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L_0009 | relative ages of rocks | T_0075 | Sediments were deposited in ancient seas in horizontal, or flat, layers. If sedimentary rock layers are tilted, they must have moved after they were deposited. | text | null |
L_0009 | relative ages of rocks | T_0076 | Rock layers may have another rock cutting across them, like the igneous rock in Figure 11.9. Which rock is older? To determine this, we use the law of cross-cutting relationships. The cut rock layers are older than the rock that cuts across them. | text | null |
L_0009 | relative ages of rocks | T_0077 | Geologists can learn a lot about Earths history by studying sedimentary rock layers. But in some places, theres a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity. Look at the rock layers in Figure 11.10. They show a feature called Huttons unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited. Huttons discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually. | text | null |
L_0009 | relative ages of rocks | T_0077 | Geologists can learn a lot about Earths history by studying sedimentary rock layers. But in some places, theres a gap in time when no rock layers are present. A gap in the sequence of rock layers is called an unconformity. Look at the rock layers in Figure 11.10. They show a feature called Huttons unconformity. The unconformity was discovered by James Hutton in the 1700s. Hutton saw that the lower rock layers are very old. The upper layers are much younger. There are no layers in between the ancient and recent layers. Hutton thought that the intermediate rock layers eroded away before the more recent rock layers were deposited. Huttons discovery was a very important event in geology! Hutton determined that the rocks were deposited over time. Some were eroded away. Hutton knew that deposition and erosion are very slow. He realized that for both to occur would take an extremely long time. This made him realize that Earth must be much older than people thought. This was a really big discovery! It meant there was enough time for life to evolve gradually. | text | null |
L_0009 | relative ages of rocks | T_0078 | When rock layers are in the same place, its easy to give them relative ages. But what if rock layers are far apart? What if they are on different continents? What evidence is used to match rock layers in different places? | text | null |
L_0009 | relative ages of rocks | T_0079 | Some rock layers extend over a very wide area. They may be found on more than one continent or in more than one country. For example, the famous White Cliffs of Dover are on the coast of southeastern England. These distinctive rocks are matched by similar white cliffs in France, Belgium, Holland, Germany, and Denmark (see Figure 11.11). It is important that this chalk layer goes across the English Channel. The rock is so soft that the Channel Tunnel connecting England and France was carved into it! | text | null |
L_0009 | relative ages of rocks | T_0080 | Like index fossils, key beds are used to match rock layers. A key bed is a thin layer of rock. The rock must be unique and widespread. For example, a key bed from around the time that the dinosaurs went extinct is very important. A thin layer of clay was deposited over much of Earths surface. The clay has large amount of the element iridium. Iridium is rare on Earth but common in asteroids. This unusual clay layer has been used to match rock up layers all over the world. It also led to the hypothesis that a giant asteroid struck Earth and caused the dinosaurs to go extinct. | text | null |
L_0009 | relative ages of rocks | T_0081 | Index fossils are commonly used to match rock layers in different places. You can see how this works in Figure | text | null |
L_0009 | relative ages of rocks | T_0082 | Earth formed 4.5 billion years ago. Geologists divide this time span into smaller periods. Many of the divisions mark major events in life history. | text | null |
L_0009 | relative ages of rocks | T_0083 | Divisions in Earth history are recorded on the geologic time scale. For example, the Cretaceous ended when the dinosaurs went extinct. European geologists were the first to put together the geologic time scale. So, many of the names of the time periods are from places in Europe. The Jurassic Period is named for the Jura Mountains in France and Switzerland, for example. | text | null |
L_0009 | relative ages of rocks | T_0084 | To create the geologic time scale, geologists correlated rock layers. Stenos laws were used to determine the relative ages of rocks. Older rocks are at the bottom and younger rocks are at the top. The early geologic time scale could only show the order of events. The discovery of radioactivity in the late 1800s changed that. Scientists could determine the exact age of some rocks in years. They assigned dates to the time scale divisions. For example, the Jurassic began about 200 million years ago. It lasted for about 55 million years. | text | null |
L_0009 | relative ages of rocks | T_0085 | The largest blocks of time on the geologic time scale are called eons. Eons are split into eras. Each era is divided into periods. Periods may be further divided into epochs. Geologists may just use early or late. An example is late Jurassic, or early Cretaceous. Figure 11.13 shows you what the geologic time scale looks like. | text | null |
