The Challenge of Natural Hazards

How do Earthquakes occur?

  • Two plates against each other eventually getting stuck, then the pressure starts to build
  • The plates pressure gets too great, they jolt violently in opposite directions causing an earthquake
  • The center of the earthquake is called the focus, waves are released from the jolt, these are called seismic waves
  • The place where the seismic waves first hit the ground is called the epicenter. Buildings here will receive the most damage
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Constructive Plate Boundary

  • At constructive plate margins, the two tectonic plates move apart
  • Magma is injected between the two plates adding new materials to plates creating volcanoes at the surface. These volcanoes are not very dangerous or explosive. Small tremors are common. They are produced by the movement of rising magma and are more like small earthquakes than large earthquakes.
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Destructive Subduction Boundary

  • At destructive subduction plate margins, the two tectonic plates move together.
  • The oceanic plate subducts beneath the continental plate since it is denser
  • Magma forms as the oceanic plate melt. The magma rises through the continental plate and erupts at the surfaces.
  • The volcanoes are large, explosive and dangerous. Earthquakes happen because of pressure between the continental and subducting oceanic plate. These earthquakes can be shallow or deep and are often very violent.
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Conservative Plate Boundary

  • At conservative plate margins, the two tectonic plates slide past each other.
  • There are no volcanoes at conservative plate margins, and no magma erupts from the surface. Sometimes the plates cannot move past each other and get stuck.
  • Pressure builds up until the plates give way. This produces large destructive earthquakes.
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How do shield volcanoes form?

  • The plate boundaries move away from each other
  • The space left between them allows the magma to spill out easily
  • The lava becomes runny because it has a low viscous level which slowly spills out
  • The lava slowly dries and layers over the previous layers of lava creating a wide base
  • This shield volcano will continue to grow over thousands of years
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How do Composite volcanoes form?

  • The plate boundaries are moving towards each other
  • The viscous lava (meaning it is thick) has to burst through the plate margin and work its way up, some lava will dry which creates explosions of ash
  • The lava and ash will layer over each other, the volcano has a narrow base because the thick lava does not move fast.
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High & Low Pressure

  • All air exerts a weight on the earth's surface. This is known as air pressure. We measure air pressure in millibars using a barometer. The weight air depends on whether air is cold or warm. Warm air is light so it rises. As it rises, the pressure at the surface decreases leading to low pressure. Cold air is dense (heavy) so it sinks. As it sinks, the pressure. High-pressure systems are called anticyclone; low-pressure systems are called depressions.
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How do Tropical Storms form?

  • The center of the storm is called the eye - it's up to 50km cross and caused by descending air
  • There is very high pressure, light winds, no clouds, no rain and a high temperature in the eye
  • The eye is surrounded by the eyewall, where there's spiraling rising air, very strong winds, storm clouds, torrential rain, and a low temperature
  • Towards the edges of the storm the wind speed falls, the clouds become smaller and more scattered, the rain becomes less intense and temperature increases. 
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