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L_0073 | nonrenewable energy resources | T_0712 | Coal is a black or brownish-black rock that burns easily (Figure 5.3). Most coal is sedimentary rock. The hardest type of coal, anthracite, is a metamorphic rock. That is because it is exposed to higher temperature and pressure as it forms. Coal is mostly carbon, but some other elements can be found in coal, including sulfur. | text | null |
L_0073 | nonrenewable energy resources | T_0713 | Around the world, coal is the largest source of energy for electricity. The United States is rich in coal. Pennsylvania and the region to the west of the Appalachian Mountains are some of the most coal-rich areas of the United States. Coal has to be mined to get it out of the ground. Coal mining affects the environment and human health. Coal mining can take place underground or at the surface. Each method has some advantages and disadvantages. Surface mining exposes minerals that were underground to air and water at the surface. These minerals contain the chemical element sulfur. Sulfur mixes with air and water to make sulfuric acid. This acid is a highly corrosive chemical. Sulfuric acid gets into nearby streams and can kill fish, plants, and animals. Surface mining is safer for the miners. Coal mining underground is dangerous for the coal miners. Miners are sometimes killed if there is an explosion or a mine collapse. Miners breathe in coal dust and can get terrible lung diseases after a number of years in the mines. | text | null |
L_0073 | nonrenewable energy resources | T_0714 | To prepare coal for use, the coal is first crushed into powder and burned in a furnace. Like other fuels, coal releases most of its energy as heat when it burns. The heat from the burning coal is used to boil water. This makes steam. The steam spins turbines, which creates electricity. | text | null |
L_0073 | nonrenewable energy resources | T_0715 | Oil is a thick, dark brown or black liquid. It is found in rock layers of the Earths crust. Oil is currently the most commonly used source of energy in the world. | text | null |
L_0073 | nonrenewable energy resources | T_0716 | The way oil forms is similar in many ways to coal. Tiny organisms like plankton and algae die and settle to the bottom of the sea. Sediments settle over the organic material. Oxygen is kept away by the sediments. When the material is buried deep enough, it is exposed to high heat and pressure. Over millions of years, the organic material transforms into liquid oil. | text | null |
L_0073 | nonrenewable energy resources | T_0717 | The United States produces only about one-quarter as much oil as it uses. The main oil producing regions in the U.S. are the Gulf of Mexico, Texas, Alaska, and California. Geologists look for oil in folded layers of rock called anticlines. Oil moves through permeable rock and is trapped by the impermeable cap rock. | text | null |
L_0073 | nonrenewable energy resources | T_0718 | Oil comes out of the ground as crude oil. Crude oil is a mixture of many different hydrocarbons. Oil is separated into different compounds at an oil refinery (Figure 5.4). This is done by heating the oil. Each hydrocarbon compound in crude oil boils at a different temperature. We get gasoline, diesel, and heating oil, plus waxes, plastics, and fertilizers from crude oil. These fuels are rich sources of energy. Since they are mostly liquids they can be easily transported. These fuels provide about 90% of the energy used for transportation around the world. | text | null |
L_0073 | nonrenewable energy resources | T_0719 | Gasoline is a concentrated resource. It contains a large amount of energy for its weight. This is important because the more something weighs, the more energy is needed to move it. If gasoline could only provide a little energy, a car would have to carry a lot of it to be able to travel very far. Or the car would need to be filled up frequently. So a highly concentrated energy resource is a practical fuel to power cars and other forms of transportation. Lets consider how gasoline powers a car. As gasoline burns, it releases most of its energy as heat. It also releases carbon dioxide gas and water vapor. The heat makes the gases expand. This forces the pistons inside the engine to move. The engine makes enough power to move the car. | text | null |
L_0073 | nonrenewable energy resources | T_0720 | Using gasoline to power automobiles affects the environment. The exhaust fumes from burning gasoline cause air pollution. These pollutants include smog and ground-level ozone. Air pollution is a big problem for cities where large numbers of people drive every day. Burning gasoline also produces carbon dioxide. This is a greenhouse gas and is a cause of global warming. Similar pollutants come from other forms of oil. | text | null |
L_0073 | nonrenewable energy resources | T_0721 | Natural gas is mostly methane. | text | null |
L_0073 | nonrenewable energy resources | T_0722 | Natural gas is often found along with coal or oil in underground deposits. This is because natural gas forms with these other fossil fuels. One difference between natural gas and oil is that natural gas forms at higher temperatures. | text | null |
L_0073 | nonrenewable energy resources | T_0723 | The largest natural gas reserves in the United States are located in the Rocky Mountain states, Texas, and the Gulf of Mexico region. California also has natural gas, mostly in the northern Sacramento Valley and the Sacramento Delta. Natural gas must be processed before it can be used as a fuel. Poisonous chemicals and water must be removed. Natural gas is delivered to homes, where it is used for cooking and heating. Natural gas is also a major energy source for powering turbines to make electricity. Natural gas releases most of its energy as heat when it burns. The power plant is able to use this heat, either in the form of hot gases or steam, to spin turbines. The spinning turbines turn generators, and the generators create electricity. | text | null |
L_0073 | nonrenewable energy resources | T_0724 | Processing natural gas has harmful effects on the environment, just like oil. Natural gas burns cleaner than other fossil fuels. As a result, it causes less air pollution. It also produces less carbon dioxide than the other fossil fuels. Still, natural gas does emit pollutants. | text | null |
L_0073 | nonrenewable energy resources | T_0725 | Fossil fuels present many problems. These fuels are non-renewable resources, so our supplies of them will eventually run out. Safety can be a problem, too. Since these fuels burn so easily, a natural gas leak in a building or an underground pipe can lead to a deadly explosion. Using fossil fuels affects the environment in a variety of ways. There are impacts to the environment when we extract these resources. Burning these fuels causes air pollution. These fuels release carbon dioxide, which is a major factor in global warming (Figure 5.5). Many of the problems with fossil fuels are worse for coal than for oil or natural gas. Burning coal releases more carbon dioxide than either oil or natural gas. Yet coal is the most common fossil fuel, so we continue to burn large amounts of it. That makes coal the biggest contributor to global warming. Another problem with coal is that most coal contains sulfur. As it burns, the sulfur goes into the air as sulfur dioxide. Sulfur dioxide is the main cause of acid rain. Acid rain can be deadly to plants, animals, and whole ecosystems. Burning coal also puts a large number of small solid particulates into the air. These particles are dangerous to people, especially those who have asthma. People with asthma may end up in the hospital on days when particulate pollution is high. | text | null |
