Geology is the study of Earth. Modern geology includes studies in seismology (earthquake studies), volcanology, energy resources exploration and development, tectonics (structural and mountain building studies), hydrology and hydrogeology (water-resources studies), geologic mapping, economic geology (e.g., mining), paleontology (ancient life studies), soil science, historical geology and stratigraphy, geological archaeology, glaciology, modern and ancient climate and ocean studies, atmospheric sciences, planetary geology, engineering geology, and many other subfields.
Geologists study mountains, valleys, plains, sea floors, minerals, rocks, fossils, and the processes that create and destroy each of these. Geology consists of two broad categories of study. Physical geology studies the Earth's materials (erosion, volcanism, sediment deposition) that create and destroy the materials and land-forms. Historical geology explores the development of life by studying fossils (petrified remains of ancient life) and the changes in land (for example, distribution and latitude) via rocks. But the two categories overlap in their coverage: for example, to examine a fossil without also examining the rock that surrounds it tells only part of the preserved organism's history.
Physical geology further divides into more specific branches, each of which deals with its own part of Earth's materials, landforms, and/or processes. Mineralogy and petrology investigate the composition and origin of minerals and rocks, respectively. Sedimentologists look at sedimentary rocks—products of the accumulation of rock fragments and other loose Earth materials—to determine how and where they formed. Volcanologists tread on live, dormant, and extinct volcanoes checking lava, rocks and gases. Seismologists set up instruments to monitor and to predict earthquakes and volcanic eruptions. Structural geologists study the ways rock layers bend and break. Plate tectonics unifies most aspects of Physical Geology by demonstrating how and why plates (sections of Earth's outer crust) collide and separate and how that movement influences the entire spectrum of geologic events and products.
Fossils are used in Historical geology as evidence of the evolution of life on Earth. Plate tectonics adds to the story with details of the changing configuration of the continents and oceans. For years paleontologists observed that the older the rock layer, the more primitive the fossil organisms found therein, and from those observations developed evolutionary theory. Fossils not only relate evolution, but also speak of the environment in which the organism lived. Corals in rocks at the top of the Grand Canyon in Arizona, for example, show a shallow sea flooded the area around 290 million years ago. In addition, by determining the ages and types of rocks around the world, geologists piece together continental and oceanic history over the past few billions of years. For example, by matching fossil and tectonic evidence, geologists reconstructed the history and shape of the 200-300 million year-old supercontinent, Pangea (Pangaea).
Many other sciences also contribute to geology. The study of the chemistry of rocks, minerals, and volcanic gases is known as geochemistry. The physics of the Earth is known as geophysics. Paleobotanists study fossil plants. Paleozoologists reconstruct fossil animals. Paleoclimatologists reconstruct ancient climates.
Much of current geological research focuses on resource utilization. Environmental geologists attempt to minimize human impact on the Earth's resources and the impact of natural disasters on human kind. Hydrology and hydrogeology, two subdisciplines of environmental geology, deal specifically with water resources. Hydrologists study surface water whereas hydrogeologists study ground water. Both disciplines try to minimize the impact of pollution on these resources. Economic geologists focus on finding the minerals and fossil fuels (oil, natural gas, coal) needed to maintain or improve global standards of living. Extraterrestrial geology, a study in its infancy, involves surveying the materials and processes of other planets, trying to unlock the secrets of the Universe and even to locate useful mineral deposits.
Geologic employment has been traditionally dominated by the petroleum industry and related geologic service companies. In the modern world, this is no longer so. Mining and other economic geology occupations (e.g., prospecting and exploration), in former days plentiful, have also fallen away as major employers. Environmental geology, engineering geology, and ground water related jobs are more common employment opportunities today. As these fields are modern growth areas with vast potential, this trend will likely hold true well into the future. Many modern laws and regulations require that licensed, professional geologists supervise all or part of key tasks in certain areas of engineering geologic work and environmental work. It is common for professional geologists and professional engineers to work together on such projects, including construction site preparation, waste disposal, ground-water development, engineering planning, and highway construction. Many federal, state, and local agencies employ geologists, and there are geologists as researchers and teachers in most academic institutions of higher education.
Hancock P.L., and Skinner B.J., eds. The Oxford Companion to the Earth. New York: Oxford University Press, 2000.
Tarbuck, Edward D., Frederick K. Lutgens, and Tasa Dennis. Earth: An Introduction to Physical Geology. 7th ed. Upper Saddle River, NJ: Prentice Hall, 2002.
Winchester, Simon. The Map That Changed the World: William Smith and the Birth of Modern Geology. New York: Harper Collins, 2001.
Hellfrich, George, and Bernard Wood. "The Earth's Mantle." Nature. (August 2, 2001): 501–507.