Topic1 - Tectonic Processes And Hazards EQ1 Flashcards
What is a tectonic hazard?
threats caused by the movement of tectonic plates that have the potential to cause damage to life, property, the environment, human activity and/or the economy
- include earthquakes and volcanic eruptions, aswell as secondary hazards like tsunamis
- classed as seismic or volcanic
Outline the distribution of earthquakes around the world?
- main earthquake zones are found (often in clusters) alongside plate boundaries OFZ and CFZ zones
- about 70% are in the ‘Ring of fire’
- the most powerful are at convergent or conservative boundaries
- rare intra-plate earthquakes can happen near the middle of plates - associated with ancient faults
Outline the distribution of earthquakes with focus on oceanic fracture zones (OFZ)?
A belt of activity through the ocean along the mid-ocean ridges, coming ashore in Africa, the Red Sea, the Dead Sea rift and California
Outline the distribution of earthquakes with focus on continental fracture zones (CFZ)?
A belt following the mountain ranges from Spain, via the alps, to the Middle East, the Himalayas to the East Indies and then around the pacific
Outline the distribution of volcanos around the world?
- most active volcanoes occur on or near to plate boundaries
- about 75% occur around the ‘Ring of Fire’ surrounding the Pacific Ocean
- Volcanoes occur at convergent and divergent plate boundaries - also found in the middle of plates at hotspots in the middle of plates like Hawaii
Outline the distribution of tsunamis around the world?
- Over 70% of tsunamis occur around the Pacific Ocean
- 15% Mediterranean Sea, 9% Caribbean Sea and Atlantic Ocean
- caused by tectonic activity most occur due to activity at convergent boundaries
What are the three categories of convergent boundaries (plates moving towards each other)?
- Oceanic - continental
- Oceanic - oceanic
- Continental - continental (also known as a collision boundary)
forms mountains and volcanoes
Explain how volcanos happen at intra plate locations - explain mantle plumes and hotspots?
Mantle plume - especially hot areas of the mantle that move upwards underneath the crust and push it up - can cause weak points in the crust which become hot spots
Hot spots - magma from mantle breaks through crust - 2 types (one associated with individual upwelling from near the core mantle boundary - second from top of large mantle plumes)
outline the 2 plate boundaries associated with formation of volcanoes?
- Plates rip apart at a divergent plate boundary, causing volcanic activity and shallow earthquakes
- convergent plate boundary, one plate “subducts” beneath the other, resulting in a variety of earthquakes and a line of volcanoes on the overriding plate
also caused by hotspots
Outline the distribution of earthquakes with focus on continental interiors?
Scattered earthquakes in continental interiors - can also occur along old fault lines
Outline the 4 main parts of the earths structure?
- crust (oceanic and continental)
- mantle
- outer core
- inner core
Outline and explain the characteristics of the earths crust?
- surface temperature to about 400 degrees
- less dense
- composed mainly of granite (continental) and basalt (oceanic)
- solid physical state
- 7km to 70km thick
- surface and body waves able to pass through
Outline and explain the characteristics of the earths mantle?
- temperature around 870 degrees
- less dense to medium density
- composed of magnesium, aluminum, silicon, and oxygen silicates
- phases of liquid and solid state
- body waves pass through at varying rates due to differences in density
- 700 to 2890 km deep
Outline and explain the characteristics of the earths outer core?
- temperature from 4440 to 6100 degrees
- high density
- made up of mainly sulphur and iron
- liquid state - generates magnetic field
- only P waves able to pass through
- 2890 to 5150 km deep
Outline and explain the characteristics of the earths inner core?
- temperature of 7000 degrees - radioactive decay
- very dense
- made up of nickel and iron
- solid state
- only P waves reach inert core and pass through
- 5150 km deep to centre
outline the general idea of continental drift as a hypothesis for plate movement?
