We can look at Earth's surficial "layers" as being comprised of several major realms or "geospheres": the atmosphere, lithosphere, hydrosphere, biosphere and pedosphere. The reference to "sphere" applies because Earth is spherical, and hence layers around it form somewhat spherical shells at the outermost edges of the solid planet. The atmosphere, biosphere and lithosphere more completely encircle Earth, while the hydrosphere and pedosphere tend to be more discontinuous. Figure 1.01 schematically illustrates some of the processes operating between these realms. All of the realms are interconnected, with energy and materials flowing both ways between any two "spheres." Within each shell or geosphere, energy and materials are also transported from one location to another.
Though in the schematic the geospheres appear as separate isolated entities, they reside closely together at their boundaries, which overlap and blend, and the biosphere is actually contained on and within the other domains. For example, the atmosphere mixes down into the other spheres and releases precipitates; the hydrosphere overlaps onto land surfaces and water vapor rises up into the atmosphere; the biosphere reaches up into the atmosphere, down into soils and rock, and throughout the oceans, exchanging gases, water and nutrients; the lithosphere adds volcanic gases into the air and stores other gases in sediments which may ultimately become rock. Changes occurring in one geosphere affect the others through feedback mechanisms (interconnecting processes determined by dynamic physical conditions).
The atmosphere is the envelope of gases surrounding Earth that extends up to approximately 10,000 km above Earth's surface (the extreme edges of the atmosphere lie about 35,000 km above the surface). Atmospheric density decreases going further away from Earth's surface. Because of this, most (99%) of the atmosphere's mass lies within 30 km of Earth's surface.
The bottom four layers of the atmosphere (troposphere, stratosphere, mesosphere, and ionosphere) are shown in Figure 1.02. The composition of air sampled at sea level is made up of nitrogen (78%), oxygen (21%), and small amounts of other gases including argon and carbon dioxide. In addition to this mixture of gases, the atmosphere contains gaseous, liquid, and solid water, and various particulates.
The atmosphere transports matter (such as water vapor and particulates) and energy (heat) from one place to another via the movement of air masses, and also functions as an insulator preventing surface temperatures from becoming too extreme. Atmospheric processes are discussed in greater detail in Chapters 2 and 3.
The lithosphere is the outer, rigid shell of the solid Earth. It is composed of the entire crust (oceanic and continental) and the top rigid portion of the mantle lying above the partially melted, less rigid asthenosphere. Broken into major tectonic plates moving relative to one another over the asthenosphere, the lithosphere consists of rocks overlain by discontinuous surface veneers of sediment and soil. Lithospheric thickness varies from about 50 kilometers under ocean bottoms to about 100 kilometers below continental surfaces. The lithosphere forms the base layer of Earth's surficial realms.
The lithosphere is composed predominantly of oxygen and silicon, with aluminum, iron , calcium, magnesium, sodium, potassium, and other elements contributing lesser amounts. Exact lithospheric compositions vary spatially, for example between oceanic and continental vicinities. The lithosphere stores very large amounts of mass. With the exception of volcanic activity, however, transport of materials through the lithosphere is relatively slow. Physical, chemical and biological processes interact with the outermost land surfaces of the lithosphere to form soil, ultimately providing materials essential for vegetative growth.
In terms of processes, Earth is truly a water planet. On Earth, water is found in all three material states: gas, liquid, and solid. Transformation between these states captures and releases significant amounts of energy.
The hydrosphere consists of all the water girdling Earth whether it be gas, liquid or solid. Most water (97%) resides in the oceans. The remainder is found in the ice sheets of Greenland and Antarctica (2%), and in freshwater lakes, rivers, ground water below the surface, and water vapor in the atmosphere (1%).
Since water covers over 70% of Earth's surface, the hydrosphere is an important component of global climate patterns, and is a large reservoir for materials and energy. Water, materials, and energy contained in the hydrosphere are continually transported within the hydrosphere and through other spheres by ocean circulation, the hydrologic cycle of evaporation and precipitation, tectonic subduction, and biologic respiration. Some of these processes are discussed in more detail in Chapter 3.
Representing all life on Earth, the biosphere can be thought of as the space in which organisms live. It is an example of the overlapping and coexisting nature of Earth's surficial shells. Life is found pervasively on the pedosphere (soil) surface, the land surface of the lithosphere, in the hydrosphere (oceans, lakes and rivers), extending down to various depths into the pedosphere and lithosphere, and up into the lower atmosphere. Physical and chemical processes in these geospheres make life possible, and in turn the processes of life and death actively use energy and cycle matter. Chapter 3 explains these processes in more detail.
The biosphere includes both oceanic and terrestrial (land) domains. On land there are a number of different, easily recognizable communities called biomes. Biomes essentially result from the interaction of regional climate, biota (life) and substrates (soils or underlying surfaces), and are usually defined by their climax vegetation. Some examples of biomes are rainforest, savanna, grassland, desert, taiga, and tundra. Biomes can be divided into smaller, more descriptive units to provide a more detailed characterization of an area and information about the types of organisms that might exist there. Chapter 6 discusses the classification of environmental regions within the biosphere.
The pedosphere is comprised of relatively thin soil layers found on top of much of Earth's land surface. Processes interacting between the lithosphere, atmosphere, hydrosphere, and biosphere result in the formation of individual soil units with unique properties (called "pedons") across the landscape. The pedosphere contributes significantly to controls regulating the movement of air, water, mass, and energy between each of the other "geospheres" (see Figure 1.01). A number of processes operating on a variety of time scales occur in the pedosphere: heat, water, gas, and energy transfers, biological activities, weathering and erosion, compaction, salinization, biogeochemical cycling, organic matter decomposition and humus formation, and soil genesis.
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