Geoscience Reference
In-Depth Information
Chapter 7
Convection and complexity
. . . if your theory is found to be
against the second law of
thermodynamics, I can give you no
hope; there is nothing for it but to
collapse in deepest humiliation.
convection
has been taken as the classic exam-
ple of thermal convection, and the hexagonal
planform has been considered to be typical of
convective patterns at the onset of thermal con-
vection. Fifty years went by before it was real-
ized that Bénard's patterns were actually driven
from above, by surface tension, not from below by
an unstable thermal boundary layer. Experiments
showed the same style of convection when the
fluid was heated from above, cooled from below
or when performed in the absence of gravity. This
confirmed the top-down surface-driven nature of
the convection which is now called
Marangoni
or Bénard-Marangoni
convection.
Although it is not generally recognized as
such, mantle convection is a branch of the newly
renamed
science of complexity
. Plate tectonics may
be a
self-driven far-from-equilibrium system
that orga-
nizes itself by dissipation in and between the
plates, the mantle being a passive provider of
energy and material. Far-from-equilibrium sys-
tems, particularly those in a gravity field, can
locally evolve toward a high degree of order. Plate
tectonics was once regarded as passive motion of
plates on top of mantle convection cells but it
now appears that continents and plate tecton-
ics organize the flow in the mantle. But man-
tle convection and plate tectonics involve more
than geometry and space-filling considerations.
The mantle is a heat engine, controlled by the
laws of thermodynamics. One can go just so
far without physics. Conservation of mass and
energy are involved, as are balancing of forces.
Although the mantle behaves as a fluid, mineral
physics principles and classical solid-state physics
Eddington
Contrary to current textbooks ...the
observed world does not proceed
from lower to higher “degrees of
disorder”, since when all
gravitationally-induced phenomena
are taken into account the emerging
result indicates a net decrease in the
“degrees of disorder”, a greater
“degree of structuring”...classical
equilibrium thermodynamics ...has
to be completed by a theory of
'creation of gravitationally-induced
structures'...
Gal-or
Overview
In
1900 Henri Bénard
heated whale oil in a
pan and noted a system of hexagonal convec-
tion cells.
Lord Rayleigh in 1916
analyzed
thisintermsoftheinstabilityofafluidheated
from below. Since that time
Rayleigh-Bénard
