EarthStructure  An Introduction to Structural Geology and Tectonics

7. Joints and Veins

Visitors from around the world trek to Arches National Park in southeastern Utah (USA) to marvel at its graceful natural arches. These arches appear to have been carved through high, but relatively thin, free-standing sandstone walls. From the air, you can see that the park contains a multitude of such walls, making its landscape resemble a sliced-up loaf of bread. The surfaces of rock walls in Arches Park initiated as joints, which are natural fractures in rock across which there has been no shear displacement. Erosive processes through the ages have preferentially attacked the walls of the joints, so that today you can walk in the space between the walls. Though joints are not always as dramatic as those in Arches National Park, nearly all outcrops contain joints. At first glance, joints may seem to be simple and featureless geologic structures, but in fact they are well worth studying, not only because of their importance in controlling landscape morphology, but also because they profoundly affect rock strength and permeability, and because they can provide a detailed, though subtle history of stress and strain in a region. Although the basic definition of the term joint is non-genetic, most contemporary geologists who study joints believe that they form during Mode I loading (see Chapter 6), i.e., they are tensile fractures that form perpendicular to the sigma3 trajectory and parallel to the principal plane of stress that contains the sigma1 and sigma2 directions at the time they formed. Not all geologists share this viewpoint, and some researchers use the term 'joint' when referring to shear fractures as well. This second usage is ambiguous, because structures that are technically faults might also be referred to as joints and, therefore, we do not use the term 'joint' in reference to a shear fracture. In this chapter, we begin by describing the morphology of individual joints and the geometric characteristics of groups of joints. Then, we discuss how to study joints in the field, and how to interpret them. We conclude by describing veins, which are fractures filled with minerals that precipitated from a fluid. But before we begin, we offer a note of caution. The interpretation of joints and veins remains quite controversial, and it is common for field trips that focus on these structures to end in heated debate. As you read this chapter, you'll discover why.