A* Summary Notes: Unit 1 - World at Risk (Natural Hazards)
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Course
Unit 1 - World at Risk
Institution
PEARSON (PEARSON)
Detailed notes on tectonic hazards, hydrometeorological hazards (drought, cyclones), global warming and climate change, and their impacts. These notes are based on the IAL Edexcel Geography specification, but the content is also relevant to other exam boards. Contains definitions of key terms, note...
1.3.1 -
Global Enquiry question: What are global hazards and what causes them?
Hazards
Plate tectonics and volcanic processes cause geophysical hazards (earthquakes, volcanic
eruptions, tsunami).
Event: the occurrence of a hazard.
Hazard: a natural phenomenon which has the potential to cause loss of life, injury or damage to property and
infrastructure.
Disaster: a major event that causes widespread disruption on a significant scale.
Primary impacts: direct impacts of a hazard.
Secondary impacts: impacts of direct impacts of a hazard.
Classification of Hazards
Meteorological events Hydrological events Climatological events Geophysical events
Storms, including Flooding, Drought Earthquake
tropical storms, including flash Extreme heat Volcanic eruption
mid-latitude floods, river Wildfire, Mass movement,
depressions flooding and triggered by e.g., landslide or
Tornado storm surge lightning rock fall
Lightning strike, Mass movement, Tsunami
or local hailstorm including
associated with landslide and
convection avalanche
Subsidence
Plate Boundaries
Natural
hazards
are
caused
by
geophysi
cal
processes
Destructive
,Oceanic + Continental
1. Two plates converge.
2. The denser oceanic plate subducts below the
continental plate, forming a deep ocean
trench.
3. The oceanic crust is melted as it subducts
into the asthenosphere.
4. The extra magma causes pressure to build
up, which forces its way through weak areas
in the continental plate.
5. Explosive, high pressure volcanoes erupt
through the continental plate, called
composite volcanoes.
6. Fold mountains form when sediment is
pushed upwards during subduction.
7. Friction between the colliding plates causes
earthquakes in the Benioff Zone.
E.g. Nazca plate forced under the South American
plate.
Oceanic + Oceanic
1. Two plates converge.
2. The denser oceanic plate subducts, forming a
deep ocean trench.
3. The subducted oceanic plate melts into the
mantle, forming magma.
4. The extra magma causes pressure to build
up, which creates underwater volcanoes.
5. Volcanoes rise above sea level to form island
arcs.
6. Friction between colliding plates causes
earthquakes in the Benioff Zone.
E.g. the Indian plate forced under the Eurasian
plate, which caused the 2004 Indian Ocean
tsunami.
Collision
1. Two continental plates collide.
2. Both plates have the same density, so they
do not subduct.
3. Pressure between plates forces sediment
upwards to form fold mountains.
E.g. Indian plate and Eurasian plate, which
formed the Himalayas.
Constructive
Oceanic + Oceanic
1. Two oceanic plates diverge.
2. Magma rises to the surface to fill the gap left
by the two plates separating, forming new
land when it cools.
3. As magma rises, explosive submarine
volcanoes form.
4. New land forming on the ocean floor by lava
fill the gaps is known as sea floor spreading.
E.g. Mid-Atlantic Ridge, formed by the North
American and Eurasian plates diverging.
, Continental + Continental
1. When two continental plates diverge, the
crust is forced apart, forming a rift valley.
2. Magma rises to the surface, which cools to
form shield volcanoes.
E.g. East African Rift Valley, formed by the
splitting of the Somali and Nubian plates.
Further forces influencing how constructive boundaries occur:
Ridge push: the slope created when plates move apart has gravity acting upon it as it is at a higher
elevation. Gravity pushes the plates further away, widening the gap.
Slab pull: when a plate subducts, the plate sinking into the mantle pulls the rest of the plate with it,
causing further subduction.
Conservative
1. Two plates slide past each other in opposite direction,
causing friction which causes plates to become stuck.
2. This causes a major break in the crust, called
a fault.
3. The plates often get stuck, building up stress
and pressure as the convection currents
continue to push the plates.
4. When the plates slip past in a sudden
movement, the pressure is released and the
shockwaves released from the focus can be
strong.
5. There is no volcanic activity because no
plates are destroyed and no landforms are
created.
E.g. San Andreas Fault, formed by the North
American and Pacific plates.
Convection in the Mantle
Heat is radiated from the inner core through the mantle material, causing it to convect since it behaves
like a viscous liquid.
Convection cells come into contact with the base of the crust, causing friction, which drags the crust along
in the direction of the convection.
Evidence for Plate Tectonic Theory
Characteristics of the Earth’s structure
o Source of heat from the inner core that radiates outwards through other layers.
Mantle convection
o Mantle material behaves like a viscous liquid, so it can convect, providing evidence for continental
drift.
Palaeomagnetism and sea floor spreading
o Outer core is made of hot liquid iron and convects to generate a magnetic field, which changes
direction regularly (every 100,000 years).
o When new igneous rock cools to form solid rock, they trap the magnetic field direction of the time.
There are symmetrical bands of rock with alternating bands of magnetic polarity on either side of
constructive plate boundaries, providing evidence of sea floor spreading.
Subduction and slab pull
o At a destructive plate boundary, the denser oceanic plate subducts under the continental plate.
The Earth’s Structure
, Part Temperature Density Composition Physical Seismic waves
state
Crust 400°C on Less dense Granite Solid Surface and body
surface (continental) waves
Basalt (oceanic)
Mantle 870°C Mixed density – Peridotite Solid and Body waves at
less to medium liquid layers different rates due to
density changes
Outer 4400°C to Dense 88% iron Liquid Only P waves – S wave
core 6100°C shadow zone
Inner 7000°C Very dense 80% iron Solid Only P waves
core
Earthquakes
Causes Constructive Plate Boundary
May have small earthquakes) as there is not much friction as plates diverge.
Low magnitude earthquakes generated by magma movement as plates pull apart.
Destructive Plate Boundary
High magnitude earthquakes as subducting plate ‘sticks’.
Destructive plate boundaries create the largest amount of friction, with earthquake focal
points following the Benioff zone to considerable depths.
Conservative Plate Boundary
High magnitude earthquakes along complex fault zones.
Conservative plate boundaries have a large amount of friction, and when strain is
released, the shockwaves can be strong.
1. As two plates slide past each other, the movement is not smooth, friction builds which
causes the plates to become stuck.
2. This causes a major break in the crust, called a fault.
3. The plates often get stuck, building up stress and pressure as the convection currents
continue to push the plates.
4. When the plates slip past in a sudden movement, the pressure is released and the
shockwaves released from the focus can be strong.
Intraplate Earthquakes
Small movements along these old faults build up friction and strain over time, which is
suddenly released, causing low magnitude earthquakes.
Isostatic readjustment can cause earthquakes as the crust moves up or down.
Human activities can trigger tremors, e.g., the weight of water in a reservoir, fracking of
gas or large explosions.
Distributi Interplate Earthquakes
on The majority of earthquakes occur along the interplate.
Ocean fracture zone (OFZ): a belt of activity through oceans along mid-ridges, e.g. coming
ashore in Africa and the Red Sea.
The continental fracture zone (CFZ): a belt of activity following mountain ranges from
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