T1 - Tectonic Processes and Hazards
EQ1 - why are some locations more at risk from tectonic hazards?
1.1 - the global distribution of tectonic hazards can be explained by plate boundaries and other tectonic
processes
1.1.1 - the global distribution and causes of earthquakes, volcanic eruptions and tsunamis
hazard → potential threat to human life and property
- hydro-meteorological → caused by climatic processes
- geophysical → caused by land processes
geophysical hazards occur near plate boundaries
- these plates move at different speeds and directions which can cause collisions, earthquakes and volcanic
activity
usually, the most powerful earthquakes occur at convergent or conservative boundaries
earthquakes can also occur near the middle of plates (intra-plate earthquakes)
- it is assumed that this is caused by plates having pre-existing weaknesses which become reactivated, forming
seismic waves
volcanic hotspots, eg the Ring of Fire, are also situated amongst the centre of plates
- this a localised area of the lithosphere, which has an unusually high temperature due to the upwelling of hot
molten material from the core
- this was first theorized by Tuzo Wilson in 1963
- at hotspots, such as the Hawaii hotspot, magma rises as plume (hot rock)
seismic activity volcanic activity topography or bathymetry
divergent shallow focus, low effusive eruptions, low VEI ocean ridge with central rift
magnitude (5-6) scale (1-3) valley, faulting at right angles,
volcanic islands
convergent: oceanic and range of focal depths, often explosive, moderate to ocean trench, fold mountains
continental high magnitude (8-9) high VEI scale (5-6) with volcanic peaks
convergent: oceanic and range of focal depths, explosive, moderate to island arc, oceanic trench,
oceanic moderate to high magnitude high VEI scale (5-6) back arc and fore arc zones
(7-9)
collision (convergent): shallow to intermediate focal usually none fold mountains and plateaus
continent and continent depth, moderate magnitude
(6-8)
conservative shallow focus, moderate usually none, occasional ridges and scars on surface
magnitude fissure eruptions
1.1.2 - the distribution of plate boundaries resulting from divergent, convergent and conservative plate
movements
destructive/convergent plate boundaries
, continental and oceanic
- denser oceanic plate subducts below the continental
- this leaves a deep ocean trench
- the oceanic crust is melted as it subducts into the
asthenosphere
- the extra magma created causes pressure to build up
- pressurised magma forces through weak areas in the
continental plate
- explosive, high pressure volcanoes erupt through the
continental plate, known as composite volcanoes
- fold mountains occur when sediment is pushed upwards
during subduction
oceanic and oceanic
- heavier plate subducts leaving an ocean trench
- fold mountains also occur
- built up pressure causes underwater volcanoes bursting
through the oceanic plate
- lava cools and creates new land called island arcs
continental and continental
- both plates are not as dense as oceanic so lots of
pressure builds
- there is no subduction of continental crust
- pile up of continental crust on top of lithosphere due to
pressure between plates
- fold mountains formed from piles of continental crust
constructive/ divergent plate boundaries
oceanic to oceanic
- magma rises in between the gap left by the two plates
separating, forming new land when it cools
- less explosive underwater volcanoes formed as magma
rises
- new land forming on the ocean floor by lava filling the gaps
is known as sea floor spreading
continental to continental
- any land in the middle of the separation is forced apart,
causing a rift valley
- volcanoes form where the magma rises
- eventually the gap will most likely fill with water and
separate completely from the main island
- horsts → the lifted areas of rock
- graben → the valley itself
, conservative plate boundary
- between any crust, the parallel plates move in different
directions or at different speeds
- no plates are destroyed so no landforms are created
- when these plates move, a lot of pressure is built up
- on oceanic crust, this movement can displace a lot of water
- on continental crust, fault lines can occur where the ground
is cracked by the movement
1.1.3 - the causes of intra-plate earthquakes, and volcanoes associated with hot spots from mantle plumes
intraplate earthquakes
- earthquakes can happen anywhere the crust is cracked, usually ancient faults created by previous plate
movements
- small movements along these old faults build up friction and strain over time, which is then suddenly
released causing low magnitude earthquakes
- isostatic readjustment can cause earthquakes as the crust moves up or down
- isostatic readjustment → weight of ice removed after Ice Age
- human activities can also trigger tremors, for example:
- the weight of water in a reservoir
- fracking of gas
- large explosions
case study: UK
- the UK experiences minor earthquakes despite being 1500km from the nearest plate boundary (under the centre
of the atlantic ocean
- there are about 25 earthquakes in the UK that are felt by people
- the largest occurred in 1931 under the North Sea at Dogger Bank (6.1 on the Richter scale)
intraplate volcanoes
- volcanic activity can be found in both the middle of oceanic and continental plates
- some of these volcanoes may be linked to existing faults in the Earth’s crust
oceanic plates
- 7km thick
- magma rising through the mantle from the edge of the outer core is able to melt its way through, forcing molten
material to the surface
- this creates volcanoes that grow from the seabed
continental plates
- much thicker, but get stretched enough in places for the magma from a hot spot, linked to a mantle plume, to
break through
case study - Hawaii
- the Hawaiian islands have 15 volcanoes which are the youngest of 130
found along a 6000km line to their northwest
- Kilauea and Mauna Loa are the most active volcanoes on the planet
- the submarine volcano Lo’ihi is youngest in the Hawaiian chain, being
creates as the Pacific Plate moves north-westwards over the hotspot
mantle plumes and hot spots
- there are massive mantle plumes in the Earth’s mantle layer, once
centred under the Pacific plate and the other under the African plate
- molten viscous silicate material rises from the outer core/mantle boundary to about 700km beneath the
lithosphere (crust)
, - from these mantle plumes small hot spots are created where magma rises through the asthenosphere and
sometimes breaks through the crust
- isolated hot spots occur where there is an upwelling of molten material directly from the outer core/mantle
boundary to the surface
- eg the hawaiian islands
1.2 - there are theoretical frameworks that attempt to explain plate movements
1.2.1 - plate tectonics
earths internal structure
- the Earth’s structure has been established by studying the
movement of seismic waves through the planet
- seismic waves travel at different speeds through material of
different densities, and some types do not travel through
liquids
core
- central part of the Earth
- made up of an inner core and outer core
- inner core: solid, 5,100 km
- outer core: liquid, 2,900 km
- both are made from iron and nickel
- a source of radioactive heat
mantle
- semi-molten body of rock between the Earth’s crust and its
core
asthenosphere
- part of the mantle
- semi molten
- below the lithosphere which floats on top of it
lithosphere
- crust and upper mantle which form the tectonic plates
- together they are 80-90km thick
crust
- changes in thickness under oceans and continents
- oceans: 6-10km
- continents: 30-40km
mantle convection
- heat radiating from the inner core through the mantle material causes it to convect, as it behaves like a viscous
liquid
- it has phases of liquid and solid consistency under pressure
- convection may be in several layers or just one
- the convection cells make contact with the base of the crust (at the Moho), causing friction, and so drag the crust
along in the general direction of the convection
moho (mohorovicic discontinuity)
- the boundary between the crust and the mantle
- defines where there is a change of state between solid (crust) and material able to flow (mantle)
- found at various depths
- anywhere between 5km and 90km
palaeomagnetism and seafloor spreading
- the outer core is mostly liquid iron and it convects
