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Re: [Phys-L] Phys-l Talking to a geophysics socrates.

Thank you for your reply.

You wrote:
But the author does have the "gist" of the mechanism.
The newly created mid-ocean magmatic crust indeed
cools as it travels away from the midocean ridge. In
cooling the density does increase,

Hard to agree that "newly created mid-ocean magmatic crust indeed cools as
it travels away from the midocean ridge". On its birth it gets the oceanic
water temperature on its top; and the temperature of ductile material on
its bottom. On travelling away thew border conditions does not change.
Hence the plate does not cool, a socrates would say.

Anyway, he would add, only approx 5 km of crust is getting created exactly
within the divergent ridge. The rest, up to 90% of the plate thickness adds
later under the constant border conditions (oceanic water temperature on
the top and ductile temperature on the bottom). So, he would say, there is
no much need to mention the midocean ridge at all.

Indeed, he would agree, a plate may contain layers that are denser than the
layers under the plate. But, he would ask, what is the reason to select
temperature related things to explain the fact?

Sergey Sukhotinsky.


On Tue, Nov 26, 2013 at 8:48 PM, Dan Beeker <debeeker@comcast.net> wrote:

Yes, we all know it is common for science writers to convolute seemingly
simple physics. But the author does have the "gist" of the mechanism. The
newly created mid-ocean magmatic crust indeed cools as it travels away from
the midocean ridge. In cooling the density does increase, the slab gets
'heavier' per unit volume, (think isostacy as well as getting thinner).
causing the slab to "sink" and iirc thin slightly more as it moves away
from the hot ridge. Using magnetics one can accurately enough measure plate
motion over long periods of time. Elevation/ocean floor profile is easy to
measure.The elevation profile of the ocean floor corresponds quite nicely
to the mathematical model of a simple cooling plate. The explanation gets
more complicated as one looks at crustal structure far away from ridges and
is not always quite as straight forward. Hot spots, which appears to be the
subject of this article, are a topic of considerable controversy. There is
lots of data - primarily seismi
c, but the interpretation of that data is often speculative. The models
we read about in the popular press and in "news releases" from research
groups are often highly simplified and are written to promote their point
of view which may or may not stand the test of time. That means earth
science isn't just cold hard facts and the attraction to the field is we
don't yet really undersatand all the mechanisms involved in plate
techtonics. For a truly inquisitive mind of any age I would prepare them
with a basic physics, chemistry and geology courses so they could
understand the material in the following books:

Kearey, Philip, Keith A. Klepeis, and Frederick J. Vine
Global Tectonics, 3rd ed.
Hoboken, Wiley-Blackwell, 2009

Cox, Allan, and Hart, Robert B.
Plate Tectonics: How it Works
Palo Alto, Blackwell Scientific, 1986

I would also direct such people to the local geology/geophysics
department. Although many of us have the background to understand the
theories and mechanisms of earth physics it does take time and effort to be
more than superficially knowledgeable. If the person is genuinely
interested in he topic I would say - "Here's the explanation as best I can
give you. As you study the natural sciences, you will understand better."
I would then point them to the geoscientists for more information.

Keep your explanations simple and don't daze and amaze the beginning
student with things that are currently beyond their grasp. Stick to basic
physics and don't hesitate to say the story is only partly correct. It's
kind of like teaching magnetism to a grade schooler using quantum
mechanics. Perhaps you can't give a totally satiisfying explanation but
hopefully will be good enough to inspire the person to dig deeper and learn
more.

Dan Beeker
Lab Coordinator
Physics Department, Indiana University
(doodlebugger in a previous life)


On Nov 26, 2013, at 12:00 PM, phys-l-request@phys-l.org wrote:

Message: 1
Date: Mon, 25 Nov 2013 21:17:25 +0200
From: Sergey Sukhotinsky <s.sukhotinsky@gmail.com>
To: Phys-L@phys-l.org
Subject: [Phys-L] Talking to a geophysics socrates.
Message-ID:
<CAJkU4ozVxpukXMV=ai5NkL2+cQ=dT3rB5_V4bf5g_4=mQhn=
uQ@mail.gmail.com>
Content-Type: text/plain; charset=windows-1252

I just read "New model of Earth?s interior reveals clues to hotspot
volcanoes"
< http://newscenter.berkeley.edu/2013/09/05/hotspot-volcanoes/ >

The quotation from the article:
"The formation of volcanoes at the edges of plates is closely tied to the
movement of tectonic plates, which are created as hot magma pushes up
through fissures in mid-ocean ridges and solidifies. As the plates move
away from the ridges, they cool, harden and get heavier, eventually
sinking
back down into the mantle at subduction zones."

What if a young curious mind would read the text and ask the next
questions:

***
1. How a plate getting heavier makes it to sink down. The thing should
develop its density to sink down.

2. Is a plate really cooling? The top and bottom border conditions for
temperature remain the same. The plate develops its thickness, Its
"average" temperature remains aprox the same.

3. The coefficient of thermal expansion of the material a plate is built
of
is the smallest. Combined with the fact that the plate does not cool make
the plate density to be stable. Why is the plate eventually sinking back
down into the mantle when the plate's density is stable?

4. If an elementary volume of mantle develops its density on cooling and
changing its state to solid, - why does it glue to the ceiling (a plate's
bottom), why not it to drop down just on the cooling.
***

How to deal with the questions?

Thank you.

Sergey Sukhotinsky
sukhotinsky@phystech.edu
twitter.com/Sukhotinsky
sukhotinsky.blogspot.com


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