geo - hazards

• Types of Plate Boundary

  • Volcanic eruptions and earthquakes most commonly occur at or near plate boundaries
  • There are four main types of plate boundaries:
  • Constructive (divergent)
  • Destructive (convergent)
  • Collision
  • Conservative (transform)

  Constructive (Divergent) Plate Boundary

  • At the constructive boundary, the plates are moving apart
  • The Mid-Atlantic Ridge is an example of a constructive plate boundary
  • Both volcanic eruptions and earthquakes can occur at this type of plate boundary
• Destructive (Convergent) Plate Boundary

  • At a destructive (convergent) plate boundary the plates are moving together
  • The denser, heavier oceanic plate subducts under the lighter, less dense continental plate
  • The boundary between the Nazca plate and the South American plate is an example
  • Both volcanic eruptions and earthquakes occur at this type of plate boundary
• Collision Boundary

  • At a collision boundary two plates of similar density move towards each other
  • Neither is dense enough to subduct so the land is pushed upwards
  • This forms fold mountains such as the Himalayas
  • Earthquakes are the main hazard at this type of plate boundary
• Conservative (Transform) Boundary

  • At a conservative (transform) boundary the plates move passed each other in opposite directions or in the same direction at different speeds
  • Earthquakes are the only hazard at this type of boundary
• Causes of Volcanic Hazards

  • Volcanoes occur at constructive (divergent), destructive (convergent) plate boundaries and hot spots
  • Volcanoes do not occur at collision boundaries or conservative (transform) boundaries
• Volcanoes at Constructive Boundaries

  • At a constructive (divergent) boundary the tectonic plates are moving away from each other:
  • Constructive plate boundaries often occur under the sea/ocean
  • The lava escapes through the gap left as the plates move apart
  • The lava cools and hardens forming a new crust
  • At constructive plate boundaries, the lava tends to be runny and eruptions are less explosive
  • These types of eruption form shield volcanoes which have gently sloping sides
• At a destructive (convergent) boundary, the tectonic plates are moving 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.
• Friction causes heat and the plate material melts forming magma.
• The magma rises to the surface through cracks in the crust.
• Volcanoes at Hot Spots 

  • At a hot spot the tectonic plate passes over a plume of magma:
  • The magma 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
• The cooling lava and ash build up forming a volcano.
• At destructive plate boundaries, the lava tends to be sticky and produces explosive eruptions.
• These eruptions tend to form composite or stratovolcanoes.
• Volcano Primary and Secondary Hazards

  • Volcanic eruptions only become hazards when they affect people 
  • The hazards from the volcanic eruption itself are primary hazards:
  • Ash
  • Pyroclastic flow
  • Lava flow
  • Gas emissions
  • Volcanic bombs
  • The hazards created that happen as a result of the primary hazards are secondary hazards:
  • Lahars 
  • Acidification
  • Landslides
  • Climate change
  • Fires
  • Floods
• Earthquakes and Plate Boundaries

  • Earthquakes can occur anywhere but mostly occur at or near plate boundaries
  • Earthquakes happen at all plate boundaries - constructive (divergent), destructive (convergent), collision and conservative (transform)
  • At a constructive (divergent) plate boundary, earthquakes tend to be weaker as the plates are moving apart
  • At destructive (convergent), collision and conservative (transform) plate boundaries earthquakes tend to be stronger
• Earthquakes Primary and Secondary Hazards

  • The primary hazard of an earthquake is the ground shaking all other hazards then follow on from this as secondary hazards
  • Secondary hazards can include:
  • Collapse of buildings and other structures
  • Landslides
  • Gas leaks 
  • Fires
  • Soil liquifaction
  • Subsidence
  • Mudflows
  • Tsunami
• Earthquake Sequence

  • The sequence of an earthquake is the same regardless of the boundary at which it happens:
  • As the tectonic plates move, they can get stuck
  • Pressure builds as the plates continue to try to move
  • Eventually, the plates jolt free and the pressure is released as energy
  • The point at which the earthquake starts is the focus
  • The epicentre is the point directly above the focus on the earth's surface
  • The energy passes through the Earth's crust as waves which is the earthquake
  • Earthquakes can happen as a result of human activity such as drilling into the crust or mining
• Causes of Tropical Cyclone Hazards