Developed by Alfred Wegener based on numerous evidence for example - jigsaw fit, biological evidence, coal deposits and glacial evidence
- idea of a single continent (Pangea) - earths internal radioactive heat was driving force of mantle convection that moves tectonic plates
Outline and explain evidence support for continental drift?
- jigsaw fit - all continent fit together perfectly
- biological evidence - ancient fossils of same species found in different parts of the world
- geological evidence - identical geology found in different parts of the world (mountains in American geologically related to mountains in Scotland)
- glacial evidence - identical glacial till in Antartica, Africa and S America - not possible unless close together
outline the general idea of sea floor spreading as a hypothesis for plate movement?
Magma rising from mantle pushing oceanic crust inside and then coming into contact with water - forms further oceanic crust, oceanic crust pushed way comes into contact with continental crust
Oceanic crust forced under continental back into the mantle
Outline and explain evidence support for sea floor spreading?
Compelling evidence came from the Atlantic ocean sea bed which was spreading outwards from the centre at approx 5 cm a year
- idea that ocean trenches were locations were the ocean floor was destroyed and recycled (mid-ocean ridges were where oceans grew from their centre due to molten material from earths mantle oozing up)
outline the general idea of paleomagnetism as a hypothesis for plate movement?
- earths magnetic field changes every 400,000 years, switching polarity - paleomagnetism study of this
- geologists hypothesised a theory that earths lava changes orientation when polarity switches - when material from mantle rises up through mid ocean ridges and cools it preserves a record of polarity, they noticed symmetrical pattern of magnetic stripes on either sides of the mid ocean ridges, which were same age and similar distance away from ridge on each side - conforms Atlantic floor was created at the mid ocean ridges
Outline and explain evidence support for paleomagnetism?
Geologists notices that on either side of the mid ocean ridges a symmetrical pattern of magnetic stripes - once dating the basalt stripes on the ocean floor, they found the same age at similar distances from each ridge (progressively older the further you go from the ridge)
- shows the Atlantic sea floor was created at mid-Atlantic ridges and then moved sideways
outline the general idea of convection currents as a hypothesis for plate movement?
Holmes’s proposed that the mantle is heated from hotspots in the earths core (created by intense radioactive decay in the core) heat creates convection cells in mantle and causes it to rise and cool, becoming more dense and sinking before repeating the cycle
- plates move slowly due to a frictional drag on the earths lithosphere
Explain reasons for plate movement regarding convection currents?
- convection currents push force created at mid ocean ridges
- slab pull describes how oceanic crust at destructive boundaries become dense and thicker as it cools - causes crust to sink into mantle (pulls plate further down)
- small suction force when plate subducts
Outline the process of sea floor spreading?
1) upwelling convection in mantle causes oceanic crust to form a ridge
2) lateral tension develops causing rift daunting and downward, movements of the central block magma intrudes along faults giving surface lava
3) lateral movements continues with further intrusions parallel to original rift faults
4) main rifting sequence is repeated periodically as upwelling continues
Outline the 3 types of plate boundaries?
- conservative/transform
- constructive/divergent (continental/continental and oceanic/oceanic)
- destructive/convergent (continental/continental, oceanic/oceanic and oceanic/continental)
Outline what happens at a divergent/constructive plate boundary?
Earths lithosphere on 2 different plates move away from each other due to convection currents forming a gap - magma gently rises to surface to fill the gap and create new land (Low volcanoes formed)
- often found in the sea - enough lava eruption can lead to formation of island
Outline what happens at a divergent/constructive plate boundary - focus on continental and continental areas?
When convection happens beneath major land mass, the heating leads to upcoming of crust causing fracturing and rifting - as lithosphere moves apart, central sections drop down forming rift valleys (e.g. great East African rift valley
- heating of overlying plates causes expansion and bulging leading to horsts - fracturing along fault lines - this leads to fallen blocks of lowland called grabens
Outline what happens at a divergent/constructive plate boundary - focus on oceanic and oceanic areas?