L_0009 | relative ages of rocks | T_0086 | The geologic time scale may include illustrations of how life on Earth has changed. Major events on Earth may also be shown. These include the formation of the major mountains or the extinction of the dinosaurs. Figure 11.14 is a different kind of the geologic time scale. It shows how Earths environment and life forms have changed. | text | null |
L_0009 | relative ages of rocks | T_0087 | We now live in the Phanerozoic Eon, the Cenozoic Era, the Quaternary Period, and the Holocene Epoch. Phanero- zoic means visible life. During this eon, rocks contain visible fossils. Before the Phanerozoic, life was microscopic. The Cenozoic Era means new life. It encompasses the most recent forms of life on Earth. The Cenozoic is sometimes called the Age of Mammals. Before the Cenozoic came the Mesozoic and Paleozoic. The Mesozoic means middle life. This is the age of reptiles, when dinosaurs ruled the planet. The Paleozoic is old life. Organisms like invertebrates and fish were the most common lifeforms. | text | null |
L_0010 | absolute ages of rocks | T_0088 | Radioactive decay is the breakdown of unstable elements into stable elements. To understand this process, recall that the atoms of all elements contain the particles protons, neutrons, and electrons. | text | null |
L_0010 | absolute ages of rocks | T_0089 | An element is defined by the number of protons it contains. All atoms of a given element contain the same number of protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons are called isotopes. Consider carbon as an example. Two isotopes of carbon are shown in Figure 11.15. Compare their protons and neutrons. Both contain 6 protons. But carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage of carbon atoms are carbon-14. Carbon-14 is unstable. Figure 11.16 shows carbon dioxide, which forms in the atmosphere from carbon-14 and oxygen. Neutrons in cosmic rays strike nitrogen atoms in the atmosphere. The nitrogen forms carbon- 14. Carbon in the atmosphere combines with oxygen to form carbon dioxide. Plants take in carbon dioxide during photosynthesis. In this way, carbon-14 enters food chains. | text | null |
L_0010 | absolute ages of rocks | T_0089 | An element is defined by the number of protons it contains. All atoms of a given element contain the same number of protons. The number of neutrons in an element may vary. Atoms of an element with different numbers of neutrons are called isotopes. Consider carbon as an example. Two isotopes of carbon are shown in Figure 11.15. Compare their protons and neutrons. Both contain 6 protons. But carbon-12 has 6 neutrons and carbon-14 has 8 neutrons. Almost all carbon atoms are carbon-12. This is a stable isotope of carbon. Only a tiny percentage of carbon atoms are carbon-14. Carbon-14 is unstable. Figure 11.16 shows carbon dioxide, which forms in the atmosphere from carbon-14 and oxygen. Neutrons in cosmic rays strike nitrogen atoms in the atmosphere. The nitrogen forms carbon- 14. Carbon in the atmosphere combines with oxygen to form carbon dioxide. Plants take in carbon dioxide during photosynthesis. In this way, carbon-14 enters food chains. | text | null |
L_0010 | absolute ages of rocks | T_0090 | Like other unstable isotopes, carbon-14 breaks down, or decays. For carbon-14 decay, each carbon-14 atom loses an alpha particle. It changes to a stable atom of nitrogen-14. This is illustrated in Figure 11.17. The decay of an unstable isotope to a stable element occurs at a constant rate. This rate is different for each isotope pair. The decay rate is measured in a unit called the half-life. The half-life is the time it takes for half of a given amount of an isotope to decay. For example, the half-life of carbon-14 is 5730 years. Imagine that you start out with 100 grams of carbon-14. In 5730 years, half of it decays. This leaves 50 grams of carbon-14. Over the next 5730 years, half of the remaining amount will decay. Now there are 25 grams of carbon-14. How many grams will there be in another 5730 years? Figure 11.18 graphs the rate of decay of carbon-14. | text | null |
L_0010 | absolute ages of rocks | T_0091 | The rate of decay of unstable isotopes can be used to estimate the absolute ages of fossils and rocks. This type of dating is called radiometric dating. | text | null |
L_0010 | absolute ages of rocks | T_0092 | The best-known method of radiometric dating is carbon-14 dating. A living thing takes in carbon-14 (along with stable carbon-12). As the carbon-14 decays, it is replaced with more carbon-14. After the organism dies, it stops taking in carbon. That includes carbon-14. The carbon-14 that is in its body continues to decay. So the organism contains less and less carbon-14 as time goes on. We can estimate the amount of carbon-14 that has decayed by measuring the amount of carbon-14 to carbon-12. We know how fast carbon-14 decays. With this information, we can tell how long ago the organism died. Carbon-14 has a relatively short half-life. It decays quickly compared to some other unstable isotopes. So carbon- 14 dating is useful for specimens younger than 50,000 years old. Thats a blink of an eye in geologic time. But radiocarbon dating is very useful for more recent events. One important use of radiocarbon is early human sites. Carbon-14 dating is also limited to the remains of once-living things. To date rocks, scientists use other radioactive isotopes. | text | null |
