Guided Discovery and Scoring Rubric for Petrographic Analysis of a Thin Section

Dave Mogk, Montana State University.

Short description of the activity:

A guided discovery approach is used to "unpack" the methods and observations used by "master" petrographers in the petrographic analysis of a thin section. A series of spread sheets are used to direct students to make appropriate observations to systematically

a) identify minerals in thin section,
b) describe rock textures to interpret petrogenetic processes and geologic history, and
c) apply this information to address questions of geologic significance.

How does this activity lend itself to teaching the methods of geoscience?

Higher order thinking skills goals for this activity
The continuum of Bloom's taxonomy of cognitive skills can be developed in this set of exercises ranging from:

a)knowledge of petrologic phenomena,
b)ability to make independent observations and interpretations in context,
c)apply petrologic relations to address specific questions of geologic interest,
d) analyze data and synthesize the evidence in multiple working hypotheses towards a single, coherent and internally consistent explanation, and
e) evaluate these observations and interpretations in light of the larger body of geologic understanding.

Other skills goals for this activity
As students follow the spreadsheets as guides to petrographic analysis of a thin section, a learning strategy is provided that demonstrates how to systematically observe and interpret a thin section. Students should begin to ask appropriate questions of the thin sections, and begin to seek the answers using:

a) all the techniques available via the petrographic microscope,
b) making focused and purposeful observations, and
c) making interpretations that are contextual, constrained (by known geologic relations), relevant and meaningful.

The scientific method is used to: formulate hypotheses about what minerals might be expected, and the tests are provided to confirm or reject the hypotheses; taking a Popperian approach, interpretations can be continually tested against new observations to check for validity; Predictions can be made based on geologic insights and reasoning.

Sketching and photomicrographs: by sketching images of the thin sections, students must make critical observations, decide what is important and what to emphasize, and disregard "extraneous" information in the slide. Many students will have access to digital cameras; photomicrographs are also useful, but it's best to annotate the key features .

Petrographic study should be purposeful: you should have a clear idea of what questions you are trying to answer and why:

a) What is the stable mineral assemblage?
b) What alteration minerals are present?
c) What accessory minerals are present? How can these be used to interpret e.g. provenance, or to be used for further analysis e.g. geochronology? d) What is the history of the rock based on overprints and cross-cutting relations preserved in the rock fabric?

Internal inconsistencies can be revealed as new evidence is brought to bear: Checks on observations and interpretations can be made by considering if the results are:

a) possible in Nature, and
b) internally consistent with known relationships, etc.

Specific Adaptations: How do these help the activity address the methods of geoscience?

The spreadsheets provided are designed for universal application to help students do either mineral identification or petrographic analysis of thin sections. The Excel sheets can be readily modified by other instructors to include other more detailed topics if desired. This step-by-step guide leads students through the essential observations that should be made in mineral optics or petrographic analysis so that they are reminded of the type of evidence that is required to make a determination. With enough practice, students will internalize these steps and the guide will no longer be needed (i.e. a scaffolded approach to instruction).

Assessment: How are the methods of geoscience assessed?

The spreadsheets are an embedded assessment because each mineral in a thin section will have a very clear list of properties that can be readily checked. Students may miss some essential points, or they may be making inaccurate or erroneous observations--this becomes readily evident as their observations may not be internally consistent.