Planet Earth is part of the solar system, which is made up of the sun and the many objects that revolve around it. Earth's structure has been and continues to be shaped by powerful forces within the earth as well as exterior forces such as wind and water.
Planet Earth At least nine planets revolve around the sun. Earth is the third planet from the sun, one of the four inner planets closest to the sun. Planets are classified as terrestrial planets, such as Earth, or gas giant planets. Thousands of other smaller objects—asteroids, comets, and meteoroids—also revolve around the sun. Earth is the largest of the inner planets. 70 percent of the earth's surface is water, and about 30 percent of its surface is land. Seven large landmasses called continents feature landforms of varying shapes and elevations.
Forces of Change The earth is composed of layers—a super-hot, solid inner core surrounded by a liquid outer core, a layer of dense rock called the mantle, and a relatively thin, rocky shell at the surface. Both internal and external forces change the earth. Continental drift, the movement of surface plates, causes landmasses to move slowly across the earth's surface. The movement of magma within the earth may cause this plate movement, resulting in mountains, trenches, folds, and faults that form where plates meet. Earthquakes and volcanoes also occur along plate boundaries, especially along the Ring of Fire. Weathering changes the earth's surface by physically breaking down rock or by changing the chemical makeup of rock. Land is also eroded by wind, glaciers, and moving water.
Earth's Water Although the total amount of water on the earth does not change, water is constantly moving and changing form. This water cycle begins with evaporation and continues with condensation and precipitation. Most of Earth's water is salt water, circling the planet in 4 great oceans which extend into seas, gulfs, and bays. Only 3 percent of Earth's water is freshwater, most of which is frozen in glaciers and ice caps. Lakes, streams, rivers, and groundwater all supply human settlements with needed freshwater.