L_0073 | nonrenewable energy resources | T_0726 | Nuclear energy is produced by splitting the nucleus of an atom. This releases a huge amount of energy. | text | null |
L_0073 | nonrenewable energy resources | T_0727 | Nuclear power plants use uranium that has been concentrated in fuel rods (Figure 5.6). The uranium atoms are split apart when they are hit by other extremely tiny particles. These particles must be controlled or they would cause a dangerous explosion. Nuclear power plants use the energy they produce to heat water. The water turns into steam, which causes a turbine to spin. This in turn produces electricity. | text | null |
L_0073 | nonrenewable energy resources | T_0728 | Many countries around the world use nuclear energy as a source of electricity. For example, France gets about 80% of its electricity from nuclear energy. In the United States, a little less than 20% of electricity comes from nuclear energy. Nuclear energy does not pollute. If there are no accidents, a nuclear power plant releases nothing but steam into the air. But nuclear energy does create other environmental problems. Splitting atoms creates dangerous radioactive waste. These wastes can remain dangerous for hundreds of thousands of years. Scientists and engineers are still looking for ways to keep this waste safely away from people. | text | null |
L_0073 | nonrenewable energy resources | T_0729 | Nuclear power is a controversial subject in California and most other places. Nuclear power has no pollutants including carbon emissions, but power plants are not always safe and the long-term disposal of wastes is a problem that has not yet been solved. The future of nuclear power is murky. Find out more at: http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0074 | renewable energy resources | T_0730 | null | text | null |
L_0074 | renewable energy resources | T_0731 | The Sun is Earths main source of energy. The Sun gives us both light and heat. The Sun changes hydrogen into helium through nuclear fusion. This releases huge amounts of energy. The energy travels to the Earth mostly as visible light. The energy is carried through the empty space by radiation. We can use sunlight as an energy resource, called solar energy (Figure 5.7). | text | null |
L_0074 | renewable energy resources | T_0732 | Solar energy has been used on a small scale for hundreds of years. Today we are using solar energy for more of our power demands. Solar power plants are being built in many locations around the world. In the United States, the southwestern deserts are well suited for solar plants. | text | null |
L_0074 | renewable energy resources | T_0733 | Sunlight is turned into electricity at a solar power plant. These power plants use a large group of mirrors to focus sunlight on one place. This place is called a receiver (Figure 5.8). At the receiver, a liquid such as oil or water is heated to a high temperature. The liquid transfers its heat by conduction. In conduction, energy moves between two objects that are in contact. The higher temperature object transfers heat to the lower temperature object. For example, when you heat a pot of water on a stove top, energy moves from the pot to its metal handle by conduction. At a solar power plant, the energy conducted by the heated liquid is used to make electricity. | text | null |
L_0074 | renewable energy resources | T_0734 | Solar energy is used to heat homes and water, and to make electricity. Scientists and engineers have many ways to get energy from the Sun (Figure 5.9). One is by using solar cells. Solar cells are devices that turn sunlight directly into electricity. Lots of solar cells make up an individual solar panel. You may have seen solar panels on roof tops. The Suns heat can also be trapped in your home by using south facing windows and good insulation. | text | null |
L_0074 | renewable energy resources | T_0735 | Solar energy has many benefits. It does not produce any pollution. There is plenty of it available, much more than we could possibly use. But solar energy has problems. The Sun doesnt shine at night. A special battery is needed to store extra energy during the day for use at night. The technology for most uses of solar energy is still expensive. Until solar technology becomes more affordable, most people will prefer to get their energy from other sources. | text | null |
L_0074 | renewable energy resources | T_0735 | Solar energy has many benefits. It does not produce any pollution. There is plenty of it available, much more than we could possibly use. But solar energy has problems. The Sun doesnt shine at night. A special battery is needed to store extra energy during the day for use at night. The technology for most uses of solar energy is still expensive. Until solar technology becomes more affordable, most people will prefer to get their energy from other sources. | text | null |
L_0074 | renewable energy resources | T_0736 | Moving water has energy (Figure 5.10). That energy is used to make electricity. Hydroelectric power harnesses the energy of water moving down a stream. Hydropower is the most widely used form of renewable energy in the world. This abundant energy source provides almost one fifth of the worlds electricity. The energy of waves and tides can also be used to produce water power. At this time, wave and tidal power are rare. | text | null |
L_0074 | renewable energy resources | T_0737 | To harness water power, a stream must be dammed. Narrow valleys are the best for dams. While sitting in the reservoir behind the dam, the water has potential energy. Water is allowed to flow downhill into a large turbine. While flowing downhill, the water has kinetic energy. Kinetic energy makes the turbine spin. The turbine is connected to a generator, which makes electricity. | text | null |
L_0074 | renewable energy resources | T_0738 | Many of the suitable streams in the United States have been developed for hydroelectric power. Many streams worldwide also have hydroelectric plants. Hydropower is a major source of Californias electricity. It accounts for about 14.5 percent of the total. Most of Californias nearly 400 hydroelectric power plants are located in the Sierra Nevada mountains. | text | null |
