Geology is a general term for the group of sciences that study the solid Earth, its materials, and processes. The numerous subdisciplines of geology may be divided into two major categories: physical geology and historical geology. Physical geology, which includes such sciences as geophysics, geochemistry, mineralogy, and hydrogeology studies the matter of which the Earth is made and the interaction of that material with water, air, and organisms. Historical geology studies fossils and rock strata to understand the history of life forms and of the Earth itself.
The word geology comes from the Greek words ge , "Earth," and logos, "reason". The Greeks were probably not the first people to develop theories about the composition and changing form of the Earth, but they did provide us with some of the oldest written records of these endeavors. Such philosophers as Anaximander (c. 610-547 B.C.), Empedocles (c. 490-430 B.C.), and Theophrastus (c. 372-287 B.C.) believed that fossils were evidence of previous life forms, that water can shape rocks, that the Earth was a ball of rock with a hot, liquid center, and that volcanic and seismic activity can cause the Earth's surface to rise and fall. However, much of the Greeks' speculation about the Earth was based on superstition, myth, and legend rather than observations of fact. The Romans improved somewhat on Greek ideas about the Earth by gathering accurate information on mineral ores, mining, earthquakes, and volcanic activity.
During the Renaissance, such scientists as Georgius Agricola (1494-1555) and Nicolaus Steno (1638-1636) made important contributions to the study of the Earth, providing the foundation for the modern science of geology. In the sixteenth century, Agricola published major works on metallurgy, fossils, minerals, and mining that set forth ideas still considered important. In 1669, Steno established the law of superposition, which states that the oldest layers of rock strata are found at the bottom of sedimentary rock sequences, while the youngest strata of rock appear at the top of such sequences. This concept allows geologists to determine such things as the relative age of fossils and to understand the series of processes or environments responsible for a rock deposit.
In the late eighteenth century, James Hutton, a Scottish physician, developed a theory of Earth development and evolution that today is known as the principle of uniformitarianism. According to this theory, the Earth's present landscapes came about through natural processes acting over long periods of time to enact gradual changes to the landscape, and would continue to change indefinitely in the same gradual manner. Hutton's theory was monumental as it forced geologists to examine their preconceptions about the age of the Earth, and the significance of everyday Earth processes. For his contributions to early geologic thought, Hutton is often considered the father of geology.
During the last two centuries many major theories have been developed that contributed significantly to the theory and practice of geology. However, none is probably more important or far-reaching than the theory of plate tectonics. This theory states that the rocky surface of the Earth is composed of half a dozen or major blocks, plates, and about as many minor ones. Through their interactions, these plates are responsible for most of the major features on Earth's surface, including mountains, deep ocean trenches, volcanic islands, and mid-oceanic ridges and rifts.
In addition to producing many large-scale features, plate tectonics has explained many previously inexplicable phenomena, and allowed scientists to see relationships between activities that they had thought were unrelated. Prior to the development of the theory of plate tectonics, geology lacked a conceptual framework or foundation upon which new ideas could build. Plate tectonics has stimulated tremendous amounts of research in traditional areas as well as provoking totally new ones. As a result, plate tectonics is considered by many to be the dominant unifying concept in geology.
Geology has diversified into a number of cross-disciplinary sciences that are not concerned solely with the study of rocks. Biologists and geologists, for example, study fossils together to understand the changing nature of life on Earth. Historians and geologists may cooperate to date items found in archeological digs. Collaboration between physicists and geologists has produced a variety of methods to determine the age of Earth materials, and has contributed to a greater understanding of such phenomena as earthquakes and volcanism. Engineering geology has important commercial applications, including designing more earthquake resistant structures and assessing land-use with respect to flooding, volcanic and earthquake hazards. Economic geologists explore for, and assess deposits of petroleum and coal, industrial minerals, and metallic ores. Few aspects of life today are untouched by the activities of geologists in their quest to understand the Earth.
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