Geoscience Reference
In-Depth Information
measured using tiltmeters. The movements can be fast
or slow, positive or negative. Differences in movement
can occur over short distances. Large, sudden tilts
generally herald a violent eruption while slight tilts of
increasing frequency foreshadow the movement of
magma closer to the Earth's surface. Tiltmeters on the
northern side of Mt St Helens measured dramatic infla-
tion at the rate of 0.5-1.5 m day
-1
preceding the
eventual eruption. Tiltmeters have also been used in
Hawaii to accurately forecast eruptions of Kilauea.
Studies of seismology have dominated volcanology
since the early 1900s. As magmas flow through sub-
terranean channels, they apply stress to rocks, which
can fracture and set off seismic activity. This applies
mostly to volcanoes characterized by fluid magma.
Earthquakes set off by volcanoes differ from tectonic
ones in that they occur at depths of less than 10 km, and
are low in magnitude. Volcanic tremors can be divided
into two groups. The first group consists of prolonged,
continuous volcanic vibrations due to resonance of fluid
magma flowing through cracks. Some explosive volca-
noes, notably Mt St Helens, were beset with this type of
tremor. These long period vibrations have proved highly
successful at forecasting impeding eruptions. The most
notable success was the prediction twenty-four hours in
advance of the eruption of Mt Redoubt in Alaska on
2 January 1990. The second group consists of spasmodic
or regular vibrations, the frequency of which indicates
the origin and nature of the magma. All forecast erup-
tions of Kilauea, Hawaii, have been based upon tilting
and earthquake precursors. However, not all seismic
activity associated with volcanism can be used with
certainty to predict subsequent activity. While most
eruptions are usually, but not always, preceded by
swarms of earthquakes, the presence of tremors can
also represent collapse of rock into emptying magma
channels, a process that may indicate cessation of
volcanic activity. In a few cases, the subsidence
of earthquake activity may also signal an eruption.
Active volcanoes are dominated by temporally and
spatially variable geomagnetic fields. Volcanoes contain
a high content of ferromagnetic minerals, which can set
up changes in the local magnetic field. Magnetization,
however, is reduced by increasing temperature, vanish-
ing completely above 600°C. The magnetic field of a
volcano is thus reliant upon the proximity and temper-
ature of molten magma near the surface. As hot magma
between 200 and 600°C approaches the surface,
the
Magnetization can also be enhanced by increasing
pressure and stress exerted by flowing magma as it
approaches the surface. This process is termed
piezomagnetism
, and at present is being researched
extensively as a new technique for prediction. Geo-
electric measurements involving the resistivity of the
subsurface layers of a volcano and the change in the
telluric currents can give an image of the behavior of
magma at depth. Resistivity depends upon the nature of
the rock, its water content, salinity, and temperature.
Telluric currents depend upon geological structure,
lithology
, moisture content, and temperature. At
present, these methods are used to define the structure
of the volcano, including the presence of natural
conduits, which may become the preferred pathways
for continued magma movement.
The analysis of gaseous constituents exhaled from
a volcano constitutes one of the best techniques
for understanding and forecasting eruptive activity.
Unfortunately, while the technique is so informative, it
is restricted by the need to chemically analyze the gas
constituents immediately they are vented from the
volcano. Many gas studies are still in their infancy,
mainly because techniques for sampling and analyzing
gases from venting volcanoes are still being developed.
The most common gases vented by a volcano are H
2
O,
CO
2
, SO
2
, H
2
, CO, CH
4
, COS, CS
2
, HCl, H
2
S, S
2
,
HF, N
2
and the rare gases helium, argon, xenon, neon
and krypton. The chemical nature of these gases
depends upon the maturity of the magma melt,
because not all gas components have the same solubil-
ity at a given pressure. The type of gas also depends
upon the amount of crustal material relative to original
magma that is incorporated into the melt. If the
magma is chemically stable, the
partial pressure
ratios
between various gases can give an indication of the
pressure and temperature of the magma en route to
the surface. For example, the partial pressure ratio of
CO:CO
2
, HF:HCl and H
2
O:CO
2
can all be used
as qualitative geothermometers. The first two ratios
increase, and the last one decreases, as temperature
increases. The ratios of CO:CO
2
, H
2
:H
2
O, and
H
2
S:SO
2
are sensitive to changes in the conditions that
control the thermodynamic equilibrium of the gas
phase, and can be used to distinguish between
hydrothermal
and magma flows. The ratios SO
2
:CO
2
and S:Cl, plus the absolute amount of HCl, increase
immediately prior to eruptions, while the ratio He:CO
2
decreases. These changes result from the different
geomagnetic field should therefore decrease.
