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Chapter 2 , Section 2 page 31 Land, Air, and Water • What forces shape the land? • What are the Earth’s major landforms? Chapter 2 , Section 2 Forces Inside the Earth What is the Earth made of? The Earth’s surface is made up of 75 percent water and 25 percent land. Continents are unique, in part because of their landforms, which include mountains, hills, plateaus, and plains. Pangaea: The Supercontinent Geographers theorize that millions of years ago the Earth had only one huge landmass, which they call Pangaea. They believe that 200 million years ago, some force made Pangaea split into several pieces and begin to move apart, forming separate continents. The theory of plate tectonics explains why the continents separated. Continents are part of plates that shift over time. Chapter 2 , Section 2 The Movement of the Continents When geographers first began to study world maps, they realized that the continents look like pieces of a jigsaw puzzle. Chapter 2 , Section 2 Volcanoes, Volcanoes, Earthquakes, Earthquakes, and and Shifting Shifting Plate Plate Forces Forces on on the the Earth's Earth's Surface Surface The world’s plates move in different directions, causing a variety of effects: Ridges: In some places, plates move apart, and magma leaks through the cracks in the crust. In the oceans, over time, the cooling rock builds up to form lines of underwater mountains called ridges. Volcanoes: In other places, the plates push against one another, forcing one plate under the other. Tremendous pressure and heat builds up causing molten rock to explode on the surface. Earthquakes: Along plate boundaries, there are many weak places in the Earth’s crust. When plates push against each other, the crust cracks and splinters from pressure. These cracks are called faults. When the crust moves along faults, it releases great amounts of energy in the form of earthquakes. Chapter 2 , Section 2 Air and Water: Two Ingredients for Life Air • The thick layer of gases that surrounds the Earth is called the atmosphere. • It provides life-giving oxygen for people and animals and lifegiving carbon dioxide for plants. • It also acts as a blanket, holding in enough heat from the sun to make life possible. • Winds distribute this heat around the globe. Water • Roughly 97 percent of Earth’s water is found in the oceans. • The rest is fresh water, or water without salt. Most of that is frozen at the poles. • Fresh water comes from lakes, rivers, and rain. Much fresh water, called groundwater, is stored in the soil. • People need fresh water—the Earth has enough, but some places have too much, and others have too little. Chapter 2 , Section Chapter 2 , Section 2 The World: Wind Patterns Chapter 2 , Section 2 Land, Air, and Water–Assessment Why are there earthquakes and volcanoes? Weak places in the Earth’s crust crack and splinter from pressure, causing earthquakes, or break apart, allowing liquid rock from beneath the surface to rise as volcanoes. What forces on the Earth’s surface break down rocks? Wind, rain and ice Why is the atmosphere important? It makes it possible for life to exist on the Earth.