Experimental rock deformation

To understand how and why rocks deform, experiments are done using laboratory apparatus that simulate some of the conditions found in the earth's crust and mantle. These experiments have shown that the mechanical behavior of rocks is complex but can be deciphered. The results help to develop an intuition that leads to meaningful interpretations of field situations.

Brittle vs. Ductile Behavior

Experiments in rock deformation aim to develop an understanding of how rocks behave mechanically. Under conditions found at the surface of the earth, most rocks behave as brittle solids, but when subjected to the conditions found at depth within the earth, those same rocks behave in a very ductile manner. If a chalk-sized piece of rock were squeezed between the jaws of a vise, it would fail in a sudden fashion, almost like an explosion. The pieces of rock recovered from such an experiment would be sharp shards similar to the remnants of a rock that was smashed with a hammer. The breaking strength, pore pressure, and internal angle of friction determine the failure criterion for a given type of rock. The failure criterion encompasses the combinations of factors that can cause the rock to fail. Engineers can apply such criteria to determine the stability of a slope, for example, or the spacing required for pillars in an underground mine.

Often a rock will deform without breaking. Rock layers are sometimes buckled into folds that can vary from angular kink bands to smoothly undulating surfaces. Other rocks seen in roadcuts and outcrops show, by the patterns of their banding and textures, that they have deformed in a very ductile way, flowing much like a fluid. To simulate this behavior in a laboratory experiment, the conditions under which deformation occurred must be considered.

Deformation Conditions

The most striking examples of ductile deformation come from deep crustal or upper mantle depths. What are the pressure, temperature, and strain rate like at depths of 20 kilometers? The pressure produced by 20 kilometers of rock with a density of approximately 2.7 grams per cubic centimeter is about 5.2 kilobars, which is approximately 75,000 pounds per square inch, or 5,000 atmospheres of pressure. By studying minerals found in volcanic rocks that come from deep sources, scientists have learned that the temperatures at a depth of 20 kilometers are 250-500 degrees Celsius. If an object deforms so that its length changes by 1 percent in a second, it is undergoing a deformation with a strain rate of 0.01 per second. Geologists have found examples of deformation that incorporate datable features, and from these have learned that strain rates of 10⁻¹³ to 10⁻¹⁴ per second are typical for geological processes. At such rates, the length of an object would change by about one part per million per year. This is too slow to study in the laboratory, so experiments are run at strain rates on the order of 10⁻⁵ per second. The results are extrapolated to estimate how rocks would behave at very low strain rates. An extrapolation over nine orders of magnitude is risky but is supported by theoretical considerations.

Experiments have been designed to study the effects of pressure, temperature, and strain rate, as well as pore pressure, anisotropy, and water content. The emerging picture reveals that rocks exhibit a complex mechanical behavior. At low confining pressures, low temperatures, and high strain rates, they behave as elastic solids when subjected to stresses up to their breaking strength, then fail in a brittle fashion. At high confining pressures, high temperatures, and low strain rates, they deform in a ductile fashion, with yield strengths and viscosities that are functions of temperature and strain rate. One result of this behavior is that when subjected to the very low strain rates associated with convection, mantle rocks flow easily; when subjected to the high strain rates resulting from the passage of those seismic waves called shear waves, mantle rocks respond like elastic solids.

Dislocations

Results from flow experiments indicate that at high temperatures and pressures and low strain rates, rocks deform by the movement of offsets in crystal lattices called dislocations. This is similar to the way a caterpillar moves forward—only a few of its legs are in motion at any one time, but the movements propagate along as waves, and the whole animal moves forward. As dislocations move through a crystal, only a few bonds are broken at a time, but the entire crystal deforms as a result.

The study of dislocations and how they move has resulted in a better understanding of ductile deformation of rocks and other materials. Several different mechanisms, such as power law creep and diffusion creep, have been found to be active in different substances under different conditions. Flow laws have been formulated, and maps have been constructed that show, for a given mineral, which flow law will dominate the deformation for a given stress difference and temperature. Because rocks are aggregates of different minerals, their behavior is more complex than that of any single mineral. Progress is being made, however, and eventually the behavior of the material of the crust and upper mantle will be better understood.

Studying Lower Mantle and Core Behavior

To study lower mantle and core behavior, experiments have been designed using diamonds as platens (flat plates that exert or receive pressure). Diamonds can withstand very high pressures and are transparent to visible light. This transparency permits visual observations of phase changes and allows samples to be heated to very high temperatures using lasers. Conditions similar to those within the earth's core can be simulated in such experiments, but the measurements that are possible are limited by the need to use small samples.

