Earth's Water is in Constant Motion
When astronauts see Earth from space, this is how it looks. Notice how blue the planet appears. That’s because oceans cover much of Earth’s surface. The water in those oceans is in constant motion. Water is also found in the clouds that rise above the planet and travel across the sky. The water may precipitate out as rain or snow. Water passes through soil and into groundwater. Or it is taken up by the leaves of plants and then enters the atmosphere. Water moves into and out of animals. Water may remain as ice in a glacier for thousands of years. This chapter is about Earth's most dynamic substance - water.
Water is found on Earth as a gas, liquid and solid. Water as a gas is mostly in the atmosphere. Water as a solid is found as ice floating on ponds, as frost on a car, or in glaciers, among other places. Glaciers move, by definition, although they do it very slowly. Liquid water moves around the planet through streams, groundwater, lakes, ponds. Most of Earth's liquid water is in the oceans. But it doesn't remain stagnant there. Seawater moves in waves, tides, and currents. Some ocean water changes state and becomes sea ice. Water may evaporate into the atmosphere and become water vapor. Water sets Earth apart from the other planets in the solar system. In fact, water in all three states is probably exceedingly rare everywhere in the galaxy. Human activities pollute water. Pollutants come from industrial, municipal and agricultural sources. Water is protected from pollution by acts and rules that guide people to protect water. Water can also be cleaned up once it is polluted.