GEOGRAPHY REVISION
Tectonics
EQ1- why some locations are more at risk from tectonic
hazards?
1.1- distribution of hazards- plate boundaries + other
processes
Global distribution of tectonic hazards:
Earthquakes
- Distribution- most earthquakes found at tectonic plate boundaries. - 95%
- 70% of all earthquakes are found in the Ring of Fire i pacific
- But some earthquakes don’t occur at boundaries- intra plate due to hotspots
Cause (in general)
- Friction betwee. Plates as they do not fit perfectly together
- As two plates move past each other- they get ‘stuck’
- This allows strain and pressure to build over time since convection currents are still
pushing the plates - increasing stress
- A sudden release of stored energy- jolt movement
- So earthquakes are generated due to the release of that pressure, when the plates
finally slip and continue moving- releasing seismic waves in all directions from the
focus
Volcanoes
- Distribution: also occurs near or on tectonic plate boundaries
- Exceptio of hotspots- Hawaii
Cause of the eruption itself
- Magma- molten rock- lighter than surrounding rock so rises up
- Collects in magma chambers eventually when enough has built up, magma pushes
through the vents up to earth surface
- Often causing eruptions of lava
,Tsunamis
- distribution: 90% of all events occurring within Pacific basin- associated with plate
boundary activity
- Most are generated at subduction/Benioff zones
Causes
- usually generated when a sub-marine earthquake displaces sea bed vertically due
to movement along fault line at subduction
- The motion- displaces large volume of water in the water column- moves outwards
in all directions
- BUT they can also form due to landslides, volcanic activities and certain types of
weather
- Produces series of long waves- may be 60 miles long, reach heights of 30m
Distribution of plate boundaries
1. Divergent ( constructive boundaries)
EARTHQUAKE + regular VOLCANIC ERUPTIONS
SHALLOW FOCUS- but if it happens underwater- less risk to humans AS IT IS MINOR,
and underwater
two plates moving apart generally, leading to formation of new crust
- In oceans- this leaves mid- ocean ridges
- On continents- it forms rift valleys
Eruptions are small and effusive, as the erupted basalt lava has a low gas content and
high viscosity.
Earthquakes are shallow, less than 60 km deep, and have low magnitudes of under 5.0.
Ocean - oceanic
- Rising convection currents bring magma to surface, resulting in small eruptions-
SUBMARINE VOLCANO
- Minor quakes
- PROCESS- SEAFLOOR SPREADING
- E.g Mid-Atlantic ridge in Iceland (create new islands)
Continent-continent plates
- Mantle plume sit in between continental plate
- Creates new basin- as land between faults collapses
- Volcanoes and minor quake
- E.g African Rift Valley OR RED SEA
2. Conservative
, Oceanic-continental
Powerful quake, no volcano
- slide past each other, results in major break in the cruise between them
- The break= a fault
- No crust made or destroyed- no volcanic activity
- But VERY TECTONICALLY ACTIVE- most powerful earthquakes
- Stick and slip - releasing pressure
- Shallow focus- more risk
- E.g San Andreas fault California - 1,200km
3. Convergent (Destructive )
- Plates move towards each other
Types of this boundary
Oceanic- oceanic
- one plate (denser, faster) is subducted under the other
- Deep ocean trenches form
- Subducted plate melts- magma
- This rises up from benioff to form underwater volcanoes
- Can over time form island volcanoes- arcs
- Earthquake from subduction- shallow focus VERY POWERFUL- e.g caused 2004
Indian Ocean tsunami
Continent-continent
- At collision margins
- Both plates- same density, so neither plate subducted
- So landmasses crumple- forced up to form fold mountains (tectonic uplift)
- Infrequent major earthquakes due to shallow focus- but strong- 2005 Kashmir quake
+ Nepal
- No volcanic activity- rare as magma would cool beneath surface
- E.g Himalayas- indo- Australian and Eurasian plates
Oceanic-continental
Mantle convection pulls plates towards subduction zones, whilst other end of plate is being
pulled AWAY at divergent plate boundaries/margins (one end subduction, one end is
constructive/divergent margin)
- oceanic denser, older so when plates collide, it subducted beneath continental into
mantle and melts at depth
- Generates magma w high gas and silica content- so ERUPTIONS ARE EXPLOSIVE
- Deep ocean trenches form due to subduction
Tectonics
EQ1- why some locations are more at risk from tectonic
hazards?
