of the surface experience an intense shaking motion
Point of pressure release is known as focus ­ point immediately above on earth's surface is
called epicentre
Depth of focus is significant and 3 broad categories of earthquake are recognised:
o Shallow focus (0-70km deep) ­ tend to cause the greatest damage / account for 75% of
all earthquake…read more

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Seismic records enable earthquake frequency to be observed ­ records only date back to 1848
when instrument to record seismic waves was first developed
Effects of earthquakes
Initial effect is ground shaking
Severity depends on magnitude of earthquake, distance from epicentre, local geological
conditions
Secondary effects:
o Soil liquefaction when violently shaken, soils with high water content lose mechanical
strength / start to behave like fluid
o Landslides/avalanches slope failure as a result of ground shaking
o Effects of people and built environment
Collapsing buildings…read more

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