The outer shells of Earth - New Theory of the Earth - page 35

Chapter 4

The outer shells of Earth

When I use a word, it means just
what I choose it to mean -- neither
more nor less.

at suture zones (former plate boundaries), frac-
ture zones and subplate boundaries, often gener-
ating volcanic chains in the process. A plate can
be rigid in the sense that relative plate motions
can be described by rotations about Euler poles
on a sphere but can still have meter-wide cracks,
which is all that is needed to create volcanic
chains from the hot underlying mantle and its
low-viscosity magmas. What is meant by rigid-
ity is relative coherence in motions, not absolute
strength.
Because rocks are weak under tension, the
conditions for the existence of a plate proba-
bly involve the existence of lateral compressive
forces. Plates have been described as rigid but
this implies long-term and long-range strength.
They are better described as coherent entities
organized by stress fields and rheology. The corol-
lary is that volcanic chains and plate boundaries
are regions of extension. Plates possibly organize
themselves so as to minimize dissipation.
The term plate itself has no agreed-upon for-
mal definition. If plate is defined operationally as
that part of the outer shell that moves coherently
then several interpretations are possible.

The question is, whether you CAN
make words mean so many different
things.

The question is, which is to be
master -- that's all.

Humpty Dumpty and Alice

Plate tectonics involves the concepts of plates,
lithospheres, cratonic keels and thermal bound-
ary layers; these are not equivalent concepts.
Lithosphere means rocky shell or strong layer . It can
support significant geologic loads, such as moun-
tains, and can bend, as at trenches, without sig-
nificant time-dependent deformation; it behaves
elastically. Plates are not necessarily strong or
elastic. Thermal boundary layers conduct heat out
of the underlying regions; they are defined by a
particular thermal gradient.

Plates

(1) Plates are strong and rigid (the conventional
interpretation).
(2) Plates are those regions defined by lat-
eral compression since plate boundaries are
formed by lateral extension.
(3) Plates

A plate is a region of the Earth's surface that
translates coherently. The word plate as in plate
tectonics implies strength, brittleness and perma-
nence and often the adjectives rigid and elas-
tic are appended to it. But plates are collages,
held together by stress and adjacent portions
rather than by intrinsic strength. Plates break

move coherently because the parts

experience similar forces or constraints.

With the first definition, if there is to be spread-
ing and volcanism, local heating or stretching