Basaltic rocks

“Basalt” is the term applied to dark, iron-rich volcanic rocks that occur everywhere on the ocean floors, as oceanic islands, and in certain areas on continents. It is the parent material from which nearly any other igneous rock can be generated by various natural processes.

Basalt Composition

Basalt is a dark, commonly black, volcanic rock. It is sometimes called “trap” or “traprock,” from the Swedish term trapp, which means “steplike.” On cooling, basalt tends to form hexagonal columns, which in turn form steplike structures after erosion. Excellent examples can be seen at Devils Postpile, in the Sierra Nevada in central California, and the Giant’s Causeway, in County Antrim, Ireland. The term “trap,” however, is used mostly by miners and nonspecialists; scientists prefer the word “basalt” for the fine-grained, volcanic rock that forms by the solidification of lava flows. Basaltic magma that crystallizes more slowly below the earth’s surface, thus making larger mineral grains, is called “gabbro.”

The importance of basalt to the evolution of planets like Earth cannot be overemphasized. Basalt is considered to be a “primary silicate liquid,” in that the first liquids to form by the melting of the original minerals that made up all the so-called rocky planets (those composed mostly of silicates) were basaltic in composition. In turn, basalt contains all the necessary ingredients to make all the other rocks that may eventually form in a planet’s crust. Furthermore, many meteorites (which are believed to represent fragments of planetoids or asteroids) are basaltic or contain basalt fragments, and the surfaces of the moon and the planets Mercury, Venus, and Mars are known to be covered to various degrees by basaltic lava flows.

Like most rocks in the earth’s crust, basalt is composed of silicate minerals, substances whose principal component is the silica molecule. Compared with other silicate rocks, basalts contain large amounts of iron and magnesium and small amounts of silicon. This characteristic is reflected in the minerals in basalts, which are mostly pyroxene minerals (dark-colored, calcium-iron-magnesium silicates such as augite) and certain feldspar minerals called plagioclase (light-colored, calcium-sodium-aluminum silicates). Pyroxenes and plagioclase are essential minerals in basalts, but some types of basalt also contain the mineral olivine (a green, iron-magnesium silicate). The high abundance of dark green or black pyroxene and, in some cases, olivine gives basalts their characteristic dark color. This color can be mainly attributed to the high iron content of pyroxene and olivine.

Oceanic Basalt Distribution

Basalt is the most common type of igneous rock (that is, rock formed by the crystallization of magma) on or near the surface of the earth. It is the principal rock in the ocean floors and is common, though less abundant, on the continents as well. Drilling into the sea floor by specially designed oceanographic research ships reveals that basalt invariably lies just below a thin cover of fine, sedimentary mud. Ocean floor basalt flows out of midocean ridges, a system of underwater mountain ranges that spans the globe. These ridges commonly trend roughly down the middle of ocean basins, and they represent places where the earth’s lithospheric plates are being literally split apart. In this process, basalt magma is generated below what is referred to as rift mountains; it flows onto the cold ocean floor and solidifies. Although oceanic ridges are normally hidden from view under the oceans, a segment of the Mid-Atlantic Ridge emerges above the waves as the island of Iceland.

Oceanic islands are also composed of basalt. The islands of Hawaii, Fiji, Mariana, Tonga, and Samoa, among others, are large volcanoes or groups of volcanoes that rise above water from the ocean floor. Unlike the basalts that cover the ocean floors, however, these volcanoes do not occur at oceanic ridges but instead rise directly from the sea floor. Basalt is also a fairly common rock type on island arcs, volcanic islands that occur near continental margins. These curvilinear island chains arise from melting along subduction zones, areas where the earth’s lithospheric plates are colliding. This process generally involves material from the ocean basins diving under the more massive continents; andesite (a light-colored rock) volcanoes are the main result, but some basalt erupts there as well. The Japanese, Philippine, and Aleutian island chains are examples of island arcs, as are the island countries of New Zealand and Indonesia.

Continental Basalt Distribution

Basaltic lava flows are not nearly as common on continents as in oceanic areas. Andesites, rhyolites, and related igneous rocks are far more abundant than basalt in continental settings. In North America, basaltic lava flows and volcanoes are most exposed to view in the western United States and Canadian provinces, western Mexico, Central America, and western South America. The greatest accumulations of basaltic lava flows in the United States are in the Columbia Plateau of Washington, Oregon, and Idaho. In this large area, a series of basaltic lava flows have built up hundreds of feet of nearly flat-lying basaltic flows over a few million years. These “fissure flows” result from lava pouring out of long cracks in the crust, or fissures. They are similar in many respects to the basalt flows produced at oceanic ridges, because no actual volcanic cones are produced—only layer after layer of black basalt. The Snake River Plain in southern Idaho has a similar origin, and other extensive basalt plateaus occur in the Deccan area of southwestern India, the Karroo area of South Africa, and Paraná State in Brazil.

