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What You Will Learn Lesson 1: Earth and the Sun’s Energy 41 The Big Idea Earth’s movement and the sun’s energy interact to create day and night, temperature changes, and the seasons. Lesson 2: Water on Earth 46 The Big Idea Water is a dominant feature on Earth’s surface and is essential for life. Lesson 3: The Land 52 The Big Idea Processes below and on Earth’s surface shape the planet’s physical features. Lesson 4: Weather and Climate 62 The Big Idea The sun, location, wind, water, and mountains affect weather and climate. Lesson 5: World Climates 68 The Big Idea Earth’s five major climate zones are identified by temperature, precipitation, and plant life. Lesson 6: Environments and Biomes 77 The Big Idea Plants, animals, and the environment, including soil, interact with and affect one another. Lesson 7: Natural Resources 82 The Big Idea Earth’s natural resources have many valuable uses, and their availability affects people in many ways.
In this module, you will learn about Earth’s resources, and the processes that cause the seasons, shape landmasses and climates, and support life. You will also learn how humans use and interact with these resources and processes.
Document-Based Investigations Graphic Organizers Interactive Games Animation: The Water Cycle Channel One News Video: Hunting Hurricanes Channel One News Video: Earthquake Preparedness Interactive Map: Major Climate Regions Image with Rich Media: Desert Ecosystem
- The Ring of Fire
- Plate Tectonics and Continental Drift
Essential Question Why do we study Earth and its physical systems?
The Physical World
Module 2
About the Photo: Many of Earth’s features are visible from space. This photo, taken from a satellite orbiting the planet, shows part of the North American continent.
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Land Forces on and under Earth’s surface have shaped the different landforms on our planet. Geographers study how mountains and other landforms were made.
Water on Earth Water is essential for life on Earth. Much of the planet’s water supply is stored in Earth’s oceans and ice caps.
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Environments Living things, such as this koala, depend on their surroundings.
Lesson 1
Earth and the Sun’s Energy
The Big Idea Earth’s movement and the sun’s energy interact to create day and night, temperature changes, and the seasons.
Main Id eas ■ ■ (^) Earth’s movement affects the amount of energy we receive from the sun.
■ ■ (^) Earth’s seasons are caused by the planet’s tilt.
Key Terms and Places solar energy rotation revolution tropics
If YOU lived there...
You live in Chicago and have just won an exciting prize—a trip to Australia during winter vacation in January. As you prepare for the trip, your mother reminds you to pack shorts and a swimsuit. You are confused. In January you usually wear winter sweaters and a heavy jacket.
Why is the weather so different in Australia?
Earth’s Movement Energy from the sun helps crops grow, provides light, and warms Earth. It even influences the clothes we wear, the foods we eat, and the sports we play. All life on Earth requires solar energy, or energy from the sun, to survive. The amount of solar energy places on Earth receive changes constantly. Earth’s rotation, revolution, and tilt, as well as latitude, all affect the amount of solar energy parts of the planet receive from the sun.
Rotation Imagine that Earth has a rod running through it from the North Pole to the South Pole. This rod represents Earth’s axis—an imaginary line around which a planet turns. As Earth spins on its axis, different parts of the planet face the sun. It takes Earth 24 hours, or one day, to complete this rotation. A rotation is one complete spin of Earth on its axis. As Earth rotates during this 24-hour period, it appears to us that the sun moves across the sky. The sun seems to rise in the east and set in the west. The sun, however, does not move. It is actually Earth’s rotation that creates the sense of the sun’s movement. Earth’s rotation also explains why day changes to night. As you will see in the illustration on the next page, solar energy strikes only the half of Earth facing the sun. Warmth and light from the sun create daytime. At the same time, the half of the planet facing away from the sun experiences the cooler temperatures and darkness of night. Earth’s rotation causes regular shifts from day to night. As a result, levels of solar energy on Earth constantly change.
The Physical World 41
North Pole
Sun’s Rays
South Pole
E q u a t o r
(^23). (^5) °
Revolution As Earth spins on its axis, it also follows a path, or orbit, around the sun. Earth’s orbit around the sun is not a perfect circle. Some- times the orbit takes Earth closer to the sun, and at other times the orbit takes it farther away. It takes 365¼ days for Earth to complete one revolu- tion, or trip around the sun. We base our calendar year on the time it takes Earth to complete its orbit around the sun. To allow for the fraction of a day, we add an extra day—February 29—to our calendar every four years.
