Anyone who studies Earth soon learns that our planet is a dynamic body with many separate but interacting parts, or spheres. The hydrosphere, atmosphere, biosphere, and geosphere and all of their components can be studied separately. However, the parts are not isolated. Each is related in some way to the others, producing a complex and continuously interacting whole that we call the Earth system.

Earth’s Spheres

The images in the figure below ▼, taken in the mid- and late-twentieth century, are considered to be classics because they allowed humanity to see Earth differently than it had ever before. The accompanying video commemorates the 45th anniversary of Apollo 8’s historic flight by re-creating the moment when the crew first saw and photographed Earth rising from behind the Moon. These early views profoundly altered our conceptualizations of Earth and remain powerful images decades after they were first viewed. Seen from space, Earth is breathtaking in its beauty and startling in its solitude. The photos remind us that our planetary home is, after all, a planet—small, self-contained, and in some ways even fragile.

As we look closely at our planet from space, it becomes apparent that Earth is much more than rock and soil. In fact, the most conspicuous features of Earth in part A ▲ are swirling clouds suspended above the surface of the vast global ocean. These features emphasize the importance of water on our planet.

The closer view of Earth from space shown in part B ▲helps us appreciate why the physical environment is traditionally divided into three major parts: the water portion of our planet, the hydrosphere; Earth’s gaseous envelope, the atmosphere; and, of course, the solid Earth, or geosphere. It needs to be emphasized that our environment is highly integrated and not dominated by rock, water, or air alone. Rather, it is characterized by continuous interactions as air comes in contact with rock, rock with water, and water with air. Moreover, the biosphere, which is the totality of all life on our planet, interacts with each of the three physical realms and is an equally integral part of the planet. Thus, Earth can be thought of as consisting of four major spheres: the hydrosphere, atmosphere, geosphere, and biosphere. All four spheres are represented in the chapter-opening photo.

The interactions among Earth’s spheres are incalculable. The figure below ▼ provides one easy-to-visualize example. The shoreline is an obvious meeting place for rock, water, and air. In this scene, ocean waves created by the drag of air moving across the water are breaking against the rocky shore.

Hydrosphere

Earth is sometimes called the blue planet. Water, more than anything else, makes Earth unique. The hydrosphere is a dynamic mass of water that is continually on the move, evaporating from the oceans to the atmosphere, precipitating to the land, and running back to the ocean again. The global ocean is certainly the most prominent feature of the hydrosphere, blanketing nearly 71 percent of Earth’s surface to an average depth of about 3682 meters (12,080 feet, or approaching 2.5 miles). Impressively, the global ocean accounts for about 97 percent of Earth’s water ▼. However, the hydrosphere also includes the freshwater found underground and in streams, lakes, and glaciers. Moreover, water is an important component of all living things.

Even though freshwater constitutes only a small fraction of Earth’s hydrosphere, it plays an outsized role in Earth’s external processes. Streams, glaciers, and groundwater sculpt many of our planet’s varied landforms, and freshwater is vital for life on land.

Atmosphere

Earth is surrounded by a life-giving gaseous envelope called the atmosphere ▼. When we watch a high-flying jet plane cross the sky, it seems that the atmosphere extends upward for a great distance. However, when compared to the thickness (radius) of the solid Earth (about 6400 kilometers [4000 miles]), the atmosphere is a very shallow layer. Despite its modest dimensions, this thin blanket of air is an integral part of the planet. It not only provides the air we breathe but also protects us from the Sun’s intense heat and dangerous ultraviolet radiation. The energy exchanges that continually occur between the atmosphere and Earth’s surface and between the atmosphere and space produce the effects we call weather and climate. Climate has a strong influence on the nature and intensity of Earth’s external processes. When climate changes, these processes respond.

If, like the Moon, Earth had no atmosphere, our planet would be lifeless, and many of the processes and interactions that make the surface such a dynamic place could not operate. Without phenomena of the atmosphere resulting in weathering and erosion of the geosphere, the face of our planet might more closely resemble the lunar surface, which has not changed appreciably in nearly 3 billion years.

Biosphere

The biosphere includes all life on Earth ▼. Ocean life is concentrated in the sunlit upper waters. Most life on land is also concentrated near the surface, with tree roots and burrowing animals reaching a few meters underground and flying insects and birds reaching a kilometer or so into the atmosphere. A surprising variety of life-forms are also adapted to extreme environments. For example, on the ocean floor, where pressures are extreme and no light penetrates, there are places where vents spew hot, mineral-rich fluids that support communities of exotic life-forms, as in part B of the figure below ▼. On land, some bacteria thrive in rocks as deep as 4 kilometers (2.5 miles) and in boiling hot springs. Moreover, air currents can carry microorganisms many kilometers into the atmosphere. But even when we consider these extremes, life still must be thought of as being confined to a narrow band very near Earth’s surface.

