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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 Ideas • Earth’s movement affects the amount of energy we receive from the sun. • Earth’s seasons are caused by the planet’s tilt. Holt McDougal, Main Idea 1: Earth’s movement affects the amount of energy we receive from the sun. • All life on Earth requires solar energy, or energy from the sun, to survive. – Amount of solar energy received changes constantly – Earth’s rotation, revolution, tilt, and latitude all affect the amount of solar energy Earth receives. Holt McDougal, Rotation and Revolution Rotation Revolution • Earth spins on its axis— an imaginary line that runs through the center of the planet around which it turns. • Earth follows an orbit, or path, around the sun. • It takes Earth 24 hours to make one rotation, or complete spin on its axis. • As Earth spins, different parts of the planet face the sun, thus causing the changes from day to night. Holt McDougal, • Orbit is not a perfect circle. • It takes Earth 365¼ days to complete one revolution, or trip around the sun. • Our calendar year is based on the time it takes Earth to complete its orbit. Tilt and Latitude Tilt Latitude • Earth’s axis is tilted at an angle of 23½ degrees from vertical. • Latitude is the distance north or south of Earth’s equator. • At any give time of year, some places on Earth tilt toward the sun, and others tilt away. • Low-latitude areas, those nearest the equator, receive direct rays from the sun all year. • Places tilting toward the sun receive more solar energy and have warmer temperatures than those that tilt away. • High-latitude areas, those farther from the equator, receive indirect rays from the sun and have colder temperatures. Holt McDougal, Main Idea 2: Earth’s seasons are caused by the planet’s tilt. • Seasons are periods during the year that are known for a particular type of weather. – Many places experience four seasons—winter, spring, summer, and fall. – In some parts of the world, seasons are based on the amount of rainfall Holt McDougal, The Seasons Winter and Summer Spring and Fall Rainfall and Seasons • Earth tilts away from sun in winter and towards the sun in summer. • In spring, Earth begins to tilt toward sun, solar energy increases, temperatures rise, and days grow longer. • In the tropics, regions close to the equator, seasons are marked by rainfall rather than temperature. • In fall, the opposite occurs. • At certain times of year, winds bring either dry or moist air to the tropics, creating wet and dry seasons. • Because of the Earth’s tilt, the Northern and Southern hemispheres experience opposite seasons. Holt McDougal, Water on Earth The Big Idea Water is a dominant feature on Earth’s surface and is essential for life. Main Ideas • Salt water and freshwater make up Earth’s water supply. • In the water cycle, water circulates from Earth’s surface to the atmosphere and back again. • Water plays an important role in people’s lives. Holt McDougal, Main Idea 1: Salt water and freshwater make up Earth’s water supply. Earth’s Water • Water covers some two-thirds of the planet. • About 97 percent of the Earth’s water • Unsafe to drink because of high levels of salt Salt Water • In general, found in Earth’s oceans, which cover some 71 percent of the planet’s surface • Also found in some of Earth’s lakes Freshwater • Water without salt • Makes up only 3 percent of our water supply Holt McDougal, Freshwater • Much of Earth’s freshwater is locked in glaciers, large areas of slow moving ice, and in the ice of the Arctic and Antarctic regions. • Surface water is water that is found in Earth’s streams, rivers, and lakes. – Less than one percent of Earth’s water supply – Streams, rivers, and lakes are common sources. • Precipitation is water that falls to Earth’s surface as rain, snow, sleet, or hail. – Streams form when precipitation collects in a narrow channel and flows toward the ocean. • Most available freshwater is groundwater, water found below Earth’s surface. – Some naturally bubbles from the ground to the surface as a spring. – Most obtained by digging wells Holt McDougal, Main Idea 2: In the water cycle, water circulates from Earth’s surface to the atmosphere and back again. • Water is the only substance on Earth that occurs naturally as a solid, a liquid, and a gas, or water vapor. • The water cycle is the movement of water from Earth’s surface to the atmosphere and back ; it is driven by the sun’s energy. – Evaporation—water turns from liquid to gas. – Condensation—the rising gas cools and condenses, or changes from a vapor into tiny liquid droplets, to form