10: Earthquakes and Volcanoes

• which plate boundaries do volcanoes occur at?

  divergent (constructive), convergent (destructive) and hot spots
• what happens at a hot spot

  the tectonic plate passes over a plume of magma which rises to the surface through cracks in the crust. As the tectonic plate moves slowly over the magma plume a line of islands may form e.g. Hawaii
• what is a destructive plate boundary?

  a.k.a. a convergent plate boundary. The plates move towards each other; the heavier, denser oceanic plate subducts under the lighter continental plate. In the subduction zone, the two plates come together causing friction which causes heat and the plate material melts forming magma which rises to the surface through cracks in the crust. The cooling lava and ash build up forming a volcano; at which the eruptions tend to form composite or stratovolcanoes

  Ex: Nazca and South American plate; earthquakes also occur at this type of plate boundary
• what is a composite volcano

  a.k.a stratovolcanoes; steep-sided, has sticky (viscous) lava, more explosive eruptions, formed from alternating layers of ash and lava
• what is a divergent plate boundary?

  a.k.a constructive plate boundary often occurs under the sea/ocean and lava escapes through the gap left as the plates move apart. The lava cools and hardens forming a new crust which tends to be runny, and eruptions are less explosive; which form shield volcanoes. Earthquakes can also occur at this type of plate boundary

  Ex: Mid Atlantic Ridge
• what are shield volcanoes

  gently sloping sides, runny/thin lava, less explosive (gentle eruptions), can also form on hot spots and has frequent eruptions
• what is the difference between continental and oceanic crust?

  continental crust is thick, old and not as dense as ocenic crust

  oceanic crust is thinner, heavier and denser than continental crust. It is continually being created and destroyed as a result of plate movement where it is denser and so subducts under the continental crust
• what is the process of convection currents?

  1. Magma (semi-molten rock) near the outer core is heated
  2. As the magma warms it expands and becomes less dense
  3. The less dense magma then starts to rise towards the crust
  4. As the magma nears the crust it begins to cool
  5. The cooling magma becomes less dense and begins to sink
  6. The rising and falling magma creates circular currents within the mantle which create friction with the crust above and causes it to move.
• what is a collision boundary

  two plate of similar density move towards each other; neither is dense enough to subduct, so the land is pushed upwards forming fold mountains such as the Himilayas. Earthquakes are the main hazard at this type of boundary
• what is a transform boundary

  a.k.a. conservative boundary: the plates move past each other in opposite directions or the same direction at different speeds. Earthquakes are the only hazard at this type of boundary
• how does earthquakes work?

  the epicentre is the point on the Earth's surface directly above the focus which is the point at which the earthquake starts below the Earth's surface. The magnitude (amount of energy released) by earthquakes is measured on the Moment Magnitude Scale which replaced the Richter scale. The damage caused by earthquakes is measured on the Mercalli scale.
• Earthquakes can occur anywhere, but they mostly occur ar to near all types of plate boundaries. Most occur along the Pacific 'Ring of Fire'

  • at constructive (divergent) plate boundaries, earthquakes tend to be weaker as the plates are moving apart
  • at destructive (convergent), collisions and conservative (transform) plate boundaries earthquakes tend to be stronger
• what are primary hazards of earthquakes and volcanoes?

  primary hazards are a direct result of the earthquake or eruption these include

  • ground shaking
  • surface rupture (when movement on a fault deep within the earth breaks through to the surface)
  • pyroclastic and lava flow
  • volcanic bombs
  • lahars (mudflow)
  • earthquakes
  • direct ash fall
• what are secondary hazards

  these occur as a result of the primary effects

  • liquefaction (when loose material including soil and rocks acts like a liquid due to an earthquake
  • landslides
  • tsunami
  • fires
  • acid rain
  • ash fallout from the atmosphere
• what human factors affect the impacts of earthquakes?