Tilt and Latitude Another factor affecting the amount of solar energy we receive is the planet’s tilt. As the illustration shows, Earth’s axis is not straight up and down. It is actually tilted at an angle of 23.5 degrees from vertical. At any given time of year, some locations on Earth are tilting away from the sun, and others are tilting toward it. Places tilting toward the sun receive more solar energy and experience warmer temperatures. Those tilting away from the sun receive less solar energy and experience cooler temperatures. A location’s latitude, the distance north or south of Earth’s equator, also affects the amount of solar energy it receives. Low-latitude areas, those near the equator like Hawaii, receive direct rays from the sun all year. These direct rays are more intense and produce warmer temperatures. Regions with high latitudes, like Antarctica, are farther from the equator. As a result, they receive indirect rays from the sun and have colder temperatures.
Academic Vocabulary factor cause
Earth’s tilt and rotation cause changes in the amount of energy we receive from the sun. As Earth rotates on its axis, energy from the sun creates periods of day and night. Earth’s tilt causes some locations, especially those close to the equator, to receive more direct solar energy than others.
Analyze Visuals Is the region north or south of the equator receiving more solar energy? How can you tell?
Solar Energy
Reading Check Find Main Ideas What factors affect the solar energy Earth receives?
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Because of Earth’s tilt, the Northern and Southern Hemispheres experi- ence opposite seasons. As the North Pole tilts toward the sun in summer, the South Pole tilts away from it. As a result, the Southern Hemisphere experiences winter. Likewise, when it is spring in the Northern Hemi- sphere, it is fall in the Southern Hemisphere.
Spring and Fall As Earth orbits the sun, there are periods when the poles tilt neither toward nor away from the sun. These periods mark spring and fall. During the spring, as part of Earth begins to tilt toward the sun, solar energy increases. Temperatures slowly start to rise, and days grow longer. In the fall the opposite occurs as winter approaches. Solar energy begins to decrease, causing cooler temperatures and shorter days.
Focus on Culture
Predict How might people’s daily lives be affected by the midnight sun?
The Midnight Sun
Can you imagine going to sleep late at night with the sun shining in the sky? People who live near the Arctic and Antarctic Circles experience this every summer, when they can receive up to 24 hours of sunlight a day. The time-lapse photo below shows a typical sunset during this period—except the sun never really sets! This phenomenon is known as the midnight sun. For locations like Tromso, Norway, this means up to two months of constant daylight each summer. People living near Earth’s poles often use the long daylight hours to work on outdoor projects in preparation for winter, when they can receive 24 hours of darkness a day.
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Rainfall and Seasons Some regions on Earth have seasons marked by rainfall rather than temperature. This is true in the tropics, regions close to the equator. At certain times of year, winds bring either dry or moist air to the tropics, creating wet and dry seasons. In India, for example, sea- sonal winds called monsoons bring heavy rains from June to October and dry air from November to January.
Summary and Preview Solar energy is crucial for all life on the planet. Earth’s position and movements affect the amount of energy we receive from the sun and determine our seasons. The Northern and Southern Hemispheres have opposite seasons. Some regions experience four seasons while others experience only two. In the next lesson you will learn about Earth’s water supply and its importance to us.
Review Ideas, Terms, and Places
1. a. Identify What is solar energy, and how does it affect Earth? b. Analyze How do rotation and tilt each affect the amount of solar energy that different parts of Earth receive? c. Predict What might happen if Earth received less solar energy than it currently does? 2. a. Describe Name and describe Earth’s seasons. b. Contrast How are seasons different in the Northern and Southern Hemispheres? c. Elaborate How might the seasons affect human activities?
Critical Thinking
3. Identify Cause and Effect Use your notes and the chart to identify the causes of seasons.
Lesson 1 Assessment
Reading Check Identify Cause and Effect What causes the seasons to change?
Cause Cause^ Effect: Earth’s changing seasons
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surface. Earth’s oceans are made up of smaller bodies of water such as seas, gulfs, bays, and straits. Altogether, Earth’s oceans cover some 139 million square miles (360 million square km) of the planet’s surface. Some of Earth’s lakes contain salt water. The Great Salt Lake in Utah, for example, is a saltwater lake. As salt and other minerals have collected in the lake, which has no outlet, the water has become salty.
