The purpose of geology is to understand the earth and all its aspects—its origin, its varied features, the composition and structure of the material that composes it, and its impact on the life upon it. It is concerned with all the forces that have acted upon the earth and the effects of those forces. It attempts to reconstruct the history of the earth, particularly as it is recorded in the rocks of the outer crust.

What is the earth made of? How has the earth changed over time? What causes the earth to change? How can we predict changes in the earth? How can we use what we know about the physical environment in making decisions?

Geologists primarily study rocks and derivative materials that make up the earth’s crust, as well as information about physical forces that affect the earth’s development. For example, they use information about the earth’s water in relation to geological processes. They use maps of the earth, as well as knowledge from geodesy (the branch of applied math concerned with measuring, or determining the shape of the earth, or with locating points on the earth). Geologists gather information about landforms and other surface features of the earth, as well as information
about minerals within the earth. Geologists study the geomagma field, paleomagnetism in rocks and soils, heat flow phenomena within the earth, chemicals within the earth, sediments, oil, coal, fossils, and geothermal energy.

Geologists make judgments about the physical properties of the earth and its internal composition. They make judgments about the causes of change in the earth, the chemical makeup of the earth, the origin, structure, history and composition of rocks and minerals. They make judgments about prehistoric life, about how the earth is altered due to external forces, about how best to utilize and exploit the earth’s natural resources, about how to design human-made structures given the earth’s processes and makeup, and about problems caused by human use and exploitation of the physical environment.

Key geological concepts include: the geological time scale (obtained from four major rock types, each produced by a different kind of crustal activity; endogenetic processes (processes originating within the earth) exogenetic processes (those that originate externally); and the plate tectonics hypothesis; (that the earth’s crust is divided into a number of plates that move about, collide, and separate over geological time). In addition, geologists use principles from other sciences to understand the earth. For example, biology is needed to explain life records of the past; the remote history of the earth’s beginnings are interwoven with astronomy; theories in physics must be used to explain tides, earth heat, interior rigidity and many other phenomena; chemistry is needed to analyze the materials within the earth; theories in meteorology and climatology are needed to explain how external forces impact the earth’s surface.

That geology is interwoven with many other branches of science and therefore, that geologists must rely on theories and laws from other scientific branches to think geologically; that the history of the earth is best interpreted in terms of what is known about geological processes at work in the present, rather than supposed processes in the past (principle of uniformitarianism); that the structures and forces within the earth, as well as those affecting the earth, are interrelated and must be understood in relationship to one another; that the physical structures of the earth and the ways in which they function are predictable, though there is much about the earth that we cannot yet predict.

There is almost unlimited practical value in applying geological knowledge. By studying geology, for example, there are implications for determining and predicting water, coal, and oil supply, stone quarrying, and locating ore. Geology aids in identifying geologically stable environments for human constructions. It can also help in forecasting natural hazards associated with geodynamic forces including volcanoes and earthquakes. Geologists can make a significant contribution in illuminating the effects of human exploitation on the earth’s surface and resources.

Geologists see the earth as a physical structure containing predictable structures, influences and forces which, when systematically studied, can improve the quality of life.