Studying Fracture and Flow

The methods used to study rock deformation in the laboratory depend on whether fracture or flow is the subject of investigation. Many studies of fracture are motivated by the desire to understand how earthquakes occur, and, if possible, to develop means of predicting them. Because damaging earthquakes frequently occur in rock near the surface, these experiments are done at low temperatures and confining pressures. Studying the flow of rock at high temperatures and high confining pressures develops insights into mantle convection and plate tectonics. The general procedure is to prepare a sample of the rock to be studied; attach strain gauges to the sample to monitor changes in strain during the course of the experiment; insert the sample between the platens of the press; adjust confining pressure, temperature, and pore pressure to the conditions of interest; and then squeeze the sample while recording data from the sensors.

An experiment with no confining pressure is called a “uniaxial” test, because all the stress is applied along one axis. A hydrostatic confining pressure can be applied to the sample by immersing it within a medium and then compressing that medium. Although the terminology is not quite correct, this kind of experiment is usually called a “triaxial” experiment. The confining medium can be a solid (such as talc), a fluid (commonly kerosene), or a gas.

During experiments exploring the fracturing process, the sample may be instrumented by gluing small microphones to it. Just before the rock fails catastrophically, several small acoustic events often can be located within the sample by triangulation from a number of microphones. These noises are thought to be produced by the extension of small, naturally occurring fractures as they grow in response to the increasing stress. Determining the relationship between these events and the fracture that finally forms offers a promising means of earthquake prediction: Foreshocks are commonly recorded in the vicinity where a large earthquake is about to occur and may be similar to the acoustic events observed in the laboratory.

High confining pressures, high temperatures, and low strain rates are needed to study the flow of rock. Such experiments are technically difficult, particularly if they run for long periods of time. Commonly, the temperature is increased above that typically expected to occur within the earth at the pressures being studied, so as to increase the rate of deformation. At the completion of the experiment, the sample may be recovered, sliced into thin sections, and studied under a microscope. If the textures produced in the laboratory resemble those observed in samples from the crust and mantle, it is likely that similar processes are active. The flow laws operative during the experiment can be determined and then adjusted for differences in temperature and strain rate, which permits the extrapolation to the conditions present within the earth to be conducted with more confidence.

Understanding Formation of Giant Structures

A topographic map from the Valley and Ridge Province of the Appalachian Mountains shows sinuous ridges tracing out elaborate folds in a coherent pattern extending for hundreds of miles. The landscape south of San Francisco is dominated by long, linear valleys parallel to the San Andreas fault. Roadcuts near the Thousand Islands reveal swirling, flowing patterns that appear to have formed as if the marble there behaved like a fluid.

Each of these phenomena is a striking demonstration of how rocks deform when subjected to the mammoth stresses involved in mountain building and plate tectonics, yet they are all very different from one another. To understand how such giant structures are formed, scientists have performed experiments in the laboratory on small samples of the rocks from which the structures are made. They have learned that the behavior of rock is a function of its environment at the time it is being deformed, the size of the stresses applied to it, and the rate at which those stresses are applied.

Fracture Orientations and Patterns

Some of this behavior can be compared to that of three familiar materials: modeling clay, beeswax, and Silly Putty. Modeling clay shows a behavior that varies with its environment, particularly temperature. A piece of cold modeling clay is difficult to work with. Most people spend a few minutes kneading it in their warm hands; its behavior changes noticeably as it warms. A piece of very cold modeling clay may shatter if it is dropped on the floor, unlike a piece that has been warmed. If the pieces were reassembled, there would be a pattern of fractures related to the orientation of the rock within the vise. Much larger, but similar, fractures occur within the crust of the earth, which are called joints or faults. Theoretical considerations and data from laboratory experiments are used to interpret the orientations and patterns produced by these brittle fractures.

If a tennis ball were put in the vise, it would shorten in the direction it was squeezed and would get fatter in the plane of the jaws of the vise. A rock deforms elastically, just like a tennis ball, but at a much smaller scale. Sensors, called strain gauges, attached to the rock sample will record these tiny changes in shape. Careful monitoring of the stress applied by the vise and the strain experienced by the rock would help to determine the elastic constants that describe the behavior of the rock before failure begins. These constants, called Young's modulus for compressional stresses and Poisson's ratio for shearing stresses, can be used to calculate seismic wave velocities. As failure occurs, fractures grow across the sample. In the rock-in-a-vise example, these fractures will usually form perpendicular to the jaws of the vise, corresponding to what are called extension joints. If the sample were enclosed in a jacket that provided pressure on its sides, the experiment would be conducted with a confining pressure present. Under these conditions, many fractures might form at an angle to the jaws of the vise, producing a set of what are called conjugate shear joints. Alternatively, one fracture might develop, and the sample might slip in opposite directions on both sides of this fracture. Such a fracture corresponds to a fault in the field. Measuring the angles at which these fractures form would show that they are somewhat constant for fractures produced in the same material. By increasing the confining pressure, the stress needed to break the sample also increases. Graphing the results permits the determination of another material constant, called the internal angle of friction. A comparison of this angle with the angle at which conjugate fractures and faults form shows that they are simply related.

