Physical properties of rocks

Rocks and rock products are used so widely in everyday life that physical properties of rocks affect everyone. Major properties of rocks fall into two categories: those properties that compose the exterior nature of the rock, such as color and texture, and those properties that make up the rock’s internal nature, such as strength, resistance to waves, and toughness.

Color

Physical properties of rocks are important not only to geologists and geophysicists but also to construction engineers, technicians, architects, builders, and highway planners. In fact, rock properties affect everyone. Major physical properties include color, hardness, toughness, density, compressive strength, shear strength, tensional and bending (transverse) strength, elasticity, coefficient of thermal expansion, electrical resistivity, absorption rate of fluids, rates of weathering, chemical activity, response to freeze-thaw tests, texture, spacing between fractures, ability to propagate waves, radioactivity, and melting point.

Color is one of the most interesting physical properties of rocks. It is determined by the colors of the component minerals, arrangement of these minerals, and weathering pigments. Color and patterns of colors are important in assessing the aesthetic qualities of ornamental and building stone. The unweathered colors must be pleasing to the eye, and staining that is detrimental to the appearance must not occur in appreciable amounts. The quality and beauty of many marbles are renowned when used in ornamental stone and sculpture. Building limestone must have a pleasing gray or tan color and not contain iron or manganese minerals, which would stain the rock brown or black during weathering. The pink-and-black or white-and-black mottling of attractive granites, diorites, and syenites is a familiar sight in business, school, university, and other public buildings. It is not, therefore, necessary or even always desirable that a color be uniform throughout.

Hardness, Toughness, and Density

Hardness, or resistance to abrasion, can be found by using the Mohs scale, which was derived by Friedrich Mohs in 1822 to measure relative resistance to abrasion in minerals and rocks. Any substance will be able to scratch any substance softer than or as hard as itself. Ten standard minerals are used: Talc is the softest and designated as having a hardness of 1, and diamond is the hardest and designated as having a hardness of 10. Rock hardness is of particular importance to those people who use rocks or rock products in building façades, monuments, tombstones, patios, and other structures. A variety of tests are used to determine resistance to abrasion over a time interval.

Toughness differs from hardness. This property is defined as the degree of resistance to brittle fracture or plastic deformation of a particular substance. Toughness is determined by a test using repeated impacts of a heavy object. A hammer is dropped from a specified height upon a sample, and this height is continually increased. The height of the fall in centimeters upon breakage is then defined as toughness. An example of a very tough material is jade, whereas brittle substances include rock salt. The so-called French coefficient of wear is also used to measure toughness. This test measures the amount of material worn off a sample by tumbling in a drum under standard conditions. Toughness is a very important property in rocks that are used as crushed rock in building roads and airstrips, which are subject to repeated stresses.

The density of a rock is its mass or weight divided by unit volume (amount of space). More frequently, the rock is compared with an equal volume of water that has a density of almost exactly 1 gram per cubic centimeter. This number, which is dimensionless, is known as specific gravity. There is a considerable variation in specific gravity among rock types and even within them. Most limestone and dolomite rocks range from about 2.2 to 2.7 in specific gravity, whereas basalt and traprock are considerably heavier, at 2.8 to 3.0. Density is not synonymous with strength and toughness. Some low-density materials are strong, and some very dense materials cut by fractures are weak.

Compressive, Shearing, and Bending Strengths

Compressive strength, or bulk modulus, is measured on the basis of the highest pressure or stress that a rock can withstand per square unit, usually measured in pascals or pounds per square inch. One pound per square inch, or psi, is about 7,000 pascals, and 1,000 psi is about 7 million pascals or 7 megapascals, abbreviated MPa. This strength is generally greater than transverse, tensional, or shearing strength. Limestones average about 100 MPa (15,000 psi), granites about 175 MPa (25,000 psi), quartzite about 200 MPa (30,000 psi), and basalts and traprock about 350 MPa (48,000 psi). This strength will vary considerably from rock to rock of the same composition because of variation in structural properties, as most rocks have fractures and voids and differ in grain size and shape.

