Date: October 17, 2013
To: Professor Thacker
From: Keith Piersol
Subject: Definition and Description Memo
Purpose. The purpose of this memo is to describe the audience, need, placement, and
rhetorical moves of the attached definition and description of Bowen's Reaction Series.
Audience. The audience of the attached definition and description are beginning students in
the field of Geology. Presumably these new students are currently taking their introductory 101
(rocks for jocks) or 111 course.
Need. New students in the field of geology are required to understand basic chemical
characteristics of the formation of minerals from melts. An understanding of Bowen's Reaction
series goes a long way towards building a foundation for other classes such as IgMet (Igneous
and Metamorphic Petrology) to build upon. Though basic, the concept is applied throughout
the rest of further classes dealing with igneous petrology.
Placement. The definition attached is to be found in a glossary of a textbook or at the beginning
of a chapter. It is meant to be simple enough for it to be quickly absorbed and understood
without getting into too much detail until other relevant information is presented.
The description is found either on an online websource or later in the chapter that the
definition is given in. It is a detailed explanation of the information provided by the reaction
series and what can be done with that information.
Rhetorical Moves. The definition is short and to the point. It explains where Bowen's Reaction
Series came from and the basic information that it provides. It is not meant to give a thorough
understanding of the concept since it is either in a glossary or chapter preface and is kept short
so that it is easy for someone to glean information from it.
I structured the description like a large sentence with the aid of the diagram itself. Context and
information was given in the beginning of the description and the emphasis was placed on what
can be done with that information. I held the uses of this information until last because I
wanted the reader to already grasp the basic information within the diagram. Understanding
the way minerals form from a melt and the way those basic concepts can be applied is key to
understanding further classes in the field of geology.
Bowen's Reaction Series
Bowen's Reaction Series is a diagram developed by Normal L Bowen from experimental results.
It describes the order in which minerals crystallize out of molten material. It demonstrates the
two ways minerals can crystallize out of a melt as it cools. It also shows that different types of
minerals tend to crystallize exclusively from others. Since some minerals cannot form at the
same time as others, one can use this diagram to determine the conditions under which a
mineral formed.
Understanding Bowen's Reaction Series
Bowen's Reaction Series describes the order and conditions under which minerals crystallize
out of a melt. By matching the minerals found within a sample against the diagram, it is
possible to determine the composition and temperate/pressure conditions under which the
sample formed.
On the left side of this graph is the discontinuous series of minerals, with the continuous series
being on the right. The discontinuous series of minerals are minerals that react discontinuously
with the changes in temperature and pressure as the melt cools or rises in the mantle and
crust. For example, as a melt that had crystallized olivine moved to a lower temperature the
olivine would disassociate and
form pyroxene. The right side
is continuous and instead has
the minerals formed in the
melt constantly shifting in
chemistry as the temperature
decreases and calcium is taken
out of the melt by newly
forming
discontinuous
minerals.
As the melt moves from mafic
(iron
rich)
through
intermediate and into felsic
(silica rich) the two branches merge. At this point as intermediate and mafic minerals have
differentiated themselves from the melt, felsic rocks such as granites begin to form. There will
still be some biotites or other less felsic minerals within the granites but it will be a small
fraction of the total composition. Light colored silica rich minerals such as Quartz and
Muscovite will form at this point in the cooling of the melt.
This diagram explains why minerals such as olivine (Mg/Fe2Si04) and quartz (Si04) are rarely
found in the same place. The chart shows the relative stability each mineral has and what
conditions they are most stable at. Minerals tend to be most stable under the conditions they
were formed in, which makes quartz most stable on or near the earth's surface, and olivine
least stable. Using this information one can understand why there are no olivine sand deserts
even though quartz sand deserts are plentiful despite their similar composition.
Works Cited
Strickler, Mike. " Bowen's Reaction Series." GeoMania. N.p., n.d. Web. 18 Oct. 2013.
<http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry32.html>.