Convection currents create high temps - causing up-doming of crust and tension that pull it apart producing a Mid-oceanic ridge or in continental areas rift valleys
- as gap forms, it is filled by magma rising which cools, solidifies and forms dense new basaltic rock and new oceanic crust formed at submarine volcanoes which may become ridges or islands
Outline what happens at a convergent/destructive plate boundary (subduction)?
Heavy oceanic crust moves towards lighter continental crust and is forced under it (subducted)
- down warping of oceanic plate forms deep ocean trench
- uplifting along edges of the continental plate form fold mountains (eg Andes)
- ocean crust moving down increases in pressure and differences in rates of movement can trigger major earthquakes (may be shallow or deep) line of the subducting plate called the Wadati-Benioff zone
- composite explosive volcanoes form
Explain fully, how volcanoes form at convergent/destructive plate margins?
Oceanic plate subducted under continental - as rock descends, together with the increasing heart and friction it begins to partially melt into magma
- due to it being less dense than surroundings, the molten magma rise as plutons of magma reaching the surface and forming volcanoes
- viscous Andesitic lava, due to high levels of silica which cause gas bubbles to be trapped in the magma - these suddenly expand and are released when they are erupted - causes the explosiveness
Outline what happens at a convergent/destructive plate boundary (subduction) focus mainly on oceanic and oceanic?
two pieces of oceanic crust on different plates collide, one is forced under the other
and the processes involved with subduction begin
- resulting magma from Benioff zone forms crescents of submarine volcanoes along plate margins (may grow into island arcs)
- produce frequent shallow-to-deep focus earthquakes which can be very powerful
Outline what happens at a convergent/destructive plate boundary (collision) focus on continental and continental?
Continental plates are of lower density than the asthenosphere beneath them (no subduction - no volcanic activity)
- when they collide any sediment deposited between them is uplifted and buckle - forms fold mountains
- earthquakes tend to have shallow to intermediate focus depth and are moderate in magnitude
Outline what happens at a transform/conservative plate boundary?
2 plates slide past each other horizontally - results in major break (fault) in lithosphere - plates sometimes stick as they move past each other, causing build up of stress and pressure which is suddenly released as a strong earthquake at a shallow focus (eg San Andreas fault)
- no volcanic activity- but associated with powerful earthquakes
Outline the seismic activity present at plate boundary’s?
Divergent - shallow focus (usually low magnitude 5-6)
Convergent (oceanic and continental) - range of focal depths, shallow to 700km along wadati-Benioff zone (high mag 8-9)
Convergent (oceanic and oceanic) - range of focal depths, shallow to 700km along wadati-Benioff zone (moderate/high mag 7-9)
Convergent (collision) - shallow to intermediate focal depth (moderate mag 6-8)
Conservative/transform - shallow focus (moderate mag 6-8)
Outline the 3 types of destructive plate margins?
- oceanic and oceanic
- oceanic and continental (subduction)
- continental and continental (collision)
Outline the volcanic activity present at plate boundary’s?
Divergent - effusive eruptions (low VEI scale)
Convergent (oceanic and continental) - explosive (moderate to high VEI scale)
Convergent (oceanic and oceanic) - explosive (moderate to high VEI scale)
Convergent (collision) - usually none
Conservative/transform - usually none, occasional fissure eruptions
Define earthquake?
The ground shaking caused by shockwaves emanating from a sudden slip on a fault (huge amounts of pressure builds up when plates get stuck causing rocks to fracture along cracks, called faults - leads to sudden release of energy as seismic waves)
Focus - point inside crust, which the strain is released
Epicentre - point on earths surface directly above focus
Explain what a locked fault is and what it causes?
fault which is stuck
Frictional resistance on the fault is greater than the stress across the fault - when frictional resistance is overcome the stored strain is eventually released as a large magnitude earthquake
Explain why earthquakes at convergent boundaries tend to have a greater depth focus?
