Volcanic Hazards

A detailed volcanic hazards worksheet on the following issues/features:

  • Gases
  • Lahars
  • Landslides
  • Lava Flows
  • Pyroclastic Flows
  • Tephra
    Suitable for A2 Geography
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James Fortson
Magma contains dissolved gases that are
released into the atmosphere during eruptions.
Gases are also released from magma that
either remains below ground (e.g. As an
intrusion), or is rising towards the surface.
The volcanic gases that pose the greatest
potential hazard to people, animals,
agriculture, and property are sulphur dioxide,
carbon dioxide, and hydrogen fluoride. The concentration of CO2 gas in these areas
can be lethal to people, animals, and vegetation.
Case study: Mammoth Mountain (Long Valley Caldera), California
A Lahar is a hot or cold mixture of water and rock fragments,
flowing down the slopes of a volcano and/or river valleys.
When moving, a lahar looks like a mass of wet concrete that
carries rock debris ranging in size from clay to boulders more
than 10 m in diameter. Lahars can vary in both size and
As a lahar rushes downstream from a volcano, its size,
speed, and the amount of water and rock debris it carries
constantly change. Eruptions may trigger one or more lahars
directly by quickly melting snow and ice on a volcano, or
ejecting water from a crater lake. Sometimes lahars are
created by intense rainfall during/after an eruption -
rainwater can erode loose volcanic rock and soil on hillsides
and in river valleys.
Lahars almost always occur on or near stratovolcanoes because these volcanoes tend
to erupt explosively and have tall & steep cones.
They can crush almost anything in the path of a lahar; e.g. buildings and valuable land
may become partially or completely buried by layers of rock debris. By destroying
bridges and key roads, lahars can also trap people in areas vulnerable to other
hazardous volcanic activity, especially if the lahars leave deposits that are too deep,
too soft, or too hot to cross.
Case Study: Mount St. Helens, Washington (Muddy River, May 18, 1980).

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James Fortson
Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the
force of gravity. These mixtures of debris move in a wet or dry state, or both. Volcano
landslides range in size from less than 1 km3 to more than 100 km3.
A landslide typically destroys everything in its path and may generate a variety of
related activity.…read more

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James Fortson
km per hour, Pyroclastic flows knock down, shatter, bury or carry away nearly all objects
and structures in their way.
Pyroclastic flows vary considerably in size and speed, but even relatively small flows
that move <5 km from a volcano can destroy buildings, forests, and farmland. And on
the margins of pyroclastic flows, death and serious injury to people and animals may
result from burns and inhalation of hot ash and gases.
Case Study: Mount St. Helens, Washington (1980).…read more


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