• Geology means "the study of Earth." The two broad areas of the science of geology are (1) physical geology, which examines the materials composing Earth and the processes that operate beneath and upon its surface; and (2) historical geology, which seeks to understand the origin of Earth and its development through time.

• During the seventeenth and eighteenth centuries, catastrophism influenced the formulation of explanations about Earth. Catastrophism states that Earth's landscapes have been developed over short time spans primarily as a result of great catastrophes. By contrast, uniformitarianism, one of the fundamental principles of modern geology advanced by James Hutton in the late 1700s, states that the physical, chemical, and biological laws that operate today have also operated in the geologic past. The idea is often summarized as "the present is the key to the past." Hutton argued that processes that appear to be slow-acting could, over long spans of time, produce effects that were just as great as those resulting from sudden catastrophic events.

  

• Using the principles of relative dating, the placing of events in their proper sequence or order without knowing their age in years, scientists developed a geologic time scale during the nineteenth century. Relative dates can be established by applying such principles as the law of superposition and the principle of fossil succession.

• All science is based on the assumption that the natural world behaves in a consistent and predictable manner. The process by which scientists gather facts and formulate scientific hypotheses and theories is called the scientific method. To determine what is occurring in the natural world, scientists often (1) collect facts, (2) develop a scientific hypothesis, (3) construct experiments to test the hypothesis, and (4) accept, modify, or reject the hypothesis on the basis of extensive testing. Other discoveries represent purely theoretical ideas that have stood up to extensive examination. Still other scientific advancements have been made when a totally unexpected happening occurred during an experiment.

  

• Earth's physical environment is traditionally divided into three major parts: the solid Earth; the water portion of our planet, the hydrosphere; and Earth's gaseous envelope, the atmosphere. In addition, the biosphere, the totality of life on Earth, interacts with each of the three physical realms and is an equally integral part of Earth.

• Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that we call the Earth system. Changing one part of the Earth system can produce changes in any or all of the other parts.

• The two sources of energy that power the Earth system are (1) the Sun, which drives the external processes that occur in the atmosphere, hydrosphere, and at Earth’s surface, and (2) heat from Earth’s interior that powers the internal processes that produce volcanoes, earthquakes, and mountains.

  

• The rock cycle is one of many cycles or loops of the Earth system in which matter is recycled. The rock cycle is a means of viewing many of the interrelationships of geology. It illustrates the origin of the three basic rock groups and the role of various geologic processes in transforming one rock type into another.

• Earth’s internal structure is divided into layers based on differences in chemical composition and on the basis of changes in physical properties. Compositionally, Earth is divided into a thin outer crust, a solid rocky mantle, and a dense core. Based on physical properties, the layers of Earth are (1) the lithosphere, the cool, rigid outermost layer that averages about 100 kilometers thick, (2) the asthenosphere, a relatively weak layer located in the mantle beneath the lithosphere, (3) the more rigid mesosphere, where rocks are very hot and capable of gradual flow, (4) the liquid outer core where Earth’s magnetic field is generated, and (5) the solid inner core.

  

• The theory of plate tectonics provides a comprehensive model of Earth's internal workings. It holds that Earth's rigid outer lithosphere consists of several segments called plates that are slowly and continually in motion relative to each other. Most earthquakes, volcanic activity, and mountain building are associated with the movements of these plates.

• The three distinct types of plate boundaries are (1) divergent boundaries—where plates move apart; (2) convergent boundaries—where plates move together, causing one to go beneath the other, or where plates collide, which occurs when the leading edges are made of continental crust; and (3) transform fault boundaries—where plates slide past each other.

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