Shear strength, or modulus of rigidity, is measured on the basis of the highest lateral stress in pounds a rock can withstand per square inch. An example of shear stress is the stress exerted by a car sideswiping another car. Values for limestones average about 14 MPa (2,000 psi) and granites about 20 MPa (3,000 psi). Shear strength is generally much less than compressive strength.

Bending, or transverse, strength is defined as the strength of a slab loaded at the center and supported only by adjustable knife edges. This strength is determined by the “modulus of rupture,” which is a function of the rupture load in pounds, the length of a slab, the width or breadth of a slab, and the thickness of a slab.

Elasticity and Linear Expansion

The so-called modulus of torsional rigidity, or elasticity (Young’s modulus), is also measured in pascals or pounds per square inch. This property is a measurement of how difficult it is to deform a material. Young’s modulus is the force that would have to be applied to deform a material by 100 percent of its original size. The amount that a material deforms under pressure is the ratio of the applied force to Young’s modulus. This modulus is extremely variable, ranging typically from about 200,000 to 400,000 MPa (3 × 10⁶ psi to 6 × 10⁶ psi) for limestone, and about 400,000 to 560,000 MPa (6 × 10⁶ psi to 8 × 10⁶ psi) for granite; that is, a stress of 20 MPa applied to limestone will deform it by about 1/10,000 of its length. Elasticity refers to the ability of a material to spring back to its original shape without plastic deformation or rupture. Beyond a certain value of stress, rupture or flow will occur.

The coefficient of thermal expansion measures expansion along a line through the rock with increase in temperature. It is also termed linear expansion. This property is expressed as a ratio of change in length divided by unit length times change in temperature. Typical values for common rocks used in crushed stone or building stone range from about 4 × 10^-6 (four millionths) to 12 × 10^-6 (twelve millionths) per degree Celsius. Knowledge of the coefficient of thermal expansion is very important to engineers who design structures such as highways and bridges. On bridges, expansion-contraction joints are commonly used in consideration of this property.

Resistivity and Fluid Absorption Rate

A measurement important to petroleum geologists, geophysicists, and engineers is electrical resistivity of rocks. The resistivity may be defined as the reciprocal of electrical conductivity. Resistivity is measured in ohm-centimeters—that is, electrical resistance along a centimeter’s length. It will vary greatly depending on whether a rock is dry or contains saline water, organic compounds such as petroleum or natural gas, or fresh water in its pores or cracks. Igneous rocks such as basalts and traprock will vary from about 1 × 10⁴ to 4 × 10⁵ ohm-centimeters, and granites about 10⁷ to 10⁹ ohm-centimeters. Basalts have less resistivity and greater conductivity than granite because they generally contain more abundant amounts of dark, nonmetallic minerals than granites. Limestones and sandstones commonly range from 10³ to 10⁵ ohm-centimeters, but those containing much saline water will have much smaller values, because water with dissolved salts is a good conductor of electricity. Metallic ore deposits will show very low resistivities because of the high conductivity of metals.

The absorption rate of fluids by rocks is another property often measured. A dry rock sample of known weight (or dry aggregate of rock chips) is soaked in water for twenty-four hours, dried under surface conditions, and weighed. The weight of water absorbed is given as a percentage of the dry weight of the rock. This property is extremely variable in rocks. For example, limestones may vary from 0.03 percent to 12 percent absorption rate. Very fine-grained limestones will commonly have a higher absorption rate than coarse-grained limestones.

Soundness, Texture, and Spacing Between Fractures

Soundness, or resistance to weathering, is important because rock materials are exposed to outside conditions, where temperature and moisture conditions may vary considerably. Chemicals may also attack the rock, and in arctic or temperate climates, freezing and thawing may occur. In a climate where much freezing and thawing occur, the breakage and deterioration of cement and rock materials caused by this phenomenon will be noticed. Engine-driven vehicles and equipment would not last very long in winter if antifreeze were not added to the engine block and radiator. Examples of chemical weathering can be found in an old cemetery, where gravestone inscriptions of the same age on chemically resistant rocks such as quartzite are much clearer than those on chemically reactive rocks such as marble.

Texture relates to grain size and grain shape, as well as fabric, which is the relationship of the grains to one another. Appearance, resistance to weathering, absorption rate characteristics, and strength are all related to texture.

Spacing between fractures is a significant structural property of rocks. For building stone, it is usually desirable to have massive rock or rock separated at intervals of meters by layering or bedding planes or by vertical cracks or joints. Preferably, the distance between planes will be fairly constant and the cracks flat and even. Other faces may be sawed out by means of wire saws and silica grit. For some uses of building stone, such as roofing or facing slates and paving stones, smooth, closely spaced fractures are desirable, provided the stone is relatively strong and free from fractures in between.

Wave Transmission Ability

The ability to transmit waves is an important rock property to petroleum geologists and engineers, as well as construction engineers regarding the design of buildings, roads, bridges, and dams in earthquake-prone areas. This property depends on the density and the strength of the rock, especially its compressibility and its rigidity (shear) modulus. The property is useful to engineers in predicting how strongly rocks will shake in an earthquake, and to petroleum geologists for interpreting the results of seismic surveys. Basically, there are two types of waves that result from earthquakes or human-made explosions: longitudinal and transverse waves. Longitudinal waves, similar to sound waves, create pushing and pulling effects on molecules while traveling along a straight line. Transverse waves move sideways as they travel. Compressive strength (pushing strength) is important in determining the speed of longitudinal waves, whereas modulus of rigidity or shear strength is more important in determining the speed of transverse waves. In general, the greater the strength, the greater the wave speed through a material; the greater the density, the less the wave speed through a material or rock under a given temperature and pressure. The density of darker igneous rocks is generally greater than that of lighter-colored igneous and sedimentary rocks. Also, the density of rocks increases toward the earth’s center, but velocity is usually greater, because strength tends also to increase. This factor generally outweighs the density factor and produces a net increase in velocity or speed.

Radioactivity and Melting Point

Radioactivity, another physical-chemical property, is demonstrated by those rocks that spontaneously emit radiant energy (which results from the disintegration of unstable atomic nuclei). Scientists use the heat generated by absorption of the radiation emitted by these rocks to establish the temperature of the earth’s interior and its thermal evolution. Radioactivity has been of particular interest since the early 1980’s because of the health hazard posed by radon. Radon is an intermediate product of radioactive decay, and it can seep into basements and cause lung cancer. Proper designing or repair of basement interiors may be necessary to reduce rates of radon infiltration.

Rock melting point is a rock property that is generally of interest to petrologists, engineers on deep-drilling projects, and engineers designing furnaces or kilns that require rock or brick that has a melting point higher than that of the materials being processed in the kiln. Except in the case of dry, completely one-mineral rocks, melting will occur over a range of temperatures rather than at merely one temperature. This type of melting occurs for three reasons. First of all, several types of minerals, each with a different melting point when pure, usually occur in rocks. Second, each mineral mutually affects the melting points of the other minerals, usually lowering the melting points and extending them over a range of temperatures. Third, water and other fluids may mutually lower the melting point of a rock.

Physical Property Tests

A great variety of tests are used to determine the degree or types of physical properties of rocks. Hardness may be determined by comparing a rock with known hardness points made from minerals of known hardness or from ceramic cones of various known hardnesses, either on the Mohs ten-point scale or on a more elaborate 1,000-point industrial scale. Another test is called the Dorry hardness test. Rock cones 2.5 centimeters in diameter are loaded with a total weight of about 1,000 or 1,250 grams and then subjected to the abrasive action of a fine quartz of known size fed upon a rotating cast-iron disk. The loss in weight of the core after 1,000 revolutions is used to compute value of hardness. One of the most widely used machines to test hardness is the Los Angeles Rattler (abrasion testing machine). It has a steel drum 70 centimeters in diameter and 50 centimeters long. The sample is inserted with a certain number of steel balls, and the drum is rotated five hundred revolutions at thirty revolutions per minute. The material is then sized into further grades, and the coarser sets of remaining fragments are subjected to one thousand further revolutions using various numbers of steel balls weighing about 5,000 grams. Another test, the Brinell test, determines hardness by pressing a small ball of hard material on the sample and measuring the size of the depression.

Toughness is tested on samples with a diameter of about 2.5 centimeters. The sample is held in a test cylinder on an anvil and subjected to the fall of a steel hammer or plunger weighing 2 kilograms. The first fall is from a 1-centimeter height; this height is progressively increased by 1 centimeter until breakage occurs. Height of fall is then expressed as toughness.

Specific gravity is measured by placing the sample in water. It is measured, dried to obtain dry weight, then immersed in water for twenty-four hours, surface dried, and weighed. Finally, it is weighed again in water. Loss of weight from dried weight initially is true specific gravity. The second step, involving the twenty-four-hour immersion, is necessary to ascertain that pore spaces in the sample are filled with water so that a true measure of buoyancy can be taken in the third step.

Compressive strength is measured on test rock cylinders 5 centimeters in diameter and 5.6 centimeters long. The sample is placed in a cylindrical casing in a press, and the press is lowered at a specified rate. Amount of compression in pounds per square inch at failure is the compressive strength.

The so-called soundness test measures resistance to chemical weathering or, alternatively, resistance to a major type of mechanical weathering, called ice or frost wedging. In the first case, the sample (usually about 1,000 grams) is covered with a saturated solution of sodium or magnesium sulfate for eighteen hours. Then it is oven-dried. The test is repeated usually five times. If the sample decomposes, it is called unsound; if it shows signs of decomposition but is still intact, it is called questionable. If it shows no signs or extremely minor signs of decomposition, it is called sound. In place of ice, the test uses expansion of the sulfate to measure resistance to wedging. In the case of freeze-thaw tests, different-sized materials are tested by freezing in water to an air temperature of -22 degrees Celsius, then thawed at room temperature (about 20 degrees Celsius). A first cycle lasts twenty-four hours and may be repeated. Damage to the sample is then noted.

Seismographic Study

Wave velocities, or speeds through rocks, are dependent on density and strength. These velocities can be determined theoretically or in the laboratory if density and strength can be measured. Conversely, compressive and shearing strength may be measured knowing rock density and velocity of wave transfer.

Geophysicists and engineering geologists may determine wave speeds through the earth’s crust by setting up a seismograph station in the field. Explosives are set off, and travel time and velocity of waves are monitored by a system of geophones that relay the wave energies to the seismograph, which then measures the waves on a recording drum with a stylus. Characteristic wave reflections occur at certain depths. By knowing the times of explosive detonation and the time of arrival of the two kinds of earth body waves (primary and secondary waves), and by understanding that distance traveled for the two waves is the same, scientists can determine the wave speeds. This information is vital when drilling for oil or gas and digging deep foundations, especially in limestone sinkhole areas or in areas prone to earthquakes.

Engineering and Construction Applications

Knowledge of particular rock physical properties is important to geologists, engineers, geophysicists, hydrologists, builders, architects, industrial city and regional planners, and the general public. Rock properties are especially important in regard to the construction of buildings, dams, roads, airstrips, human-made lakes, and tunnels; drilling for petroleum and natural gas; mining and quarrying; monitoring earthquake hazards; measuring properties of aggregate and construction materials; determining rates of heat flow and mechanical strain in the earth; monitoring radioactive hazards; studying chemical makeup and physical structure of the earth’s interior; and evaluating aesthetic properties of ornamental construction materials.

Within earthquake-prone areas (as in California) or areas affected by wind shear (as in the tops of high buildings), the utmost importance is granted to the nature and strength of materials surrounding foundations, the foundations themselves, groundwater behavior and its effects on building strength, and the ability of building materials to withstand vibrations. Engineering codes have been enacted within some of these areas so that earthquake-resistant or wind-shear-resistant buildings, dams, tunnels, and highways may be constructed.

Principal Terms

coefficient of thermal expansion: the linear expansion ratio (per unit length) for any particular material as the temperature is increased

compressive strength: the ability to withstand a pushing stress or pressure, usually given in pascals or pounds per square inch

density: mass per unit volume

elasticity: the maximum stress that can be sustained without suffering permanent deformation

hardness: the resistance to abrasion or surface deformation

shear, or shearing, strength: the ability to withstand a lateral or tangential stress

toughness: the degree of resistance to fragmentation or resistance to plastic deformation

Bibliography

Birch, Francis, J. F. Schairer, and H. Cecil Spicer. Handbook of Physical Constants. Geological Society of America Special Paper 36. Baltimore: Waverly Press, 1942. Covers physical and chemical properties of rocks and minerals. Offers prefatory discussion with most of the tables. Especially important are discussions and tables concerning rock strength and wave propagation. Contains an extensive section on chemical properties of rocks and minerals. Suitable for college-level students.

Carmichael, Robert S., et al. Practical Handbook of Physical Properties of Rocks and Minerals. Boca Raton, Fla.: CRC Press, 1989. A compilation of major physical and chemical properties of rocks and minerals. Of special interest are the sections on radioactive and electrical properties of rocks. Most of the tables are preceded by discussion of the particular property illustrated. Indispensable for geologists and engineers.

Dorn, Ronald I. Rock Coatings. New York: Elsevier, 1998. Covers the physical properties of rocks and their distribution patterns on the surface of the earth. Illustrations and maps are particularly useful, as is the extensive, detailed bibliography.

Fettes, Douglas, and Jacqueline Desmons, eds. Metamorphic Rocks: A Classification and Glossary of Terms. New York: Cambridge University Press, 2007. Discusses the classification of metamorphic rocks. Feldspars are mentioned throughout in reference to the various types of metamorphic rocks and basic classifications.

Haussühl, Siegfried. Physical Properties of Crystals: An Introduction. Weinheim, Germany: Wiley-VCH, 2007. Begins with foundational information and discusses tensors. Contains some mathematical equations. Best suited for more advanced students, professionals, and academics.

Jerram, Dougal, and Nick Petford. The Field Description of Igneous Rocks. 2d ed. Hoboken, N.J.: Wiley-Blackwell, 2011. Begins with a description of field skills and methodology. Contains chapters on lava flow and pyroclastic rocks. Designed for student and scientist use in the field.

Pellant, Chris. Smithsonian Handbooks: Rocks and Minerals. New York: Dorling Kindersley, 2002. An excellent resource for identifying minerals and rocks. Contains colorful images and diagrams, a glossary, and an index.

Potts, P. J., A. G. Tindle, and P. C. Webb. Geochemical Reference Material Compositions: Rocks, Minerals, Sediments, Soils, Carbonates, Refractories, and Ore Used in Research and Industry. Boca Raton, Fla.: CRC Press, 1992. Focuses on the composition of rocks, determinative mineralogy, and analytical geochemistry as they apply to scientific research and industrial use. Includes a useful bibliography.

Schon, J. H. Physical Properties of Rocks. Amsterdam: Elsevier, 2004. Contains information on geophysics useful to a number of professions. Discusses the physical properties of rocks, geophysics theories, and experiments. Suited for engineers, geologists, geophysicists, and well loggers.

Vernon, Ron H. A Practical Guide to Rock Microstructure. New York: Cambridge University Press, 2004. Investigates the microscopic structures of sedimentary, igneous, metamorphic, and deformed rocks. Discusses techniques and common difficulties with microstructure interpretations and classifications. Includes color illustrations, references, and indexing.

Wenk, Hans-Rudolf, and Andrei Bulakh. Minerals: Their Constitution and Origin. Cambridge, England: Cambridge University Press, 2004. Covers the structure of minerals, physical characteristics, processes, and mineral systematic. Multiple chapters devoted to non-silicates, followed by chapters discussing silicates.