L_0010 | absolute ages of rocks | T_0093 | The isotopes in Table 11.1 are used to date igneous rocks. These isotopes have much longer half-lives than carbon- 14. Because they decay more slowly, they can be used to date much older specimens. Which of these isotopes could be used to date a rock that formed half a million years ago? Unstable Isotope Decays to At a Half-Life of (years) Potassium-40 Uranium-235 Uranium-238 Argon-40 Lead-207 Lead-206 1.3 billion 700 million 4.5 billion Dates Rocks Aged (years old) 100 thousand - 1 billion 1 million - 4.5 billion 1 million - 4.5 billion | text | null |
L_0011 | the origin of earth | T_0094 | Our solar system began about 5 billion years ago. The Sun, planets and other solar system objects all formed at about the same time. | text | null |
L_0011 | the origin of earth | T_0095 | The Sun and planets formed from a giant cloud of gas and dust. This was the solar nebula. The cloud contracted and began to spin. As it contracted, its temperature and pressure increased. The cloud spun faster, and formed into a disk. Scientists think the solar system at that time looked like these disk-shaped objects in the Orion Nebula (Figure | text | null |
L_0011 | the origin of earth | T_0096 | Temperatures and pressures at the center of the cloud were extreme. It was so hot that nuclear fusion reactions began. In these reactions hydrogen fuses to make helium. Extreme amounts of energy are released. Our Sun became a star! Material in the disk surrounding the Sun collided. Small particles collided and became rocks. Rocks collided and became boulders. Eventually planets formed from the material (Figure 12.2). Dwarf plants, comets, and asteroids formed too (Figure 12.3). | text | null |
L_0011 | the origin of earth | T_0096 | Temperatures and pressures at the center of the cloud were extreme. It was so hot that nuclear fusion reactions began. In these reactions hydrogen fuses to make helium. Extreme amounts of energy are released. Our Sun became a star! Material in the disk surrounding the Sun collided. Small particles collided and became rocks. Rocks collided and became boulders. Eventually planets formed from the material (Figure 12.2). Dwarf plants, comets, and asteroids formed too (Figure 12.3). | text | null |
L_0011 | the origin of earth | T_0096 | Temperatures and pressures at the center of the cloud were extreme. It was so hot that nuclear fusion reactions began. In these reactions hydrogen fuses to make helium. Extreme amounts of energy are released. Our Sun became a star! Material in the disk surrounding the Sun collided. Small particles collided and became rocks. Rocks collided and became boulders. Eventually planets formed from the material (Figure 12.2). Dwarf plants, comets, and asteroids formed too (Figure 12.3). | text | null |
L_0011 | the origin of earth | T_0097 | Material at a similar distances from the Sun collided together to form each of the planets. Earth grew from material in its part of space. Moons origin was completely different from Earths. | text | null |
L_0011 | the origin of earth | T_0098 | Earth formed like the other planets. Different materials in its region of space collided. Eventually the material made a planet. All of the collisions caused Earth to heat up. Rock and metal melted. The molten material separated into layers. Gravity pulled the denser material into the center. The lighter elements rose to the surface (Figure 12.4). Because the material separated, Earths core is made mostly of iron. Earths crust is made mostly of lighter materials. In between the crust and the core is Earths mantle, made of solid rock. | text | null |
L_0011 | the origin of earth | T_0099 | This model for how the Moon formed is the best fit of all of the data scientists have about the Moon. In the early solar system there was a lot of space debris. Asteroids flew around, sometimes striking the planets. An asteroid the size of Mars smashed into Earth. The huge amount of energy from the impact melted most of Earth. The asteroid melted too. Material from both Earth and the asteroid was thrown out into orbit. Over time, this material smashed together to form our Moon. The lunar surface is about 4.5 billion years old. This means that the collision happened about 70 million years after Earth formed. | text | null |
L_0011 | the origin of earth | T_0100 | An atmosphere is the gases that surround a planet. The early Earth had no atmosphere. Conditions were so hot that gases were not stable. | text | null |
L_0011 | the origin of earth | T_0101 | Earths first atmosphere was different from the current one. The gases came from two sources. Volcanoes spewed gases into the air. Comets carried in ices from outer space. These ices warmed and became gases. Nitrogen, carbon dioxide, hydrogen, and water vapor, or water in gas form, were in the first atmosphere (Figure 12.5). Take a look at the list of gases. Whats missing? The early atmosphere had almost no oxygen. | text | null |