Freshwater Since the water in Earth’s oceans is too salty to use, we must rely on other sources for freshwater. Freshwater , or water without salt, makes up only about 3 percent of our total water supply. Much of that freshwater is locked in Earth’s glaciers , large areas of slow-moving ice, and in the ice of the Antarctic and Arctic regions. Most of the freshwater we use every day is found in lakes, in rivers, and under Earth’s surface.
Glaciers Tremendous bodies of ice consist of frozen freshwater. This cruise ship looks like a toy next to San Rafael Glacier at Laguna San Rafael National Park on the Pacific coast of southern Chile.
Freshwater Freshwater from lakes, rivers, and streams makes up only a fraction of Earth’s water supply.
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One form of freshwater is surface water. Surface water is water that is found in Earth’s streams, rivers, and lakes. It may seem that there is a great deal of water in our lakes and rivers, but only a tiny amount of Earth’s water supply—less than 1 percent—comes from surface water. Streams and rivers are a common source of surface water. Streams form when precipitation collects in a narrow channel and flows toward the ocean. Precipitation is water that falls to Earth’s surface as rain, snow, sleet, or hail. In turn, streams join together to form rivers. Any smaller stream or river that flows into a larger stream or river is called a tributary. For exam- ple, the Missouri River is the largest tributary of the Mississippi River. Lakes are another important source of surface water. Some lakes were formed as rivers filled low-lying areas with water. Other lakes, like the Great Lakes along the U.S.-Canada border, were formed when glaciers carved deep holes in Earth’s surface and deposited water as they melted. Most of Earth’s available freshwater is stored underground. As precipita- tion falls to Earth, much of it is absorbed into the ground, filling spaces in the soil and rock. Water found below Earth’s surface is called groundwater. In some places on Earth, groundwater naturally bubbles from the ground as a spring. More often, however, people obtain groundwater by digging wells, or deep holes dug into the ground to reach the water.
The Water Cycle When you think of water, you probably visualize a liquid—a flowing stream, a glass of ice-cold water, or a wave hitting the beach. But did you know that water is the only substance on Earth that occurs naturally as a solid, a liquid, and a gas? We see water as a solid in snow and ice and as a liquid in oceans and rivers. Water also occurs in the air as an invisible gas called water vapor. Water is always moving. As water heats up and cools down, it moves from the planet’s surface to the atmosphere, or the mass of air that sur- rounds Earth. One of the most important processes in nature is the water cycle. The water cycle is the movement of water from Earth’s surface to the atmosphere and back. The sun’s energy drives the water cycle. As the sun heats water on Earth’s surface, some of that water evaporates, or turns from liquid to gas, or water vapor. Water vapor then rises into the air. As the vapor rises, it cools. The cooling causes the water vapor to condense, or change from a vapor into tiny liquid droplets. These droplets join together to form clouds. If the droplets become heavy enough, precipitation occurs—that is, the water falls back to Earth as rain, snow, sleet, or hail. When that precipitation falls back to Earth’s surface, some of the water is absorbed into the soil as groundwater. Excess water, called runoff, flows over land and collects in streams, rivers, and oceans. Because the water cycle is constantly repeating, it allows us to maintain a fairly constant sup- ply of water on Earth.
Reading Check Contrast Ideas How is salt water different from freshwater?
Reading Check Find Main Ideas What is the water cycle?
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Even where water is plentiful, it may not be clean enough to use. If chemicals and household wastes make their way into streams and rivers, they can contaminate the water supply. Polluted water can carry diseases. These diseases may harm humans, plants, and animals. Flooding is another water problem that affects people around the world. Heavy rains often lead to flooding, which can damage property and threaten lives. One example of dangerous flooding occurred in Bangladesh in 2004. Severe floods there destroyed roads and schools, affecting about 25 million people. Water dramatically impacts the physical environment in other ways as well. In Florida, where limestone is plentiful, sinkholes can arise with little warning. Over time, water dissolves and weakens limestone, leaving the surface with little support. The ground layer collapses. The resulting hole in the ground can swallow cars and houses. The Devil’s Millhopper is one example of a well-known sinkhole.
Water’s Benefits Water does more than just quench our thirst. It provides us with many benefits, such as food, power, and even recreation. Water’s most important benefit is that it provides us with food to eat. Everything we eat depends on water. For example, fruits and vegetables need water to grow. Animals also need water to live and grow. As a result, we use water to farm and raise animals so that we will have food to eat. Water is also an important source of energy. Using dams, we harness the power of moving water to produce electricity. Electricity provides power to air-condition or heat our homes, to run our washers and dryers, and to keep our food cold. Water also provides us with recreation. Rivers, lakes, and oceans make it possible for us to swim, to fish, to surf, or to sail a boat. Although recre- ation is not critical for our survival, it does make our lives richer and more enjoyable.