Geosphere

Lying beneath the atmosphere and the ocean is the solid Earth or geosphere, extending from the surface to the center of the planet at a depth of nearly 6400 kilometers (4000 miles)—by far the largest of Earth’s spheres. Much of our study of the solid Earth focuses on the more accessible surface and near-surface features, but it is worth noting that many of these features are linked to the dynamic behavior of Earth’s interior. Earth’s interior is layered. As the figure below ▼ shows, we can think of this layering as being due to differences in both chemical composition and physical properties. On the basis of chemical composition, Earth has three layers: a dense inner sphere called the core; the less dense mantle; and the crust, which is the light and very thin outer skin of Earth. The crust is not a layer of uniform thickness. It is thinnest beneath the oceans and thickest where continents exist. Although the crust may seem insignificant when compared with the other layers of the geosphere, which are much thicker, it was created by the same general processes that formed Earth’s present structure. Thus, the crust is important in understanding the history and nature of our planet.

The layering of Earth in terms of physical properties reflects the way Earth’s materials behave when various forces and stresses are applied. The term lithosphere refers to the rigid outer layer of Earth that includes the crust and uppermost mantle. Beneath the rigid rocks that compose the lithosphere, the rocks of the asthenosphere are weak and able to slowly flow in response to the uneven distribution of heat deep within Earth.

The two principal divisions of Earth’s surface are the continents and the ocean basins. The most obvious difference between these two provinces is their relative vertical levels. For the continents —the seven largest land masses on the globe, including North America, South America, Europe, Asia, Africa, Australia, and Antarctica—the average elevation above sea level is about 840 meters (2750 feet), whereas the average depth of the ocean basins is about 3700 meters (12,000 feet). Thus, the continents stand on average 4540 meters (about 4.5 kilometers, or 2.8 miles) above the level of the ocean floor.

Soil, the thin veneer of material at Earth’s surface that supports the growth of plants, may be thought of as part of all four spheres. The solid portion is a mixture of weathered rock debris (geosphere) and organic matter from decayed plant and animal life (biosphere). The decomposed and disintegrated rock debris is the product of weathering processes that require air (atmosphere) and water (hydrosphere). Air and water also occupy the open spaces between solid particles in soil.

Earth System Science

A simple example of the interactions among different parts of the Earth system occurs every winter, as moisture evaporates from the Pacific Ocean and subsequently falls as rain in the mountains of Washington, Oregon, and California, triggering destructive debris flows. The processes that move water from the hydrosphere to the atmosphere and then to the solid Earth have a profound impact on the plants and animals (including humans) that inhabit the affected regions ▼.

Scientists have recognized that in order to more fully understand our planet, they must learn how its individual components (land, water, air, and life-forms) are interconnected. This endeavor, called Earth system science, aims to study Earth as a system composed of numerous interacting parts, or subsystems. Rather than look through the limited lens of only one of the traditional sciences—geology, atmospheric science, chemistry, biology, and so on—Earth system science attempts to integrate the knowledge of many academic fields. Using an interdisciplinary approach, those engaged in Earth system science attempt to achieve the level of understanding necessary to comprehend and solve many of our global environmental problems.

A system is a group of interacting, or interdependent parts that form a complex whole. Most of us hear and use the term system frequently. We may service our car’s cooling system, make use of the city’s transportation system, and be a participant in the political system. A news report might inform us of an approaching weather system. Further, we know that Earth is just a small part of a larger system known as the solar system, which in turn is a subsystem of an even larger system called the Milky Way Galaxy.

The Earth System

The Earth system has a nearly endless array of subsystems in which matter is recycled over and over. One familiar loop, or subsystem, is the hydrologic cycle. It represents the unending circulation of Earth’s water among the hydrosphere, atmosphere, biosphere, and geosphere ▼. Water enters the atmosphere through evaporation from Earth’s surface and transpiration from plants. Water vapor condenses in the atmosphere to form clouds, which, in turn, produce precipitation that falls back to Earth’s surface. Some of the rain that falls onto the land sinks in and then is taken up by plants or becomes groundwater, and some flows across the surface toward the ocean.

Viewed over long time spans, the rocks of the geosphere are constantly forming, changing, and re-forming. The loop that involves the processes by which one rock changes to another is called the rock cycle. The cycles of the Earth system are not independent of one another. To the contrary, there are many places where the cycles come in contact and interact.