Not all basalt is erupted as lava flows. If the lava is particularly rich in volatiles such as water and carbon dioxide, it will be explosively ejected from the volcano as glowing fountains of incandescent particles that rain down on the surrounding area. Conical volcanoes composed almost exclusively of basalt ejecta particles are called cinder cones. Good examples of cinder cones are Sunset Crater in northern Arizona, and the numerous cinder cones in Hawaii and Iceland.

Basalt Classification

Although basaltic rocks may all look alike to the nonspecialist, there are actually many different kinds of basalt. They are arranged by scientists into a generally accepted classification scheme based on chemistry and, to some extent, mineralogy. To begin with, basalt can be distinguished from the other major silicate igneous rocks by its relatively low silica content (about 50 percent). Within the basalt clan itself, however, other means of classification are used. Basalts are divided into two major groups: the alkaline basalts and the subalkaline basalts. Alkaline basalts contain large amounts of the alkali metal ions potassium and sodium but relatively small amounts of silica. In contrast, subalkaline basalts contain less potassium and sodium and more silica. As might be expected, this chemical difference translates into differences in the mineral content of the basalt types as well. For example, all alkaline basalts contain one or more minerals called “feldspathoids” in addition to plagioclase feldspar. They also commonly contain significant olivine. Subalkaline basalts, in contrast, do not contain feldspathoids, although some contain olivine, and they may be capable of crystallizing very tiny amounts of the mineral quartz. The presence of this very silica-rich mineral reflects the relatively high silica content of subalkaline basalt magmas versus alkaline basalt magmas. Within these two major groups are many subtypes, too numerous to discuss here.

Study Techniques

Like other igneous rocks, basalts are analyzed and studied by way of many techniques. Individual studies may include extensive field mapping, in which the distribution of various types of basalt is plotted on maps. The history of magma generation and its relation to tectonic history (earth movements) can be reconstructed for a particular area by correlating. Geologic maps of basalt types with absolute ages are determined through radiometric dating techniques. Good examples of such studies are those conducted in recent years on the Hawaiian Islands. These studies indicate that the alkaline basalts on any given island are generally older than the subalkaline basalts, showing that magma production has moved upward, to lower-pressure areas, in the mantle with time. This finding supports the idea that oceanic island basalts such as those in Hawaii are generated within so-called mantle plumes—roughly balloon-shaped, slowly rising masses of mantle material made buoyant by localized “hot spots.”

Samples of basalt are also analyzed in the laboratory. The age of crystallization of basalt is obtained by means of radiometric dating techniques that involve the use of mass spectrometers to determine the abundances of critical isotopes, such as potassium-40 and argon-40, or rubidium-87 and strontium-86 and -87. To obtain information on how basalt magma is generated and how it subsequently changes in composition before extrusion as a lava flow, scientists place finely powdered samples in metallic, graphite, or ceramic capsules and subject them to heating and cooling under various conditions of pressure. Such procedures are known as experimental petrology. Such studies prove that nearly any other igneous magma composition can be derived from basalt magma by the process of crystal fractionation. Widely believed to be the major factor influencing chemical variation among igneous rocks, this process results in ever-changing liquid compositions, as the various silicate minerals crystallize and are thus removed from the liquid over time. Basalt’s parental role gives it enormous importance in the discipline of igneous petrology.

Trace Element Analysis

Another useful avenue of research is the trace element analysis of basalts. Trace elements occur in such low abundances in rocks that their concentrations must usually be expressed in terms of parts per million or even parts per billion. Among the most useful substances for tracing the history of basalt are the rare-earth elements. The elements chromium, vanadium, nickel, phosphorus, strontium, zirconium, scandium, and hafnium are also used. There are many methods for measuring these elements, but the most common, and most accurate, is neutron activation. This method involves irradiating samples in a small nuclear reactor and then electronically counting the gamma-ray pulses generated by the samples. Since different elements tend to emit gamma rays at characteristic energies, these specific energies can be measured and the intensity of gamma pulses translated into elemental concentrations.

Once determined for a particular basalt sample, trace element abundances are sensitive indicators of events that have transpired during the evolution of the basalt. There are two reasons for this sensitivity. First, trace elements are present in such low concentrations (as compared with major elements—iron, aluminum, calcium, silicon, and the like) that any small change in abundance caused by changes in the environment of basalt production will be readily noticed. Second, different minerals, including those crystallizing in the magma and those in the source peridotite, incorporate a given trace element into their structures or reject it to the surrounding liquid to widely varying degrees. Therefore, trace element concentrations can be used to show the minerals that were involved in producing certain observed chemical signatures in basalts and those that were likely not involved.