Water’s Benefits: for Life Without water, plants won’t grow. This modern irrigation system, used for large agricultural projects, ensures that plants get the water they need and humans get the food they need.
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Recognizing all of these benefits, some communities work together to man- age freshwater supplies. For example, in Florida, long-term population growth strains the freshwater supply. That, combined with drought conditions, inspired the creation of the Central Florida Water Initiative (CFWI). This organization works to protect and conserve water resources in a 5,300-square- mile (13,727-sq-km) area. The CFWI works with businesses, utilities, environ- mental groups, agricultural groups, and others to meet people’s water needs. Many regions around the world have similar organizations.
Summary and Preview In this lesson you learned that water is essential for life on Earth. Next, you will learn about the shapes on Earth’s surface.
Water’s Benefits: for Fun This surfer rides a wave at the beach. Many people enjoy swimming, boating, fishing, and other recreational activities on water.
Reading Check Summarize How does water affect people’s lives?
Review Ideas, Terms, and Places
1. a. Describe Name and describe the different types of water that make up Earth’s water supply. b. Analyze Why is only a small percentage of Earth’s freshwater available to us? c. Elaborate In your opinion, which is more important—surface water or groundwater? Why? 2. a. Recall What drives the water cycle? b. Make Inferences From what bodies of water do you think most evaporation occurs? Why? 3. a. Define What is a drought? b. Analyze How does water support life on Earth? c. Evaluate What water problem do you think is most critical in your community? Why?
Critical Thinking
4. Sequence Draw the graphic organizer. Then use your notes and the graphic organizer to identify the stages in Earth’s water cycle.
Lesson 2 Assessment
Heat from the sun evapo- rates water on Earth.
The water cycle repeats.
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Pacific plate
North American plate
Australian plate
South American plate
African plate
Indian plate
Antarctic plate
mg7vs_ear000014aa 8th pass 08/18/ cmurphy
Nazca plate
Eurasian plate
Forces below Earth’s Surface Geographers often study how landforms are made. One explanation for how landforms have been shaped involves forces below Earth’s surface.
Earth’s Plates To understand how these forces work, we must examine Earth’s structure. The planet is made up of three layers. A solid inner core is surrounded by a liquid layer, or mantle. The solid outer layer of Earth is called the crust. The planet’s continents , or large landmasses, are part of Earth’s crust. Geographers use the theory of plate tectonics to explain how forces below Earth’s surface have shaped our landforms. The theory of plate tectonics suggests that Earth’s surface is divided into a dozen or so slow- moving plates, or pieces of Earth’s crust. As you can see in the image above, some plates, like the Pacific plate, are quite large. Others, like the Nazca plate, are much smaller. These plates cover Earth’s entire surface. Some plates are under the ocean. These are known as ocean plates. Other plates, known as continental plates, are under Earth’s continents. Why do these plates move? Energy deep inside the planet puts pressure on Earth’s crust. As this pressure builds up, it forces the plates to shift. Earth’s tectonic plates all move. However, they move in different direc- tions and at different speeds.
Academic Vocabulary structure the way something is set up or organized
Earth’s Plates
The theory of plate tectonics states that the plates that make up Earth’s crust are moving, usually only a few inches per year. As Earth’s plates collide, separate, and slide past each other, they create forces that shape many of Earth’s landforms.
Analyze Visuals Looking at the map, what evidence indicates that plates have collided or separated?
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Plate B Plate A
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The Movement of Continents Earth’s tectonic plates move slowly—up to several inches per year. The continents, which are part of Earth’s plates, shift as the plates move. If we could look back some 200 million years, we would see that the continents have traveled great distances. This idea is known as continental drift. The theory of continental drift, first developed by Alfred Wegener, states that the continents were once united in a single supercontinent. Wegener’s inspiration came from the similarity he observed between the western coast of Africa and the eastern coast of South America. According to this theory, Earth’s plates shifted over millions of years. As a result, the continents slowly separated and moved to their present positions. Earth’s continents are still moving. Some plates move toward each other and collide. Other plates separate and move apart. Still others slide past one another. Over time, colliding, separating, and sliding plates have shaped Earth’s landforms.