The Parts Are Linked

The parts of the Earth system are linked so that a change in one part can produce changes in any or all of the other parts. For example, when a volcano erupts, lava from Earth’s interior may flow out at the surface and block a nearby valley. This new obstruction influences the region’s drainage system by creating a lake or causing streams to change course. The large quantities of volcanic ash and gases that can be emitted during an eruption might be blown high into the atmosphere and influence the amount of solar energy that can reach Earth’s surface. The result could be a drop in air temperatures over the entire hemisphere.

Where the surface is covered by lava flows or a thick layer of volcanic ash, existing soils are buried. This causes the soil-forming processes to begin anew to transform the new surface material into soil ▼. The soil that eventually forms will reflect the interactions among many parts of the Earth system—the volcanic parent material, the climate, and the impact of biological activity. Of course, there would also be significant changes in the biosphere. Some organisms and their habitats would be eliminated by the lava and ash, and new settings for life, such as a lake formed by a lava dam, would be created. The potential climate change could also impact sensitive life-forms.

Time and Space Scales

The Earth system is characterized by processes that vary on spatial scales from fractions of millimeters to thousands of kilometers. Time scales for Earth’s processes range from milliseconds to billions of years. As we learn about Earth, it becomes increasingly clear that despite significant separations in distance or time, many processes are connected, and a change in one component can influence the entire system.

Energy for the Earth System

The Earth system is powered by energy from two sources. The Sun drives external processes that occur in the atmosphere, in the hydrosphere, and at Earth’s surface. Energy from the Sun drives weather and climate, ocean circulation, and erosional processes. Earth’s interior is the second source of energy. The internal processes that produce volcanoes, earthquakes, and mountains are powered by heat remaining from when our planet formed and heat that is continuously generated by radioactive decay.

People and the Earth System

Humans are part of the Earth system, a system in which the living and nonliving components are entwined and interconnected. Therefore, our actions produce changes in all the other parts. When we burn gasoline and coal, dispose of our wastes, and clear the land, we cause other parts of the system to respond, often in unforeseen ways. Throughout this book, you will learn about many of Earth’s subsystems, including the hydrologic system, the tectonic (mountain-building) system, the rock cycle, and the climate system. Remember that these components, and we humans, are all part of the complex interacting whole we call the Earth system.

The organization of this text involves traditional groupings of chapters that focus on closely related topics. Nevertheless, the theme of Earth as a system keeps recurring through all major units of this text. It is a thread that weaves through the chapters and helps tie them together.

• Earth’s physical environment is traditionally divided into three major parts: the solid Earth, called the geosphere; the water portion of our planet, called the hydrosphere; and Earth’s gaseous envelope, called the atmosphere.

• A fourth Earth sphere, called the biosphere, is the totality of life on Earth. It is concentrated in a relatively thin zone that extends a few kilometers into the hydrosphere and geosphere and a few kilometers up into the atmosphere.

• Of all the water on Earth, about 97 percent is in the oceans, which cover nearly 71 percent of the planet’s surface.

• Although each of Earth’s four spheres can be studied separately, they are all related in a complex and continuously interacting whole that is called 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.

• 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.

• asthenosphere: A subdivision of the mantle situated below the lithosphere. This zone of weak material exists below a depth of about 100 kilometers (60 miles) and in some regions extends as deep as 700 kilometers (430 miles). The rock within this zone is easily deformed.

• atmosphere: The gaseous portion of a planet; the planet’s envelope of air. One of the traditional subdivisions of Earth’s physical environment.

• biosphere: The totality of life on Earth; the parts of the solid Earth, hydrosphere, and atmosphere in which living organisms can be found. One of the traditional subdivisions of the Earth system.

• continents: Large, continuous areas of land that include the adjacent continental shelf and islands that are structurally connected to the mainland.

• core: The innermost layer of Earth, located beneath the mantle. The core is divided into an outer core and an inner core.

• crust: The very thin, rocky, outermost layer of Earth.

• Earth system: Earth viewed as a dynamic system of interacting parts and processes, including the geosphere, hydrosphere, biosphere, and atmosphere.

• Earth system science: An interdisciplinary study that seeks to examine Earth as a system composed of numerous interacting parts or subsystems.

• geosphere: The solid Earth, the largest of Earth’s four major spheres.

• hydrosphere: The portion of our planet that includes water in its liquid, gaseous, and solid forms; one of the traditional subdivisions of Earth’s physical environment.

• lithosphere: The rigid outer layer of Earth, including the crust and upper mantle.

• mantle: The 2900-kilometer- (1800-mile-) thick layer of Earth located below the crust and above the core.

• system: Any size group of interacting parts that form a complex whole.