Image credit: Scott Brande
Identification and Classification
We will observe and evaluate unknown rock samples to identify them by name, according to a classification.
Identification and classification are not the same thing.
- classification - an arrangement of relationships among categories based upon similarities and differences, e.g., marble is a category of metamorphic rock defined by its composition - calcite (Ca CO3)
- identification - a process of assigning an object to a previously named group or category, e.g., "this particular rock sample is a marble because I tested the rock and it fizzes strongly under cold, dilute HCl, therefore, it must belong to the category marble
There are numerous classifications of metamorphic rock that serve different purposes (e.g., used by experts, industry, students). A simplified classification of metamorphic rock is included here for the purposes of
- learning how to observe features
- distinguishing some of the more common metamorphic rock
- assigning a name to an unknown sample.
Classification of Metamorphic Rock
What is a classification?
Nature on Earth has produced an amazing diversity of chemical compounds. Changing environments and the passage of time results in a wide variety of rock. The burial of rock and action of plate tectonics subjects rock to increased temperature and pressure which cause metamorphism.
Geologists accommodate the diversity of metamorphic rock origin, composition, texture and other features with a myriad of metamorphic rock names not typically relevant to learning objectives for introductory (non-science) students (e.g., pelite, granofels, eclogite, skarn, etc.). For the advanced student, textbooks and online resources provide additional detail.
Simplified Classification of Common Metamorphic Rock
In this introductory level treatment, we will omit the professional complexity of metamorphic rocks in favor of simplicity.
The simplified classification table presented below focuses on features that may be observable without additional equipment (e.g., no hand-lenses, thin-sections, polarizing microscopes) and without advanced technical knowledge.
The small number of metamorphic rocks included tend to exhibit easily recognizable features that reduce complexity and uncertainty in the identification process for the introductory student.
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