GL3 Revision



The Hazard:

  • blast/explosion, 
  • ashfall, pyroclastic flows (nuéesardentes), and gases 
  • lava flows
  • debris flows and mudflows (lahars)

The nature of the hazard depends on the composition, viscosity and gas content of the magma.

Prediction and Control:

Indicators of magma movement include: ground deformation, gravity and thermal anomalies, gas emissions and seismic activity.

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The Hazard:

  • There is a relationship between earthquakes and active fault zones. 
  • The magnitude of an earthquake event is measured on the Richter scale. 
  • The intensity of earthquake damage around an event is measured on the modified
  • Mercalli scale and is related to earthquake size, depth, distance, local ground conditions and building standards. 
  • Tsunamis can cause devastation in coastal areas following an undersea earthquake or volcanic eruption.

Prediction and Control:

Methods used to predicting earthquakes include monitoring changes in: seismic activity, groundwater levels and pressure, tilting and ground movement, radon gas emissions, electromagnetic signals, electrical resistivity, animal behaviour, and the presence of earthquake lights.

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Mass Movement

The Hazard:

The mechanism and triggering of rock avalanches, landslides and debris flows are linked to angle of slope, lithology, weathering, load, groundwater regime,rainfall, shrinkage/expansion, ground vibration.

Prediction and Control:

  • Sites of potential slope failure can be monitored by: mapping, surveying, measurement of creep, strain, groundwater pressures.
  • The risk of damage to property or loss of life is related to population density, building type and density and human activity (social and economic) in the area of the hazard.
  • Slope stabilisation, drainage control, retaining structures.
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Case Studies


1883 Krakatoa; 1980 Mt St Helens; 1991 Pinatubo; late 1990s Montserrat.


1906 San Francisco; 1964 Alaska; 1985 Mexico; 1996 Kobe; 2004 Indian Ocean.

Mass Movement

1915 Folkestone Warren; 1988 Anatolia; 1999 Beachy Head.


1883 Krakatoa; 1999 Philippines; 2004 Indian Ocean.

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Waste Disposal

Problems of ground contamination, including groundwater pollution and methane gas production, can be controlled by good geological site selection and engineering practice.
Use of former landfill sites for development can pose problems of ground instability and subsidence. There are special problems with the disposal of highly toxic and radioactive waste.

Leachate is the fluid generated by water dissolving soluble chemicals from landfill sites

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Dams and reservoirs

  • Valley shape and rock structure.
  • Foundation strength; porosity and permeability; zones of structural weakness and high permeability. 

Cuttings and rock tunnels

  • Rock strength; stable and unstable patterns of geological structures (bedding, jointing, faulting, cleavage). 


  • Foundation strength; depth to water table and rockhead; problems of radon gas in buildings.
  • Sources and pathways to surface, surface geology of high-risk areas.
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Groundwater & Aquifers

  • Groundwater is the water retained in the pore spaces of rocks below the water table. 
  • The water table is the surface seperating unsaturated rock above from saturated rock below.
  • An aquifer is a body of porous rock and permeable rock capable of storing and yielding significant amounts of water.
  • The re-charge zone is the area of an aquifer open to the atmosphere, allowing repleshiment of water.
  • Porosity is the volume of pore spaces.
  • Permability is the rate at which a fluid flows through a rock.
  • Artesian wells hold water under hysrostatic perssure, which rises up the well on release.
  • A borehole is drilled down into an aquifer, with an average diameter of 200mm.
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