Pore Pressure

These results characterize some of the mechanical behavior of the rock from which the sample was taken. Young's modulus, Poisson's ratio, breaking strength, and the internal angle of friction are material constants that vary little among different samples from the same rock. Different types of rocks have different elastic constants and strengths, just as they have different densities.

Fluids within the pores of a rock play a significant role in its brittle behavior. Experiments that control the pressure of such pore fluids show that the strength of the rock decreases as the pore pressure increases. Some of a rock's resistance to failure is provided by the pressure of one grain against the next. Pore pressures reduce this pressure and so weaken the rock, which helps to explain why most catastrophic landslides have occurred after heavy rainfall. Slopes that are stable when dry can weaken as the pore pressure within them increases to become unstable and to fail. High pore pressure may also facilitate movements on thrust faults deep within the earth.

Size and Rate of Stresses

The behavior of beeswax varies with the size of the stresses applied to it. A chunk of beeswax feels hard and makes a sharp, rapping sound when struck against a table. The fact that hives and statues in wax museums maintain their shape for years attests to the ability of beeswax to resist the forces of gravity over long periods of time. Yet it is easy to stick a thumbnail into a chunk of beeswax. The stress produced by the edge of a nail is greater than the strength of the beeswax, and it deforms, whereas the stresses produced by gravitational force are less than the strength of the beeswax and are unable to cause it to deform.

Silly Putty shows a behavior that varies with the rate at which stresses are applied to it. Throw a sphere of Silly Putty onto the floor, and it bounces. But pull on it slowly, and it will stretch. In response to a rapid application of stress, Silly Putty behaves like a brittle, elastic solid. But when subjected to a slowly applied force, its behavior is much more like that of a fluid.

Principal Terms

brittle behavior: the sudden failure of a sample by catastrophic loss of cohesion

confining pressure: pressure acting in a direction perpendicular to the major applied stress in a rock deformation experiment

dislocation: a defect in a crystal caused by misalignment of the crystal lattice; the presence of dislocations greatly reduces the stress necessary to produce permanent deformation

ductile behavior: permanent, gradual, nonrecoverable deformation of a solid; sometimes called plastic deformation

elastic behavior: recoverable deformation where the strain is proportional to the stress

pore pressure: the pressure in the fluid within the pores of a rock

strain: a measure of deformation including translation, rotation, dilatation, and distortion; it is usually measured as a percentage or ratio and results from stress

stress: the intensity of forces (force per unit area) acting within a body; may refer to a particular stress acting in a particular direction on a particular plane or to the collection of all stresses acting on all planes at that point

Bibliography

Billings, Marland P. Structural Geology. 3rd ed. Englewood Cliffs, N.J.: Prentice-Hall, 1972.

Dahlen, F. A., and Jeroen Tromp. Theoretical Global Seismology. Princeton: Princeton University Press, 1998.

Davis, George H. Structural Geology of Rocks and Regions. 2d ed. New York: John Wiley & Sons, 1996.

Doyle, Hugh A. Seismology. New York: John Wiley, 1995.

Fossen, Haakon. Structural Geology. New York: Cambridge University Press, 2010.

Guo, Xiaoxiao, et al. "Experimental Study of Rock Fracture Behavior under Direct Tension Using Three-Dimensional Digital Image Correlation." Scientific Reports, vol. 14, no. 19211, 19 Aug. 2024, doi.org/10.1038/s41598-024-70252-6. Accessed 9 Feb. 2025.

Hobbs, Bruce E., Winthrop D. Means, and Paul F. Williams. An Outline of Structural Geology. New York: John Wiley & Sons, 1976.

Jackson, Ian, ed. The Earth's Mantle: Composition, Structure, and Evolution. Cambridge: Cambridge University Press, 2000.

Jaeger, John Conrad, Neville George Wood Cook, and Robert Zimmerman. Fundamentals of Rock Mechanics. 4th ed. New York: John Wiley & Sons, 2007.

Mancktelow, N. S. “Fracture and Flow in Natural Rock Deformation.” Trabajos de Geologia 29 (2009): 29-35.

Marshak, Stephen, and Gautam Mitra. Basic Methods of Structural Geology. Englewood Cliffs, N.J.: Prentice-Hall, 1988.

Paterson, Mervyn S., and Teng-fong Wong. Experimental Rock Deformation: The Brittle Field. 2d ed. New York: Springer, 2005.

Suppe, John. Principles of Structural Geology. Englewood Cliffs, N.J.: Prentice-Hall, 1985.