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Chapter 8: Water on Earth

Difficulty Level: At Grade Created by: CK-12


Pale Blue Dot.

"From this distant vantage point, the Earth might not seem of any particular interest. But for us, it's different. Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every 'superstar,' every 'supreme leader,' every saint and sinner in the history of our species lived there – on a mote of dust suspended in a sunbeam." —Carl Sagan, Pale Blue Dot: A Vision of the Human Future in Space, p. 6

Chapter Outline

Chapter Summary


What makes the dot blue, of course, is water. Water cycles through multiple reservoirs — glaciers, lakes, oceans, groundwater, and life, among others. As it cycles, it changes state between solid, liquid, and gas. Water is almost always moving, imperceptibly slowly as an ice crystal in a glacier or at rapid speeds in flooding stream or an ocean current. Surface water cycles through streams and into ponds and lakes. When too much water falls, the stream or even lake may flood. Water from the surface may filter through the ground to enter an aquifer. Streams eventually enter the ocean, which has motions of its own. Water travels in surface currents and may undergo downwelling to cycle through the deep ocean. Ocean currents are important to the planet, for example they moderate climate by bringing warm equatorial water toward the poles and cold polar water toward the Equator.

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Difficulty Level:

At Grade



Date Created:

Feb 24, 2012

Last Modified:

Jan 05, 2015
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