L_0074 | renewable energy resources | T_0739 | Water power does not burn a fuel. So it causes less pollution than many other kinds of energy. Water power is also a renewable resource. Water keeps flowing downhill. Although we use some of the energy from this movement, we are not using up the water. Water power does have problems. A large dam stops a streams flow, which floods the land upstream. A beautiful location may be lost. People may be displaced. The dams and turbines also change the downstream environment. Fish and other living things may not be able to survive. Dams slow the release of silt. Downstream deltas retreat and beaches may be starved of sand. Seaside cities may become exposed to storms and rising sea levels. Tidal power stations may need to close off a narrow bay or estuary. Wave power plants must withstand coastal storms and the corrosion of seawater. | text | null |
L_0074 | renewable energy resources | T_0740 | Although not yet widely used, many believe tidal power has more potential than wind or solar power for meeting alternative energy needs. Quest radio looks at plans for harnessing power from the sea by San Francisco and along the northern California coast. Learn more at: http://science.kqed.org/quest/audio/harnessing-power-from-the-sea/ MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0074 | renewable energy resources | T_0741 | The energy from the Sun creates wind (Figure 5.11). Wind energy moves by convection. The Sun heats some locations more than others. Warm air rises, so other air rushes in to fill the hole left by the rising air. This horizontal movement of air is called wind. | text | null |
L_0074 | renewable energy resources | T_0742 | Wind power uses moving air as a source of energy. Some types of wind power have been around for a long time. People have used windmills to grind grain and pump water for hundreds of years. Sailing ships have depended on wind for millennia. Wind is now used to generate electricity. Moving air can make a turbine spin, just like moving water can. Moving air has kinetic energy. When wind hits the blades of the turbine, the kinetic energy makes the blades move. The turbine spins and creates electricity. | text | null |
L_0074 | renewable energy resources | T_0743 | Wind power has many advantages. It is clean: it does not release pollutants or carbon dioxide. It is plentiful almost everywhere. The technology to harness wind energy is being developed rapidly. Wind power also has problems. Wind does not blow all of the time, so wind energy must be stored for later use. Alternatively, another energy source needs to be available when the wind is not blowing. Wind turbines are expensive. They can wear out quickly. Finally, windmills are not welcomed by residents of some locations. They say that they are unattractive. Yet even with these problems, wind turbines are a competitive form of renewable energy. Many states are currently using wind power. Wind turbines are set up in mountain passes. This is common in California, where cool Pacific Ocean air is sucked across the passes and into the warmer inland valleys. | text | null |
L_0074 | renewable energy resources | T_0744 | Biomass is another renewable source of energy. Biomass includes wood, grains, and other plant materials or waste materials. People can burn wood directly for energy in the form of heat. Biomass can also be processed to make biofuel. Biofuel is a fairly new type of energy that is becoming more popular. Biomass is useful because it can be made liquid. This means that they can be used in cars and trucks. Some car engines can be powered by pure vegetable oil or even recycled vegetable oil. Sometimes the exhaust from these cars smells like French fries! By using biofuels, we can cut down on the amount of fossil fuel that we use. Because living plants take carbon dioxide out of the air, growing plants for biofuel can mean that we will put less of this gas into the air overall. This could help us do something about the problem of global warming. | text | null |
L_0074 | renewable energy resources | T_0745 | Geothermal energy comes the Earths internal heat. Hot springs and geysers are produced by water that is heated by magma or hot rock below the surface. At a geothermal power plant, engineers drill wells into the hot rocks. Hot water or steam may come up through the wells. Alternatively, water may be put down into the well to be heated. It then comes up. The hot water or steam makes a turbine spin. This makes electricity. | text | null |
L_0074 | renewable energy resources | T_0746 | Because the hot water or steam can be used directly to make a turbine spin, geothermal energy can be used without processing. Geothermal energy is clean and safe. It is renewable. There will always be hot rocks and water can be pumped down into a well. There, the water can be heated again to make more steam. Geothermal energy is an excellent resource in some parts of the world. Iceland is gets about one fourth of its electricity from geothermal sources. In the United States, California leads all states in producing geothermal energy. Geothermal energy in California is concentrated in the northern part of the state. The largest plant is in the Geysers Geothermal Resource Area. Geothermal energy is not economical everywhere. Many parts of the world do not have underground sources of heat that are close enough to the surface for building geothermal power plants. | text | null |
L_0074 | renewable energy resources | T_0747 | Where Earths internal heat gets close to the surface, geothermal power is a clean source of energy. In California, The Geysers supplies energy for many nearby homes and businesses. Learn more at: http://science.kqed.org/ques MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0076 | continental drift | T_0757 | Alfred Wegener was an early 20th century German meteorologist. Wegener believed that the continents were once all joined together. He named the supercontinent Pangaea, meaning all earth. Wegener suggested that Pangaea broke up long ago. Since then, the continents have been moving to their current positions. He called his hypothesis continental drift. | text | null |
L_0076 | continental drift | T_0758 | Wegener and his supporters collected a great deal of evidence for the continental drift hypothesis. Wegener found that this evidence was best explained if the continents had at one time been joined together. | text | null |
L_0076 | continental drift | T_0759 | Wegener found rocks of the same type and age on both sides of the Atlantic Ocean. He thought that the rocks formed side by side. These rocks then drifted apart on separate continents. Wegener also matched up mountain ranges across the Atlantic Ocean. The Appalachian Mountains were just like mountain ranges in eastern Greenland, Ireland, Great Britain, and Norway. Wegener concluded that they formed as a single mountain range. This mountain range broke apart as the continents split up. The mountain range separated as the continents drifted. | text | null |