Benioff zone is an area of deep seismicity due to a plate being thrust deep down (usually thinner, denser oceanic plate) during subduction - different speeds and movement of rock at this point produce numerous earthquakes
- subduction - more likely to be locked plates and frictional stress - can result in a mega-thrust earthquake
- convergent - more likelihood for a locked fault lasting hundreds of years beguiling enormous stress as frictional resistance is greater than shear stress across Benioff zone
Give the 3 different kinds of seismic waves released during an earthquake?
- P (fast)
- S (slower)
- L (surface)
*severity of earthquake determined by amplitude and frequency of these waves**
S and L waves more destructive - cause crustal fracturing, ground shaking and secondary hazards
Explain the characteristics of P waves (primary waves)?
- waves from focus, caused by compression - fastest wave, first to reach surface
- travel through both solid and liquid
- push in forwards/backwards motion
only damaging in very powerful earthquakes
Explain the characteristics of S waves (secondary waves)?
- slower waves (half speed of P) from focus
- can only travel through solids
- move in an up/down motion - do more damage than P waves
Explain the characteristics of L waves (surface waves)?
- surface waves (2 types - Rayleigh and Love) both shake the earths crust away from the epicentre
Rayleigh waves - only travel through surface of crust in rolling motion - responsible for most shaking felt
Love wave - only travel through surface of crust (fastest surface wave) and moves forwards from side to side
can only move along surface of planet
Give the 2 types of surface waves?
- Love waves
- Rayleigh waves
Explain the significance of L and S waves?
- more destructive due to larger amplitude and energy force
- cause crustal fracturing and ground shaking
Responsible for secondary hazards
- liquefaction
- landslides
- tsunamis
Explain the magnitude of earthquakes - how is it measured?
Compares size of earthquake by examining amount of energy released at epicentre (moment magnitude scale, MMS used)
- MMS better than Richter scale because that only uses P and S waves for calculation
Explain the intensity of earthquakes - how is it measured?
Intensity - shows the perceptible shaking and amount of damage and effect on people, structures and natural environment
Modified Mercalli intensity scale takes observations from people who experienced the earthquake and rates them from 1 (hardly noticed) to XII (catastrophic)
- local conditions can affect intensity - eg development level, proximity to urban areas, soil types ext)
Outline the primary hazards resulting from earthquakes?
- ground shaking - causes buildings, bridges, roads and infrastructure to collapse
- crustal fracturing - when energy released during earthquakes causes the earths crust to crack, leaving gaps
Outline the secondary hazards resulting from earthquakes - give examples?
- landslides - for example Santa Tesla in El Salvador was hit by a landslide causing 500 houses to be buried and 800 people reported missing
- liquefaction - occurs due to soft sediment behaving like quicksand during an earthquake - shaking brings water to the surface - can causes buildings to topple over or sink into the ground
- avalanches and mudslides - common in mountain areas eg Nepal 2015 earthquake
Explain what the VEI measures for?
Volcanic explosivity index measures the size of eruptions based upon volume of ejecta (gas, ash, tephra, lava) erupted and how high this is thrown into the atmosphere
- begins at 0 but can go up to 8
Give the 3 types of lava?
- basaltic lava
- andesitic lava
- rhyolitic lava
Andesitic and rhyolitic - typical of destructive plate margins - where oceanic crust is destroyed adds silica to magma
Outline the characteristics of basaltic lava?
- hottest temp (1000 - 1200)
- low silica - formed by melting of mantle minerals specially upper zone
- low gas content - gentle eruption energy
- thin and runny
ocean hotspots, mid-oceanic ridges and shield volcanoes
Outline the characteristics of andesitic lava?
- temp (800- 1000)
- intermediate silica - formed by subducted oceanic plate melting and mixing with seawater
- low gas content (3 to 4%) - violent eruption moderately explosive
- slow and viscous
composite cone volcanoes - subduction zones
Outline the characteristics of rhyolitic lava?
