Geoscience Reference
In-Depth Information
Part II
Earth: the dynamic planet
This is the fourth time that I have
taken part in a public discussion of
this theory. In each previous one a
distinguished biologist or geologist has
presented the case for drift, and has
been followed by equally distinguished
ones who have pointed out facts that
it would render more difficult to
explain...Thepresent impasse
suggests that some important factor
has been overlooked.
phrases in the same typeface, is a Googlet; see
Preface or type it into a search engine]. In this
view the plates are driven by thermal and density
variations in the mantle. Cooling plates and sink-
ing slabs can also be regarded as driving them-
selves,
and
driving convection in the underlying
mantle; they create chemical, thermal and den-
sity anomalies in the mantle.
Plate tectonics qualifies as a branch of
complex-
ity theory
. Plate tectonics may be a
far-from-
equilibrium self-organized system
powered by heat and gravity from the mantle
and organized by dissipation in and between the
plates. Mantle convection, below the plates, may
not drive or organize the plates; it may be the
other way around. Plate buoyancy and dissipa-
tion control plate motions, stresses, and locations
of plate boundaries, intraplate extensional zones
and volcanic chains. The cold stiff outer shell of
Earth is the active element and the template; the
underlying
convective mantle
is passive.
The outer shell of the Earth is not just a ther-
mal boundary layer or a cold strong layer. It is, in
part, the accumulated buoyant residue of man-
tle differentiation including the on-going pro-
cess of seafloor spreading and building of island
arcs. It is composed of fertile melts, dikes, sills
and cumulates, and infertile refractory residues.
It is, in part, isolated from the low-viscosity
fertile interior. Earthquakes and volcanoes not
Sir Harold Jeffreys,1951
Overview
Plate tectonics on Earth, at present, consists of
about a dozen large semi-coherent entities --
called plates -- of irregular shape and size that
move over the surface, separated by boundaries
that meet at triple junctions. There are also many
broad zones of deformation.
Plate tectonics is often regarded as
sim-
ply the surface, or the most impor-
tant, manifestation of thermal con-
vection in the mantle
[this phrase, and