L_0076 | continental drift | T_0760 | Wegener also found evidence for continental drift from fossils (Figure 6.7). The same type of plant and animal fossils are found on continents that are now widely separated. These organisms would not have been able to travel across the oceans. Fossils of the seed fern Glossopteris are found across all of the southern continents. These seeds are too heavy to be carried across the ocean by wind. Mesosaurus fossils are found in South America and South Africa. Mesosaurus could swim, but only in fresh water. Cynognathus and Lystrosaurus were reptiles that lived on land. Both of these animals were unable to swim at all. Their fossils have been found across South America, Africa, India and Antarctica. Wegener thought that all of these organisms lived side by side. The lands later moved apart so that the fossils are separated. | text | null |
L_0076 | continental drift | T_0761 | Wegener also looked at evidence from ancient glaciers. Glaciers are found in very cold climates near the poles. The evidence left by some ancient glaciers is very close to the equator. Wegener knew that this was impossible! However, if the continents had moved, the glaciers would have been centered close to the South Pole. | text | null |
L_0076 | continental drift | T_0762 | Coral reefs are found only in warm water. Coal swamps are also found in tropical and subtropical environments. Wegener discovered ancient coal seams and coral reef fossils in areas that are much too cold today. Wegener thought that the continents have moved since the time of Pangaea. | text | null |
L_0076 | continental drift | T_0763 | Some important evidence for continental drift came after Wegeners death. This is the magnetic evidence. Earths magnetic field surrounds the planet from pole to pole. If you have ever been hiking or camping, you may have used a compass to help you find your way. A compass points to the magnetic North Pole. The compass needle aligns with Earths magnetic field (Figure 6.8). Some rocks contain little compasses too! As lava cools, tiny iron-rich crystals line up with Earths magnetic field. | text | null |
L_0079 | stress in earths crust | T_0793 | Stress is the force applied to a rock. There are four types of stresses: Confining stress happens as weight of all the overlying rock pushes down on a deeply buried rock. The rock is being pushed in from all sides, which compresses it. The rock will not deform because there is no place for it to move. Compression stress squeezes rocks together. Compression causes rocks to fold or fracture (Figure 7.1). When two cars collide, compression causes them to crumple. Compression is the most common stress at convergent plate boundaries. Tension stress pulls rocks apart. Tension causes rocks to lengthen or break apart. Tension is the major type of stress found at divergent plate boundaries. Shear stress happens when forces slide past each other in opposite directions (Figure 7.2). This is the most common stress found at transform plate boundaries. The amount of stress on a rock may be greater than the rocks strength. In that case, the rock will change and deform (Figure 7.3). Deep within the Earth, the pressure is very great. A rock behaves like a stretched rubber band. When the stress stops, the rock goes back to its original shape. If more stress is applied to the rock, it bends and flows. It does not return to its original shape. Near the surface, if the stress continues, the rock will fracture and break. | text | null |
L_0079 | stress in earths crust | T_0794 | Sedimentary rocks are formed in horizontal layers. This is magnificently displayed around the southwestern United States. The arid climate allows rock layers to be well exposed (Figure 7.4). The lowest layers are the oldest and the higher layers are younger. Folds, joints and faults are caused by stresses. Figure 7.5 shows joints in a granite hillside. If a sedimentary rock is tilted or folded, we know that stresses have changed the rock (Figure 7.6). | text | null |
L_0079 | stress in earths crust | T_0794 | Sedimentary rocks are formed in horizontal layers. This is magnificently displayed around the southwestern United States. The arid climate allows rock layers to be well exposed (Figure 7.4). The lowest layers are the oldest and the higher layers are younger. Folds, joints and faults are caused by stresses. Figure 7.5 shows joints in a granite hillside. If a sedimentary rock is tilted or folded, we know that stresses have changed the rock (Figure 7.6). | text | null |
L_0079 | stress in earths crust | T_0795 | Deep within the Earth, as plates collide, rocks crumple into folds. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. In sedimentary rocks, you can easily trace the folding of the layers. In the Figure 7.6, the rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. Once rocks are folded, they do not return to their original shape. There are three types of folds: monoclines, anticlines, and synclines. A monocline is a simple one step bend in the rock layers (Figure 7.7). In a monocline, the oldest rocks are still at the bottom and the youngest are at the top. An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure 7.8). The oldest rocks are found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks are exposed at the center. A syncline is a fold that bends downward (Figure 7.9). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a basin. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin. | text | null |
L_0079 | stress in earths crust | T_0795 | Deep within the Earth, as plates collide, rocks crumple into folds. You can model these folds by placing your hands on opposite edges of a piece of cloth and pushing your hands together. In sedimentary rocks, you can easily trace the folding of the layers. In the Figure 7.6, the rock layers are no longer horizontal. They tilt downhill from right to left in a monocline. Once rocks are folded, they do not return to their original shape. There are three types of folds: monoclines, anticlines, and synclines. A monocline is a simple one step bend in the rock layers (Figure 7.7). In a monocline, the oldest rocks are still at the bottom and the youngest are at the top. An anticline is a fold that arches upward. The rocks dip away from the center of the fold (Figure 7.8). The oldest rocks are found at the center of an anticline. The youngest rocks are draped over them at the top of the structure. When upward folding rocks form a circular structure, that structure is called a dome. If the top of the dome is eroded off, the oldest rocks are exposed at the center. A syncline is a fold that bends downward (Figure 7.9). In a syncline, the youngest rocks are at the center. The oldest rocks are at the outside edges. When rocks bend downward in a circular structure, it is called a basin. If the rocks are eroded, the youngest rocks are at the center. Basins can be enormous, like the Michigan Basin. | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0796 | With enough stress, a rock will fracture, or break. The fracture is called a joint if the rock breaks but doesnt move, as shown in Figure 7.10. If the rocks on one or both sides of a fracture move, the fracture is called a fault (Figure 7.11). Faults can occur alone or in clusters, creating a fault zone. Earthquakes happen when rocks break and move suddenly. The energy released causes an earthquake. Slip is the distance rocks move along a fault, as one block of rock moves past the other. The angle of a fault is called When compression squeezes the crust into a smaller space, the hanging wall pushes up relative to the footwall. This creates a reverse fault. A thrust fault is a type of reverse fault where the angle is nearly horizontal. Rocks can slip many miles along thrust faults (Figure 7.13). | text | null |