For example, it is well known that the mineral garnet readily accepts the rare-earth element lutetium into its structure but tends to reject most lanthanum to any adjacent liquid, even though these elements are very similar chemically. Basalts with very little lutetium but much lanthanum were therefore probably derived by the melting of garnet-bearing mantle rocks. Since garnet-bearing mantle rocks can exist only at great depths, basalts with such trace element patterns must have originated by melting at these depths in the mantle. In fact, that is one of the most important lines of evidence to support that alkaline basalts originate at high-pressure regions in the mantle.

Economic Applications

Basaltic islands, particularly in the Pacific basin, are some of the most popular tourist stops in the world. More importantly, however, basalt magma contains low concentrations of valuable metals that, when concentrated by various natural processes, provide the source for many important ores. Copper, nickel, lead, zinc, gold, silver, and other metals have been recovered from ore bodies centered in basaltic terrains. Some of the richest mines of metallic ores in the world are located in Canada, where ores are found associated with extremely old basaltic rocks, called “greenstones,” from long-vanished oceans. The richest of these mines is Kidd Creek in northern Ontario. These ore-bearing basalts were first extruded more than 2 billion years ago, during what geologists call Precambrian times (the period from 4.6 billion to about 600 million years ago). Other notable ore deposits include the native, or metallic, copper in late Precambrian basalts that were mined for many years in the Keweenaw Peninsula of northern Michigan. The island of Crete in the Mediterranean Sea has copper mines that were mined thousands of years ago during the “copper” and “bronze” ages of human history. The basalt enclosing these ores is believed to have erupted from an ancient midocean ridge formerly located between Africa and Europe.

Basalt can also be used as a building stone or raw material for sculptures, but its high iron content makes it susceptible to rust stains. It is also ground up to make road gravel, especially in the western United States, and it is used as decorative stone in yards and gardens.

Principal Terms

augite: an essential mineral in most basalts, a member of the pyroxene group of silicates

crust: the upper layer of the earth and the other “rocky” planets; it is composed mostly of relatively low-density silicate rocks

lithospheric plates: giant slabs composed of crust and upper mantle; they move about laterally to produce volcanism, mountain building, and earthquakes

magma: molten silicate liquid, including any crystals, rock inclusions, or gases trapped in that liquid

mantle: a layer beginning at about 5 to 50 kilometers below the crust and extending to the earth’s metallic core

oceanic ridges: a system of mostly underwater rift mountains that bisect all the ocean basins; basalt is extruded along their central axes

olivine: a silicate mineral found in mantle and some basalts, particularly the alkaline varieties

peridotite: the most common rock type in the upper mantle, where basalt magma is produced

plagioclase: one of the principal silicate minerals in basalt, a member of the feldspar group

subduction zones: areas marginal to continents where lithospheric plates collide

Bibliography

Ballard, Robert D. Exploring Our Living Planet. Rev. ed. Washington, D.C.: National Geographic Society, 1993. Covers every aspect of the earth’s volcanic and tectonic features; lavishly illustrated with color photographs, illustrations, and diagrams. The sections on “spreading” and “hot spots” largely deal with basalt volcanism and its relationship to plate tectonic theory. Well written and indexed, the text will be easily understood and appreciated by specialists and laypersons alike.

Brooks, Kent. "Rocks Explained 2: Basalt." Geology Today, 15 Dec. 2022, onlinelibrary.wiley.com/doi/abs/10.1111/gto.12414. Accessed 25 July 2024.

Decker, Robert, and Barbara Decker. Volcanoes. 4th ed. New York: W. H. Freeman, 2005. Gives a comprehensive treatment of volcanic phenomena. Illustrated with numerous black-and-white photographs and diagrams. Chapters 1, 2, 3, and 6 deal almost exclusively with basalt volcanism. The last four chapters deal with human aspects of volcanic phenomena, such as the obtaining of energy and raw materials, and the effect of volcanic eruptions on weather. Includes an excellent chapter-by-chapter bibliography. Suitable for high school and college students.

Faure, Gunter. Origin of Igneous Rocks: The Isotopic Evidence. New York: Springer-Verlag, 2010. Discusses the chemical properties of igneous rocks and the isotopes within these rock formations. Specific locations of igneous rock formations and the origins of these rocks are provided. Includes many diagrams and drawings, along with an overview of isotope geochemistry in the first chapter, to make this accessible to professionals and undergraduate students.