clouds. – Precipitation—if the droplets in clouds become heavy enough, they fall back to Earth as rain, snow, sleet, or hail. – Runoff—excess water that isn’t absorbed as groundwater flows over land and collects in streams, rivers, and oceans. Holt McDougal, Main Idea 3: Water plays an important role in people’s lives. Water Problems Water Benefits • Lack of available freshwater, which can be caused by droughts or overuse • Provides us with food to eat • Contaminated, or polluted, water can harm humans, plants, and animals. • Provides us with recreation, including swimming, fishing, surfing, and sailing • Flooding can damage property and threaten lives. Holt McDougal, • Important source of energy The Land The Big Idea Processes below and on Earth’s surface shape the planet’s physical features. Main Ideas • Earth’s surface is covered by many different landforms. • Forces below Earth’s surface build up our landforms. • Forces on the planet’s surface shape Earth’s landforms. • Landforms influence people’s lives and culture. Holt McDougal, Main Idea 1: Earth’s surface is covered by many different landforms. • Landforms, or shapes on the planet’s surface, make up the landscapes that surround us. • Earth’s surface is covered with landforms of many different shapes and sizes. – Mountains, land that rises higher than 2,000 feet – Valleys, areas of low land located between mountains or hills – Plains, stretches of mostly flat land – Islands, areas of land completely surrounded by water – Peninsulas, land surrounded by water on three sides Holt McDougal, Main Idea 2: Forces below Earth’s surface build up our landforms. Earth’s Plates • The planet’s continents, or large landmasses, are part of Earth’s crust—the solid outer layer of the planet. • 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. Movement of Continents • The idea that continents have traveled great distances over millions of years is known as continental drift. • Theory, developed by Alfred Wegener, states that the continents were once united in a single supercontinent and over time, slowly separated and moved to their present positions. • As plates collide, separate, and slide, they shape Earth’s landforms Holt McDougal, Plates Collide, Separate, and Slide Plates Collide • Two ocean plates: one pushes under the other, creating ocean trenches, or deep valleys in ocean floor • Ocean and continental plate: ocean plate drops beneath continental plate, forcing land above to crumple and form mountain range • Two continental plates: land pushes up to form mountains Plates Separate Plates Slide • As plates move apart, gaps between plates allow magma to rise to Earth’s crust. • As plates pass by each other, they sometimes grind together, producing earthquakes, sudden, violent movements of Earth’s crust. • Lava, magma that reaches the Earth’s surface, emerges from the gap. • As lava cools, it builds a mid-ocean range, or underwater mountain, that can rise above the surface of the ocean to form islands. Holt McDougal, • Earthquakes often take place along faults, or breaks in Earth’s crust where movement occurs. • The region around the Pacific plate, called the Ring of Fire, is home to most of the world’s earthquakes and volcanoes. Main Idea 3: Forces on the planet’s surface shape Earth’s landforms. • Weather, water, and other forces change Earth’s landforms by wearing them away or reshaping them. • Weathering is the process by which rock is broken down into smaller pieces called sediment. – Heating and cooling can cause rocks to crack. – Expansion of water as it freezes can cause cracks to expand. – Roots of trees can pry rocks apart. • Erosion is the movement of sediment from one location to another. – Can wear away or build up landforms Holt McDougal, Types of Erosion Wind Erosion • Winds lift sediment into air and carry it across great distances . • On beaches and in deserts, deposits can form dunes. • Blowing sand can wear down rock. Glacial Erosion Water Erosion • Glaciers, or large, slow-moving sheets of ice, erode the land by carving valleys and mountain peaks. • Waves and flowing water can cut through rock, carry sediment, and deposit sediment in new locations. • Crush rock into sediment and move it great distances • Floodplains are created when rivers flood their banks and deposit sediment. • Sediment carried by a river all the way to the sea creates a delta. Holt McDougal, Main Idea 4: Landforms influence people’s lives and culture. Landforms can influence where people settle. Landforms often influence what jobs are available in a region. Landforms can affect language. People sometimes change landforms to suit their needs. Holt McDougal,