L_0079 | stress in earths crust | T_0797 | A strike-slip fault is a dip-slip fault where the dip of the fault plane is vertical. Strike-slip faults result from shear stresses. If you stand with one foot on each side of a strike-slip fault, one side will be moving toward you while the other side moves away from you. If your right foot moves toward you, the fault is known as a right-lateral strike-slip fault. If your left foot moves toward you, the fault is a left-lateral strike-slip fault (Figure 7.14). | text | null |
L_0079 | stress in earths crust | T_0798 | The San Andreas Fault in California is a right-lateral strike-slip fault (Figure 7.15). It is also a transform fault because the San Andreas is a plate boundary. As you can see, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northeastward, while the North American plate moves southwest. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco! | text | null |
L_0079 | stress in earths crust | T_0798 | The San Andreas Fault in California is a right-lateral strike-slip fault (Figure 7.15). It is also a transform fault because the San Andreas is a plate boundary. As you can see, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northeastward, while the North American plate moves southwest. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco! | text | null |
L_0079 | stress in earths crust | T_0798 | The San Andreas Fault in California is a right-lateral strike-slip fault (Figure 7.15). It is also a transform fault because the San Andreas is a plate boundary. As you can see, California will not fall into the ocean someday. The land west of the San Andreas Fault is moving northeastward, while the North American plate moves southwest. Someday, millions of years from now, Los Angeles will be a suburb of San Francisco! | text | null |
L_0079 | stress in earths crust | T_0799 | Many processes create mountains. Most mountains form along plate boundaries. A few mountains may form in the middle of a plate. For example, huge volcanoes are mountains formed at hotspots within the Pacific Plate. | text | null |
L_0079 | stress in earths crust | T_0800 | Most of the worlds largest mountains form as plates collide at convergent plate boundaries. Continents are too buoyant to get pushed down into the mantle. So when the plates smash together, the crust crumples upwards. This creates mountains. Folding and faulting in these collision zones makes the crust thicker. The worlds highest mountain range, the Himalayas, is growing as India collides with Eurasia. About 80 million years ago, India was separated from Eurasia by an ocean (Figure 7.16). As the plates collided, pieces of the old seafloor were forced over the Asian continent. This old seafloor is now found high in the Himalayas (Figure 7.17). | text | null |
L_0079 | stress in earths crust | T_0801 | Volcanic mountain ranges form when oceanic crust is pushed down into the mantle at convergent plate boundaries. The Andes Mountains are a chain of coastal volcanic mountains. They are forming as the Nazca plate subducts beneath the South American plate (Figure 7.18). | text | null |
L_0079 | stress in earths crust | T_0802 | Mid-ocean ridges form at divergent plate boundaries. As the ocean floor separates an enormous line of volcanoes is created. When continental crust is pulled apart, it breaks into blocks. These blocks of crust are separated by normal faults. The blocks slide up or down. The result is alternating mountain ranges and valleys. This topography is known as basin-and-range (Figure 7.19). The area near Death Valley, California is the center of a classic basin-and-range province (Figure 7.20). | text | null |
L_0079 | stress in earths crust | T_0802 | Mid-ocean ridges form at divergent plate boundaries. As the ocean floor separates an enormous line of volcanoes is created. When continental crust is pulled apart, it breaks into blocks. These blocks of crust are separated by normal faults. The blocks slide up or down. The result is alternating mountain ranges and valleys. This topography is known as basin-and-range (Figure 7.19). The area near Death Valley, California is the center of a classic basin-and-range province (Figure 7.20). | text | null |
L_0079 | stress in earths crust | T_0802 | Mid-ocean ridges form at divergent plate boundaries. As the ocean floor separates an enormous line of volcanoes is created. When continental crust is pulled apart, it breaks into blocks. These blocks of crust are separated by normal faults. The blocks slide up or down. The result is alternating mountain ranges and valleys. This topography is known as basin-and-range (Figure 7.19). The area near Death Valley, California is the center of a classic basin-and-range province (Figure 7.20). | text | null |
L_0086 | igneous landforms and geothermal activ | T_0863 | Extrusive igneous rocks cool at the surface. Volcanoes are one type of feature that forms from extrusive rocks. Several other interesting landforms are also extrusive features. Intrusive igneous rocks cool below the surface. These rocks do not always remain hidden. Rocks that formed in the crust are exposed when the rock and sediment that covers them is eroded away. | text | null |
L_0086 | igneous landforms and geothermal activ | T_0864 | When lava is thick, it flows slowly. If thick lava makes it to the surface, it cannot flow far from the vent. It often stays right in the middle of a crater at the top of a volcano. Here the lava creates a large, round lava dome (Figure | text | null |
L_0086 | igneous landforms and geothermal activ | T_0865 | A lava plateau is made of a large amount of fluid lava. The lava flows over a large area and cools. This creates a large, flat surface of igneous rock. Lava plateaus may be huge. The Columbia Plateau covers over 161,000 square kilometers (63,000 square miles). It makes up parts of the states of Washington, Oregon, and Idaho. Thin, fluid lava created the rock that makes up the entire ocean floor. This is from multiple eruptions from vents at the mid-ocean ridge. While not exactly a lava plateau, its interesting to think about so much lava! | text | null |
L_0086 | igneous landforms and geothermal activ | T_0866 | New land is created in volcanic eruptions. The Hawaiian Islands are shield volcanoes. These volcanoes formed from fluid lava (Figure 8.21). The island grows as lava is added on the coast. New land may also emerge from lava that erupts from beneath the water. This is one way that new land is created. | text | null |
L_0086 | igneous landforms and geothermal activ | T_0867 | Magma that cools underground forms intrusions (Figure 8.22). Intrusions become land formations if they are exposed at the surface by erosion. | text | null |
