Associated terms:
Hydrovolcanic eruptions
Phreatic eruptions are violent explosions triggered by steam.
The steam is produced as rising magma heats groundwater already
in the volcano. The explosions blast out fragments of cold,
old rock, which may travel several kilometers and may also generate
cold pyroclastic surges. Phreatic eruptions commonly occur in
the period shortly before new magma breaches the surface, but
may also occur at later stages in the eruptive cycle.
Fig 7. Phreatic explosion
Associated terms:
Fumarole
Gases
Soufrière
Fumaroles are vents where volcanic gases and vapourised groundwater
(heated by magma) escape from the ground. The most common emissions
are steam, carbon dioxide (CO2) and sulphur dioxide (SO2). Gases
such as carbon dioxide may also leak invisibly out of the soil
on the flanks of a volcano. Dense gases such as carbon dioxide
may accumulate in depressions and can be lethal to humans and
livestock. Volcanic gases may also damage crops, sterilize soils
and lead to acid rain even at considerable distances from the
volcano.
Volcanic gases are virtually colourless and difficult for the
human eye to detect. |
Some gases sting (e.g. the acidic SO2) or have distinctive
smells (e.g. hydrogen sulphide, H2S, which smells of rotten
eggs). Others, such as CO2 are odourless.
All are dangerous if sufficiently concentrated.
The presence of fumaroles does not necessarily mean that a
volcano is about to erupt.
Established fumaroles may become more active and new ones
may appear before an eruption.
Fumarolic activity is likely to continue long after an eruption
ceases.
Fig 8. Fumarole activity
During a volcanic crisis, scientists use several methods to
monitor a volcano and attempt to determine if and when it
will erupt. No volcano erupts without warning signs. Magma
must open a pathway for itself before it can reach the surface.
This it does by cracking open the rock as it rises, a process
that triggers earthquakes. As it accumulates beneath the surface
prior to eruption, the magma causes the surface to bulge,
and also produces tiny changes in the Earth's gravity field
and in the electrical and magnetic properties of the rock.
The proximity of magma may also cause changes in the temperature
and composition of fumaroles and in their compositions.
Methods for forecasting volcanic eruptions are largely based
upon measuring how rising magma changes the rates of earthquake
generation, ground swelling, and gas release as
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