- coolest (650 -800)
- high silica - formed by melting of lithospheric mantle
- gas content (4to 6%) - very violent eruption
- thick and stiff and highly viscous
composite cone volcanoes or supervolcanoes
Explain how the silica level impacts the viscosity of volcanoes?
Andesitic and rhyolitic lava have a high silica level which results in high viscosity - which means that the gas bubbles struggle to expand and the lava often solidifies before reaching the surface, leading to a build-up of pressure and ultimately a violent explosion
- basaltic lava has low silica level and has low viscosity - water and other volatiles can more easily and gradually escape from lava
Give some examples of primary hazards caused by earthquakes?
- lava flow
- pyroclastic flow
- tephra
- gas release
- flank collapse
Explain the primary hazard - lava flow and give an example?
fast moving lava poses a big threat to human life - viscosity is determined by the silica dioxide content
- on steep slopes flows can reach 15m/sec.
- greatest lava-related disaster was in 1873 where molten material spilled from Lakagigar fissure, Iceland (estimated 22% of the total population died)
resulting famine.
Explain the primary hazard - pyroclastic flow and give an example?
result from frothing of molten magma in the vent - bubbles burst explosively, ejecting hot gases and pyroclastic materials
- account for majority of volcano deaths - due to dense downslope surges of very hot ash and gas moving at very fast speeds
Explain the primary hazard - tephra?
An eruption sometimes ejects rock fragments (tephra) into the atmosphere - size can vary from bombs to fine dust
- can lead to death on impact and cause ash to build up on floors leading to them collapsing
Explain the primary hazard - gas release?
associated with explosive eruptions and lava flows - mix normally includes gases like sulphur dioxide and carbon monoxide
- most deaths are associated with carbon dioxide (colourless and odourless gas) which can accumulate in valleys undetected
Explain the primary hazard - flank collapse and give an example?
Very viscous magma can form an immovable ‘plug’ within a volcano - pressure from magma below can cause the side of a volcano to bulge and eventually collapse, leading to a sideways eruption or lateral blast
- example St Helens eruption 1980
Explain the secondary hazard - mudflows?
Composed of fine sand and silt material - degree of hazard depends on steepness of slope, volume of material and particle size
- associated with heavy rainfall which triggers old tephra deposits
Explain what a tsunami is?
sequence of waves with deep troughs in between, they’re created by water column displacement, mostly by undersea plate
movements where part of the seabed is thrust upwards (or downwards) very quickly OR by explosive volcanic eruptions on a volcanic island
Explain how a tsunami occurs?
- when an earthquake occurs beneath the sea bed this can lead to a tsunami - as the sea bed jolts water is displaced and forced upwards creating a wave (wave slows as it approaches land)
- as water reaches the shore a vacuum is created and the water recedes rapidly out to sea - leaving sea bed exposed
Outline some of the environmental factors caused by the Tohoku tsunami?
- estimated 23,600 hectares of farmland (rice paddies) damaged by tsunami - salt water impacts soil fertility
- surge of water carried estimated 5 million tons of debris out to sea
- 300 tons of radioactive water continues to leak from Fukushima nuclear plant every day
Outline some of the economic factors caused by the Tohoku tsunami?
- total damage from earthquake - estimated by 300bn
- 12,000 fishing boats destroyed/damaged - with 440 fishermen killed/missing - damage to fishing industry was 5bn
- earthquake had immediate impacts on businesses such as Toyota, Nissan and Honda which temporarily suspended production
- 300,000 buildings destroyed - 1 million damaged, 4000 roads, 78 bridges and 29 railways
Outline some of the social factors caused by the Tohoku tsunami?
- 80,000 refugees moved due to damage to houses
- 140,000 residents moved due to radioactive leaks
- number of confirmed deaths - 15,000 with 2500 missing
Explain the factors which impact the nature of a tsunamis wave - give 4?
- cause of wave - eg eruption or earthquake
- distance travelled from source - as energy is lost as they travel
- offshore topography and coastline orientation
- water depth over route affects energy loss through friction
