>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.
>
I am aware of this non-magmatic tectonism but that is irrelevant to the
problem of cooling a batholith because we find batholiths in tectonically
inactive areas like the East Coast of the United State. The Great Stone
Dome that I mentioned a while back as well as diabase sills found on Georges
Bank offshore Massachusetts, are in areas that did not suffer from much
regional tectonism. The entire problem with the oil potential of the east
coast was that there was too little faulting, which means too little
tectonism, yet there are still some batholiths.
>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
>off."
>
>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
>pressures.
>
>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.
But at the point that the magma chamber ceases motion, the rind can no
longer swell. If the magma chamber never went through this phase, there
would be no cool batholiths. At some point the rind must never again swell
and break. this is the point at which the long cooling time begins to happen.
>
>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.
Here is my reasoning. Since the solid rock above the magma chamber has a
low thermal conductivity, 1000 degree water should quickly heat the walls of
the conduit to near 1000 deg,so that there would then be little means for
the water to cool on the way up. The water travels so fast that it would
have little time to cool on its way up. It moves at 1-5 m/s (Mcdonald et
al, Earth and Planetary Science Letters 48:1-7 p. 2) At these speeds, the
water could come from a depth of 1 kilometer in 16 minutes.
>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
>lower.
I will agree that hydrothermal convection will cool an area more rapidly
than an area cools without it.The reason is that the water circulation
avoids part of the conduction pathway. But there still would be some of the
conduction pathway required, and that would make the cooling quite long.
Hydrothermal activity effectively acts to remove SOME of the overburden
through which conduction must take place otherwise.
glenn
Foundation, Fall and Flood
http://www.isource.net/~grmorton/dmd.htm