The movement of water around, over, and through the Earth is called the water cycle, a key process of the hydrosphere. In physical geography, the term hydrosphere (Greek hydro means "water") describes the collective mass of water found on, under, and over a planet's surface. The Earth's hydrosphere consists chiefly of the oceans, but technically includes clouds, inland seas, lakes, rivers, and underground waters. Hydrosphere refers to that portion of Earth that is composed of water. The hydrosphere represents one component of Earth's system, operating in conjunction with the solid crust (lithosphere) and the air that envelopes the planet (atmosphere). The derivation of the term hydrosphere, from the Greek words for water and ball, is truly descriptive of our world, as it reflects the abundance and importance of water on Earth. On Earth, water exists in the three primary states of matter; liquid, solid, and gas. The distance of Earth from the Sun, by fortunate coincidence, is such that the amount of energy arriving at the surface of most of the planet is sufficient to elevate the ambient temperature to levels above the freezing point of water, yet insufficient to cause all of the water to evaporate into the gaseous state. The capacity of water to store large quantities of heat energy heavily influences the nature of the global climate. The presence of large bodies of liquid water and the atmosphere restrict the range of temperature fluctuations on Earth. These conditions have allowed the existence of the fourth component of Earth's system, the biosphere. Water is constantly being cycled through its various manifestations and through the components of Earth's systems by means of the hydrologic cycle. Driven by solar energy, water is evaporated from the ocean surface and distributed over the earth as water vapor. Precipitation returns the water, in liquid and solid forms, to other parts of the globe. Throughout the cycle, water may exist in a number of forms, interact with the atmosphere and lithosphere, or may be utilized by organisms within the biosphere. One commonly cited statistic asserts that 71% of the surface area of our planet is covered by water, with the largest part covered by oceans. The total volume of sea-water, amounting to 97.2% of all the water on the planet, is 295,000,000 mi3 (1,230,000,000 km3). Usable freshwater constitutes less than 0.5% of all water on Earth. Water in all rivers, lakes and streams totals only 29,800 mi3 (124,200 km3). The amount of groundwater that is within 0.5 mi (0.8 km) of the surface is 960,000 mi3 (4,000,000 km3). Water also exists on Earth in the solid form as icecaps and glaciers, occupying a volume of 6,900,000 mi3 (28,600,000 km3). Straddling the division between hydrosphere and atmosphere is water vapor. A volume of 3,000 mi3 (12,700 km3) of water can be found in the atmosphere. The abundance of water on Earth is a unique feature that distinguishes our "blue planet" from others in the solar system. Approximately 70.8 percent of the Earth is covered by water and only 29.2 percent is terra firma. The average depth of the Earth's oceans is 3,794 m (12,447 ft)—more than five times the average height of the continents. The mass of the oceans is approximately 1.35 × 1018 tons, or about 1/4400 of the total mass of the Earth. The hydrosphere plays a key role in the development and sustenance of life. It is thought that the earliest living organisms probably emerged in a watery soup. In addition, each human life begins in the watery environment of its mother's womb, our cells and tissues are mostly water, and most of the chemical reactions that are part of life's processes take place in water. History
There are several theories regarding the formation of the Earth's hydrosphere. This planet contains proportionately more surface water than comparable bodies in the inner solar system. Outgassing of water from the Earth's interior is not sufficient to explain the...
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