Rocks - Real-life applications
Rocks and Human Existence
Rocks are all around us, especially in our building materials but also in everything from jewelry to chalk. Then, of course, there are the rocks that exist in nature, whether in our backyards or in some more dramatic setting, such as a national park or along a rugged coastline. Indeed, humans have a long history of involvement with rocks—a history that goes far back to the aptly named Stone Age.
The latter term refers to a period in which the most sophisticated human tools were those made of rock—that is, before the development of the first important alloy used in making tools, bronze. The Bronze Age began in the Near East in about 3300 B.C. and lasted until about 1200 B.C. , when the development of iron-making technology introduced still more advanced varieties of tools.
These dates apply to the Near East, specifically to such areas as Mesopotamia and Egypt, which took the lead in ancient technology, followed much later by China and the Indus Valley civilization of what is now Pakistan. The rest of the world was even slower in adopting the use of metal: for instance, the civilizations of the Americas did not enter the Bronze Age for almost 4,000 years, in about A.D. 1100. Nor did they ever develop iron tools before the arrival of the Europeans in about 1500.
THE STONE AGE.
In any case, the Stone Age, which practically began with the species Homo sapiens itself, was unquestionably the longest of the three ages. The Stone Age is divided into two periods: Paleolithic and Neolithic, sometimes called Old and New Stone Age, respectively. (There was also a middle phase, called the Mesolithic, but this term is not used as widely as Paleolithic or Neolithic.) Throughout much of this time, humans lived in rock caves and used rock tools, including arrowheads for killing animals and (relatively late in prehistory) flint for creating fire.
The Paleolithic, characterized by the use of crude tools chipped from pieces of stone, began sometime between 2.5 and 1.8 million years ago and lasted until last ice age ended (and the present Holocene epoch began), about 10,000 years ago. The Neolithic period that followed saw enormous advances in technology, so many advances that historians speak of a "Neolithic Revolution" that included the development of much more sophisticated, polished tools. The mining of gold, copper, and various other ores began long before the development of the first alloys (bronze is formed by the mixture of copper and tin). Yet even after humans discovered metals, they continued to use stone tools.
THE PYRAMIDS AND OTHER STONE STRUCTURES.
Indeed, the great pyramids of Egypt, built during the period from about 2600-2400 B.C. , were constructed primarily with the use of stone rather than metal tools. The structures themselves, of course, also reflect the tight connection between humans and rocks. Built of limestone, the pyramids are still standing some 4,500 years later, even as structures of clay and mud built at about the same time in Mesopotamia (a region poor in stone resources) have long since dwindled to dust.
Incidentally, the great pyramids once had surfaces of polished limestone, such that they gleamed in the desert sun. Centuries later, Arab invaders in the seventh century A.D. stripped this limestone facing to use it in other structures, and the only part of the facing that remains today is high atop the pyramid of Khafre. For this reason, Khafre's pyramid is slightly taller than the structure known as the Great Pyramid, that of Cheops, or Khufu, which was originally the largest pyramid.
The centuries that have followed the building of those great structures likewise are defined, at least in part, by their buildings of stone. The Bible is full of references to stones, whether those used in building Solomon's temple or the precious gemstones said to form the gates of the New Jerusalem described in the Book of Revelation. Greece and Rome, too, are known for their structures of stone, ranging from marble (lime-stone that has undergone metamorphism) to unconsolidated stones in early forms of concrete, pioneered by the Egyptians.
Still later, medieval Europe built its cathedrals and castles of stone, though it should be noted that the idea of the castle came from the Middle East, where the absence of lumber for fortresses caused Syrian castle builders to make use of abundant sandstone instead. Other societies left behind their own great stone monuments: the Great Wall of China, Angkor Wat in southeast Asia, the pyramids of Central America and Machu Picchu in South America, the great cliffside dwellings of what is now the southwestern United States, and the stone churches of medieval Ethiopia.
Certainly there were civilizations that created great structures of wood, but these structures were simply not as durable. The oldest wood building, a Buddhist temple at Horyuji in Japan, dates back only to A.D. 607, which, of course, is quite impressive for a wooden structure. But it hardly compares to what may well be the oldest known human structure, a windbreak discovered by the paleobiologist Mary Leakey (1913-1996) in Tanzania in 1960. Consisting of a group of lava blocks that form a rough circle, it is believed to be 1.75 million years old.