  • Low-pressure areas (less than 950mb)
  • Tropical Cyclones require specific conditions to form:
  • Sea surface temperatures over 27°C 
  • Between 5° and 20° north and south of the equator
  • Low wind shear
  • A deep layer of humid air
• In the right conditions, a tropical cyclone can form rapidly.
• A tropical cyclone follows a number of stages: Warm, moist air rapidly rises forming an area of low pressure.
• Air from high-pressure areas rushes in to take the place of the rising air.
• The air then rises forming a continuous flow of rising air.
• As the air rises it cools and condenses, releasing heat energy which helps to power the tropical cyclone.
• Some of the air sinks in the middle of the storm forming the cloudless, calm eye.
• Air at the top of the storm goes outwards away from the centre of the storm.
• The Coriolis force causes the rising air to spiral around the centre.
• When a tropical cyclone makes landfall or moves over an area of cold water it no longer has a supply of warm, moist air and it loses speed and temperature.
• The tropical cyclone moves westwards from its source.
• Features of Tropical Cyclone

  • Tropical cyclones have a number of features:
  • Heavy rainfall
  • High wind speeds (over 74kmph)
  • Storm surges 
  • Calm eye
  • Highest winds and heaviest rain in the wall of the eye
  • Diameter up to 800km 
• • A hazard is an event which has the potential to cause harm to the environment, people or the economy
  • A natural hazard is an event caused by environmental processes
  • A disaster occurs when harm actually occurs to the environment, people or the economy
  • Natural hazards can be categorised by their causes
• • Natural events only become hazards and disasters due to their impact on people, the environment or the economy
  • Natural hazards can also be categorised in a range of other ways:
  • Magnitude - the strength/power of the event
  • Frequency how often the event occurs
  • Size - the area covered by the hazard
  • Duration - the time a hazard event lasts
  • Location - where a hazard event occurs
• Tropical Cyclones

  • Tropical cyclones are rotating, low pressure systems (below 950mb)
  • They are known as hurricanes, cyclones and typhoons in different areas of the world
  • Characteristics include:
  • Heavy rainfall
  • High wind speeds (over 119 kmph)
  • High waves and storm surges
  • Measuring between 100-2000km across the rotating clouds surround a central, calm eye
  • The magnitude of tropical cyclones is measured on the Saffir-Simpson Scale from 1 to 5
  • They develop in tropical regions between 5o and 30o north and south of the equator
• Earthquakes

  • A sudden, violent shaking of the ground
  • Earthquakes occur at all types of plate boundaries
  • Earthquakes are the result of pressure building when tectonic platesmove
  • The epicentre is the point on the Earth's surface directly above the focus
  • The focus is the point at which the earthquake starts below the Earth's surface
  • The magnitude of earthquakes is measured on either the Richter Scale or the Moment Magnitude Scale
  • The damage caused by earthquakes is measured on the Mercalli Scale
• Volcanoes

  • When magma erupts onto the Earth's surface as lava
  • Most volcanoes occur at constructive (divergent) and destructive (convergent) plate boundaries
  • The majority of active volcanoes are located around the rim of the Pacific Ocean called the 'Ring of Fire'
  • Hotspots occur away from plate boundaries and are plumes/columns of magma which escape through the Earth's crust
  • The magnitude of a volcanic eruption is measured on the Volcanic Explosivity Index (VEI)
• • Warm, moist air rapidly rises forming a area of low pressure
  • Air from high-pressure areas rushes in to take the place of the rising air
  • This air then rises forming a continuous flow of rising air
  • As the air rises it cools and condenses releasing heat energy shich helps to power the trp cyc.
  • Air at the top of the storm goes outwards away from the centre of the storm
  • The Coriolis force causes the rising air to spiral around the centre.
  • Some of the air sinks in the middle of the storm forming the cloudless, calm eye.
  • The tropical cyclone moves westwards from its source 
• • The tropical cyclone moves westwards from its source 
  • When a tropical cyclone makes landfall or moves over an area of cold water it no longer has a supply of warm, moist air and it loses speed and temperature. Rainfall and winds decrease