Tectonics
When do natural hazards become disasters?
- risk = hazard x vulnerability / capacity to cope
- PAR model = root causes – dynamic pressures – unsafe conditions = disaster = natural hazard
- a natural disaster when more than 10 dead
Distribution of tectonic hazards
- convergent, divergent and transform
- 70% of all earthquakes in Ring of Fire
- OFZ (Oceanic Fracture Zone) – belt of activity through constructive boundaries in oceans along mid-ocean ridges
- CFZ (Continental Fracture Zone) – belt of activity following mountain ranges from Spain, Alps, Middle East, Himalayas
- 500 active volcanoes and 50 erupt each year
- 5% of earthquakes are intra-plate earthquakes - due to old faults and large faults
Hotspot volcanoes
- heat rises as a hot thermal plume from deep Earth
- high heat and lower pressure at the base of lithosphere enables melting of rock
- rises and forms active volcanoes
- as the plate moves away from stationary hotspot the volcanoes are rafted away and new ones form in their place
- the old volcanoes cool and subside producing older islands, atolls and seamounts
Theory of tectonics
- inner core = 6000 Degrees Celsius, solid and mainly consists of iron
- outer core = semi molten, 4500 – 6000 Degrees Celsius
- mantle = widest layer, 870 Degrees Celsius, phases of liquid and solid in layers
- crust – oceanic = basalt, density: 3.3 g/cm3 and continental = granite, density: 2.7 g/cm3
- evidence = earthquake depths, geological fit, continental fit, fossil records, glaciation evidence, palaeomagnetism etc
Why do plates move?
- convection currents = heat from radioactive decay drives convection currents which moves plates
- slab pull = convection and gravity (denser plate pulled down) driving tectonic plate movement
- subduction = as a new crust is being created, it’s being destroyed in another by subduction
- seafloor spreading = magma forced up from the asthenosphere and hardens forming new crust, pushes plates apart
Plate boundaries
Divergent
- magma rises due to the decomposition in asthenosphere, causes gap to be filled by lava from fissures
- rift valley: when plates move apart crust breaks into parallel cracks, land between faults collapse forming steep valleys
- small and effusive eruptions – erupted basalt lava has a low gas content and high viscosity
- shallow earthquakes, less than 60km deep and low magnitude
Convergent – 2 continental
- plates move together and are pushed up
- subduction caused when the compressed sediments result in plate subduction beneath them
- shallow, magnitude, infrequent earthquakes – cut by huge thrusts
Convergent – 2 oceanic
- denser oceanic plate subducted creating friction causing earthquakes in the Benioff Zone
- magma rises to surface and breaks friction – causing volcanoes
, - frequent earthquakes and violent eruptions from curving chain of volcanic islands
Convergent – 1 oceanic 1 continental
- oceanic plate subducted and melts – deep ocean trenches mark the place where the oceanic plume starts to sink
- plumes of melted magma rise under pressure
- volcanic eruptions created by melting oceanic plate pushes up through the surface faults
- frequent large earthquakes and violent eruptions as generates magma with high gas and silica content
Transform
- 2 plates either pass each other in opposite directions or at different speeds
- friction builds up and stress released – sends shock waves through Earth’s crust
- shallow focus earthquakes = high magnitude
Volcanoes
- Caldera, Shield volcano, stratovolcano, dome, fissure, cinder cone
- andesitic lava = slow, violent, 800-1000 Degrees Celsius
- basaltic lava = thin + runny, gentle, 1000-1200 Degrees Celsius
- rhyolitic lava = thick + stiff, violent, 650-800 Degrees Celsius
Volcanic hazards
- lava flows = streams of lava that have erupted from a volcano
- pyroclastic flows = mixture of dense rocks, lava, ash and gases ejected from volcano – travel at 100km per hour
- tephra + ash falls = ejected rock fragments - can cause poor visibility, slippery roads, stop engines
- gas eruptions = magma contains dissolved gases that are released i.e. water vapour, sulphur dioxide
- lahars = volcanic mudflows composed of fine sand and silt – if the eruption melts ice or heavy rainfall which erodes
rock and soil
- Jokulhlaup = heat of eruption can melt the snow and ice in a glacier causing heavy and sudden floods – the water
carries glacial moraines and blocks of ice
Volcanic Explosivity Index – VEI
- used to describe and compare size and magnitude of eruptions
- 0 to 8 on logarithmic scale
- 0 to 3 = constructive plate margins and mid-plate hotspots
- 4 to 7 = destructive plate boundary
- 8 = supervolcano
Why are some volcanoes more hazardous than others?
- greater viscosity = more explosive eruption – viscosity determined by temperature, dissolved gases and chemistry
- proximity to population centres
- predictions, forecasts and reactions used – i.e. geologists use evidence from past and create hazard maps
- plate margins – more hazardous at destructive i.e. 80% of world’s eruptions occur at destructive
Measures of earthquakes
Richter Scale
- based on amplitude of the lines made on a seismograph using largest wave amplitude recorded
- pros = accurate, objective, can compare
- cons = scale goes up to 8, logarithmic – may lose accuracy, doesn’t take into consideration impacts
Mercalli
- intensity
- scale I to XII
When do natural hazards become disasters?