L_0086 | igneous landforms and geothermal activ | T_0868 | Water works its way through porous rocks or soil. Sometimes this water is heated by nearby magma. If the water makes its way to the surface, it forms a hot spring or a geyser. | text | null |
L_0086 | igneous landforms and geothermal activ | T_0869 | When hot water gently rises to the surface, it creates a hot spring. A hot spring forms where a crack in the Earth allows water to reach the surface after being heated underground. Many hot springs are used by people as natural hot tubs. Some people believe that hot springs can cure illnesses. Hot springs are found all over the world, even in Antarctica! | text | null |
L_0086 | igneous landforms and geothermal activ | T_0870 | Geysers are also created by water that is heated beneath the Earths surface. The water may become superheated by magma. It becomes trapped in a narrow passageway. The heat and pressure build as more water is added. When the pressure is too much, the superheated water bursts out onto the surface. This is a geyser. There are only a few areas in the world where the conditions are right for the formation of geysers. Only about 1,000 geysers exist worldwide. About half of them are in the United States. The most famous geyser is Old Faithful at Yellowstone National Park (Figure 8.23). It is rare for a geyser to erupt so regularly, which is why Old Faithful is famous. | text | null |
L_0087 | weathering | T_0871 | Weathering changes solid rock into sediments. Sediments are different sizes of rock particles. Boulders are sedi- ments; so is gravel. At the other end, silt and clay are also sediments. Weathering causes rocks at the Earths surface to change form. The new minerals that form are stable at the Earths surface. It takes a long time for a rock or mountain to weather. But a road can do so much more quickly. If you live in a part of the world that has cold winters, you may only have to wait one year to see a new road start to weather (Figure | text | null |
L_0087 | weathering | T_0872 | Mechanical weathering breaks rock into smaller pieces. These smaller pieces are just like the bigger rock; they are just smaller! The rock has broken without changing its composition. The smaller pieces have the same minerals in the same proportions. You could use the expression a chip off the old block to describe mechanical weathering! The main agents of mechanical weathering are water, ice, and wind. | text | null |
L_0087 | weathering | T_0873 | Rocks can break apart into smaller pieces in many ways. Ice wedging is common where water goes above and below its freezing point (Figure 9.2). This can happen in winter in the mid-latitudes or in colder climates in summer. Ice wedging is common in mountainous regions. This is how ice wedging works. When liquid water changes into solid ice, it increases in volume. You see this when you fill an ice cube tray with water and put it in the freezer. The ice cubes go to a higher level in the tray than the water. You also may have seen this if you put a can of soda into the freezer so that it cools down quickly. If you leave the can in the freezer too long, the liquid expands so much that it bends or pops the can. (For the record, water is very unusual. Most substances get smaller when they change from a liquid to a solid.) | text | null |
L_0087 | weathering | T_0874 | Abrasion is another type of mechanical weathering. With abrasion, one rock bumps against another rock. Gravity causes abrasion as a rock tumbles down a slope. Moving water causes abrasion it moves rocks so that they bump against one another (Figure 9.3). Strong winds cause abrasion by blasting sand against rock surfaces. Finally, the ice in glaciers cause abrasion. Pieces of rock embedded in ice at the bottom of a glacier scrape against the rock below. If you have ever collected beach glass or pebbles from a stream, you have witnessed the work of abrasion. | text | null |
L_0087 | weathering | T_0875 | Sometimes biological elements cause mechanical weathering. This can happen slowly. A plants roots grow into a crack in rock. As the roots grow larger, they wedge open the crack. Burrowing animals can also cause weathering. By digging for food or creating a hole to live in the animal may break apart rock. Today, human beings do a lot of mechanical weathering whenever we dig or blast into rock. This is common when we build homes, roads, and subways, or quarry stone for construction or other uses. | text | null |
L_0087 | weathering | T_0876 | Mechanical weathering increases the rate of chemical weathering. As rock breaks into smaller pieces, the surface area of the pieces increases. With more surfaces exposed, there are more places for chemical weathering to occur. Lets say you wanted to make some hot chocolate on a cold day. It would be hard to get a big chunk of chocolate to dissolve in your milk or hot water. Maybe you could make hot chocolate from some smaller pieces like chocolate chips, but it is much easier to add a powder to your milk. This is because the smaller the pieces are, the more surface area they have. Smaller pieces dissolve more easily. | text | null |
L_0087 | weathering | T_0877 | Chemical weathering is different than mechanical weathering. The minerals in the rock change. The rock changes composition and becomes a different type of rock. Most minerals form at high pressure or high temperatures deep within Earth. But at Earths surface, temperatures and pressures are much lower. Minerals that were stable deeper in the crust are not stable at the surface. Thats why chemical weathering happens. Minerals that formed at higher temperature and pressure change into minerals that are stable at the surface. Chemical weathering is important. It starts the process of changing solid rock into soil. We need soil to grow food and create other materials we need. Chemical weathering works through chemical reactions that change the rock. There are many agents of chemical weathering. Remember that water was a main agent of mechanical weathering. Well, water is also an agent of chemical weathering. That makes it a double agent! Carbon dioxide and oxygen are also agents of chemical weathering. Each of these is discussed below. | text | null |
L_0087 | weathering | T_0878 | Water is an amazing molecule. It has a very simple chemical formula, H2 O. It is made of just two hydrogen atoms bonded to one oxygen atom. Water is remarkable in terms of all the things it can do. Lots of things dissolve easily in water. Some types of rock can even completely dissolve in water! Other minerals change by adding water into their structure. | text | null |
L_0087 | weathering | T_0879 | Carbon dioxide (CO2 ) combines with water as raindrops fall through the air. This makes a weak acid, called carbonic acid. This happens so often that carbonic acid is a common, weak acid found in nature. This acid works to dissolve rock. It eats away at sculptures and monuments. While this is normal, more acids are made when we add pollutants to the air. Any time we burn any fossil fuel, it adds nitrous oxide to the air. When we burn coal rich in sulfur, it adds sulfur dioxide to the air. As nitrous oxide and sulfur dioxide react with water, they form nitric acid and sulfuric acid. These are the two main components of acid rain. Acid rain accelerates chemical weathering. | text | null |
