Geoscience Reference
In-Depth Information
picked from an ongoing debate about the
source of oceanic magmas and
large igneous
provinces
.
therefore an ancient undegassed reservoir, therefore
thedeepmantle.
Fallacy of irrelevant conclusion
Numerical simulations of plumes reproduce many of
the geophysical observations, such as the rate of
magma production and the topography and gravity
anomalies produced by plume material as it spreads
beneath the lithosphere. Therefore, . . .
Circulus in demonstrando and affirming
the consequent
Absolute plate motions agree well with the fixed
hotspot hypothesis;
therefore . . .
Argumentum ad populum
For many geoscientists, the mantle-plume model is as
well established as plate tectonics.
Fallacy of irrelevant conclusion, affirming
the consequent and permissivity
Theoretical and laboratory studies of fluids predict
that plumes should form in the deep Earth because
the core is much hotter than the mantle. Therefore
hotspots are caused by plumes from the core--mantle
boundary.
False dilemma and bifurcation
A limited number of options (usually two) is
given, while in reality there are more options (or
perhaps only one). A false dilemma is an illegit-
imate use of the 'or' operator. Putting issues or
opinions into 'black or white' terms is a common
instance of this fallacy.
Confusion of 'should' with 'do' or 'must'.
Ignoratio elenchi and circulus in
demonstrando
Continental flood basalts (CFB) erupt a million cubic
kilometers of basalt or more in 1 million years or less.
Therefore plumes erupt a million cubic kilometers of
basalt or more in 1 million years or less.
Melting anomalies (oceanic islands, hotspots) are due
to either high absolute temperatures or high degrees
of melting
Plumes either come from the core--mantle
boundary or the transition region.
This circular argument is often used as a basis
for supporting a plume model for CFB.
Some invalid deductive arguments are so
common that logicians have tabulated them, and
given some of them names; the following are just
afew:
Red herring and fallacy of irrelevant
conclusion
The upwelling mantle under Hawaii must also be 200
to 300 K hotter than the surrounding mantle to
achieve the required large melt fractions . . . such hot
rock must come from a thermal boundary layer . . .
the core--mantle boundary is the most likely source,
unless there is another interface within the mantle
between compositionally distinct layers.
If P then Q
Q
Therefore
,
P
These are requirements of the plume hypothesis,
not general requirements. Large melt volumes
have a variety of explanations and thermal
boundary layers are not essential for creating
upwellings.
If P then Q
Not-P
Therefore, Not-Q
If P then Q
Therefore, if Not-P then Not-Q
The ratio fallacy and the slippery
slope fallacy
The chemistry and isotopic composition of hotspot
lavas indicate that the hotspots sample a part of the
mantle distinct from that sampled by midocean-ridge
basalts. High ratios imply high
The following statements can all be written in
these forms:
We have a midocean-ridge basalt (MORB). It is from
the upper mantle. We have a sample from the upper
mantle. It is therefore MORB.
3
He contents,
