Tectonics Flashcards
geophysical hazards
caused by land processes, near plate boundaries. these plates move at different speeds and directions which can cause colllisions, earthquakes and volcanic activity.
intra plate earthquakes
assumed that plates have pre existing weaknesses which become reactivated, forming seismic waves. for example, an intra plate earthquake may occur if solid crust, which has weakened over time, cracks under pressure
volcanic hotpots
localised area of the lithosphere which has an unusually high temperature due to upwelling of got molten material from the core (first theorised by tuzo wilson in 1963)
where do the most powerful earthquakes occur at
convergent or conservative boundaries
OFZ
(ocean fracture zone) a belt of activity through the ocean and along the mid ocean ridges through africa, the red sea and the dead sea
CFZ
(continental fracture zone) a belt of activity along mountain ranges from spain through the alps to the middle east and to the himalayas
tectonic trend since the 1960
- total number of recored hazards increased
- number of fatalities decreased
- amount of people affected increased (population growth)
- economic costs has significantly increased (more developed infrastructure) increasing number of insurance policies (higher cost)
why is reporting disaster impacts (e.g. fatalities) controversial
- direct deaths or indirect deaths as a cause (some take time to be apparent)
- hard to collect data from certain locations (rural/isolated or high population density)
- different methods produce different results
- bias from governments to show they are doing a good job (2004 indian ocean tsunami the burmese government claimed they had 0 deaths)
hot and cold magma
cooler magma is denser so sinks and hot magma is less dense so rises (particles spread out)
why does the innder core experience high temperatures
- primordial heat left over from earths formation
- radiogenic heat produced from radioactive decay
landforms created from destructive plate margins
(plates move towards each other)
continental/continental (earthquakes/fold mountaines)
oceanic/oceanic (ocean trench/island arc/earthquakes/volcanoes)
continental/oceanic (fold mountains/volcanoes/earthquakes
landforms created from constructive plate margins
(plates move away from each other)
continental/continental (rift valleys/earthquakes/volcanoes)
oceanic/oceanic (ocean ridge/earthquakes/volcanoes)
landforms creates from conservative plate margins
(plates move parallel to each other)
only create earthquakes
continental to oceanic destructive plate boundaries
- denser oceanic plate subducts
- subduction creates deep ocean trench
- oceanic crust is melted as it is subducted into asthenosphere
- extra magma causes pressure to build
- pressurised magma forces through weak areas in continetal plate
- explosive high pressure volcanoes (composite voclanoes)
- fold mountains occur when sediment pushed up during subduction
oceanic to oceanic destructive plate boundaries
- heavier plate subducts (ocean trench and fold mountains occur)
- built up pressure causes underwater volcanoes
- lava cools and creates new land called island arcs
continental to continental destructive plate boundaries
- high pressure due to low density plates
- no subduction of continental crust
- pile up of continental crust ontop of lithosphere due to pressure between plates
fold mountains from piles of continental crust
oceanic to oceanic constructive plate boundaries
- magma rises in between the gap left by two plates seperating forming new land once cooled
- less explosive underwater volcanoes formed as magma rises
- new land forming on ocean floor by lava filling gap is known as sea floor spreading
evidence for seafloor spreading
theorised by Harry Hess in the 1940s
as a new rock forms and cools the magnetic grains within the rock align with the magnetic poles (the north and south poles switch periodically
there are symmetrical bands of rock with alternating banfs of magnetic polarity (evidence for seafloor spreading)
continental to continental constructive plate boundaries
- any land in the middle of the seperation is forced apart causing a rift valley
- volcanoes form where magma rises
- eventually gap will fill with water and seperate completely from mainland
- lifted areas of rocks are known as horsts whereas the valley itself is known as a graben
further forces influences how convergent boundaries occur
ridge push - the slope created when plates moving apart has gravity acting upon it as it is at a high elevation. gravity pushes the plates further away, widening the gap (gravitational sliding)
slap pull - when a plate subducts, the plate sinking into the mantle pulls the rest of the plate with it, causing further subduction
conservative plate boundary
between any crust, the parallel plates move in different directions or at different speeds. no plates are destroyed and so no landforms created. when plates move pressure builds up. on oceanic crust this movement can displace alot of water. on continental crust, fault lines can occur where the ground is cracked by the movement
different mechanisms that cause plate movement
mantle convection - radioactive elements in the core of the earth decay which produce alot of thermal energy. this causes the lower mantle to heat up and rise, as the magma rises it cools down and becomes more dense and begins to sink back down to the core. these are convection current. these convection currents push the plates
slap pull - old oceanic crust will submerge into the mantle. pulling action drags the rest of the plate with it (first theorised by Dan McKenzie)
why are tectonic movements not fully understood
previously convection current were thought to be the primary cause of plate movement. however it is now believed that slab bull is the primary mechanism for plate movement; convection currents seem too weak to move massively dense plates
primary waves (p waves)
- travel through solids
- compressional
- vibrates in the direction of travel
- travels at 4-8km/s
secondary waves (s waves)
- vibrate at right angles to direction of travel
- travels only through solid rocks
- travels at 2.5-4km/hr
love waves
- near to ground surface
- rolling motion producing vertical ground movement
- travels at 2-6km/hr
rayleigh waves
- vertical and horizontal displacement
- travels at 1-5km/hr
- compressional
what type of waves are the most destructive and why
secondary and love waves are the most destructive as they have large amplitudes.