L_0011 | the origin of earth | T_0102 | Earths atmosphere slowly cooled. Once it was cooler, water vapor could condense. It changed back to its liquid form. Liquid water could fall to Earths surface as rain. Over millions of years water collected to form the oceans. Water began to cycle on Earth as water evaporated from the oceans and returned again as rainfall. | text | null |
L_0012 | early earth | T_0103 | The earliest crust was probably basalt. It may have resembled the current seafloor. This crust formed before there were any oceans. More than 4 billion years ago, continental crust appeared. The first continents were very small compared with those today. | text | null |
L_0012 | early earth | T_0104 | Continents grow when microcontinents, or small continents, collide with each other or with a larger continent. Oceanic island arcs also collide with continents to make them grow. | text | null |
L_0012 | early earth | T_0105 | There are times in Earth history when all of the continents came together to form a supercontinent. Supercontinents come together and then break apart. Pangaea was the last supercontinent on Earth, but it was not the first. The supercontinent before Pangaea is called Rodinia. Rodinia contained about 75% of the continental landmass that is present today. The supercontinent came together about 1.1 billion years ago. Rodinia was not the first supercontinent either. Scientists think that three supercontinents came before Rodina, making five so far in Earth history. | text | null |
L_0012 | early earth | T_0106 | Since the early Earth was very hot, mantle convection was very rapid. Plate tectonics likely moved very quickly. The early Earth was a very active place with abundant volcanic eruptions and earthquakes. The remnants of these early rocks are now seen in the ancient cores of the continents. | text | null |
L_0012 | early earth | T_0107 | For the first 4 billion years of Earth history there is only a little evidence of life. Organisms were tiny and soft and did not fossilize well. But scientists use a variety of ways to figure out what this early life was like. | text | null |
L_0012 | early earth | T_0108 | Life probably began in the oceans. No one knows exactly how or when. Life may have originated more than once. If life began before the Moon formed, that impact would have wiped it out and it would have had to originate again. Eventually conditions on Earth became less violent. The planet could support life. The first organisms were made of only one cell (Figure 12.6). The earliest cells were prokaryotes. Prokaryotic cells are surrounded by a cell membrane, but they do not have a nucleus. The cells got their nutrients directly from the water. The cells needed to use these nutrients to live and grow. The cells also needed to be able to make copies of themselves. To do this they stored genetic information in nucleic acids. The two nucleic acids are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Nucleic acids pass | text | null |
L_0012 | early earth | T_0109 | Early cells took nutrients from the water. Eventually the nutrients would have become less abundant. Around 3 billion years ago, photosynthesis began. Organisms could make their own food from sunlight and inorganic molecules. From these ingredients they made chemical energy that they used. Oxygen is a waste product of photosynthesis. That first oxygen combined with iron to create iron oxide. Later on, the oxygen entered the atmosphere. Some of the oxygen in the atmosphere became ozone. The ozone layer formed to protect Earth from harmful ultraviolet radiation. This made the environment able to support more complex life forms. | text | null |
L_0012 | early earth | T_0110 | The first organisms to photosynthesize were cyanobacteria. These organisms may have been around as far back as 3.5 billion years and are still alive today (Figure 12.7). Now they are called blue-green algae. They are common in lakes and seas and account for 20% to 30% of photosynthesis today. | text | null |
L_0012 | early earth | T_0111 | Eukaryotes evolved about 2 billion years ago. Unlike prokaryotes, eukaryotes have a cell nucleus. They have more structures and are better organized. Organelles within a eukaryote can perform certain functions. Some supply energy; some break down wastes. Eukaryotes were better able to live and so became the dominant life form. | text | null |
L_0012 | early earth | T_0112 | For life to become even more complex, multicellular organisms needed to evolve. Prokaryotes and eukaryotes can be multicellular. Toward the end of the Precambrian, the Ediacara Fauna evolved (Figure 12.8). These are the fossils discovered by Walcott in the introduction to the next section. The Ediacara was extremely diverse. They appeared after Earth defrosted from a worldwide glaciation. The Ediacara fauna seem to have died out. Other multicellular organisms appeared in the Phanerozoic. | text | null |
L_0014 | water on earth | T_0131 | Water is a simple chemical compound. Each molecule of water contains two hydrogen atoms (H2 ) and one oxygen atom (O). Thats why the chemical formula for water is H2 O. If water is so simple, why is it special? Water is one of the few substances that exists on Earth in all three states of matter. Water occurs as a gas, a liquid and a solid. You drink liquid water and use it to shower. You breathe gaseous water vapor in the air. You may go ice skating on a pond covered with solid water ice in the winter. | text | null |
