Summary

The following statements summarize and describe many of the key terms and concepts presented in this chapter.

• Earth science is the name for all the sciences that collectively seek to understand Earth and its neighbors in space. It includes geology, oceanography, meteorology, and astronomy. Geology is traditionally divided into two broad areas—physical and historical.

• Environment refers to everything that surrounds and influences an organism. These influences can be biological, social, or physical. When applied to Earth science today, the term environmental is usually reserved for those aspects that focus on the relationships between people and the natural environment.

• Resources are an important environmental concern. The two broad categories of resources are (1) renewable, which means that they can be replenished over relatively short time spans, and (2) nonrenewable. As population grows, the demand for resources expands as well.

• Environmental problems can be local, regional, or global. Human-induced problems include urban air pollution, acid rain, ozone depletion, and global warming. Natural hazards include earthquakes, landslides, floods, and hurricanes. As world population grows, pressures on the environment also increase.

• 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 through observation and careful measurement 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 validate 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.

• One of the challenges for those who study Earth is the great variety of space and time scales. The geologic time scale subdivides the 4.5 billion years of Earth history into various units.

 

• The nebular hypothesis describes the formation of the solar system. The planets and Sun began forming about 5 billion years ago from a large cloud of dust and gases. As the cloud contracted, it began to rotate and assume a disk shape. Material that was gravitationally pulled toward the center became the protosun. Within the rotating disk, small centers, called protoplanets, swept up more and more of the cloud's debris. Because of their high temperatures and weak gravitational fields, the inner planets were unable to accumulate and retain many of the lighter components. Because of the very cold temperatures existing far from the Sun, the large outer planets consist of huge amounts of lighter materials. These gaseous substances account for the comparatively large sizes and low densities of the outer planets.

• Earth's physical environment is traditionally divided into three major parts: the solid Earth or geosphere; 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.

• 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 lower mantle, where rocks are very hot and capable of very gradual flow, (4) the liquid outer core, where Earth's magnetic field is generated, and (5) the solid inner core.

• Two principal divisions of Earth's surface are the continents and ocean basins. A significant difference is their relative levels. The elevation differences between continents and ocean basins is primarily the result of differences in their respective densities and thicknesses.

• The largest features of the continents can be divided into two categories: mountain belts and the stable interior. The ocean floor is divided into three major topographic units: continental margins, deep-ocean basins, and oceanic ridges.

• 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. Earth system science uses an interdisciplinary approach to integrate the knowledge of several academic fields in the study of our planet and its global environmental problems.

• A system is a group of interacting parts that form a complex whole. Closed systems are those in which energy moves freely in and out, but matter does not enter or leave the system. In an open system, both energy and matter flow into and out of the system.

• 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, which powers the internal processes that produce volcanoes, earthquakes, and mountains.