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The materials of Earth’s planetary system can be separated into spheres, or parts. These five major spheres of Earth’s system are illustrated in Figure 4-1:

 Atmosphere: The atmosphere is a layer of gas that surrounds the entire planet. It serves the important role of protecting everything on Earth from being destroyed by the heat and radiation from the sun and makes life possible on Earth. Within the atmosphere, gases interact with water, forming weather systems that circulate air and clouds around the globe.

 Hydrosphere: The hydrosphere includes all the water on Earth. The hydrologic cycle is the rotation of water through the hydrosphere: flowing as liquid (streams and rivers), evaporating into the atmosphere as gas (clouds), and falling to the surface as rain or snow.

 Cryosphere: The cryosphere is composed of all the solid water, or ice, found on Earth’s surface. While closely tied to the hydrologic system, the cryosphere can be examined separately because of how massive amounts of surface ice affect the weather and climate systems.

 Biosphere: All the organic materials on Earth — both living and dead organisms — are part of the biosphere.

 Geosphere: The solid, rocky layers of the Earth, from the outermost crust to the very center, compose the planet’s geosphere. Within the geosphere, scientists have further divided the layers of rock material, which I describe in the next section of this chapter.

FIGURE 4-1: The five major spheres of Earth’s planetary system.

The earth’s spheres are connected to one another through a series of interactions. For example, rainfall from the hydrosphere causes movement of surface materials in the geosphere (a process called erosion, which I describe in Part 4). Rainfall also provides water for plants to grow in the biosphere. The interactions among spheres can be studied as subsystems of the earth. An example of a subsystem is the climate system, which is influenced by the interaction of atmosphere, biosphere, hydrosphere, and even geosphere.

Every system needs energy to fuel its processes. Systems on Earth’s surface are fueled by heat energy from the sun, whereas other systems (particularly those in the geosphere) are fueled by heat energy that originates deep within the earth.

Because Earth is one giant system, geologists study not only the rock materials on Earth but also how rocks in the geosphere interact with all the other spheres.