The natural systems that we will encounter in the study of physical geography operate within the four realms, or spheres, of the Earth. These are the atmosphere, the lithosphere, the hydrosphere, and the biosphere.
The atmosphere is a gaseous layer that surrounds the Earth. It receives heat and moisture from the surface and redistributes them, returning some heat and all moisture to the surface. The atmosphere also supplies vital elements carbon, hydrogen, oxygen, and nitrogen that are needed to sustain life.
The outermost solid layer of the Earth, or lithosphere , establishes the platform for most Earthly life-forms. The solid rock of the lithosphere bears a shallow layer of soil in which nutrient elements become available to organisms.
The surface of the lithosphere is sculpted into landforms.These feature such as mountains, hills, and plains provide varied habitats for plants, animals, and humans.
The liquid realm of the Earth is the hydrosphere , which principally comprises the mass of water in the world ’s oceans. It also includes solid ice in mountain and continental glaciers, which, like liquid ocean and freshwater, is subject to fl ow under the influence of gravity. Within the atmosphere, water occurs as gaseous vapor, liquid droplets, and solid ice crystals. In the lithosphere, water is found in the uppermost layers in soils and in groundwater reservoirs.
The biosphere encompasses all living organisms of the Earth. Life-forms on Earth utilize the gases of the atmosphere, the water of the hydrosphere, and the nutrients of the lithosphere, and so the biosphere is dependent on all three of the other great realms.
Most of the biosphere is contained in the shallow surface zone called the life layer . It includes the surface of the lands and the upper 100 m or so (about 300 ft) of the ocean. On land, the life layer is the zone of interactions among the biosphere, lithosphere, and atmosphere, with the hydrosphere represented by rain, snow, still water in ponds and lakes, and running water in rivers. In the ocean, the life layer is the zone of interactions among the hydrosphere, biosphere, and atmosphere, with the lithosphere represented by nutrients dissolved in the upper layer of seawater.