EQ1: Why are some locations more at risk from tectonic hazards?
Tectonic hazard: is a threat to humans and infrastructure because of
tectonic processes.
Most earthquakes happen at or on boundaries -> most powerful at
conservative.
The global distribution of tectonic hazards: earthquakes:
- The main earthquake zones are found along plate boundaries with
about 70% falling into the pacific ring of fire.
- The most powerful earthquakes are associated with convergent or
conservative boundaries, although rare, intra-plate earthquakes can
occur. This distribution includes:
- The Ocean fracture zone: a belt of activity of earthquakes in mid
ocean ridges.
- The continental fracture zone: earthquake activity along
mountain ridges.
- Scattered small earthquakes in continental interiors as intra-plate
earthquakes often due to ancient fault lines and the hazard is
associated with the reactivation of this such as the Church Stretton
Fault in Shropshire
The global distribution of tectonic hazards: volcanoes:
- About 500 active volcanoes around the world.
- Volcanic hotspots: Fracture in crust or magma plume e.g., Hawaii
-> plates move but the magma doesn’t.
The global distribution of tectonic hazards: tsunamis:
- Distribution: The global distribution of tsunami is predictable in
terms of source areas, with around 90% of all events occurring
within the Pacific Basin, associated with activity at the plate
margins. Most occurring in subduction zones.
Plate boundary types and their distribution:
- Divergent: (constructive) margins, most clearly displayed at mid-
ocean ridges. Large numbers of shallow focus and generally low
magnitude earthquake events. Most are submarine e.g., Mid-Atlantic
Ridge with basaltic magma.
- Convergent: (where plates move together): these are actively
deforming collision locations with plate material melting in the
mantle, causing frequent earthquakes and volcanoes. E.g.,
destructive margins where a subduction zone is formed or a
continental collision, large mountain ranges like the Himalayas can
also be formed.
- Conservative: (sliding past each other) lithosphere is never created
nor subducted and while they do not cause volcanic activity, they
cause extensive shallow focus earthquakes occasionally of
significant magnitude e.g., San Andreas Fault.
, Hotspot volcanoes: are found in the middle of tectonic plates and are
thought to be fed by underlying mantle plumes such as the Hawaiian
volcanoes within the Pacific Plate. As plates move more volcanoes can
form in their place, as oceanic volcanoes move away from the hotspot,
they cool and subside to create islands, atolls and seamounts such as the
chain of volcanoes seen in Hawaii.
The Earth’s Structure:
There are two types of crust, thin oceanic crust which underlies the ocean
basins and thicker continental crust which underlies the continents, lower
density continental crust can ‘float’ much higher.
- Crust (0-100km thick)
- Lithosphere (contains upper crust and mantle)
- Asthenosphere (upper mantle, convection currents)
- Mantle (molten rock and high temperatures)
- Outer core (liquid)
- Inner core (solid)
Convection currents = internal heat engine as heat rises from the mantle
and operate as cell, moving plates in several directions.
Sea floor spreading occurs at divergent plate boundaries under the
oceans. This is a continuous input of magma forming a mid-ocean ridge
like the Mid-Atlantic Ridge. A technique involving the reconstruction of
palaeomagnetism can be used to date the age of new tectonic crust.
Key theories:
- Alfred Wegner’s Continental Drift 1912 = Pangea and now
moved apart into the continents.
- Arthur Holmes 1930’s = Mantle convection was the driving force.
- The discovery in 1960s of the asthenosphere, a weak,
deformable layer beneath the rigid lithosphere which moves.
- Palaeomagnetism theory 1960s = signals past reversals of
Earth’s magnetic field to prove new ocean crust and sea floor
spreading as magnetic strips on the seabed.
Subduction = denser is subducted to create earthquakes. Convergent
(destructive) plate margins.
Gravitational sliding = constructive, rising heat creates slight slope
between plates, natural slide so gap opens which oceanic plate slides at
convergent (destructive) margin
Slap pull = cold dense oceanic subducts continental, density pulls itself
into the mantle. Occurs at destructive.