- risk = hazard x vulnerability / capacity to cope
- PAR model = root causes – dynamic pressures – unsafe conditions = disaster = natural hazard
- a natural disaster when more than 10 dead
Distribution of tectonic hazards
- convergent, divergent and transform
- 70% of all earthquakes in Ring of Fire
- OFZ (Oceanic Fracture Zone) – belt of activity through constructive boundaries in oceans along mid-ocean ridges
- CFZ (Continental Fracture Zone) – belt of activity following mountain ranges from Spain, Alps, Middle East, Himalayas
- 500 active volcanoes and 50 erupt each year
- 5% of earthquakes are intra-plate earthquakes - due to old faults and large faults
Hotspot volcanoes
- heat rises as a hot thermal plume from deep Earth
- high heat and lower pressure at the base of lithosphere enables melting of rock
- rises and forms active volcanoes
- as the plate moves away from stationary hotspot the volcanoes are rafted away and new ones form in their place
- the old volcanoes cool and subside producing older islands, atolls and seamounts
Theory of tectonics
- inner core = 6000 Degrees Celsius, solid and mainly consists of iron
- outer core = semi molten, 4500 – 6000 Degrees Celsius
- mantle = widest layer, 870 Degrees Celsius, phases of liquid and solid in layers
- crust – oceanic = basalt, density: 3.3 g/cm3 and continental = granite, density: 2.7 g/cm3
- evidence = earthquake depths, geological fit, continental fit, fossil records, glaciation evidence, palaeomagnetism etc
Why do plates move?
- convection currents = heat from radioactive decay drives convection currents which moves plates
- slab pull = convection and gravity (denser plate pulled down) driving tectonic plate movement
- subduction = as a new crust is being created, it’s being destroyed in another by subduction
- seafloor spreading = magma forced up from the asthenosphere and hardens forming new crust, pushes plates apart
Plate boundaries
Divergent
- magma rises due to the decomposition in asthenosphere, causes gap to be filled by lava from fissures
- rift valley: when plates move apart crust breaks into parallel cracks, land between faults collapse forming steep valleys
- small and effusive eruptions – erupted basalt lava has a low gas content and high viscosity
- shallow earthquakes, less than 60km deep and low magnitude
Convergent – 2 continental
- plates move together and are pushed up
- subduction caused when the compressed sediments result in plate subduction beneath them
- shallow, magnitude, infrequent earthquakes – cut by huge thrusts
Convergent – 2 oceanic
- denser oceanic plate subducted creating friction causing earthquakes in the Benioff Zone
- magma rises to surface and breaks friction – causing volcanoes
, - frequent earthquakes and violent eruptions from curving chain of volcanic islands
Convergent – 1 oceanic 1 continental
- oceanic plate subducted and melts – deep ocean trenches mark the place where the oceanic plume starts to sink
- plumes of melted magma rise under pressure
- volcanic eruptions created by melting oceanic plate pushes up through the surface faults
- frequent large earthquakes and violent eruptions as generates magma with high gas and silica content
Transform
- 2 plates either pass each other in opposite directions or at different speeds
- friction builds up and stress released – sends shock waves through Earth’s crust
- shallow focus earthquakes = high magnitude
Volcanoes
- Caldera, Shield volcano, stratovolcano, dome, fissure, cinder cone
- andesitic lava = slow, violent, 800-1000 Degrees Celsius
- basaltic lava = thin + runny, gentle, 1000-1200 Degrees Celsius
- rhyolitic lava = thick + stiff, violent, 650-800 Degrees Celsius
Volcanic hazards
- lava flows = streams of lava that have erupted from a volcano
- pyroclastic flows = mixture of dense rocks, lava, ash and gases ejected from volcano – travel at 100km per hour
- tephra + ash falls = ejected rock fragments - can cause poor visibility, slippery roads, stop engines
- gas eruptions = magma contains dissolved gases that are released i.e. water vapour, sulphur dioxide
- lahars = volcanic mudflows composed of fine sand and silt – if the eruption melts ice or heavy rainfall which erodes
rock and soil
- Jokulhlaup = heat of eruption can melt the snow and ice in a glacier causing heavy and sudden floods – the water
carries glacial moraines and blocks of ice
Volcanic Explosivity Index – VEI
- used to describe and compare size and magnitude of eruptions
- 0 to 8 on logarithmic scale
- 0 to 3 = constructive plate margins and mid-plate hotspots
- 4 to 7 = destructive plate boundary
- 8 = supervolcano
Why are some volcanoes more hazardous than others?
- greater viscosity = more explosive eruption – viscosity determined by temperature, dissolved gases and chemistry
- proximity to population centres
- predictions, forecasts and reactions used – i.e. geologists use evidence from past and create hazard maps
- plate margins – more hazardous at destructive i.e. 80% of world’s eruptions occur at destructive
Measures of earthquakes
Richter Scale
- based on amplitude of the lines made on a seismograph using largest wave amplitude recorded
- pros = accurate, objective, can compare
- cons = scale goes up to 8, logarithmic – may lose accuracy, doesn’t take into consideration impacts
Mercalli
- intensity
- scale I to XII