Mineralogy and Petrology
Not surprisingly, mineralogy is concerned with minerals—their physical properties, chemical makeup, crystalline structures, occurrence, distribution, and physical origins. Researchers whose work focuses on the physical origins of minerals study data and draw on the principles of physics and chemistry to develop hypotheses regarding the ways minerals form. Other mineralogical studies may involve the identification of a newly discovered mineral or the synthesis of mineral-like materials for industrial purposes.
The study of rocks is called petrology, from a Greek root meaning "rock." (Hence also the words petroleum and petrify. ) Its areas of interest with regard to rocks are much the same as those of mineralogy as they relate to minerals: physical properties, distribution, and origins. It includes two major subdisciplines, experimental petrology, or the synthesis of rocks in a laboratory as a means of learning the conditions under which rocks are formed in the natural world, and petrography, or the study of rocks observed in thin sections through a petrographic microscope, which uses polarized light.
Owing to the fact that most rocks contain minerals, petrology draws on and overlaps with mineralogical studies to a great extent. At the same time, it goes beyond mineralogy, inasmuch as it is concerned with materials that contain organic substances, which are most likely to appear within the realm of sedimentary rock. Petrologists also are concerned with the other two principal types of rock, igneous and metamorphic.
Igneous Rocks
Igneous rock is rock formed by the crystallization of molten materials. It most commonly is associated with volcanoes, though, in fact, it comes into play in the context of numerous plate tectonic processes, such as seafloor spreading (see Plate Tectonics). The molten rock that becomes igneous rock is known as magma when it is below the surface of the earth and lava when it is at or near the earth's surface. Its most notable characteristic is its interlocking crystals. For the most part, igneous rocks do not have a layered texture.
When igneous rocks form deep within the Earth, they are likely to have large crystals, an indication of the fact that a longer period of time elapsed while the magma was cooling. On the other hand, volcanic rocks and others that form at or near Earth's surface are apt to have very small crystals. Obsidian (which, as we have noted, is not truly a mineral owing to its lack of crystals) is formed when hot lava comes into contact with water; as a result, it cools so quickly that crystals never have time to develop. Sometimes called volcanic glass, it once was used by prehistoric peoples as a cutting tool.
CLASSIFYING AND IDENTIFYING IGNEOUS ROCKS.
Igneous rocks can be classified in several ways, referring to the means by which they were formed, the size of their crystals, and their mineral content. Extrusive igneous rocks, ejected by volcanoes to crystallize at or near Earth's surface, have small crystals, whereas intrusive igneous rocks, which cooled slowly beneath the surface, have larger crystals. Sometimes the terms plutonic and volcanic, which roughly correspond to intrusive and extrusive, respectively, are used.
Igneous rocks made of fragments from volcanic explosions are known as pyroclastic, or "fire-broken," rocks. Those that consist of dense, dark materials are known as mafic igneous rocks. On the other hand, those made of lightly colored, less-dense minerals, such as quartz, mica, and feldspar, are called felsic igneous rocks. Among the most well known varieties of igneous rock is granite, an intrusive, felsic rock that includes quartz, feldspar, mica, and amphibole in its makeup. Also notable is basalt, which is mafic and extrusive.
Sedimentary Rocks
Earlier, we touched on the subject of consolidation, which can be explained in more depth within the context of sedimentary rock. Consolidation is the compacting of loose materials by any number of processes, including recrystallization and cementation. The first of these processes is the formation of new mineral grains as a result of changes in temperature, pressure, or other factors. In cementation, particles of sediment (material deposited at or near Earth's surface from several sources, most notably preexisting rock) are cemented together, usually with mud.
Compaction, recrystallization, and other processes, such as dehydration (which also may contribute to compaction), are collectively known as diagenesis. The latter term refers to all the changes experienced by a sediment sample under conditions of low temperature and low pressure following deposition. If the temperature and pressure increase, diagenesis may turn into metamorphism, discussed later in the context of metamorphic rock.
FORMATION OF SEDIMENTARY ROCKS.
Sedimentary rock is formed by the deposition, compaction, and cementation of rock that has experienced weathering (breakdown of rock due to physical, chemical, or biological processes) or as a result of chemical precipitation. The latter term refers not to "precipitation" in terms of weather but to the formation of a solid from a liquid, by chemical rather than physical means. (The freezing of water, a physical process, is not an example of precipitation.)
