MadSci Network: Earth Sciences

Re: Volcanos, years or months before they erupt, Is it possible or

Date: Sun Jun 3 18:33:56 2001
Posted By: David Scarboro, Faculty, Earth Sciences, The Open University
Area of science: Earth Sciences
ID: 990444340.Es

Dear John,

The answer is certainly no.  The magma beneath or inside a volcano is 
under tremendous pressure.  When that pressure is released the result is 
an eruption.  Depressurization normally occurs when the strength of the 
overlying rock confining a magma chamber is overcome by the pressure 
within the magma.  Magma contains dissolved water and other volatile gases 
under pressure, rather like the carbon dioxide dissolved in a fizzy 
drink.  The mechanisms of an explosive eruption are complex, but 
essentially the release of the confining pressure causes the gases 
dissolved in the magma to exsolve suddenly.  If I can quote the late Peter 
Francis, a leading authority on volcanoes, “… explosive eruption columns 
are driven by the thermal energy stored in the magma.  Thermal energy is 
transferred into the kinetic energy of the eruption column through the 
expansion of the gases diffusing into growing vesicles” in the magma 
[Peter Francis, “Volcanoes:  A Planetary Perspective” (London:  Clarendon 
Press, 1993) p. 167].  Even if we could drill a hole into a magma chamber 
it would run the risk of depressurising the magma, causing it to erupt.  
An example of something very like this occurred naturally in the 1980 
eruption of Mt. St. Helens.  In this case the rise of magma into the 
volcano caused the north side of the mountain to swell outward until it 
became unstable.  Rising magma triggers earthquakes, and one of these 
tremors caused the north face of Mt. St. Helens to collapse in a 
landslide.  The landslide removed the confining pressure from the magma 
chamber inside the volcano, causing it to erupt sideways.

The volatile content of magmas varies greatly.  Basaltic magmas such as 
those produced on mid-ocean ridges and hot spots (Iceland and Hawaii are 
good examples) normally contain less volatile content by volume than 
magmas at destructive plate boundaries, such as Mt. St. Helens.  But even 
basaltic magmas contain enough volatile content to make eruptions 
dangerous.  The fire fountains associated with eruptions on Hawaii, 
Iceland and Mt. Etna are driven by the exsolution of dissolved gases.

A volcano is merely the surface expression of a complex magma system, 
which will consist of numerous conduits and chambers fed with magma from 
great depth, often from the upper mantle from which magma can rise from 
depths of 60 km or more. The ultimate source of magma is therefore beyond 
the reach of any conceivable technology.  By the time magma has risen to 
shallow depths beneath a volcano it will be too late to stop any eruption 
because the magma chamber will explode when the confining pressure is 
released, however the release occurs.

This is not to say that nothing can be done to intervene in eruptions, 
especially the effusive eruptions of basaltic lavas in which lava flows 
threaten property.  Both on Mt. Etna and in Iceland lava flows have been 
diverted away from threatened areas, for example by blasting alternative 
channels in the path of advancing lava.  When an explosive eruption is 
threatening to occur, however, the best thing to do is evacuate the area.

I hope this answers your question.

Best wishes,

David Scarboro

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