Geoscience Reference
In-Depth Information
standard model is more successful than its oppo-
site, and whether paradoxes are the results of
assumptions.
We have a sample that is not MORB. It therefore
cannot come from the upper mantle.
These
are
all
variants
of
the
modus
moron
fallacy
.
Language: a guide to the literature
Hypothesis testing
We think only through the medium of words -- The
art of reasoning is nothing more than a language
well arranged. Thus, while I thought myself employed
only in forming a Nomenclature, and while I
proposed to myself nothing more than to improve the
chemical language, my work transformed itself by
degrees, without my being able to prevent it, into a
treatise upon the Elements of Chemistry.
. . . we cannot improve the language of any science
without at the same time improving the science itself;
neither can we, on the other hand, improve a science,
without improving the language or nomenclature
which belongs to it. However certain the facts of any
science may be, and, however just the ideas we may
have formed of these facts, we can only communicate
false impressions to others, while we want words by
which these may be properly expressed.
Antoine Lavoisier
In discussing the early history of quantum
mechanics, Werner Heisenberg noted how diffi-
cult it is when we try to push new ideas into
an old system of concepts belonging to an ear-
lier philosophy and attempt to put new wine
into old bottles.
Paradigms
differ from
theories
and
hypotheses
in that they represent a culture,
a way of viewing the world. They come with
sets of assumptions and dogmas that are shared
by the community, but which are generally
not stated. The
standard models of geody-
namics and mantle geochemistry
combine
the independent paradigms of
rigid plate tecton-
ics
,
the convective mantle
and accessible but iso-
lated
reservoirs
. These, in turn, spawned the
plume
paradigm
. The assumptions hidden in '
the convec-
tive mantle'
are that the upper mantle is homoge-
nous, it is vigorously stirred by convection,
and that MORB is homogenous and requires a
homogenous source.
Seismic tomography uses inverse methods to
convert seismic data to models of the interior.
Because the sampling is so sparse and the data
represent large-scale averages, and because the
local properties of the interior are unknown,
tomography is best used as a
hypothesis tester
rather
than a method for determining definitive and
unique structures.
Probabilistic tomog-
raphy maps
areonewaytodisplaytheresults.
The parameters are all given
prior probabilities
,as
in bayesian statistics. Tomography is a form of
large-scale averaging. Magmatism is another.
The technique of testing a hypothesis by
assuming the opposite, is called
reductio ad absur-
dum
. A simpler more powerful theory often emer-
ges when we drop the adjectives -- such as
rigid,
accessible, homogenous
-- and reverse the assump-
tions. This technique is useful if an alterna-
tive to a popular reigning paradigm is not yet
fully developed. One can at least test whether a
The study of the Earth's interior involves a large
number of parameters, some much more impor-
tant than others, and a specialized language.
Once we leave the familiar parameter range of
the surface of the Earth and of the laboratory,
our intuition often fails. Temperature is one of
the key parameters of the Earth's interior but it
is often over-used and abused; it is not an expla-
nation of everything. But what could be more
natural than to assume that volcanic islands are
the result of hotter than average mantle? Or to
assume that low seismic velocities in the man-
tle are caused by high mantle temperatures? The
words
hotspots
and
upwelling thermal plumes
have
been applied to these features. But the mantle is
heterogenous; it is variable in composition and
melting point and is not simply a homogenous
ideal fluid with variable temperature. Pressure
increases with depth and this changes the role
and importance of temperature.
The study of the Earth's interior also involves
a number of assumptions, often unstated. The
unwary reader can be misled by semantics and
assumptions and the following is a guide to both
