Joints (earth science)

Joints form when rocks undergo brittle failure, usually during expansion. Their orientations, physical features, and patterns of occurrence can be used to help infer the physical conditions present at the time of failure. Because joints are important conduits for fluids, particularly oil and water, that move beneath the surface, understanding their formation and occurrence has economic benefits.

Characteristics of Joints

Joints are the ubiquitous cracks found in nearly every outcrop. They are unquestionably the most common structure at the Earth's surface. They vary in size from microscopic fractures visible only within an individual grain in a rock to fractures kilometers in length, some of which are responsible for the magnificent scenery of Arches National Park in Utah. If one smashes a rock with a hammer, one produces joints. Joints also form as molten rocks solidify and cool, as weathering alters the volume of the outer layers of a rock, and even as erosion removes overlying layers, reducing the weight on the layers below and permitting them to expand and crack.

There is no appreciable slip across a joint. (Failure surfaces accommodating large amounts of slip are faults.) Often, the same mineral grain can be observed on both sides of a joint, neatly cut in two but otherwise not disturbed. This indicates failure in extension; such joints are sometimes called extension joints. The sides of the fracture moved away from each other but did not slide past each other at all. The forces producing such joints were literally pulling the rock apart. An engineer might call these tensile forces. Geologists work with rocks that are nearly always in a state of compression, however, and true tension is uncommon. Therefore, geologists usually call such forces the forces of least compression. The direction of least compression is the direction in which extension occurs. That is, as the crack opens and the sides move away from each other, they will move in the direction of the least compressive force. Consequently, the plane of an extension joint will be perpendicular to the direction of the least compressive force.

Formation of Joints

Such extension may be produced in many ways. Most obvious are mechanisms involving large-scale deformation of the rock. The folding of a unit of rock causes extension on the outside of the fold. Often, extension joints are found fanning around the “nose” of a fold. The injection of molten rock into cooler rock, and its subsequent cooling, can fracture the cooler rock, producing joints. Extension is also produced during weathering when, because of chemical reactions at the surface of a rock, the surface layer expands more than the interior does. This expanded surface pulls away from the rest of the rock much as an onion skin pulls off the outside of an onion. This process is called exfoliation or sheeting. One classic example of this is Half Dome in Yosemite National Park, California.

Contraction, too, can cause joints. The familiar cracks that form in dried mud puddles are an example: Moist mud contracts as it dries, and cracks in the surface result when the forces involved in this contraction overcome the cohesive strength of the mud. As rock cools, such as when a molten, igneous rock body solidifies and then cools further, it contracts. The cooling and contraction are greatest at the surface of such a body. The polygonal patterns of cracks that form on such surfaces are very similar to those seen in dried mud. As the hot rock continues to cool, these cracks extend into the interior of the body. This may result in spectacular columns, such as those seen at Devils Postpile National Monument in California. The process is called columnar jointing.

The most common way joints form, however, is probably when rocks that equilibrated at depth are brought to the surface, either by mountain-building forces or when erosion removes the overlying rocks. This means of forming joints was proposed by Neville Price in his 1966 book Fault and Joint Development in Brittle and Semi-Brittle Rock. Although the model cannot be allied directly at any particular location because the deformation history, local topography, and other factors vary too much from place to place, it is instructive to consider the process in general terms.

How can vertical uplift produce horizontal extension? Consider large suspension bridges such as the Verrazano Narrows in New York City or the Golden Gate Bridge in San Francisco. The vertical towers supporting these bridges diverge from one another by about one-hundredth of a degree because of the curvature of the Earth. The tops of these towers are farther apart than their bases, so a rope that exactly reached between the bases would have to stretch a bit if it were raised to their tops. If it were unable to stretch, it would break.

Would uplift of 5 kilometers be sufficient to produce joints in a typical rock? If Earth's circumference is 40,074 kilometers, the circumference of a circle lying 5 kilometers beneath the surface would be 40,043 kilometers. If that circle were brought to the surface, it would have to be stretched by 31 kilometers, or by 0.078 percent. This amount of stretch might not seem like much, as a block of rock 100 meters long would need to extend only 7.8 centimeters. However, if one tried to stretch the block of rock by attaching a gigantic pulling apparatus on it, experimental data show that the block would break before stretching that much.

In addition to this geometric extension, the rock would generally cool as it came up to the surface, contracting and making more joints in the process. Because the state of compression at depth would likely be different from that at the surface, the changes that occur during uplift might encourage or inhibit joint formation, depending on local conditions. Still, if all the joints currently at the Earth's surface are considered, it appears as if the majority of extension joints may form by uplift. The fact that joints form during uplift and erosion does not mean, however, that they are necessarily unrelated to the structural history of the rocks in which they occur. Deformed rocks often contain stored-up energy, much like the energy stored in a spring, which was caused by the deformation. This energy, usually called residual stress, can influence the development of joints. Thus, joints that form hundreds of millions of years after a rock was initially deformed will often occur in patterns and orientations clearly related to that deformation.

