Natural Hazards: Causes and Effects - Disaster Management Center ...

explosions had built up to a maximum before suddenly stopping. The danger of a sudden break
in activity in the potentially explosive silica-rich volcanoes is now recognized. In 1902 Mount
Pelee in the Caribbean built up to a crescendo of activity before suddenly stopping; for four
hours the gas pressure built up inside the volcano before it exploded in its now famous and
catastrophic fashion. A similar sequence of events today would be taken as a sign for
immediate evacuation of the area—as happened at the nearby Mount Soufriere in 1976. Some
volcanoes appear to erupt more commonly at times dictated by external processes, such as
climatic changes or earth-tide effects, and Chile’s Puyehue erupted in 1960 just 48 hours after a
major earthquake centered 290 kilometers (180 miles) away. Such trigger mechanisms are
unfortunately so little understood as yet that they can only be useful in alerting volcanologists to
intensify their search for more precise methods of prediction.
As magma starts to rise towards the vent inside a volcano, some of its heating effects may be
detectable before the actual eruption. Thermally sensitive infrared air photographs have been
used to detect surface heating of this nature, but as yet, recognizable increases in temperature
give too little notice of any impending eruption to be of much use. This same heating may
cause the waters of perennial springs and fumaroles to rise in temperature, and this effect is
noticeable rather earlier than the rock heating. The 1965 eruption of Taal, in the Philippines,
was predicted because of a temperature rise in the crater lake water; consequently there was a
rapid evacuation of the area and only 190 people died in what proved to be a very violent
eruption.
Heating also has the effect of demagnetizing rock. This effect can be monitored by surface
magnetic surveys. On the Japanese volcano of Oshima a marked magnetic loss prior to an
eruption has been recorded from a pool of relatively shallow magma. Magnetic effects have not
been found significant on the Hawaiian volcanoes, which are seemingly fed from deeper magma
accumulations.
The gases from fumaroles may yet provide one of the most useful means of prediction. On
some Japanese volcanoes, fumarole or vent gases have been found to contain much higher
contents of chlorine and sulphur dioxide just before eruptions—though this has not been
confirmed on other similar volcanoes. As the gases of a volcano are so clearly related to the
details of the mechanism of any eruption, their study may yet reveal very useful methods of
prediction.
Since the upward movement of magma involves the upward displacement and deformation of
millions of tons of surrounding rock, it is possible to detect the movement before an eruption
actually takes place. This can be done in two ways: by measuring small surface
displacements, and also by seismic recording of shocks and tremors from underground
movements. Magma is generated at considerable depths within the earth’s crust and then
moves upwards due to its low density compared with surrounding, cooler, solid rocks. Just
before its eruption on the surface, its movement into the heart of the volcano causes a regional
tumescence—a doming and uplift of the volcano itself. Accurate surveying of the surface can
detect and measure this by measuring either elevation, distances between points across the
tumescence, or tilting. The latter is usually measured because tiltmeters consisting of fluid-filled
pipes connecting two reservoir containers are relatively simple and inexpensive to make, with a
sensitivity to detect a change in slope of one in a million.
The other effect of magma’s upward movement in a volcano is the generation of small
earthquakes, and these can be detected on standard seismographs. Many small tremors shook
the area around Vesuvius for 16 years before its eruption of A.D. 79. The significance was not