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Planet Earth Our Solar System • Earth is part of a large physical system containing countless objects, all of which revolve around the sun. • Eight spheres, called planets, are the largest objects in the solar system. • Terrestrial planets have solid, rocky crusts; these four inner planets are Mercury, Venus, Earth, and Mars. • Gas giant planets are more gaseous and less dense then the other planets; these four outer planets are Jupiter, Saturn, Uranus, and Neptune. Planet Earth Getting to Know Earth • The atmosphere, hydrosphere, and lithosphere form the biosphere, the part of Earth that supports life for all people, animals, and plants. • Atmosphere is a thin layer of gases that surround Earth and contains the air we breathe. • Hydrosphere includes the liquid and frozen surface water, groundwater, and water vapor in and around oceans, lakes, and rivers on Earth. • Lithosphere is the land or surface areas on Earth, including the continents, islands, and ocean basins. Forces of Change Earth’s Structure • For millions of years, the surface of the Earth has been moving. • The Earth is composed of three layers—the core at Earth’s center, the mantle layer of dense rock on the outer core, and the crust forming Earth’s surface. • Many scientists believe Earth was once a single land mass called Pangaea, but then continental drift slowly spread the continents apart. • Plate tectonics describes the activities of continental drift and magma flow; referring to the physical processes that create and shape continents, islands, oceans, and mountain ranges. Forces of Change Internal Forces of Change • Mountains are formed when Earth’s giant continental and oceanic plates collide. • Moving plates sometimes cause Earth’s surface to buckle forming folds; in other cases the moving plates form cracks called faults. • Violent movements of Earth’s crust along fault lines are called earthquakes, which dramatically change the surface of the land and the floor of the ocean. • Volcanoes are mountains formed by lava or magma that breaks Earth’s surface. Forces of Change External Forces of Change • Wind and water break down the Earth’s surface through weathering and erosion. • Physical weathering happens when large masses of rock are broken down into smaller pieces. • Glaciers are large bodies of ice that move across the Earth’s surface, changing the landscape as they flow. • Soil building takes thousands of years of weathering, erosion, and biological activity and is influenced by five factors; climate, topography, geology, biology, and time. Earth’s Water The Water Cycle • Almost all of the hydrosphere is saltwater found in oceans, seas, and some lakes. • The amount of water on Earth never changes, but it is constantly moving through the processes of evaporation, condensation, and precipitation. • Evaporation is when the sun’s energy causes water to change into vapor or gas. • Condensation is an excess of water vapor that changes into liquid water after warm air cools. • Precipitation is the release of moisture through rain, snow, or sleet. Earth’s Water Bodies of Salt Water • Seventy percent of the Earth’s surface is water, primarily salt water. • Salt water cannot be used for drinking, farming, or manufacturing. • Due to a growing population and the need for more freshwater, some countries are focusing on the desalination process, which removes the salt from seawater. • Desalination is expensive and is also controversial; critics believe the process has negative environmental and economic impacts. Earth’s Water Bodies of Freshwater • More than two-thirds of the world’s scarce freshwater is frozen in glaciers. • Less than one-third of a percent of freshwater is found in lakes, rivers, and streams and their tributaries. • Aquifers and groundwater are important sources of freshwater found underground within the Earth.