teaching - Earth Science Teachers' Association

TEACHING EARTH SCIENCES ● Volume 30 ● Number 1, 2005
Bowen’s Reaction Series should not be
Taught to Introductory Geology Students
COLIN H DONALDSON
It is over 80 years since Bowen introduced the reaction principle to igneous petrology, emphasising
the role of various concurrently operating crystal-liquid reaction series during magma
crystallization. From the principle he argued that fractional crystallization caused by the prevention
of reactions, e.g. by settling of crystals to isolate them from melt, is the main reason why suites of
volcanic rocks commonly show a variety of related compositions, i.e. are differentiated.
Colin H Donaldson
These are key ideas in igneous petrology. Generations
of students have usually been taught them
with reference to the reaction series diagram that
Bowen published in 1922 and which is re-examined in
his 1928 book, The Evolution of the Igneous Rocks.
For a variety of reasons it will be argued that beginning
students of geology should not be introduced to
Bowen’s reaction series diagram. These include:
● Weakness in or fear of chemistry means that many
students do not understand the concept of reaction;
these students cheerfully accept that olivine changes
to pyroxene but cannot grasp why this happens, and
they are puzzled as to why quartz does not react.
● Magma is not a familiar substance and hence its
chemical behaviour on cooling is not intuitive –
that means students readily erect false impressions/understanding
of how crystallization influences
magma composition.
● Language problems complicate understanding (such
as the confusion wrought by the terms continuous
reaction and continuous zoning).
● While some can grasp the meaning of a continuous
reaction, most students are especially puzzled by
‘discontinuous reaction’.
● The Bowen diagram is a simple representation of
what happens in nature and readily leads to misunderstanding,
such as the common student suggestion
that “all magmas initially crystallize olivine”.
● These days just providing a survey of geology leads
to a very full syllabus in introductory courses – the
time does not exist at this stage to develop descriptions
and explanations of crystal-liquid reaction and
of why the prevention of reaction by fractional crystallization
causes differentiation.
It is proposed that detailed explanation of how magma
composition evolves by fractional crystallization should
be delayed until second year of an undergraduate degree
when magma is a more familiar geological material and
chemical confidence has been developed.
On the other hand, new students should be introduced
to the concept that a suite of lavas does normally
vary in composition according to a pattern, and to the
idea that this is due to differentiation involving extraction
of crystals from a magma. They should be persuaded
that crystal extraction will do this. A means of
convincing a class is take an analogue which students
can relate to. A solution of common salt in water works
well as a ‘mind experiment’. Students can represent for
themselves on a scale from 100% NaCl to 100% H2O
how the composition of the remaining solution evolves
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