Interpret Geologic Rock Diagrams

Geologists use rock diagrams to graphically express every aspect of geology and earth sciences.

Geologists, amateur rock hounds and Earth Sciences students all need to use and interpret a variety of geologic charts, graphs, maps and cross sectional diagrams. The making and interpretation of these assorted graphic tools are essential to geologic work in the field, in the laboratory, and reporting in the scientific literature. Oil and gas exploration and civil engineering are two disciplines which rely heavily upon interpreting geologic graphics and rock diagrams. You can learn to recognize, understand, and interpret a few of the basic and fundamental diagram types if you attack each graphic with a common logical approach.


1. Decide if you have seen this type of rock diagram before. Recall when and where you saw a diagram like this and try to categorize what type of diagram it is… map, cross section, block, fencepost, fault movement, quake epicenter or ternary diagram. Read a brief review of each of these main geologic rock diagram types in the “Tips” section below. Be familiar with their appearances and the parameters and concepts each one expresses.

2. Determine what is the main idea or purpose behind this graphic. Decide what the author is trying to tell you. Read and understand the title and any captions included with the geologic diagram. If the title says “cross section”, then you know that you are looking at a cut-away view of the Earth. If the title says “block diagram”, then you know you are looking at a three-dimensional slice of the Earth.

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3. Figure out what variables this diagram expresses or demonstrates. Ask yourself, “What are the things that are changing?” Each dimension on paper represents a dimension in the real world. You must decide what each dimension on the diagram represents in the real world. For example, maps represent distances and directions. Ternary diagrams represent mineral or sediment compositions.

4. Decide what units of measurement the diagram uses. If it is a map, are the units in feet, meters, kilometers or miles? If it is a block diagram or a fencepost diagram, what is the scale of the vertical dimension? In what units are the surface dimensions represented? Is there exaggeration of any of the dimensions? Geologists often exaggerate the vertical dimension scale on cross section diagrams to accentuate topography and to make sure that mountains appear as actual mountains not just as low hills.

5. Determine the range of values expressed by each variable in the rock diagram. Decide what the maximum and minimum values are for each measured parameter. Be sure to remember the units associated with the values given as explained in the previous step.

6. Ascertain the overall significance of the information presented on each diagram. As yourself, “What does this mean and why is it important?” The answer to this question is the key to correct interpretation of the geologic diagram. Understanding what information is presented is very important; but knowing what the information means is vital to grasping the concepts presented by the graphic.

7. Predict the usefulness of the information by thinking of ways that you or another geologist might be able to use the information in similar situations. Decide how you might use a rock diagram similar to this one in other applications. Try to think of ways that this information might be presented in other formats. As your analysis deepens, so does your comprehension and ability to interpret other similar geological graphics and the concepts they represent.

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