Sedimentary rock usually forms at or near the surface of the earth, as the erosive action of wind, water, ice, gravity, or a combination of these forces moves sediment. Yet this formation also may occur when chemicals precipitate from seawater or when organic material, such as plant debris or animal shells, accumulate. Evaporation of saltwater, for instance, produces gypsum, a mineral noted for its lack of thermal conductivity; hence its use in drywall, the material that covers walls in most modern homes. (Ancient peoples made alabaster, a fine-grained ornamental stone, from gypsum.)
CLASSIFICATION AND SIZES.
Sedimentary rock is classified with reference to the size of the particles from which the rock is made as well as the origin of those particles. Clastic rock comes from fragments of preexisting rock (whether igneous, sedimentary, or metamorphic) and organic matter, while nonclastic sedimentary rock is formed either by precipitation or by organic means. Examples include gypsum, salts, and other rocks formed by precipitation of saltwater as well as those created from organic material or organic activity—coal, for example.
Ranging in size from fine clay (less than 0.00015 in., or 0.004 mm) to boulders (defined as any rock larger than 10 in., or 0.254 m), sedimentary rock bears a record of the environment in which the original sediments were deposited. This record lies in the sediment itself. For example, rocks containing conglomerate, material ranging in size from clay to boulders (including the intermediate categories of silt, sand, gravel, pebbles, and cobble), come from sediment that was deposited rapidly as the result of slides or slumps. (Slides and slumps are discussed in Mass Wasting.)
Sedimentary rocks are of particular interest to paleontologists, stratigraphers, and others working in the field of historical geology, because they are the only kinds of rock in which fossils are preserved. The pressure and temperature levels that produce igneous and metamorphic rock would destroy the organic remnants that produce fossils; on the other hand, sedimentary rock—created by much less destructive processes—permits the formation of fossils. Thus, the study of these formations has contributed greatly to geologists' understanding of the distant past. (See the essays Historical Geology, Stratigraphy, and Paleontology. For more about sedimentary rock, see Sediment and Sedimentation.)
Metamorphic Rocks
Metamorphic rock is formed through the alteration of preexisting rock as a result of changes in temperature, pressure, or the activity of fluids (usually gas or water). These changes in temperature must be extreme (figures are given later), such that the preexisting rock—whether igneous, sedimentary, or metamorphic—is no longer stable.
Often formed in mountain environments, metamorphic rocks include such well-known varieties as marble, slate, and gneiss—metamorphosed forms of limestone, shale, and granite, respectively. Also notable is schist, composed of various minerals, such as talc, mica, and muscovite. There is not always a one-to-one correspondence between precursor rocks and metamorphic ones: increasing temperature and pressure can turn shale progressively into slate, phyllite, schist, and gneiss.
The presence of mica in a rock—or of other minerals, including amphibole, staurolite, and garnet—is a sign that the rock might be metamorphic. These minerals, typical of metamorphic rocks, are known as metamorphic facies. Also indicative of metamorphism are layers in the rock, more or less parallel lines along which minerals are laid as a result of the high pressures applied to the rock in its formation. Metamorphism, the process whereby metamorphic rock is created, also may produce characteristic formations, such as an alignment of elongate crystals or the separation of minerals into layers.
METAMORPHISM.
Given the conditions described for metamorphism, one might conclude that in terms of violence, drama, and stress, it is a process somewhere between sedimentation and the formation of igneous rock. That, in fact, is precisely the case: the temperature and pressure conditions necessary for metamorphism lie between those of diagenesis, on the one hand, and the extreme conditions necessary for the production of igneous rock, on the other hand. Specifically, metamorphism occurs at temperatures between 392°F (200°C) and 1,472°F (800°C) and under levels of pressure between 1,000 and 10,000 bars. (A bar is slightly less than the standard atmospheric pressure at sea level. The latter, equal to 14.7 lb. per square inch, or 101,325 Pa, is equal to 1.01325 bars.)
There are several types of metamorphism: regional, contact, dynamic, and hydrothermal. Regional metamorphism results from a major tectonic event or events, producing widespread changes in rocks. Contact or thermal metamorphism results from contact between igneous intrusions and cooler rocks above them, which recrystallize as a result of heating. Dynamic metamorphism takes place in the high-pressure conditions along faults. Finally, hydrothermal metamorphism ensues from contact with fluids heated by igneous rock. Reacting with minerals in the surrounding rock, the fluids produce different minerals, which, in turn, yield metamorphic rocks.
TYPES OF METAMORPHIC ROCKS.