L_0087 | weathering | T_0880 | Oxygen strongly reacts with elements at the Earths surface. You are probably most familiar with the rust that forms when iron reacts with oxygen (Figure 9.4). Many minerals are rich in iron. They break down as the iron changes into iron oxide. This makes the red color in soils. Plants and animals also cause chemical weathering. As plant roots take in nutrients, elements are exchanged. | text | null |
L_0087 | weathering | T_0881 | Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills. Devils Tower in Wyoming shows how different types of rock weather at different rates (Figure 9.5). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind. Minerals also weather differently. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rocks surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock. | text | null |
L_0087 | weathering | T_0881 | Each type of rock weathers in its own way. Certain types of rock are very resistant to weathering. Igneous rocks tend to weather slowly because they are hard. Water cannot easily penetrate them. Granite is a very stable igneous rock. Other types of rock are easily weathered because they dissolve easily in weak acids. Limestone is a sedimentary rock that dissolves easily. When softer rocks wear away, the more resistant rocks form ridges or hills. Devils Tower in Wyoming shows how different types of rock weather at different rates (Figure 9.5). The softer materials of the surrounding rocks were worn away. The resistant center of the volcano remains behind. Minerals also weather differently. Some minerals completely dissolve in water. As less resistant minerals dissolve away, a rocks surface becomes pitted and rough. When a less resistant mineral dissolves, more resistant mineral grains are released from the rock. | text | null |
L_0089 | acid rain | T_0900 | Acid rain is caused by sulfur and nitrogen oxides emanating from power plants or metal refineries. The smokestacks have been built tall so that pollutants dont sit over cities (Figure 1.1). As they move, these pollutants combine with water vapor to form sulfuric and nitric acids. The acid droplets form acid fog, rain, snow, or they may be deposited dry. Most typical is acid rain (Figure 1.2). | text | null |
L_0089 | acid rain | T_0901 | Acid rain water is more acidic than normal rain water. Acidity is measured on the pH scale. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline) (Figure 1.3). Natural rain is somewhat acidic, with a pH of 5.6; acid rain must have a pH of less than 5.0. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic. Regions with a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet-bowl cleaner. In arid climates, such as in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are five to ten times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need. A pH scale goes from 1 to 14; numbers are shown with the pH of some common substances. A value of 7 is neutral. The strongest acids are at the low end of the scale and the strongest bases are at the high end. | text | null |
L_0089 | acid rain | T_0901 | Acid rain water is more acidic than normal rain water. Acidity is measured on the pH scale. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline) (Figure 1.3). Natural rain is somewhat acidic, with a pH of 5.6; acid rain must have a pH of less than 5.0. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic. Regions with a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet-bowl cleaner. In arid climates, such as in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are five to ten times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need. A pH scale goes from 1 to 14; numbers are shown with the pH of some common substances. A value of 7 is neutral. The strongest acids are at the low end of the scale and the strongest bases are at the high end. | text | null |
L_0089 | acid rain | T_0901 | Acid rain water is more acidic than normal rain water. Acidity is measured on the pH scale. Lower numbers are more acidic and higher numbers are less acidic (also called more alkaline) (Figure 1.3). Natural rain is somewhat acidic, with a pH of 5.6; acid rain must have a pH of less than 5.0. A small change in pH represents a large change in acidity: rain with a pH of 4.6 is 10 times more acidic than normal rain (with a pH of 5.6). Rain with a pH of 3.6 is 100 times more acidic. Regions with a lot of coal-burning power plants have the most acidic rain. The acidity of average rainwater in the northeastern United States has fallen to between 4.0 and 4.6. Acid fog has even lower pH with an average of around 3.4. One fog in Southern California in 1986 had a pH of 1.7, equal to toilet-bowl cleaner. In arid climates, such as in Southern California, acids deposit on the ground dry. Acid precipitation ends up on the land surface and in water bodies. Some forest soils in the northeast are five to ten times more acidic than they were two or three decades ago. Acid droplets move down through acidic soils to lower the pH of streams and lakes even more. Acids strip soil of metals and nutrients, which collect in streams and lakes. As a result, stripped soils may no longer provide the nutrients that native plants need. A pH scale goes from 1 to 14; numbers are shown with the pH of some common substances. A value of 7 is neutral. The strongest acids are at the low end of the scale and the strongest bases are at the high end. | text | null |
L_0089 | acid rain | T_0902 | Acid rain takes a toll on ecosystems (Figure 1.4). Plants that are exposed to acids become weak and are more likely to be damaged by bad weather, insect pests, or disease. Snails die in acid soils, so songbirds do not have as much food to eat. Young birds and mammals do not build bones as well and may not be as strong. Eggshells may also be weak and break more easily. As lakes become acidic, organisms die off. No fish can live if the pH drops below 4.5. Organic material cannot decay, and mosses take over the lake. Wildlife that depend on the lake for drinking water suffer population declines. Crops are damaged by acid rain. This is most noticeable in poor nations where people cant afford to fix the problems with fertilizers or other technology. Acid rain has killed trees in this forest in the Czech Republic. Acid rain damages cultural monuments like buildings and statues. These include the U.S. Capitol and many buildings in Europe, such as Westminster Abbey. Carbonate rocks neutralize acids and so some regions do not suffer the effects of acid rain nearly as much. Limestone in the midwestern United States protects the area. One reason that the northeastern United States is so vulnerable to acid rain damage is that the rocks are not carbonates. Because pollutants can travel so far, much of the acid rain that falls hurts states or nations other than ones where the pollutants were released. All the rain that falls in Sweden is acidic and fish in lakes all over the country are dying. The pollutants come from the United Kingdom and Western Europe, which are now working to decrease their emissions. Canada also suffers from acid rain that originates in the United States, a problem that is also improving. Southeast Asia is experiencing more acid rain between nations as the region industrializes. Click image to the left or use the URL below. URL: Click image to the left or use the URL below. URL: | text | null |
