Laboratory Manual for Introductory Geology 4e

FIGURE 5.13 Impacts of stratovolcano eruptions.
(a) Trees from a forest scattered like pick-up sticks by the shock
wave of an explosive eruption.
(b) Plaster casts of bodies of Roman citizens of Pompeii,
preserved in the very fine-grained ash that buried them.
(c) Pyroclastic flow from a stratovolcano.
(d) A volcano-generated mudflow (lahar) destroys homes near a
stratovolcano.
To understand the severity of the hazard, we need to understand the many different
ways the eruption of a stratovolcano can affect the local area and areas as far
as thousands of miles away (FIG. 5.13):
■ The explosive blast: The sound effects reported above are an indication of the
energy that can be released suddenly when a stratovolcano erupts explosively. When
Mt. St. Helens erupted in 1980, the blast devastated areas up to 15 miles from the
volcano, tossing entire forests around like pick-up sticks (Fig. 5.13a). If the blast is
underwater, it may trigger devastating tsunamis.
■ Ash fall: Enormous volumes of ash may be blasted into the air and cover the
ground hundreds of miles from the volcano (FIG. 5.14). Entire villages may disappear,
buried beneath tens of feet of ash that fall quietly but inexorably (see Exercise 5.15 I
and Fig. 5.13b). The area affected by ash fall from the 1980 Mt. St. Helens is impressive
(red zone in Figure 5.14), but pales in comparison with those from older eruptions.
Some “super-eruptions” have global impacts. In 1815, the Mt. Tambora (Indonesia)
eruption put so much ash into the atmosphere that the summer of 1816 never happened.
Temperatures were far below normal, crops failed, and food shortages were
widespread. Modern technology creates new problems associated with volcanic ash. An
ash cloud from the 2010 eruption of Eyjafjallajökull (go ahead—try to pronounce it)
in Iceland disrupted air traffic between North America and Europe for weeks because
trans-Atlantic planes had to be diverted around it to avoid damaging their engines.
■ Pyroclastic flows (sometimes called nuées ardentes, French for burning clouds):
Red-hot, glowing clouds of ash, steam, and toxic gases erupted from stratovolcanoes
5.6 VOLCANOES AND VOLCANIC HAZARDS
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