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Summary Edexcel A Level Geography - Dynamic Landscapes - Tectonic Tectonic Processes and Hazards
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Tectonic processes and Hazards1. Locations at risk from tectonic hazards
1.1. The global distribution of tectonic hazards
Global distribution of earthquakes:
Main earthquake zones are found often in clusters along plate boundaries
Approx. 70% of all earthquakes are found in the ‘Ring of Fire’ in the Pacific Ocean
The most powerful earthquakes are associated with convergent or conservative boundaries
The distribution of earthquakes reveals the following pattern of tectonic activity:
o The oceanic fracture zone (OFZ) – belt of activity through the oceans along mid-
ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California
o The continental fracture zone (CFZ) – belt of activity following the mountain ranges
from Spain, via the Alps to the Middle East, the Himalayas to the East indies
o Small earthquakes in continental interiors.
Global distribution of volcanoes:
Violence of volcanoes is determined by the amount of dissolved gas in the magma and how
easily the gases can escape
There are about 500 active volcanoes and on average 50 of them erupt each year
Plate boundary types and their distribution:
1. Divergent (constructive) – at these locations there are large numbers of shallow focus and
generally low magnitude earthquakes
2. Convergent (plates move together) – actively deforming collision locations with plate
material melting in the mantle, causing frequent earthquakes & volcanoes
3. Conservative – one plates slides against another. Lithosphere is nor created or destroyed.
They are sites of extensive shallow focus earthquakes
,Plate movement and earthquake type:
Places where tectonics move away from each other are the
divergent ‘spreading ridges’ in the oceans. New oceanic crust,
which is thinner and denser than continental crust is formed.
Earthquakes seen at this boundary tend to be frequent, small
and low risk
Locations where plates slide past each other can present more
risk.
Plate boundaries that generate the most damaging earthquakes
are when two plates move towards each other (convergent)
o Typically when this happens, one starts sliding under
another
o As the strain builds over time in the subduction zone,
the friction between the two masses of rock is
overcome and releases energy
o E.g. tsunami-generating earthquakes in Japan 2011
Plate movement and volcanic activity:
Destructive plate boundaries – these occur where two plates
are moving together. Here they form either a subduction zone
or a continental collision.
o When a dense oceanic plate collides with a less-dense
continental plate, the oceanic plate subducts under the
continental crust, forming a subduction zone
o During collision of 2 continental plates, large mountain ranges form e.g. the
Himalayas
Divergent – creates rift volcanoes where plates diverge from one another.
o These are generally less explosive and more effusive, especially when they occur
under water e.g. the Mid-Atlantic ridge
Hotspot volcanoes – found in the middle of tectonic plates and are thought to be fed by
underlying mantle plumes that are unusually hot
Hotspot volcanoes and mantle plumes:
The presence of a hotspot is inferred by anomalous volcanism e.g. Hawaiian volcanoes
within the pacific plate
A volcanic hotspot is an area in the mantle from which heat rises as a hot thermal plume
from deep in the Earth
High heat and lower pressure at the base of the lithosphere enables melting o the rock
The molten magma rises through cracks and erupts to form active volcanoes
As the tectonic plate moves over the stationary hotspot, the volcanoes are rafted away and
new ones form in their place
As oceanic volcanoes move away from the hotspot, the cool and subside, producing older
island or a chain of volcanoes e.g. Hawaiian Islands
, 1.2 Theoretical frameworks and plate movements
There are two different types of crust
o Thin oceanic crust, composed of primarily basalt
o Thicker continental crust, composed primarily of granite
Low density of the thick continental crust allows it to ‘float’ high on much higher density
mantle below
The earth’s mantle has a temperature gradient (geothermal gradient)
The highest temp. occur where the mantle material is in contact with the heat-producing
core so there is a steady increase of temp. with depth
Rocks in the upper mantle are cool and brittle
Rocks in lower mantle are hot and plastic, but not molten
Heat which is derived form the Earth’s core rises within the mantle to drive convection
currents.
These currents operate as cells
There is likely to be a combined force of convection and gravity driving tectonic plate
movement
Gravity in particular causes the denser oceanic crust to be pulled down at the site of
subduction
The Benioff Zone and subduction processes:
Key terms:
Subduction zones – where two plates moving together, often with the thinner, more dense oceanic
plate descending beneath a continental place. Where the plates are locked together, frictional stress
builds. When that stress exceeds a threshold, a ‘mega-thrust’ earthquake occurs, releasing a strain
of energy
Locked fault – a fault that is not slipping because the frictional resistance on the fault is greater than
the shear stress across the fault. Such faults may store strain for extended periods that is eventually
released in a large magnitude earthquake
The Benioff Zone is an area of seismicity corresponding with the slab being thrust
downwards in a subduction zone
The different speeds and movements of rock at this produce numerous earthquakes
It is the site of intermediate/deep-focused earthquakes
It is important in determining earthquake magnitude, since it determines the position and
depth of the hypocentre
1.3 Physical processes and tectonic hazards
Hypocentre – focus point within the ground where the strain energy of the earthquake in the rock is
first released. The distance between this and the epicentre on the surface is called the focal length
A sequence of events occur in the generation of an earthquake:
o Movements are preceded by a gradual build-up of tectonic strain, which stores
elastic energy in crustal rocks
o When the pressure exceeds the strength of the fault, the rock fractures
o This produces sudden release of energy, creating seismic waves
o Brittle crust rebounds either side of the fracture causing ground shaking
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