  • poorly constructed buildings with poor-quality materials are more likely to collapse. Higher buildings amplify the shockwaves and are more likely to collapse
  • where buildings are close together, they are more likely to damage or destroy other buildings if they fall
  • the higher the density population the more people are likely to be injured or killed
  • earthquake drills can reduce the impact
• physical factors that affect the impact of earthquakes

  • magnitude - the amount of energy released by the earthquake. Usually, the higher the magnitude the greater the impact
  • geology - some rocks are more prone to liquefaction while others provide stronger foundations for buildings
  • distance from the epicentre - the closer somewhere is to the epicentre the stronger the shock waves will be
  • depth - shallower earthquakes mean less energy is absorbed by the crust before the shockwaves reach the surface
• volcanic eruptions (humans)

  • how close populations are to the volcano. The nearer that people are the more likely they are to be affected by lava flow and ash
  • where authorities have planned exclusion zones and evacuation plans the impact may be reduced
• volcanic eruptions (physical)

  • Magnitude - the size and strength of the eruption. Usually, the higher the magnitude the greater the impact
  • Type of volcano - composite (strato-volcanoes) tend to be more explosive and less predictable than shield volcanoes
  • Amount of ash ejected into the atmosphere - ash in the atmosphere can impact on areas 100s of km away and can affect the global climate
• effects of earthquakes and volcanic eruptions

  • loss of life and injury - may be immediate through falling buildings or ash fall. Alternatively it may be in the days and weeks after the event due to unclean water or disease
  • collapse or destruction of buildings leads to people being homeless for months or even years after the event
  • transport network - roads, bridges and railways may be damaged or destroyed impacting the speed at which aid can get to affected areas
  • loss of jobs and businesses - this impacts the economy when factories and offices are destroyed or damaged
• effects pt. 2

  • loss of crops leads to food shortages and affects farmers income
  • power and water supply damathe ged affecting supply of clean water
  • damage to the environment - loss of vegetation and habitats, effects on climate of ash
  • closure of airports - it is dangerous for jet planes to fly through ash clouds due to possible engine failure so flights are cancelled
• what opportunities are associated with living near volcanoes?

  fertile soils-the ash and cooled lava contain many minerals which lead to high yields of crops

  tourism - many people want to visit areas where there are more active volcanoes, this increases the number of jobs available

  minerals and precious stones - these can be mined and sold providing work for local people

  geothermal energy - as the magma is closer to the surface the heat can be used to generate geothermal energy

  creating new land area - once the lava and ash have cooled this forms new land area
• what is the difference between short and long term responses to earthquakes and volcanoes?

  short term responses are those that happen straight after an event and include aid and disaster relief

  long term responses aim to reduce the impact of future hazard events and include risk assessments and hazard mapping
• what are long-term responses to earthquakes?

  Methods used to monitor earthquakes include...

  tiltmeters, which monitor ground changes

  clusters of small earthquakes

  changes in radon gas emissions and animal behavior

  remote sensing of ground movement using satellites

  
  

  in many countries, earthquake drills are regularly carried out which help people to prepare for what to do in an earthquake to protect themselves. Education about how to prepare homes means that people are less likely to be injured by falling objects and furniture
• what are the building regulations and codes for new buildings to be earthquake resistant?

  shutters on windows to prevent falling glass

  cross-bracing or diagonal bracing of steel frames

  flexible building materials

  foundations sunk deep into the bedrock

  frames which sway with the earthquake tremors

  rubber shock absorbers to reduce tremors moving through the building

  reinforce walls/pillars with concrete

  shatter proof/reinforced glass

  fire resitant materials

  automatic cut off for gas/electricity
• what examples of planning is used in both areas vulnerable to earthquakes and volcanic eruptions?

  remote sensing of the Earth's surface by satellites such as Sentinel 1 provides data about changes in ground movement

  geographic information systems (GIS) provides layers of data such as

  vulnerable areas

  land use

  and infrastructure (e.g. roads)

  GIS data is then used to assist with land use planning
• what is land use planning?

  mapping of areas most at risk from earthquakes

  planning can ensure that valuable services such as the fire service and hospitals are not built in high risk areas

  densely populated housing can be located away from high-risk areas
• what can be done to reduce the impacts of eruptions on people who live near volcanoes?

  observe/monitor the volcano

  create an exclusion zone around the volcano

  evacuation

  spray water on lava

  dig diversion channels for lava

  strengthen roofs of houses

  wear masks for protection from ash

  train emergency services

  educate residents on action to take if the volcano erupts

  store emergency provisions
• why do Earthquakes of the same magnitude may have different impacts?

  variation in

  population density/if high there will be more impact

  amount of education/awareness about earthquake survival/evacuation plans/First Aid kits

  ability to monitor/study faultline movements

  quality/cost of buildings/poorer quality buildings in LEDCs

  emergency services/rescue teams/health care will be better in MEDCs