Re: Volcanic cooling - Physics anyone?

David J. Tyler (
Thu, 6 Mar 1997 13:24:57 GMT

Continuing the exchange with Glenn Morton (Wed, 05 Mar 1997) on
the cooling of batholiths and time implications.

Re: tectonic disturbance reopening channels...

GM: "I guess I don't fully understand your neocatastrophist
views. The continuing tectonic activity may or may not
continue..... But eventually, the magma runs out of its punch
and the tectonic activity ceases. At this time, the mechanisms
we have discussed, like the scaling problem and the solidified
rind around the magma chamber come into effect. What continues
the tectonic push?"

I think you are linking tectonic activity too closely to the
intrusion of magmas. Undoubtedly there is a connection, as areas
which are magmatically active are also seismically active.
However, the continental blocks bear the marks of tectonic
activity which is not geographically close to magma intrusions.
So, for example, England was affected by the Alpine orogeny:
major faults and folds affecting the southern parts which are
thought to be caused by compressive forces much further south in
Europe. I think your argument is viewing the intrusion of a
magma as a closed system - whereas I would want to explore the
thought that the system is open and there are many inter-related
and interdependent disturbances.

Re: my use of Shen and Keppler (1997): "Direct observation of
complete miscibility in the albite-H2O system", Nature, 385(20
February), 710-712....

GM: "But I am not sure that this increase in solubility of silica
with high pressure water is anything new.... My point is that
silica dissolution may work at depth, but once again as the
silica rich waters are carried to the surface, the silica
scaling should occur at a shallower level sealing the deeper part

It is true that the "increase in solubility of silica with high
pressure water" is not new. What is new is that the albite-water
system is completely miscible at higher temperatures and
pressures, so it becomes inappropriate to talk about the melting
point of the magma. There will be a continuum between hydrous
fluids and silicate melts. This seemed to me to be relevant to
the model you were advocating of a magma surrounded by a solid
margin, with conductive heat flow to a convective system. My
point was that this model applies at lower pressures and
temperatures but becomes inappropriate at higher temperatures and

Regarding models of batholith formation

GM: "My points are not undermined at the point that the batholith
quits moving and begins cooling. At that point the tectonic
motion ceases. If the water was contacting the magma, the same
thing that happens to lava when it contacts the sea should
happen. Immediately, the outer portion cools creating a rind
around it. This can be seen in pillow lavas."

Again, you appear to regard the system as closed: an assumption
I would not wish to defend. The analogy of pillow lavas is a
good one as far as the chilling of the magma is concerned, but
pillow lavas also illustrate aspects of open systems: as the
magma moves inside the pillows, the "rind" may swell and
fracture, or may collapse and implode.

GM: "Tapping the HEAT of a magma chamber is quite different from
tapping the MAGMA CHAMBER itself. I would fully agree that the
hydrothermal waters are tapping into the heat of the magma
chamber. The low temperatures of the hydrothermal waters argue
against the tapping into the magma. Apparently most of the vents
are between 200 and 500 deg C (and I am being generous on the
upper end. (see Heinrich Holland, The Chemistry of Atmospheres
and Oceans, p. 196). since this is far below the temperature of
the magma, it would seem to argue no contact. the May 1981
Scientific American article you cite, gives a temp of 350 C for
the east pacific rise smokers. (p. 101) If this were actually
in contact with molten rock, 900+ C, I would think the
temperature would be higher."

On this point, I think we are in basic agreement. I would not
use the 350 deg C temperature in the way that you have, because
a distance of 2-3 km of rock is between the black smoker and the
magma chamber. I accept that with black smokers, contact between
water and magma is momentary - the bulk of the heat will pass by
conduction through to the convective system. However, the rates
of cooling are so large that the effective surface area of the
magma chamber greatly exceeds that of a smooth dome.

This exchange has, I hope, reinforced the thought I expressed
earlier - "calculated" cooling rates are model dependent. I am
not saying these magma bodies can cool within one year, but I am
suggesting that assertions of long cooling times based on
conduction represent the extreme upper-limits of possible values
and, because convective cooling is so much more efficient than
conduction, the actual times of cooling are orders of magnitude

Best wishes,
David Tyler.