1.1- distribution of hazards- plate boundaries + other
processes
Global distribution of tectonic hazards:
Earthquakes
- Distribution- most earthquakes found at tectonic plate boundaries. - 95%
- 70% of all earthquakes are found in the Ring of Fire i pacific
- But some earthquakes don’t occur at boundaries- intra plate due to hotspots
Cause (in general)
- Friction betwee. Plates as they do not fit perfectly together
- As two plates move past each other- they get ‘stuck’
- This allows strain and pressure to build over time since convection currents are still
pushing the plates - increasing stress
- A sudden release of stored energy- jolt movement
- So earthquakes are generated due to the release of that pressure, when the plates
finally slip and continue moving- releasing seismic waves in all directions from the
focus
Volcanoes
- Distribution: also occurs near or on tectonic plate boundaries
- Exceptio of hotspots- Hawaii
Cause of the eruption itself
- Magma- molten rock- lighter than surrounding rock so rises up
- Collects in magma chambers eventually when enough has built up, magma pushes
through the vents up to earth surface
- Often causing eruptions of lava
,Tsunamis
- distribution: 90% of all events occurring within Pacific basin- associated with plate
boundary activity
- Most are generated at subduction/Benioff zones
Causes
- usually generated when a sub-marine earthquake displaces sea bed vertically due
to movement along fault line at subduction
- The motion- displaces large volume of water in the water column- moves outwards
in all directions
- BUT they can also form due to landslides, volcanic activities and certain types of
weather
- Produces series of long waves- may be 60 miles long, reach heights of 30m
Distribution of plate boundaries
1. Divergent ( constructive boundaries)
EARTHQUAKE + regular VOLCANIC ERUPTIONS
SHALLOW FOCUS- but if it happens underwater- less risk to humans AS IT IS MINOR,
and underwater
two plates moving apart generally, leading to formation of new crust
- In oceans- this leaves mid- ocean ridges
- On continents- it forms rift valleys
Eruptions are small and effusive, as the erupted basalt lava has a low gas content and
high viscosity.
Earthquakes are shallow, less than 60 km deep, and have low magnitudes of under 5.0.
Ocean - oceanic
- Rising convection currents bring magma to surface, resulting in small eruptions-
SUBMARINE VOLCANO
- Minor quakes
- PROCESS- SEAFLOOR SPREADING
- E.g Mid-Atlantic ridge in Iceland (create new islands)
Continent-continent plates
- Mantle plume sit in between continental plate
- Creates new basin- as land between faults collapses
- Volcanoes and minor quake
- E.g African Rift Valley OR RED SEA
2. Conservative
, Oceanic-continental
Powerful quake, no volcano
- slide past each other, results in major break in the cruise between them
- The break= a fault
- No crust made or destroyed- no volcanic activity
- But VERY TECTONICALLY ACTIVE- most powerful earthquakes
- Stick and slip - releasing pressure
- Shallow focus- more risk
- E.g San Andreas fault California - 1,200km
3. Convergent (Destructive )
- Plates move towards each other
Types of this boundary
Oceanic- oceanic
- one plate (denser, faster) is subducted under the other
- Deep ocean trenches form
- Subducted plate melts- magma
- This rises up from benioff to form underwater volcanoes
- Can over time form island volcanoes- arcs
- Earthquake from subduction- shallow focus VERY POWERFUL- e.g caused 2004
Indian Ocean tsunami
Continent-continent
- At collision margins
- Both plates- same density, so neither plate subducted
- So landmasses crumple- forced up to form fold mountains (tectonic uplift)
- Infrequent major earthquakes due to shallow focus- but strong- 2005 Kashmir quake
+ Nepal
- No volcanic activity- rare as magma would cool beneath surface
- E.g Himalayas- indo- Australian and Eurasian plates
Oceanic-continental
Mantle convection pulls plates towards subduction zones, whilst other end of plate is being
pulled AWAY at divergent plate boundaries/margins (one end subduction, one end is
constructive/divergent margin)
- oceanic denser, older so when plates collide, it subducted beneath continental into
mantle and melts at depth
- Generates magma w high gas and silica content- so ERUPTIONS ARE EXPLOSIVE
- Deep ocean trenches form due to subduction
