The Present Situation

Many volcanoes could cause trouble. For example, it is well known that the city of Naples could be in trouble should there be an eruption of Vesuvius. Another situation was highlighted recently on television, as follows.

A BBC Horizon programme described the discovery of a quiet, or dormant, volcano under the US Yellowstone National Park. (At the time of writing there is a full transcript of the programme, together with some relevant discussions and links, available from the BBC Web Site at the Page:-

http://www.bbc.co.uk/science/horizon/supervolcanoes_script.shtml - downloadable in 9 off A4 pages.)

Both the timing and the size of the Yellowstone volcano are startling. As regards time, it has been found that eruption may occur every 600,000 years with the last eruption just over 600,000 years ago. Currently there is movement at the ground surface. So, we may wonder for how much longer the volcano is going to remain dormant, or quiet! As regards size, the Yellowstone situation is said to be on such a scale, that, if in fact the volcano did erupt, the total effect could be world threatening. Some scientists consider that much of central North America could be covered by debris. They consider also that the rest of the world could suffer the darkness of a nuclear winter from the dust. The number of people surviving in the whole world may be only a few thousand.

The Yellowstone type of volcano is referred to as a “supervolcano”.

The recent showing of the programme was a ‘repeat’ – on April 1st! However, there seem no other suggestions of programme makers’ humour and the original showing was in February 2000.

One may conclude that we need a method for preventing the explosion of volcanoes and supervolcanoes.

Readers who would like to know more of the background of volcanoes would find the BBC book “Earth Story”, by Simon Lamb & David Sington, 1998, of great help. The Yellowstone feature happens to be included in the map of page 108. The BBC web site is also very helpful.

With the present proposals the writer suggests that it should be possible to prevent volcanoes and supervolcanoes from exploding.

The causes of volcanic explosions

At the centre of a volcano or supervolcano is magma - which is a hot liquid rock containing a large quantity of dissolved gas. It is the combination of hot liquid and a large amount of dissolved gas which presents the problem.

The magma at Yellowstone is in a huge reservoir, which reaches upwards, from great depths in the earth, to within about eight kilometres of the earth’s surface. The magma is formed at the bottom of the reservoir. The extremely high pressures and temperatures that exist at the bottom of the reservoir force gas to dissolve in the rock. It is therefore not surprising that, even now, when the upper part of the magma has moved much closer to the earth’s surface than the position at which it was formed, very great pressures can still be needed to keep the magma gas in solution. The pressures are provided by the weight of the overlying rock. Consequently, a very large weight of overlying rock can still be needed to prevent the magma gas from coming out of solution. Or, put another way, a very large weight of overlying rock can be needed to ‘bottle-up’ the magma gas. The explosive energy so stored can be extremely high.

When movements of the magma and overlying rock occur in a particular volcano or supervolcano such that in one area the weight of the overlying rock ceases to be sufficient to hold down the magma, the gas comes out of solution and lifts the overlying rock. The gas release then spreads through the magma so rapidly that it causes the explosion. The gas release and explosion continue until sufficient of the overlying and surrounding rock falls in and blankets the remaining magma.

One further aspect of the magma behaviour can be clarified by noting that the magma is impermeable, as explained to the writer in a very helful private discussion by Prof. Steve Sparks of Bristol University. As the magma is impermeable, the dissolved gas in any part of the magma can be removed only by the disintegration of that particular part of the magma. In contrast, one might expect that if gas were dissolved in a permeable material it could gradually become dissipated through perhaps porosity or faults in surrounding rock.

Preliminary discussion

Suppose that a tube of constant diameter, containing air, and with an open upper end open to the atmosphere and a closed lower end, were in some way inserted into the ground with the lower end reaching into the magma. With the specified set-up as described, suppose that the lower end closure were in some way suddenly removed. Then, momentarily, the condition achieved in the magma at its suddenly exposed surface at the lower end of the tube would be a condition which would simulate rather closely, but on a very small scale and in potentially a controlled manner, the conditions in the exploding magma of an exploding supervolcano. In particular, the magma material at the magma surface at the opening at the lower end would disintegrate and release its gas. Simultaneously, that gas with its suspension of atomised non-gas material would start to flow into the tube. The momentary rate of flow of the gas suspension starting to move into the tube, per unit of cross sectional area, would seem likely to be at about the same sort of flow rate as in the exploding supervolcano, as the local conditions seem likely to be very similar with the tube as in the explosion, other than for potentially the fully controlled nature of the tube experiment.

The above momentary situation could therefore be of great interest if it could be sustained continuously. For example, it might be used in a tube of modest diameter, with the tube reproduced in an array of such tubes, on a tube pitch of perhaps, say, 1 to 2 km for the Yellowstone magma depth below the surface of about 8km. Each tube would have a high magma evacuation velocity, and, in a sense, it would be automatic and continuous.

There seem to be various problems to converting a momentary evacuation to a continuous process.

(i) Some of the non-gaseous material, in the flow of the suspension which is produced by the disintegration of the magma, would become deposited on the internal wall of the tube by the action of the turbulence in the flow. (ii) Perhaps almost all the non-gaseous material would be so deposited, given the great length of the tube. (iii) Given, also, the very high proportional non-gas content and the very high total flow rate, the total rate of deposition would be extremely high. (iv) Moreover the material seems likely to be hot and sticky. Consequently,(v) it would seem that the tube would soon become blocked.

Discussion

The throat of the evacuation tube limits the flow of magma. Given, then, the sudden and large expansion in the passage area immediately after the throat, and provided the tube can be kept clear of blockage, the magma after the throat will be exposed to a static pressure that could be a lot closer to the atmospheric pressure at the surface than it is to the pressure before the throat. The static pressure of the magma after the throat could be only a small fraction of its pressure in the the ambient magma field from which it has been drawn. The magma would therefore be expected to disintegrate immediately after the throat, if it has not already done so during its flow up to that position.

Sufficient of the coolant would then be mixed into the magma to convert the non-gaseous part of the magma into small particles of non-sticky solid. These would be carried to the surface and to a treatment plant by the combined flow of magma gas and coolant.

An apparently better system than shown above uses heating to prevent excessive stickyness in the evacuation tube.  Further details for both systems are discussed at length in recent patents by the present author and are available from the UK and US Patent Offices.

The writer would be very pleased to receive your comments.



Written and published by
Brian Stratford,
brian@brianstratford.com
5th September, 2001 and later.

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