Shear Joints

Some joints are not formed strictly by extension. These joints develop as a series of cracks, called shear joints, which break the rock into diamond-shaped pieces. These pieces slide slightly past one another, accommodating the deformation. Careful examination of these joints may show some slight offset of grains across the joint, but the displacement across any one joint is small. The cumulative effect across hundreds of joints, however, can be considerable. Often, these joints occur in two parallel sets, with angles of about 60 degrees and 120 degrees between them. Such sets are called conjugate shear sets.

Shear joints are not perpendicular to the direction of least compression, but it is possible to determine the direction of compression from the orientation of the conjugate shear sets. The direction of intermediate compression is indicated by the line of intersection of the joints. The direction of least compression bisects the obtuse angle between the sets, and the direction of maximum compression bisects the acute angle. In terms of a diamond-shaped piece, the direction of least compression is the short way across the diamond, and the direction of greatest compression is the long way across the diamond. It is not uncommon for conjugate shear sets to occur in conjunction with extension joints. In this case, the extension joints will be parallel to the long axes of the diamonds.

Consider the joints that might be associated with a fold that forms in a horizontal layer of rock not too far beneath the surface. The fold will form as the layer buckles in response to forces acting along it—in the north-south direction, for example—which is similar to the way a playing card flexes when one squeezes the edges between one's fingers. Early in the deformation process, least compression in the east-west direction results in extension joints running north-south and shear joints running northeast and northwest at a 30-degree angle. Eventually, a buckle develops, folding the layer and producing extension fractures in the east-west direction. Much later, erosion and uplift may bring parts of this layer to the surface. The direction of the least compression at that time, which may have no relation at all to the forces that originally produced the fold, will control the vertical extension joints that develop because of this uplift. Finally, weathering and the vagaries of the topography at the time the rock is exposed to weathering control the exfoliation joints that will follow the shape of the exposed surface.

Fractography

Fracture patterns—often called decorations—on the joint surfaces can yield useful information about the speed of fracture growth and the direction in which it grew. This field of study is called fractography and has been developed by ceramic engineers concerned with reconstructing the brittle failure of glass and ceramic objects to improve their design. It can be directly applied to the study of joint surfaces.

When a fracture begins to grow, it starts with a low velocity but accelerates quickly. While moving slowly, the front of the fracture is usually a smooth curve, and the decoration it leaves on the fracture surface may be perfectly smooth, called the mirror region, or slightly frosted in appearance, in which case it is called mist hackle. If the crack grows intermittently, arrest lines may result. These curves show where the crack front was at different times when it temporarily stopped growing. As it increases in speed, the fracture front divides into several fingerlike projections. These commonly move a bit beyond the initial plane of the fracture as the fracture continues to grow. The result is a pattern on the surface of the fracture that has long been called plumose structure by geologists but is known as twist hackle by fractographers. It looks like a feather. The directions in which the fracture grew are shown by the directions of each slightly offset, curving element. Many of these features can be seen on building stones, flagstones, and slate floor tiles.

Fluids and Joints

Joints provide conduits for the movement of fluids beneath the surface. Just as cracks in a pot permit water to leak through the pot, joints in the bedrock greatly enhance the rate at which water, oil, natural gas, and other fluids move through it. Near the surface, water is the fluid most likely to move through joints. As it does so, it is likely to attack the rock on both sides of the joint, chemically weathering it. This process enlarges the joint, increasing the flow of water through it, which in turn causes it to be weathered further, and the process continues. When the jointed rocks are limestone, the result may be elaborate systems of caverns, such as Carlsbad Caverns in New Mexico. Maps of such caves clearly demonstrate that joints controlled their development. In areas underlain by less soluble rock, joints may provide access to groundwater resources. By studying joint patterns displayed on geologic maps, aerial photographs, or satellite images, hydrologists can sometimes see where the natural underground flow of water may be greatest, and they can exploit this knowledge in their search for water.

Similarly, petroleum geologists seek conditions where joints may facilitate the movement of oil and gas toward potential well sites. Because the rocks of interest to them are often much deeper than those with useful water resources, petroleum geologists may be forced to guess the location of joints at depth. Although the surface traces of joints seen on maps can help, it is often necessary to apply an understanding of how and why joints form to predict where they may be at depth. Sometimes, artificial joints are produced by pumping fluids under very high pressure into the rocks.

Principal Terms

chemical weathering: changes in rocks produced by reactions with fluids near the surface of the Earth

columnar jointing: the formation of columns, often with hexagonal cross sections, as joints grow inward from the outer surfaces of cooling igneous rock bodies

conjugate shear sets: two sets of joints that make angles with each other of close to 60 degrees and 120 degrees

exfoliation: the splitting off of curving sheets from the outside of a body of rock; also called sheeting

extension: expansion, or stretching apart, of rocks

fractography: the study of fracture surfaces to determine the propagation history of the crack

joint: a fracture in a rock across which there has been no substantial slip parallel to the fracture

Bibliography

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