Volcanic hazards
Volcanic hazards

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Volcanic hazards

1.5 Test what you've learned

Now that you've reviewed this course and completed the activities, try answering these questions to test what you've learned.

Question 1

Give at least three reasons why TYVET Table 5.2 (p. 118) would be an unreliable guide to the relative proportions of fatalities from various volcanic causes during the next 100 years.


  1. The data in the table are incomplete or unreliable for many parts of the globe, especially in the first half of the 400-year period considered.
  2. Few eruptions of VEI 7 and above occurred in that period. A single such eruption could change the balance completely.
  3. Population growth is continuing to put more people at risk.

Question 2

If you were at the foot of an erupting volcano and wanted to escape with your life, would you rather see a lava flow or a pyroclastic flow heading towards you, and why?


You would prefer to see a lava flow because pyroclastic flows travel much faster than lava flows (about 100 km per hour), whereas most lava flows barely exceed walking pace (although the exceptional Nyiragongo flow in 1977 moved at 30 km per hour). So, provided you are alert and able, you could get out of the way of a lava flow (unless escape routes are blocked), whereas you would be much less likely to evade a pyroclastic flow.

Question 3

List the different ways in which a pyroclastic flow can be initiated, and for each case cite one example of a specific eruption where this occurred.


A pyroclastic flow can be initiated by:

  1. dome collapse, e.g. Mount Pelée in 1902 (also Unzen in 1991, Soufriere Hills, Montserrat in 1997 and many others)
  2. directed blast, e.g. Mount St Helens in 1980
  3. column collapse, e.g. Mount Pinatubo in 1991 (also El Chichón in 1982, Vesuvius in AD 79, and many others).

Question 4

Describe the hazards associated with ongoing airfall, and any relevant preventive measures. (One sentence per hazard)


The weight of an airfall deposit can cause roofs to collapse, so it is sensible to sweep them clean during an eruption. Large bombs can cause damage by the speed of their impact so, if you are caught in the open while bombs are falling, it is safer to watch the bombs and dodge them when necessary rather than turning and running. Fine ash can cause respiratory problems, so try to avoid stirring it up and wear a face mask. Vehicles can be ruined if ash gets into the engine or lubrication systems, or if the engine overheats because the radiator is clogged by ash, so drive slowly or not at all.

Question 5

Describe one way in which a lahar can begin during an eruption, and one way in which a lahar can begin after an eruption. Give a recent example in each case. (One sentence for each)


A lahar can begin during an eruption if it causes snow or ice to melt, as happened at Nevado del Ruiz in 1985. [You could also have suggested Ruapehu in 1953 or Mount St Helens in 1980.] A lahar can happen after an eruption when heavy rain falls on airfall ash deposits, such as after the eruption of Mount Pinatubo in 1991. [You could also have suggested heavy rainfall on weak, fumarolically altered rock, as on Casita volcano in 1998.]

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