Basic Petrography

by

Dave Emley, Graham Lees and George Rowbotham, University of Keele

This interactive courseware module is concerned with the description and systematic classification of rocks and provides instruction in the scientific methods used to identify rocks. It combines text, diagrams, animations and user-interactions with over 440 images of rock outcrops, hand specimens and thin sections to provide a comprehensive educational package covering important aspects of Petrography.

The module comprises seven sections fronted by a main menu

1. Introduction

A definition of Petrography, an introduction to classification and an outline of the three main rock types.

2. Describing a Rock

This section guides the user through the sorts of questions that need to be asked when examining a rock systematically. They include determining such properties as grain size, texture and overall colour. Guidance is given on how to estimate mineral proportions and how to use the hand-lens and the petrological microscope. These areas are covered in sufficient detail to allow the user to assign a rock to one of the three major groups.

Estimating mineral proportions in the "Describing a Rock" section

3. Igneous, Sedimentary or Metamorphic?

The results of the preliminary investigation should enable the user to decide whether a rock is igneous, sedimentary or metamorphic. This short section concentrates on the specific features that are used to distinguish between these rock groups.

One of the eight terms explained under "Characterisation of Sedimentary Rocks"

4. Some Important Rock-forming Minerals

Before any rock can be classified further it is important that the major constituent minerals are identified. This section gives information on the identification of twenty important rock-forming minerals and six mineral groupings. The properties for each mineral and details of how to identify them in hand-specimen and in thin section are given.

5. Igneous Rocks

6. Sedimentary Rocks

7. Metamorphic Rocks

The module examines each of the three major rock groups in turn. For each group there is a section dealing with characteristics such as grain size, grain shape, texture etc., followed by a section that brings together all the observations required to enable the rock to be classified.

For igneous rocks there are separate sections on coarse, medium, fine-grained and ultramafic rocks. The general format for each group is shown opposite: a classification grid appears top right, a mineral content diagram top left, and pages of text and diagrams below.

The format used for Gabbro in the "Description of Igneous Rocks" section

There are sections covering the IUGC classification scheme for igneous rocks and the Folk classification scheme for sedimentary rocks.

For metamorphic rocks a moderate amount of general and background information is provided. An introductory section explains what metamorphic rocks are and also covers the importance of pressure, temperature, and stress/strain in their formation. There is an overview of different types of metamorphism and separate sections cover the rocks produced by dynamic, thermal/contact and regional metamorphism.

Metamorphic rock types are described in this part of the "Regional Metamorphism" section

Running the Software

Navigation around the module is by means of buttons along the base of the screen and a menu system. A glossary and an explanation of how the module works is accessed by clicking on a Help button at the top of the screen.

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