Hofmann, W. “2.03: Sampling Mantle Heterogeneity Through Oceanic Basalts: Isotopes and Trace Elements.” In Treatise on Geochemistry. Volume 2. Edited by K. K. Turekian and H. D. Holland. San Diego: Elsevier Inc., 2003. Part of a ten-volume reference set on geochemistry. Chemical composition and trace elements of midocean ridge and seamount basalts are discussed. Written in a highly technical manner, requiring prior knowledge and understanding of geochemistry fundamentals. Early sections of this chapter do provide some background information on isotopes and trace elements.

Lewis, Thomas A., ed. Volcano. Alexandria, Va.: Time-Life Books, 1982. Part of the Planet Earth series; written with the nonspecialist in mind. Wonderful color photographs, well-conceived color diagrams, and a readable narrative guide the reader through the world of volcanism. Describes past eruptions and their effects on humankind. Basalt is covered mainly in the chapter on Hawaii and the chapter on Heimaey, Iceland. Has a surprisingly extensive bibliography and index for a book of this kind.

Lutgens, Frederick K., Edward J. Tarbuck, and Dennis Tasa. Earth: An Introduction to Physical Geology. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2010. Provides a clear picture of the earth’s systems and processes suitable for high school or college readers. Offers an accompanying computer disc that is compatible with either Macintosh or Windows. Bibliography and index.

Macdonald, Gordon A. Volcanoes. Englewood Cliffs, N.J.: Prentice-Hall, 1972. Written by one of the premier volcanologists in the world; ideal for those desiring a serious but not overly technical treatment. Covers every conceivable aspect of volcanic phenomena, but the sections on basalt (particularly as it occurs in Hawaii) are particularly good. Includes suggested readings, a comprehensive list of references, a very good index, and an appendix that lists the active volcanoes of the world.

Perchuk, L. L., ed. Progress in Metamorphic and Magmatic Petrology. New York: Cambridge University Press, 1991. Although intended for the advanced reader, several of the essays in this multiauthored volume will serve to familiarize the new student with the study of igneous rocks. In addition, the bibliography will lead the reader to other useful material.

Philpotts, Anthony, and Jay Aque. Principles of Igneous and Metamorphic Petrology. 2d ed. New York: Cambridge University Press, 2009. An easily accessible text for students of geology. Discusses igneous rock formations of flood basalts and calderas. Covers processes and characteristics of igneous and metamorphic rock.

Putnam, William C. Geology. 2d ed. New York: Oxford University Press, 1971. A comprehensive and accessible classic text. Covers fundamental topics still considered relevant. Chapter 4, “Igneous Rocks and Igneous Processes,” uses a vivid description of the 1883 eruption of Krakatau as a way of introducing the formation processes of igneous rocks. Other famous, historic volcanic eruptions are also discussed. Describes rock classification and composition in detail. The chapter concludes with a list of references. Illustrated.

Ross, Pierre-Simon, et al. “Basaltic to Andesitic Volcaniclastic Rocks in the Blake River Group, Abitibi Greenstone Belt: 2. Origin, Geochemistry, and Geochronology.” Canadian Journal of Earth Sciences 48 (2011): 757-777. Discusses the rock formations in the Archean Blake River Group. Physical characteristics, age relationships, and geochemistry data are measured and interpreted to determine the origins of these rock formations. The authors hypothesize the volcanic processes that resulted in these formations. Best suited for graduate students or researchers studying volcanism, petrology, or mineralogy.

Science Mate: Plate Tectonic Cycle. Fremont, Calif.: Math/Science Nucleus, 1990. Part of the Integrating Science, Math, and Technology series of manuals, intended to help teachers explain basic scientific concepts to elementary students. Provides a basic understanding of plate tectonics, earthquakes, volcanoes, and general geology for the student with no background in Earth sciences.

Sobolev, A. V., et al. “The Amount of Recycled Crust in Sources of Mantle-Derived Melts.” Science 316 (2007): 412-417. Discusses basaltic crust mixing with peridotitic mantle due to plate tectonics processes. Examines the chemical composition of melts.

Sutherland, Lin. The Volcanic Earth: Volcanoes and Plate Tectonics, Past, Present, and Future. Sydney, Australia: University of New South Wales Press, 1995. Provides an easily understood overview of volcanic and tectonic processes, including the role of igneous rocks. Includes color maps and illustrations, as well as a bibliography.

Tarbuck, Edward J., Frederick K. Lutgens, and Dennis Tasa. Earth: An Introduction to Physical Geology. 10th ed. Upper Saddle River, N.J.: Prentice Hall, 2010. Aimed at the reader with little or no college-level science experience. Includes a chapter devoted to igneous rocks and their textures, mineral compositions, classification, and formation. Illustrated with photographs and diagrams. Includes review questions and list of key terms.