Metamorphic rocks that contain elongate or platy minerals, such as mica and amphibole, are called foliated rocks. These rocks have a layered texture, which may manifest as the almost perfect arrangement of materials in slate or as the alternating patterns of light and dark found in some other varieties of rock. Metamorphic rocks without visible layers are referred to as unfoliated rocks. As a foliated metamorphic rock, slate is particularly good for splitting into thin layers—hence one of its most important applications is in making shingles for roofing. By contrast, marble, which is unfoliated, is valued precisely for its lack of tendency to split.
Petrologists attempting to determine exactly which rocks or combinations of rocks metamorphosed to produce a particular sample often face a challenge. Many metamorphic rocks are stubborn about giving up their secrets; on the other hand, it is possible to match up precursor rocks with certain varieties. For example, as noted earlier, marble comes from limestone, while gneiss usually (but not always) comes from granite. Quartzite is metamorphosed sandstone. Nonetheless, it is not as easy to trace the history of a metamorphic rock as it is to say that a raisin was once a grape or that a pickle was once a cucumber.
Where to Find Rocks
In general, one might find igneous rocks such as basalt in any place known for volcanic activity either in the recent or distant past. This would include such well-known areas of volcanism as Hawaii, the Philippines, and Italy, but also places where volcanic activity occurred in the distant past. (See, for instance, the discussion in the essay titled "Paleontology" regarding possible volcanic activity in what is now the continental United States at the conclusion of the Triassic period.)
The best place for metamorphic rock would be in areas of mountain-building and powerful tectonic activity, as for instance in the Himalayas or the Alps of central Europe. Sedimentary rock is basically everywhere, but a good place to find large samples of it would include areas with large oil deposits, which are always found in sedimentary rock.
Closer to home, a wide array of sedimentary rocks can be located in the plains and lowlands of the United States, particularly in the West and Midwest, where large samples are exposed. Igneous and metamorphic rocks can be found, predictably, in regions where mountains provide evidence of past tectonic activity: New England, the Appalachians, and the various mountain ranges of the western United States such as the Rockies, Cascades, and Sierra Nevada.
The Rock Cycle
Given what we have seen about the characteristics of the three rock varieties—igneous, sedimentary, and metamorphic—it should be clear that there is no such thing as a rock that simply is what it is, without any possibility of changing. Rocks, in fact, are constantly changing, as is Earth itself. This process whereby rocks continually change from one type to another—typically through melting, metamorphism, uplift, weathering, burial, or other processes—is known as the rock cycle.
The rock cycle can go something like this: Exposed to surface conditions such as wind and the activity of water, rocks experience weathering. The result is the formation of sediments that are eventually compacted to make sedimentary rocks. As the latter are buried deeper and deeper beneath greater amounts of sediment, the pressure and temperature builds. This process ultimately can result in the creation of metamorphic rock. On the other hand, the rock may undergo such extreme conditions of temperature that it recrystallizes to form igneous rock. Whatever the variety—igneous, sedimentary, or metamorphic—the rock likely will be in a position eventually to experience erosion, in which case the rock cycle begins all over again.
WHERE TO LEARN MORE
Atlas of Igneous and Metamorphic Rocks, Minerals, and Textures (Web site). <http://www.geosci.unc.edu/Petunia/IgMetAtlas/mainmenu.html> .
Bishop, A. C., Alan Robert Woolley, and William Roger Hamilton. Cambridge Guide to Minerals, Rocks, and Fossils. New York: Cambridge University Press, 1999.
Busbey, Arthur Bresnahan. Rocks and Fossils. Alexandria, VA: Time-Life Books, 1996.
Discovery Channel Rocks and Minerals: An Explore Your World Handbook. New York: Discovery Books, 1999.
"The Essential Guide to Rocks." BBC Education (Web site). <http://www.bbc.co.uk/education/rocks/> .
"Igneous, Sedimentary, and Metamorphic Rock Info. "University of British Columbia (Web site). <http://www.science.ubc.ca/~geol202/petrology/rock.html> .
RocksForKids.com (Web site). <http://www.rocksforkids.com/> .
Rocks and Minerals (Web site). <http://www.fi.edu/tfi/units/rocks/rocks.html> .
Symes, R. F., Colin Keates, and Andreas Einsiedel. Rocks and Minerals. New York: Dorling Kindersley, 2000.
Vernon, R.H. Beneath Our Feet: The Rocks of Planet Earth. New York: Cambridge University Press, 2000.
Comment about this article, ask questions, or add new information about this topic: