1.1 Tectonic processes and hazards Flashcards
Types of natural hazard (2)
- hydro-meteorological (caused by climatic
processes) - geophysical (caused by land processes)
Define intra-plate earthquakes
- Earthquakes near the middle of plates
- plates have pre-existing weaknesses which become reactivated, forming seismic waves.
- e.g. an intraplate earthquake may occur if solid crust, which has weakened over time, cracks under pressure.
Define inter-plate earthquakes
- an earthquake that occurs at the boundary between two tectonic plates
- usually at conservative, collision, or destructive boundaries
What is a volcanic hotspot?
- hotspots are situated amongst the centre of plates
- a localised area of the lithosphere which has an unusually high temperature due to the upwelling of hot molten material from the core
- At hotspots, magma rises as plume
- e.g. Hawaii
Define the OFZ (Oceanic Fracture Zone)
a belt of activity through the oceans and along the mid-ocean ridges through Africa, the Red Sea, the Dead Sea
Define the CFZ (Continental Fracture Zone)
a belt of activity along the mountain ranges from Spain through the Alps to the Middle East and to the Himalayas.
Describe tectonic trends since 1960 (4)
▪ total number of recorded hazards has increased
▪ number of fatalities has decreased - some spikes during mega disasters.
▪ population growth means total number of people being affected by tectonic hazards is increasing
▪ economic costs increased significantly - due to development as infrastructure in more developed countries costs more to repair, and increasing number of insurance policies
Why is reporting disaster impacts difficult? (4)
▪ Depends on whether you look at the direct deaths or indirect deaths due to secondary impacts
▪ Location: rural and isolated areas are hard to reach and so hard to collect data from them. Similarly, data may be difficult to collect in areas with very high population densities.
▪ Different methods may be used by different organisations so as a result different sources may quote different figures
▪ The number of deaths quoted by a government could be subject to bias
4 layers of the Earth’s structure
- crust (lithosphere)
- mantle (asthenosphere)
- outer core
- inner core
Describe the crust (3)
- uppermost, thinnest, least dense, and lightest layer
- Oceanic crust is only 7km thick
- continental crust can be up to 70km thick.
Describe the mantle (4)
- Largely composed of silicate rocks, rich in iron and magnesium
- semi-molten
- temperature gradient (towards the core) generates convection currents, which causes to the circulation of the mantle and may contribute to the lithosphere’s plate tectonic movement.
- from 700km to 2890km below the surface
Describe the outer core (2)
- Dense, semi-molten rocks containing iron and nickel alloys.
- 2890km to 5150km below the Earth’s surface.
Describe the inner core (3)
- Similar composition to the outer core.
- over 5150km below the Earth’s surface
- solid due to the extreme pressures it experiences
Why does the inner core experience such high temperatures? (2)
○ primordial heat left over from the earth’s formation
○ radiogenic heat produced from radioactive decay
Describe convection currrents
- hot magma rises because it becomes less dense with heat
- magma cools at the top as it is further away from the heat source
- it then becomes more dense and sinks back down
- cooler magma is reheated and begins to rise again
Describe a destructive plate boundary between a continental and oceanic plate (7)
● Denser oceanic plate subducts below the continental.
● leaves a deep ocean trench.
● oceanic crust melts as it subducts
● extra magma created causes pressure to build up.
● Pressurised magma forces through weak areas in the continental plate
● composite volcanoes erupt through the continental plate
● Fold mountains occur when sediment is pushed upwards during subduction.
Describe a destructive plate boundary between 2 oceanic plates (3)
● Heavier plate subducts leaving an ocean trench and fold mountains
● Built up pressure causes underwater volcanoes bursting through oceanic plate.
● Lava cools and creates new land called island arcs.
Describe a collision plate boundary (5)
● 2 continental plates
● 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.
Describe a constructive plate boundary between 2 oceanic plates (3)
● 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 (as the floor spreads and gets wider).
Describe a constructive plate boundary between continental plates (4)
● 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.
● The lifted areas of rocks are known as horsts whereas the valley itself is known as a graben.
Describe a conservative plate boundary (5)
- 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.
