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Summary - Unit 1 - Dynamic Landscapes (9GEO-01)
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A full in-depth summary of content notes following full specification for tectonic hazards + coastal landscapes (including all case studies)
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Topic 1 – Tectonic processes and hazards:Enquiry Question 1: Why are some locations more at risk from tectonic hazards?
1.1 Key Idea: The global distribution of tectonic hazards can be explained by plate boundary and other
tectonic processes.
1.1a) Distribution and causes
Plate boundaries: the causes of earthquake and volcanic eruptions
Constructive - Two plates move away from each e.g., North
(divergent): other. American +
- In oceans it can cause mid- Eurasian plate
Atlantic ridge
Destructive - When 2 plate move together and e.g., Nazca +
(convergent): the dene one subducts under South American
- Oceanic subducts under
continental crust
Conservative - Occurs when two plates slide e.g., North
(transform): past each other in opposite or America +
same directions Pacific plate
- Can lock due to friction for many (San Andreas
years + pressure is released with fault)
shallow focus earthquakes
Collision: - Two of the same crust move e.g., Continental
together + Continental
- Both subducts as low density e.g., Indian plate
- Crust buckles and thickens + Eurasian plate
causing fold mountains – Himalayan
mountains
Benioff Zone:
- The boundary between an oceanic plate which is undergoing subduction beneath an overriding
continental plate
- The sinking oceanic plate is colder than the crust into which it is sinking - causing stresses and may
trigger earthquakes
1.1b) Distribution of boundaries
Constructive (divergent): Low focus
Low magnitude earthquakes
Destructive (convergent): Frequent earthquakes
Volcanic eruptions
Conservative (transform): Lithosphere is neither created nor subducted, and while conservative plate
margins do not result in volcanic activity
They are sites of extensive shallow focus earthquakes, occasionally of
considerable magnitude.
,1.1c) Intra-plate
Hotspot volcanoes:
- Some volcanic eruptions are 'intra-plate' meaning there are distant from a plate boundary at locations
called mid-plate hotspots,
o E.g., Hawaii and the Galapagos Islands.
At these locations:
1. Isolated plumes of convection heat (mantle plumes) rise towards the surface, generating basaltic
volcanoes that tend to erupt continuously.
2. A mantle plume is stationary, but the tectonic plate above moves slowly over it.
3. Over millennia, this produces a chain of volcanic islands, with extinct ones most distant from the
plume's location.
Mantle plumes:
- concentrated areas of heat convection
- At plate boundaries they are sheet-like, whereas at hot spots they are column like.
1.2 Key Idea: There are theoretical frameworks that attempt to explain plate movements
1.2a&b) The theory of plate tectonics
Earth structure:
Inner core: Radius: 1200km
Temp: 6000°
Iron + nickel (solid/dense)
Outer core: Radius: 2200km
Temp: 4000-5000°
Iron + nickel (liquid) = moves to create a magnetic field
Mantle: Radius: 2900km = most of earths volume
Temp: 3000°
Iron, magnesium + oxides of silica (heat from earth’s core keeps asthenosphere malleable
with convection current)
Crust: Continental: Oceanic:
35km thick 6km thick
Thick + old Found beneath the ocean
Less dense Found at constructive plate margin
Granite Basalt
More complex
Evidence for plate movement:
1) Continental fit South America fits into Africa
theory
2) Rock + fossils Same types can be seen either side of the oceans
3) Mountains Earths being pushed up
4) Ocean trenches Deep waters
, Mechanisms for plate movement:
1) Ridge push - Gravitational force that causes a plate to move away from the crest
of an ocean ridge, and into a subduction zone.
- It works together with Slab Pull, but is much less significant.
2) Slab pull - Cold, dense oceanic plate is subducted beneath less dense
continental plate; the density of the oceanic plate pulls itself into
the mantle.
- This occurs at destructive margins.
3) Gravitational sliding - Constructive margins have elevated altitudes because of the rising
heat between them, which creates a 'slope' down which oceanic
plates slide.
- This occurs at destructive margins.
4) Convectional current - Occur within the molten rock in the mantle, act like a conveyor belt
for the plates.
- Tectonic plates move in different directions.
- The direction of movement and type of plate margin is determined
by which way the convection currents are flowing.
Palaeomagnetism:
- It results from cooling magma locking in the earth’s magnetic polarity
- Proves that new oceanic crust is created by the process of sea floor spreading at mid-ocean ridges
1.2c) Impacts on hazards
Constructive: - Volcano: Eruptions are small and effusive + Basalt lava has high viscosity
- Earthquake: Shallow earthquakes, low magnitudes
Destructive: - Volcano: violent eruptions,
- Earthquake: large earthquake, large focus depth, high magnitude
Conservative: - Earthquake: shallow focal point, high magnitude, destructive earthquakes
- Volcano: N/A
1.3 Key Idea: Physical processes explain the causes of tectonic hazards
1.3a) Hazards caused by earthquakes
Earthquake:
- A sudden release of stored energy
o A pulse of energy radiates out in all direction from the earthquake focus point
Waves: the overall severity of an earthquake is linked to the amplitude and frequency of P and S waves
P waves: primary - The fastest to arrive (8km/sec)
- Cause the least damage
- Have a large amplitude and energy force
- Vibrations caused by compression
S waves: secondary - Arrive after p waves (4km/sec)
- Shake the ground violently, causing damage
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