On Fri 28 February, Glenn Morton wrote:
DT:
>The neocatastrophist scenario opens many avenues for tectonic
>disturbance of rocks, creating channels where none existed
>before. Even without catastrophism, I do not consider that a
>batholith can be emplaced without considerable disruption to the
>country rocks.
GM: "While this is true, the high temperatures of the water allow
them to carry lots of minerals in solution. As these hot waters
rise, they deposit some of these minerals along the pathways,
effectively sealing these passages off by means of
mineralization."
True. There will be a zonation in the mineralisation, as
minerals come out of solution differently according to the
pressure/temperature conditions. Sealing may occur, but in the
neo-catastrophist scenario, continuing tectonic disturbance will
occur to reopen channels or to create new pathways.
DT: > The shapes of the metamorphic aureoles were inconsistent
> with purely convective heat loss, and yet could be explained
> by invoking convective activity.
GM: "I think you mean "inconsistent with conductive heat loss".
But once again as the batholith center begins to cool it is not
in convective communication with the surface and is only in
conductive communication with a convective system."
Yes, I do mean "inconsistent with conductive heat loss". My
apologies for the typo. I accept that in the upper lithosphere,
there will be a solid boundary to the magma chamber - although
fracturing may be repeatedly occurring. However, at higher
temperatures and pressures, systems may not behave in ways that
are "obvious" to us on the surface. Research in this area does
not have a catastrophist emphasis, so those of us who are
sympathetic with this approach have to wait for occasional papers
to appear. An example is Shen and Keppler (1997): "Direct
observation of complete miscibility in the albite-H2O system",
Nature, 385(20 February), 710-712. They note: "With increasing
pressure, the solubility of water in silicate melts and the
solubility of silicate materials in hydrous fluids increase....
The existence of complete miscibility between fluids and silicate
melts has profound consequences for the phase relations in the
mantle. At conditions below the critical point, a hydrous fluid
phase can coexist with solid mantle minerals, until the water-
saturated solidus is reached. At this temperature, a hydrous
silicate melt forms which coexists with the hydrous fluid and
mantle minerals. Beyond the critical point, however, fluid and
melt can no longer coexist as two separate phases." This
implications of this for the topic we are discussing is
uncertain, but I cite Shen and Keppler to demonstrate that
alternative perspectives on problems exist and ought to be
explored.
DT: "In addition, I am sympathetic with the expanding earth
concept - which has thermodynamic implications which favour rapid
cooling."
GM: "I know I used to advocate that view but measurements by
large base-line radiointerferometry have destroyed the concept
of earth expansion. ...
I'm not ready to give up on this approach yet - but I hope you
will not mind if I don't pursue it here. I introduced it only
to point out a variety of alternative perspectives on the topic.
On the subject of rates of crystal formation, I quoted:
> "It is frequently assumed that the presence of large
>crystals in these phases implies slow growth over long periods
>of time. Although this may be the case, the intent here is to
>demonstrate that it does not necessarily hold" (p.405).
GM: "If I can remember, let me talk to a couple if igneous
experts at work and see what they say about this. I hate
mineralogy and probably can't go much further on crystal size
on my limited mineralogical knowledge."
At this point, perhaps I should breathe a sigh of relief: I'm not
under pressure to defend my views! As a geophysicist, Glenn
should be excused for his dislike of mineralogy!
Regarding the Oceanic Ridge systems:
GM: "Do you know how much sodium is removed by these
circulations? I know that lots of the elements deposited in the
oceans by rivers are removed by chemical reactions in the MORBs
but I can't find a good number on sodium."
Sorry I can't help on this one.
GM: "An extraneous question. Do you know how many kilometers are
in the trench system? I have been looking for this value for
about 6 months and can't find it. Even the USGS didn't know,
which amazed me. I need this value."
I've seen the figure cited - but can't remember where! If I can
find it, I'll pass it on.
Regarding models of batholith formation
DT:
>> ... Large magma
>>bodies moving upwards through the crust of the earth have the
>>problem of "What creates the space into which they move?"
GM: "melting and incorporation of the rock into the magma. as
>well as uplift. Mt. St. Helen underwent an uplift and even an
>expansion of the mountain prior to its eruption."
DT:
>Are you serious?
GM: "Yes I am serious, .... [stuff re Mt St Helens deleted]
... Like a balloon, the mountain had expanded several meters
horizontally. So yes I am serious."
I think we have a misunderstanding here. Yes, Mt St Helens did
expand by several metres before it went off. Yes, many volcanoes
around the world are fitted with arrays of markers so that
changes in shape can be monitored. The point I was making is
that movements of a few metres are totally inadequate to solve
the space problem when magmas are intruded. My question should
have read: "Are you serious in asserting that such tiny movements
can solve the batholith emplacement space problem?". I also
pointed out that if you extend the argument to allow for large
displacements, then you are moving into tectonic adjustment and
catastrophism, both of which undermine points you had been making
earlier in your post.
GM: "I have no doubt that water could flow through the cracks
that must have existed in the rocks of St. Helens. But you have
still not dealt with the conductive zone which grows as the
magma chamber cools. The solid lava rind will not have cracks
through which water can flow. On a mile wide batholith, as it
cools, the water will not be able to flow to the actual lava
chamber."
Some of my earlier comments did respond to this. In addition,
I would refer again to Cann and Stiens (1982). They argue that
black smokers must be tapping the heat of magma chambers. We may
not understand the physics of the heat flows - but to insist that
the magma chamber is rapidly insulated from further heat loss
does not accord with observations today regarding hot oceanic
ridge systems.
GM: "It has been a while for me also since I have delved into
this issue. thanks for the very interesting discussion."
I am sure there are many geological issues we could profitably
discuss. The problem I find is that the positions tend to be so
polarised - and since I can't defend either of the alternatives
on offer, I keep quiet. I hope this post meets with interest.
I too am grateful to Glenn (with comments from Art) for the
exchange (and to Joel for initiating it).
References for previous post:
Anderson, R.N., Hobart, M.A. and Langseth, M.G. 1979.
Geothermal convection through oceanic crust and sediments in the
Indian Ocean. Science, 204(25 May), 828-832.
Cann, J.R. and Stiens, M.R. 1982. Black smokers fuelled by
freezing magma. Nature, 298(8 July), 147-149.
Dowty, E. 1980. Crystal growth and nucleation theory and the
numerical simulation of igneous crystallisation. In: Hargraves,
R.B. (ed). Physics of magmatic processes. Princeton University
Press, New Jersey.
Edmond, J.M., Von Damm, K.L., McDuff, R.E. and Measures, C.I.
1982. Chemistry of hot springs on the East Pacific Rise and
their effluent dispersal. Nature, 297(20 May), 187-.
Luth, W.C. 1976. Granitic rocks. In: Bailey, D.K. and
MacDonald, R. (eds). The evolution of the crystalline rocks.
Academic Press, London.
Macdonald, K.C., Becker, K., Spiess, F.N. and Ballard, R.D.
1980. Hydrothermal heat flux of the "Black Smoker" vents on the
East Pacific Rise. Earth and Planetary Science Letters, 48, 1-7.
Macdonald, K.C. and Luyendyk, B.P. 1981. The crest of the East
Pacific Rise. Scientific American, 244(May), 86-99.
Parmentier, E.M. and Schedl, A. 1981. Thermal aureoles of
igneous intrusions: some possible indications of hydrothermal
convective cooling. Journal of Geology, 89, 1-22.
Best wishes,