Most rocks are not uniform throughout. On a scale usually best measured in millimeters or centimeters, they are composed of individual mineral grains that vary in size, shape and composition. The geometric characteristics of and relationships between these small-scale rock features constitute rock texture. Rock commonly also vary on larger scales, best measured in centimeters to meters to kilometers. The individual, contrasted, larger-scale features of rocks are called 'structures'. Our task will be to see if there are rock structures that can provide clues to a rock's formational environment: whether it's igneous, sedimentary or metamorphic.

There are hundreds of distinct rock structures. Geologists find it convenient to divide them into 'primary' and 'secondary' structures.

  • Primary Structures: structures formed before or at the same as material is in the process of becoming rock. For example, formed as magma crystallizes or as sediment accumulates.

  • Secondary Structures: structures imposed on rock after it has already formed. For example, formed as a result of compression of existing rock.

We'll examine a few of each to see what they tell us.

© 2001, David J. Leveson