Geology, as the attached eNotes essay points out, is the study of the Earth. Scientists – geologists – study the Earth’s surface and, to the extent possible through drilling, as much of the Earth’s inner layers as they can. Rocks provide clues to the Earth’s origins and studying the layers of rocks is how geologists determine their age and, by extension, the age of the planet. Through drilling and digging, geologists, over time, have discovered different forms of rock that help determine the natural processes that have transpired in a particular location over millions of years.
Rocks, however, have experienced dynamic existences. The rock cycle, first postulated by 18th Century geologist James Hutton, is the pattern that geologists have discovered that helps explain the Earth’s history by illuminating the “life” of rocks. The rock cycle is usually explained in the form of a diagram that depicts each phase of a rock’s existence. The three main rock categories – igneous, metamorphic and sedimentary – represent different natural phenomena to which rock has been subjected. The Earth’s surface being a living things in terms of plate tectonics, the enormous forces of water and wind, and the catastrophic events that have occurred throughout the planet’s history, the various types of rocks provide information on how a particular region evolved over millions of years. Igneous rocks are the solidified form of once-liquid magma. When exposed to incomprehensible levels of heat and pressure, rocks become metamorphic, and once exposed to the Earth’s atmosphere and subjected to wind, rain, currents, etc., they become sedimentary. The rock cycle, then, depicts these transitions from one type of rock to another, with naturally-occurring phenomena such as volcanic and tectonic activity inserted into the diagram to reflect their role in rock formation.