Describe paleomagnetism and how it provides evidence of seafloor spreading
- As new rock is formed and cools the magnetic grains within the rock align with the magnetic poles
- Our poles switch periodically. Each time these switch the new rocks being formed at plate boundaries align in the opposite direction to the older rock.
- On the ocean floor either side of constructive plate boundaries, Geologists observed that there are symmetrical bands of rock with alternating bands of magnetic polarity.
What are different mechanisms causing plate movements? (3)
- slab-pull
- ridge-push
- convection currents
Describe ridge-push
- The slope created when plates move apart has gravity acting upon it as it is at a higher elevation.
- Gravity pushes the plates further away, widening the gap
Describe slab-pull
- When a plate subducts, the plate sinking into the mantle pulls the rest of the plate (slab) with it, causing further subduction
How has tectonic theory changed?
- tectonic movement isn’t fully understood.
- Previously, convection currents were thought to be the primary cause of plate movement.
- However, researchers now believe that Slab Pull is the primary mechanism for plate movement
- convection currents seem too weak to move massively dense plates.
What causes an earthquake?
- Plates do not perfectly fit into each other so plates can become stuck due to the friction
- the convection currents in the asthenosphere continue to push, which builds the pressure
- the plates eventually give way.
- All of this pressure is released in a sudden movement, causing a jolting motion in the plates.
- This is responsible for seismic movement spreading throughout the ground in the form of seismic waves
Define the focus (or hypocentre) of an earthquake
- the point underground where the earthquake originates from
Define the epicentre of an earthquake
- the area above ground that is directly above the focus
Name the 4 types of seismic waves
- primary
- secondary
- love
- rayleigh
Describe primary waves (P-Waves)
- Travels through solids
- Compressional
- Vibrates in the direction of travel
- Travels at 4-8 km/s
Describe secondary waves (S-waves)
- Vibrate at right angles to direction of travel
- Travels only through solid rocks
- Travels at 2.5 - 4 km/hr
Describe love waves
- Near to ground surface
- Rolling motion producing vertical ground movement
- Travels at 2-6 km/hr
Describe Rayleigh waves
- Vertical and horizontal displacement
- Travels at 1-5 km/hr
- Compressional
Which types of wave are most destructive?
- Secondary and Love waves are the most destructive as they have large amplitudes. - Due to their different speeds, these different waves will hit a location at different times.
- The aftershocks that survivors feel are these different types of waves arriving after each other.
What affects the intensity of a seismic wave?
- Intensity will decrease further from the epicentre, as waves lose energy as they travel.
- this does not mean that impacts felt or damage caused will always decrease further from the epicentre as other factors affect a location’s vulnerability
Factors affecting an area’s vulnerability to earthquakes (5)
- geology
- geographical location (whether the earthquake occurs near the sea or intraplate)
- education of locals,
- durability of buildings
- mitigation
Name 3 secondary impacts of earthquakes
- soil liquefaction
- landslides
- tnusamis
Describe liquefaction
● 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.
Describe landslides caused by earthquakes
● The shaking caused by the earthquake can weaken or damage cliff faces, hills and snow material.
● Unconsolidated material or loose rocks can collapse.
● Landslides can travel several miles and accumulate material on the way.
● Risk varies with topography rainfall, soil and land use.
Describe tsunamis
● When an oceanic crust is jolted during an earthquake, all of the water above this plate is displaced
● This water is then pulled back down due to gravity. The energy is transferred into the water and travels through it like a wave.
● The water travels fast but with a low amplitude
● As it gets closer to the coast there is friction between the sea bed and the waves.
● This causes the waves to slow down and gain height
Where do tsunamis usually occur?
- Tsunamis are generated generally 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
Physical factors affecting tsunami impacts (5)
- geographical location
- Duration of the event
- Wave amplitude and distance travelled
- Gradient of the continental shelf
- The shape of the land - bays will funnel and concentrate tsunami waves.
Human factors affecting tsunami impacts (4)
● Population density of area hit, if the population is higher than more people are likely to be affected
● Coastal defences (e.g. Tsunami walls)
● Warning & Evacuation Systems
● Level of economic and human development
Describe strato (composite) volcanoes
- steep-sided cones formed from layers of ash and acidic lava flows.
- lava is viscous so it cannot travel far down the slope of the volcano before it cools.
- usually found at destructive plate margins.
- Most composite volcanoes contain complex internal networks of lava flows which contain intrusive (below ground) igneous features
- e.g. Mount Fuji, Mount St Helens
Describe the eruption of a composite volcano
- The eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava.
- When composite volcanoes erupt they are explosive and pose a threat to nearby life and property.
- Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes.
What is a caldera?
- A caldera is a volcanic crater
- They form when an explosive eruption destroys the cone and the magma chamber below is emptied.
- Without magma providing support below a caldera is formed when the sides of the volcano collapse.
Describe a shield volcano
- Shield volcanoes are low with gently sloping sides and are formed from layers of lava.
- Eruptions are typically non-explosive. Shield volcanoes produce fast flowing basic (fluid) lava that can flow for many miles.
- Eruptions tend to be frequent but relatively gentle.
- Although these eruptions destroy property, death or injury to humans rarely occurs.
- Shield volcanoes are usually found at constructive boundaries and sometimes at volcanic hotspots.
- e.g. Mount Kilauea and Maunaloa on Hawaii.
Define strombolian eruption
- The effects are impressive but not particularly dangerous.
- They eject short bursts of lava 15 to 90 meters in the air.
- The lava has a fairly high viscosity, so gas pressure builds up before the material can be ejected from the volcano.
- Lava flows from Strombolian eruptions are not common though they do produce small amounts of ash.
Define vulcanian eruptions
- Vulcanian eruptions involve violent gas explosions that blow out sticky plugs of lava.
- These fragments build up cones of ash and pumice.
- Lava is very viscous and solidifies rapidly.
- The eruption usually clears a blocked vent and releases a significant amount of ash.
Define Vesuvian eruptions
- vesuvian eruptions involve very large blasts of gas that force large ash clouds into the sky.
- They are more violent than Vulcanian eruptions.
- Ash falls on the surrounding area and lava flows can also happen.
Define Plinian eruptions
- most explosive and violent of volcanic eruptions.
- They produce huge plumes of ash and gas that typically takes the shape of a huge mushroom cloud.
- They are highly explosive and the AD79 eruption that buried Pompeii was one of these.
- started by highly viscous magma that has high gas content.
- As the magma emerges it depressurizes and this allows the gas to expand, propelling pyroclastic material as high as 45 km in the air
- These eruptions can last for days and create a sustained and tall eruption plume, which drops a huge amount of tephra, fallen volcanic material, on surrounding areas.
- Additionally, a Plinian eruption can produce extremely fast-moving lava flows that destroy everything in their path.
Primary hazards caused by volcanic eruptions (4)
- lava flows
- pyroclastic flows
- tephra and ash flows
- volcanic gases
Describe lava flows
– Streams of lava that have erupted onto the Earth’s surface
Factors affecting magma types (3)
- silica content
- viscosity
- gas content
How does magma’s silica content, viscosity, and gas content affect an eruption’s explosivity?
- As viscosity, silica content, and gas content increase, explosivity increases
Describe pyroclastic flows
- 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, they are extremely dangerous and can cause asphyxiation for anyone caught by the flow.
Describe 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
Describe 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 caused by volcanic eruptions (3)
- lahars
- jokulhaup
- acid rain
describe 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.
Describe a jokulhaup
– Snow and ice in glaciers melt after an eruption which causes sudden floods that are very dangerous
Describe acid rain
- caused when gases such as sulfur dioxide are released into the atmosphere by an eruption and dissolve in rain
Define multiple-hazard zone
- Multiple hazard zones are places where two or more natural hazards occur, and in some cases interact to produce complex disasters.
- Examples are California, Indonesia and Japan.
Features of multiple-hazard zones (4)
- tectonically active and so earthquakes (and often eruptions) are common
- geologically young with unstable mountain zones prone to landslides
- often on major storm tracks either in the mid-latitudes or on tropical cyclone tracks
- suffer from global climate perturbations such as ENSO
