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Lecture notes - tectonics
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A Level Geography Revision – Tectonic HazardsEQ1: Why are some locations more at risk from tectonic hazards?
Key idea 1.1: The global distribution of tectonic hazards can be explained by plate boundary and
other tectonic processes.
1.1a: The global distribution and causes of earthquakes, volcanic eruptions, and tsunamis
Earthquakes Volcanoes Tsunami
Distribution Close to Close to 90% of events
tectonic plate tectonic plate happening in
boundaries boundaries the Pacific
70% found in About 500 Basin
the ‘Ring of active Most
Fire’ (Pacific volcanoes generated at
Ocean) throughout subduction
Also found the world with zones
along old fault around 50 (convergent
lines erupting each boundaries)
year
Causes Two plates Pressure Basically,
stick as they building up underwater
move past inside the earthquake
each other Earth Violent
Generated Pressure is motion
due to a released = displaces a
sudden result of plate large volume
release of movement, of water in
stress magma the ocean
Energy explodes to water column
radiated out the surface
in all
directions
1.1b: The distribution of plate boundaries resulting from divergent, convergent, and conservative
plate movements (oceanic, continental, and combined situations)
Divergent margins = two plates move apart. Mid-ocean ridges and rift valleys are formed here
Convergent margins = two plates collide. Denser oceanic crust is forced under lighter crust.
1. Oceanic plate meets continental plate = deep ocean trenches
2. Oceanic plate meets oceanic plate = (island arcs)
3. Continental plate meets continental plate = collision margin
Conservative margins = two plates sliding past each other. No volcanic activity but produces
powerful earthquakes
,Key Idea 1.2: There are theoretical frameworks that attempt to explain plate movements
1.2A and B The Theory of Plate Tectonics: The theory of plate tectonics and its key elements
(Earth’s internal structure, mantle convection, palaeomagnetism, and sea floor spreading,
subduction and slab pull)
Plate Tectonics
Plates move at a speed of 2-5 cm per year
7 large plates (African, Pacific etc.) and smaller (Nazca, Philippine) + dozen small microplates
All fit together – jigsaw of rigid lithosphere
Each plate – 100km thick (w thinner oceanic/continental crust on top)
Lower part – consists of upper mantle. Upper part – either oceanic or continental crust
Key theory – Continental Drift (Alfred Wegener)
Hypothesised that the continents were all conjoined previously called ‘Pangaea’
Suggested that ‘Pangaea’ broke up a long time ago – continents moved to their current
positions
Hypothesised as ‘Continental Drift’
Key words:
Subduction Zone: the area in the mantle where a tectonic plate melts
Gravitational Sliding: Because of the increasing heat between them, constructive margins
are elevated, creating a "slope" for oceanic plates to slide down. At destructive margins, this
happens.
Slab Pull: Newly formed oceanic crust sinks into mantle – pulling rest of plate with it
Mantle convection
Tectonic hazards = caused by Earth’s internal heat engine
Radioactive isotopes – Uranium-238 and Thorium-232 in
core and mantle – generate lots of heat flow
to the Earth’s surface
Heat generates convection currents in mantle
Earth’s interior is dynamic
, 1.2C Impacts on Hazards – Physical processes impact on the magnitude and type of volcanic
eruption, and earthquake magnitude and focal depth
1. Constructive margins:
Eruptions are small and uncontrolled, as the erupted basalt lava has a low gas content and high
viscosity. Earthquakes are shallow, less than 60km deep, and have low magnitudes of under 5.0.
Oceanic-oceanic: Minor, shallow earthquakes
Continent-continent: Basaltic volcanoes and minor earthquakes
2. Conservative margins:
Earthquakes along conservative boundaries often have shallow focal depths, meaning high
magnitude earthquakes can be very destructive. Volcanic activity is absent.
Oceanic-Continent: Frequent, shallow earthquakes, but no volcanic activity
3. Destructive margins:
Earthquakes at subduction zones occur at a range of focal depths from 10km to 400km, following
the line of the subducting plate. This is called the Benioff Zone. – can produce large earthquakes up
to 9.0 magnitude.
Key Idea 1.3: Physical processes explain the causes of tectonic hazards
1.3A: Hazards causes by earthquakes: Earthquake waves (P, S & L waves) cause crustal fracturing,
ground shaking and secondary hazards (liquefaction and landslides)
1. P-Waves (Primary waves) -– Move fastest, reach the surface first, travelling through solids
and liquids, shaking backwards and forwards. They are the least damaging
2. S-Waves (Secondary waves) – Move slower, travelling only through solids, moving with a
sideways motion, shaking at right angles to the direction of travel. More damaging than P
waves
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