Plates Collide As plates collide, the energy created from their collision produces distinct landforms. The collision of different types of plates cre- ates different shapes on Earth’s surface. Ocean trenches and mountain ranges are two examples of landforms produced by the collision of tectonic plates. When two ocean plates collide, one plate pushes under the other. This process creates ocean trenches. Ocean trenches are deep valleys in the ocean floor. Near Japan, for example, the Pacific plate is slowly moving under other plates. This collision has created several deep ocean trenches, including the world’s deepest trench, the Mariana Trench.
The movement of Earth’s tectonic plates has produced many of Earth’s landforms. For example, the Himalayas in South Asia resulted from the collision of two massive continental plates.
Analyze Visuals What type of landform is created by the collision of two continental plates?
Mountains Form When Plates Collide
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occurs. In California, for example, the Pacific plate is sliding by the edge of the North American plate. This has created the San Andreas Fault zone, an area where earthquakes are quite common. The San Andreas Fault zone is one of many areas that lie along the boundaries of the Pacific plate. The frequent movement of this plate pro- duces many earthquakes and volcanic eruptions along its edges. In fact, the region around the Pacific plate, called the Ring of Fire, is home to most of the world’s earthquakes and volcanoes.
Processes on Earth’s Surface For millions of years, the movement of Earth’s tectonic plates has been building up landforms on Earth’s surface. At the same time, other physi- cal environmental processes are working to change those very same landforms. Imagine a small pile of dirt and rock on a table. If you poured water on the pile, it would move the dirt and rock from one place to another. Like- wise, if you were to blow at the pile, the rock and dirt would also move. The same process happens in nature. Weather, water, and other forces change Earth’s landforms by wearing them away or reshaping them.
Weathering One force that wears away landforms is weathering. Weathering is the process by which rock is broken down into smaller pieces. Several factors cause rock to break down. In desert areas, daytime heating and nighttime cooling can cause rocks to crack. Water may get into cracks in rocks and freeze. The ice then expands with a force great enough to break the rock. Even the roots of trees can pry rocks apart. Regardless of which weathering process is at work, rocks eventually break down. These small pieces of rock are known as sediment. Once weathering has taken place, wind, ice, and water often move sediment from one place to another.
Erosion Another force that changes landforms is the process of erosion. Erosion is the movement of sediment from one location to another. Ero- sion can wear away or build up landforms. Wind, ice, and water all cause erosion. Powerful winds often cause ero- sion. Winds lift sediment into the air and carry it across great distances. On beaches and in deserts, wind can deposit large amounts of sand to form dunes. Blowing sand can also wear down rock. The sand acts like sandpa- per to polish and wear away at rocks. As you can see in the photo, wind can have a dramatic effect on landforms. Earth’s glaciers also have the power to cause massive erosion. Glaciers, or large, slow-moving sheets of ice, build
Reading Check Find Main Ideas What forces below Earth’s surface shape landforms?
Wind Erosion Landforms in Utah’s Canyonlands National Park have been worn away, mostly by thousands of years of powerful winds.
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up when winter snows do not melt the following summer. Glaciers can be huge. Glaciers in Greenland and Antarctica, for example, are great sheets of ice up to two miles (3 km) thick. Some glaciers flow slowly downhill like rivers of ice. As they do so, they erode the land by carving large U-shaped valleys and sharp mountain peaks. As the ice flows downhill, it crushes rock into sediment and can move huge rocks long distances. Water is the most common cause of erosion. Waves in oceans and lakes can wear away the shore, creating jagged coastlines, like those on the coast of Oregon. Rivers also cause erosion. Over many years, the flowing water can cut through rock, forming canyons, or narrow areas with steep walls. Arizona’s Horseshoe Bend and Grand Canyon are examples of canyons cre- ated in this way. Flowing water shapes other landforms as well. When water deposits sediment in new locations, it creates new landforms. For example, in a pro- cess called alluvial deposition , rivers create floodplains when they flood their banks and deposit sediment along the banks. Sediment that is car- ried by a river all the way out to sea creates a delta. The sediment settles to the bottom, where the river meets the sea. The Nile and Mississippi rivers have created two of the world’s largest river deltas.
Reading Check Compare How are weathering and erosion similar?
Water Erosion For millions of years, the Colorado River has worn away the rock at Horseshoe Bend in Arizona.
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