Exploring Our World: People, Places, and Cultures
Earth's Physical Geography
Earth has a variety of landforms, and many of which can be found both on the continents and on the ocean floors. Forces from within and the actions of wind, water, and ice have shaped Earth's surface. Inside the earth are four layers: the inner core, outer core, mantle, and crust. Scientists theorize that volcanoes, earthquakes, and continental drift are caused by the movement of tectonic plates that float on top of the liquid rock in the mantle. The forces of weathering and erosion also continually change the Earth's surface.
About 70 percent of the Earth's surface is water, but only 3 percent of the water is usable freshwater. However, much of the freshwater is frozen in ice. The total amount of water on Earth does not change. The water cycle moves water from the oceans to the air through the process of evaporation. During this process, water changes from a liquid to a gas. When water vapor reaches cool air temperatures, condensation occurs: tiny droplets of water are suspended in clouds. Water returns to the ground, and finally back to the oceans, through precipitation such as rain or snow.
Geographers call the usual patterns of weather in a region its climate. Water, sun, and wind all influence Earth's climate. Landforms also influence climate: the distance between landforms as well as their nearness to water can affect the temperatures and rainfall in a region. The effects of wind, water, latitude, and landforms combine to create what geographers identify as different climate zones on the Earth. Each zone shares common characteristics and has particular kinds of vegetation. The five major climate zones are tropical, mid-latitude, high latitude, dry, and highland.
Human actions can affect climate and the environment. A delicate balance exists in the Earth's atmosphere, lithosphere, hydrosphere, and biosphere. People burn coal or gas or release chemicals into the air, which can cause pollution in the Earth's atmosphere. The lithosphere, or the Earth's crust, is also affected by human actions. Logging, mining, and farming can have negative effects on the land. Pollution of the hydrosphere, or surface water and groundwater, also poses a threat to humans, other animals, and plants. Changes in a region's climate can lead to decreasing populations of plants and animals in some parts of the world.