why are there aftershocks after an earthquake
due to their different speeds, different waves will hit a location at different times. the aftershock that survivors feel are these different types of waves arriving after each other
does damage caused by an earthquake decrease with distance from the epicentre?
the intensity of waves decreases with distance as the waves lose energy as they travel. this does not mean that impacts felt or damages caused will always decrease, other factors can affect a locations vulnerability
what factors can affect a locations vulnerability from earthquakes
geology, geographical location (was the earthquake near the sea or intraplate), education of locals, durability of buildings, mitigation
how does soild liquefaction occur after an earthquake
- affects poorly compacted sand and silt
- water moisture within the soil separates from the soil particles and rises to the surface
- this can cause the soil to behave like a liquid, which can cause building subsidence or landslides
how do landslides occur after an earthquake
- the shaking caused by the earthquake can weaken or damage cliff faces, hills and snow material
- unconsolidated material or loose rock can collapse
- landslides can travel several miles and accumulate material on the way
- risk varies with topography of rainfall, soil and land use
how do tsunamis form from earthquakes
- when oceanic crust is jolted during an earthquake, all the water above the plate is displaced, normally upwards
- the water is pulled back down due to gravity. energy is transferred into the water and travels through like a wave
- water travels fast but with a low amplitude
- as the sea level decreases towards the coast there is friction between the sea bed and the waves
- the waves slow down and gain height, creating a wall of water that is on average 10ft high but can reach 100ft
where are most tsunamis formed
generally generated in subduction zones at convergent plate margins. most tsunamis are found along the pacific ring of fire, hence the most vulnerable countries are often located in Asia or oceania
what factors affect the impact of a tsunamis
- population density
- coastal defences
- duration of event
- wave amplitude and distance traveled
- gradient of continental shelf
- the shape of the land - bays will funnel and concentrate tsunamis waves
- warning and evacuation systems
- level of economic and human development
primary hazards from volcanoes
- lava flows
- pyroclastic flows
- tephra and ash flows
-volcanic gases
what are lava flows
streams of lava that have erupted onto the earths surface. fast flowing lava that have erupted onto the earths surface. fats flowing lava can be very dangerous which depends on the lavas viscosity (depends on the silicon dioxide content)
what are pyroclastic flows (nuée ardente)
a mixture of hot dense rock, lava, ash and gases which move very quickly along the surface of the earth. due to their high speeds, pyroclastic flows are extremely dangerous and can cause asphyxiation for anyone cause by the flow
what are tephra and ash flows
when pieces of volcanic rock and ash are blasted into the air. this can cause serious damage to buildings, which can collapse under the weight of ash or tephra
what are volcanic gases
gases like sulphur dioxide and carbon monoxide are released into the atmosphere. due to their potency, volcanic gases can travel long distances
secondary hazards of volcanoes
lahars, jokolhulaup, acid rain
what are lahars
combination of rock, mud and water which travel quickly down the sides of volcanoes. these can occur when the heat of the eruption causes snow and ice to melt or alternatively when an eruption coincides with heavy rainfall
what are jokulhlaups
snow and ice in glaciers melt after an eruption which causes sudden flood that are very dangerous
what is acid rain
caused when gases such as sulphur dioxide are released into the atmosphere
disaster
a serious disruption of the functioning of a community or society involving human, material, economic and environmental losses which exceeds the ability of the affected community or society to cope using its own resources
risk equation
risk = hazard x vulnerability / capacity to cope