Geological Structures – A Practical Introduction (Waldron and Snyder)

Structural geology

What are geological structures?

If the Earth’s crust were completely uniform and homogeneous (the same everywhere), we would have great difficulty figuring out anything about its history. Fortunately, the Earth’s crust contains structures of many kinds. Structures are variations in the properties of the Earth’s crust. Those variations may be:

• Spatial variations: the rocks of the Earth’s crust vary from place to place, either on the surface or below; or
• Directional variations: rocks look different when viewed from different directions.

For example, where one type of rock contacts another, there is a geological boundary, a type of structure. Geological boundaries include:

• faults
• bedding planes
• the edges of igneous intrusions (intrusive contacts)
• ancient erosion surfaces (unconformities)

You should have heard about all these types of boundary in your introductory courses. All these boundaries tell you something about the geological history of the area where they are found.

Even without looking at boundaries, you may be able to see structure in a rock unit: the properties of many rocks vary with direction because the mineral grains are aligned with one another: we say the rock has fabric, another type of structure.

Such structures can tell us a great deal about the history of the Earth, and are critical for those seeking resources such as water, petroleum, and minerals. Some geological structures formed at the same time as the rocks in which they are found. These are primary structures. Examples of primary structures include beds and laminae in sedimentary rocks like sandstone, or shale, and lava pillows in extrusive igneous rocks like basalt. In general, you will learn most about primary structures in courses that deal with the formation of various rocktypes, but this introduction will cover some of the more important types of primary structure, especially those that are important in figuring out Earth history.