L_0090 | adaptation and evolution of populations | T_0903 | The characteristics of an organism that help it to survive in a given environment are called adaptations. Adaptations are traits that an organism inherits from its parents. Within a population of organisms are genes coding for a certain number of traits. For example, a human population may have genes for eyes that are blue, green, hazel, or brown, but as far as we know, not purple or lime green. Adaptations develop when certain variations or differences in a population help some members survive better than others (Figure 1.1). The variation may already exist within the population, but often the variation comes from a mutation, or a random change in an organisms genes. Some mutations are harmful and the organism dies; in that case, the variation will not remain in the population. Many mutations are neutral and remain in the population. If the environment changes, the mutation may be beneficial and it may help the organism adapt to the environment. The organisms that survive pass this favorable trait on to their offspring. | text | null |
L_0090 | adaptation and evolution of populations | T_0904 | Many changes in the genetic makeup of a species may accumulate over time, especially if the environment is changing. Eventually the descendants will be very different from their ancestors and may become a whole new species. Changes in the genetic makeup of a species over time are known as biological evolution. | text | null |
L_0090 | adaptation and evolution of populations | T_0905 | The mechanism for evolution is natural selection. Traits become more or less common in a population depending on whether they are beneficial or harmful. An example of evolution by natural selection can be found in the deer mouse, species Peromyscus maniculatus. In Nebraska this mouse is typically brown, but after glaciers carried lighter sand over the darker soil in the Sand Hills, predators could more easily spot the dark mice. Natural selection favored the light mice, and over time, the population became light colored. An explanation of how adaptations de- velop. Click image to the left or use the URL below. URL: | text | null |
L_0091 | age of earth | T_0906 | During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced by the British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the first animals with shells. Today scientists know that this event occurred about 530 million years ago. In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old, which he later lowered to 20 million years. He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of Charles Darwins theory of evolution, which required an older Earth to provide time for geological and evolutionary processes to take place. Kelvins calculations were soon shown to be flawed when radioactivity was discovered in 1896. What Kelvin didnt know is that radioactive decay of elements inside Earths interior provides a steady source of heat. He also didnt know that the mantle is able to flow and so convection moves heat from the interior to the surface of the planet. Thomson had grossly underestimated Earths age. | text | null |
L_0091 | age of earth | T_0907 | Radioactivity turned out to be useful for dating Earth materials and for coming up with a quantitative age for Earth. Scientists not only date ancient rocks from Earths crust, they also date meteorites that formed at the same time Earth and the rest of the solar system were forming. Moon rocks also have been radiometrically dated. Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well- defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate. This is the modern geologic time scale with all of the ages. Click image to the left or use the URL below. URL: | text | null |
L_0092 | agriculture and human population growth | T_0908 | Every major advance in agriculture has allowed global population to increase. Early farmers could settle down to a steady food supply. Irrigation, the ability to clear large swaths of land for farming efficiently, and the development of farm machines powered by fossil fuels allowed people to grow more food and transport it to where it was needed. | text | null |
L_0092 | agriculture and human population growth | T_0909 | What is Earths carrying capacity for humans? Are humans now exceeding Earths carrying capacity for our species? Many anthropologists say that the carrying capacity of humans on the planet without agriculture is about 10 million (Figure 1.1). This population was reached about 10,000 years ago. At the time, people lived together in small bands of hunters and gatherers. Typically men hunted and fished; women gathered nuts and vegetables. Obviously, human populations have blown past this hypothetical carrying capacity. By using our brains, our erect posture, and our hands, we have been able to manipulate our environment in ways that no other species has ever done. What have been the important developments that have allowed population to grow? | text | null |
L_0092 | agriculture and human population growth | T_0910 | About 10,000 years ago, we developed the ability to grow our own food. Farming increased the yield of food plants and allowed people to have food available year round. Animals were domesticated to provide meat. With agriculture, people could settle down, so that they no longer needed to carry all their possessions (Figure 1.2). They could develop better farming practices and store food for when it was difficult to grow. Agriculture allowed people to settle in towns and cities. More advanced farming practices allowed a single farmer to grow food for many more people. When advanced farming practices allowed farmers to grow more food than they needed for their families (Figure | text | null |
L_0092 | agriculture and human population growth | T_0910 | About 10,000 years ago, we developed the ability to grow our own food. Farming increased the yield of food plants and allowed people to have food available year round. Animals were domesticated to provide meat. With agriculture, people could settle down, so that they no longer needed to carry all their possessions (Figure 1.2). They could develop better farming practices and store food for when it was difficult to grow. Agriculture allowed people to settle in towns and cities. More advanced farming practices allowed a single farmer to grow food for many more people. When advanced farming practices allowed farmers to grow more food than they needed for their families (Figure | text | null |