Earthquakes:
Tectonic hazard: is a threat to humans and infrastructure because of
tectonic processes.
Most earthquakes happen at or on boundaries -> most powerful at
conservative.
The global distribution of tectonic hazards: earthquakes:
- The main earthquake zones are found along plate boundaries with
about 70% falling into the pacific ring of fire.
- The most powerful earthquakes are associated with convergent or
conservative boundaries, although rare, intra-plate earthquakes can
occur. This distribution includes:
- The Ocean fracture zone: a belt of activity of earthquakes in mid
ocean ridges.
- The continental fracture zone: earthquake activity along
mountain ridges.
- Scattered small earthquakes in continental interiors as intra-plate
earthquakes often due to ancient fault lines and the hazard is
associated with the reactivation of this such as the Church Stretton
Fault in Shropshire
The global distribution of tectonic hazards: volcanoes:
- About 500 active volcanoes around the world.
- Volcanic hotspots: Fracture in crust or magma plume e.g., Hawaii
-> plates move but the magma doesn’t.
The global distribution of tectonic hazards: tsunamis:
- Distribution: The global distribution of tsunami is predictable in
terms of source areas, with around 90% of all events occurring
within the Pacific Basin, associated with activity at the plate
margins. Most occurring in subduction zones.
Plate boundary types and their distribution:
- Divergent: (constructive) margins, most clearly displayed at mid-
ocean ridges. Large numbers of shallow focus and generally low
magnitude earthquake events. Most are submarine e.g., Mid-Atlantic
Ridge with basaltic magma.
- Convergent: (where plates move together): these are actively
deforming collision locations with plate material melting in the
mantle, causing frequent earthquakes and volcanoes. E.g.,
destructive margins where a subduction zone is formed or a
continental collision, large mountain ranges like the Himalayas can
also be formed.
- Conservative: (sliding past each other) lithosphere is never created
nor subducted and while they do not cause volcanic activity, they
cause extensive shallow focus earthquakes occasionally of
significant magnitude e.g., San Andreas Fault.
, Hotspot volcanoes: are found in the middle of tectonic plates and are
thought to be fed by underlying mantle plumes such as the Hawaiian
volcanoes within the Pacific Plate. As plates move more volcanoes can
form in their place, as oceanic volcanoes move away from the hotspot,
they cool and subside to create islands, atolls and seamounts such as the
chain of volcanoes seen in Hawaii.
The Earth’s Structure:
There are two types of crust, thin oceanic crust which underlies the ocean
basins and thicker continental crust which underlies the continents, lower
density continental crust can ‘float’ much higher.
- Crust (0-100km thick)
- Lithosphere (contains upper crust and mantle)
- Asthenosphere (upper mantle, convection currents)
- Mantle (molten rock and high temperatures)
- Outer core (liquid)
- Inner core (solid)
Convection currents = internal heat engine as heat rises from the mantle
and operate as cell, moving plates in several directions.
Sea floor spreading occurs at divergent plate boundaries under the
oceans. This is a continuous input of magma forming a mid-ocean ridge
like the Mid-Atlantic Ridge. A technique involving the reconstruction of
palaeomagnetism can be used to date the age of new tectonic crust.
Key theories:
- Alfred Wegner’s Continental Drift 1912 = Pangea and now
moved apart into the continents.
- Arthur Holmes 1930’s = Mantle convection was the driving force.
- The discovery in 1960s of the asthenosphere, a weak,
deformable layer beneath the rigid lithosphere which moves.
- Palaeomagnetism theory 1960s = signals past reversals of
Earth’s magnetic field to prove new ocean crust and sea floor
spreading as magnetic strips on the seabed.
Subduction = denser is subducted to create earthquakes. Convergent
(destructive) plate margins.
Gravitational sliding = constructive, rising heat creates slight slope
between plates, natural slide so gap opens which oceanic plate slides at
convergent (destructive) margin
Slap pull = cold dense oceanic subducts continental, density pulls itself
into the mantle. Occurs at destructive.
Earthquakes:
