Tectonic processes and Hazards Flashcards
What is Mantle convection?
Heat produced by the decay of radioactive elements in the earths core heats the lower mantle - creating convection currents. The magma currents move in circles causing the plates to move
What is Slab pull?
Newly formed oceanic crust at mid-ocean ridges becomes denser and thicker as it cools, causing it to sink into the mantle under its own weight – pulling the rest of the plate down with it
What is Subduction?
Two oceanic plates (or oceanic + continental) move towards each other, one slides under the other into the mantle where it melts in the subduction zones
What is Seafloor spreading?
Mid-ocean ridges form when magma is forced up from the asthenosphere and hardens forming new oceanic rock. This new crust pushes the tectonic plates apart.
What is the Benioff zone?
Intermediate and deep earthquakes occur here due to the friction created between colliding plates
What happens at a convergent boundary where Oceanic meets continental?
Oceanic is denser so slides beneath the continental into the mantle and melts, marked by deep ocean trenches. Earthquakes in the Benioff zone and volcanic activity happen lots.
What happens at a convergent boundary where oceanic meets oceanic?
The denser/faster plate subducts beneath the other, deep ocean trenches form where this occurs, and the plate melts to magma. It rises from Benioff zone to form underwater volcanoes that rise above sea levels to form island arcs.
What happens at a convergent boundary when continental meets continental?
A COLLISION MARGIN occurs. Both plates equal density so neither subducts and they crumple and are forced upwards, creating fold mountains. Some subduction inevitable, so some seismic activity happens - shallow focus so higher severity e.g. Nepal
What happens at a divergent plate boundary?
Two plates are moving apart - which leads to the formation of new crust. Forming mid-ocean ridges and rift valleys.
What are the 3 types of seismic waves?
Primary, secondary, and Love
Why are primary and secondary waves known as body waves, and Love waves known as surface waves?
because they travel through the earth’s body, and ‘surface waves’ because they travel along the earths surface
What is the MMS, and what factors does it use?
Moment Magnitude scale
- Size of seismic waves
- Amount of rock movement
- Area of fault surface broken by the earthquake
- Resistance of the affected rocks
What does the Mercalli scale measure as opposed to the Richter or MMS?
Earthquake INTENSITY via observations of destruction
Name 2 primary effects of earthquakes
- Ground shaking
- Crustal fracturing
Name 3 secondary effects of earthquakes
- Liquefaction
- Landslides/avalanches
- Tsunamis
What can the 1989 Loma Prieta earthquake be used as a case study for?
- What magnitude was the quake?
- Why were some parts of San Fran so badly damaged?
- What major incident occurred because of it?
How the geology of an area can have enormous impact on the effect of an earthquake
- 6.9
- Marina area built on man made landfill (soft, sandy soil)
- amplified shaking
- liquified causing building collapse - Part of the Cypress freeway collapsed killing 42
Why do ‘aftershocks’ occur?
They’re a result of the earth ‘settling down’/readjusting along the part of the fault that slipped at the time of the main quake
Give a case study concerning aftershocks
- Why was this particular one so damaging?
The 2011 Christchurch, magnitude 6.3 aftershock after the 2010 quake.
- occurred closer to city centre and had shallower focus causing more shaking, than the 2010 original quake
What are intra plate earthquakes? Give a case study example
- Why are they problematic?
They happen in the middle of plates, far from the margins, occurring when stresses build up in ancient faults causing them to become active again.
- Don’t happen often - 1812 Missouri 7.5 mag
- Problematic as they don’t occur in well defined patterns so they’re hard to predict
Give 4 primary hazards of volcanoes
- Lava flows
- Pyroclastic flows
- Tephra and ash falls
- Gas eruptions
Name 2 secondary hazards from volcanoes
- Lahars
2. Jokulhlaup
What can the Monserrat eruption case study be used to show?
How eruptions and their secondary hazards can have enormous effects on population, even changing them permanently perhaps
Recall as much about the 1997 Montserrat eruption as you can
Started 1995 and for 5 years it erupted
- Dozens died
- 7,000 of 11,000 residents moved
- Capital, Plymouth was destroyed
- 2/3 houses, 3/4 infrastructure destroyed
- Unemployment rose as the islands tourist industry collapsed
- 2/3 island still uninhabitable
Give an index that could be used to measure volcanic eruptions
VEI, Volcanic Explosivity Index
What 3 factors does the VEI use?
- Amount and height of the volcanic material ejected
- How long the eruption lasts
- Qualitative description terms e.g. ‘gentle’, ‘explosive’
What are three signs that a volcano might erupt?
- Small quakes
- Changes to surface of volcano
- Changes to ‘tilt’ of volcano
What is the hazard-risk formula
(R)isk = (H)azard x (V)ulnerability / (C)apacity to cope
What is Vulnerability?
The ability to anticipate, cope with, resist and recover from a natural hazard
What is Resilience?
The ability to protect lives, livelihoods and infastructure from destruction, and to restore
