Everything in Earth's system can be placed into one of four major subsystems: land, water, living things, or air. These four subsystems are called "spheres." Specifically, they are the "geosphere" (land), "hydrosphere" (water), "biosphere" (living things), and "atmosphere" (air). Each of these four spheres can be further divided into sub-spheres.4
The Geosphere contains all of the cold, hard solid rocks of the planet's crust (closest to the surface), the semi-solid rocks underneath the crust, and the liquid and solid iron near the center of the Earth. The surface of the geosphere is very uneven. High mountain ranges like the Rockies and Andes, huge plains or flat areas like those in Texas, Iowa, and Brazil and deep valleys along the ocean floor. The solid, semi-solid, and liquid parts of the geosphere form layers that are physically and chemically different. If someone were to cut through Earth to its center, these layers would be revealed like the layers of an onion. The outermost layer of the geosphere consists of loose soil rich in nutrients, oxygen, and silicon. Beneath that layer lies a thin, solid crust of oxygen and silicon. Next is a thick, semi-solid mantle of oxygen, silicon, iron, and magnesium. Below that is a liquid outer core of nickel and iron. At the center of Earth is a solid inner core of nickel and iron.
The Hydrosphere contains all the solid, liquid, and gaseous water of the planet (some scientists distinguish the solid ice portion of the hydrosphere as its own “cryosphere”). It ranges from 10 to 20 kilometers in thickness. The hydrosphere extends from Earth's surface downward several kilometers into the geosphere, and upward about 12 kilometers into the atmosphere. A small portion of the water in the hydrosphere is fresh (non-salty). This water flows as precipitation from the atmosphere down to Earth's surface, as rivers and streams along Earth's surface, and as groundwater beneath Earth's surface. The majority of Earth's fresh water, however, is actually frozen, locked within vast ice caps covering Antarctica and Greenland as well as mountain glaciers.
Ninety-seven percent of Earth's water is salty. The salty water collects in the oceans. Seawater near the poles is very cold while surface water near the equator is very warm. The differences in temperature cause water to change physical states. Extremely low temperatures like those found at the poles cause water to freeze into a solid such as a polar icecap, a glacier, or an iceberg. High temperatures like those found at the equator cause water to evaporate readily into a gas.
The Biosphere contains all the planet's living things. This sphere includes all of the microorganisms, plants, and animals of Earth. Within the biosphere (which also contains a non-negligible reservoir of water, thus overlapping the hydrosphere), living things form ecological communities based on the physical surroundings of an area. These communities are referred to as biomes. Deserts, grasslands, and tropical rainforests are three of the many types of biomes that exist within the biosphere.
The Atmosphere contains all the air in Earth's system. It extends from less than 1m below the planet's surface to more than 10 km above the planet's surface. The upper portion of the atmosphere protects the organisms of the biosphere from the sun's ultraviolet radiation. It also absorbs and emits heat. When air temperature in the lower portion of this sphere changes, weather occurs. As the air in the lower atmosphere is heated or cooled, it moves around the planet. The result can be as simple as a breeze or as complex as a tornado.5
Figure 1: The Earth’s Four Spheres 6
Interactions Between the Four Spheres
Although the four systems have individual identities, important interactions occur between them. Environmental scientists study the effects of events in one sphere on the other spheres. Ten possible types of interactions could occur within the Earth system. Some of these interactions involve all four of Earth's spheres.7 For example, a volcanic eruption in the geosphere may cause profound direct and indirect effects on the hydrosphere, atmosphere, and biosphere as follows. On May 18, 1980, Mount Saint Helens, in the state of Washington, erupted. This event altered the surrounding environment and provided scientists an opportunity to study the effects of volcanic eruptions on the lithosphere, hydrosphere, atmosphere, and biosphere. Such studies are important because volcanic eruptions will continue to occur, and will have an increasing impact on humans as people continue to settle lands closer to dormant volcanoes.
The ten types of interactions that can occur within Earth spheres often do so as a series of chain reactions. This means one interaction leads to another interaction, which leads to yet another interaction--it is a ripple effect through the earth's spheres. Volcanoes (an event in the geosphere) release a large amount of particulate matter into the atmosphere. These particles serve as nuclei for the formation of water droplets (hydrosphere). Rainfall (hydrosphere) often increases following an eruption, stimulating plant growth (biosphere). Particulate matter in the air (atmosphere) falls out, initially smothering plants (biosphere), but ultimately enriching the soil (geosphere) and thereby stimulating plant growth (biosphere). Volcanoes (events in the lithosphere) may release a substantial amount of hot lava (geosphere), which causes mountain glaciers (hydrosphere) to melt. Mudflows (geosphere) and flooding may occur downstream from volcanoes and may inundate streamside communities (biosphere).
Volcanoes (events of the geosphere) release a large amount of carbon dioxide (atmosphere), the raw material for sugar production in plants (biosphere). This may increase photosynthetic production and eventually increase the amount of biomass, which, after a very long time, forms coal and oil deposits (geosphere).
Volcanoes (geosphere) may also emit large quantities of sulfur dioxide (atmosphere). When atmospheric sulfur dioxide combines with water (hydrosphere), sulfuric and sulfurous acid form. Rain (hydrosphere) may bring these acids to the Earth, acidifying soils (geosphere), lakes and rivers (hydrosphere). Acidic water leaches nutrients from the soil (geosphere) into the water table (hydrosphere), making the soil less fertile for plants (biosphere), and the subterranean water supply (hydrosphere) less potable for humans (biosphere). Acid rain falling on lakes and streams reduces the pH of the water (hydrosphere), which may result in a decrease in phytoplankton and zooplankton growth (biosphere). If photosynthesis is reduced, atmospheric concentrations of carbon dioxide can build up and stimulate global warming (atmosphere) which may contribute to increased melting of glaciers (hydrosphere).8