L_0014 | water on earth | T_0132 | Earth is often called the water planet. Figure 13.1 shows why. If astronauts see Earth from space, this is how it looks. Notice how blue the planet appears. Thats because oceans cover much of Earths surface. Water is also found in the clouds that rise above the planet. Most of Earths water is salt water in the oceans. As Figure 13.2 shows, only 3 percent of Earths water is fresh. Freshwater is water that contains little or no dissolved salt. Most freshwater is frozen in ice caps and glaciers. Glaciers cover the peaks of some tall mountains. For example, the Cascades Mountains in North America and the Alps Mountains in Europe are capped with ice. Ice caps cover vast areas of Antarctica and Greenland. Chunks of ice frequently break off ice caps. They form icebergs that float in the oceans. | text | null |
L_0014 | water on earth | T_0132 | Earth is often called the water planet. Figure 13.1 shows why. If astronauts see Earth from space, this is how it looks. Notice how blue the planet appears. Thats because oceans cover much of Earths surface. Water is also found in the clouds that rise above the planet. Most of Earths water is salt water in the oceans. As Figure 13.2 shows, only 3 percent of Earths water is fresh. Freshwater is water that contains little or no dissolved salt. Most freshwater is frozen in ice caps and glaciers. Glaciers cover the peaks of some tall mountains. For example, the Cascades Mountains in North America and the Alps Mountains in Europe are capped with ice. Ice caps cover vast areas of Antarctica and Greenland. Chunks of ice frequently break off ice caps. They form icebergs that float in the oceans. | text | null |
L_0014 | water on earth | T_0133 | Did you ever wonder where the water in your glass came from or where its been? The next time you take a drink of water, think about this. Each water molecule has probably been around for billions of years. Thats because Earths water is constantly recycled. | text | null |
L_0014 | water on earth | T_0134 | Water is recycled through the water cycle. The water cycle is the movement of water through the oceans, atmo- sphere, land, and living things. The water cycle is powered by energy from the Sun. Figure 13.3 diagrams the water cycle. | text | null |
L_0014 | water on earth | T_0135 | Water keeps changing state as it goes through the water cycle. This means that it can be a solid, liquid, or gas. How does water change state? How does it keep moving through the cycle? As Figure 13.3 shows, several processes are involved. Evaporation changes liquid water to water vapor. Energy from the Sun causes water to evaporate. Most evaporation is from the oceans because they cover so much area. The water vapor rises into the atmosphere. Transpiration is like evaporation because it changes liquid water to water vapor. In transpiration, plants release water vapor through their leaves. This water vapor rises into the atmosphere. Condensation changes water vapor to liquid water. As air rises higher into the atmosphere, it cools. Cool air can hold less water vapor than warm air. So some of the water vapor condenses into water droplets. Water droplets may form clouds. Precipitation is water that falls from clouds to Earths surface. Water droplets in clouds fall to Earth when they become too large to stay aloft. The water falls as rain if the air is warm. If the air is cold, the water may freeze and fall as snow, sleet, or hail. Most precipitation falls into the oceans. Some falls on land. Runoff is precipitation that flows over the surface of the land. This water may travel to a river, lake, or ocean. Runoff may pick up fertilizer and other pollutants and deliver them to the water body where it ends up. In this way, runoff may pollute bodies of water. Infiltration is the process by which water soaks into the ground. Some of the water may seep deep under- ground. Some may stay in the soil, where plants can absorb it with their roots. In all these ways, water keeps cycling. The water cycle repeats over and over again. Who knows? Maybe a water molecule that you drink today once quenched the thirst of a dinosaur. | text | null |
L_0015 | surface water | T_0136 | Look at the pictures of flowing water in Figure 13.4. A waterfall tumbles down a mountainside. A brook babbles through a forest. A river slowly meanders through a broad valley. What do all these forms of flowing water have in common? They are all streams. | text | null |
L_0015 | surface water | T_0137 | A stream is a body of freshwater that flows downhill in a channel. The channel of a stream has a bottom, or bed, and sides called banks. Any size body of flowing water can be called a stream. Usually, though, a large stream is called a river. | text | null |
L_0015 | surface water | T_0138 | All streams and rivers have several features in common. These features are shown in (Figure 13.5). The place where a stream or river starts is its source. The source might be a spring, where water flows out of the ground. Or the source might be water from melting snow on a mountain top. A single stream may have multiple sources. A stream or river probably ends when it flows into a body of water, such as a lake or an ocean. A stream ends at its mouth. As the water flows into the body of water, it slows down and drops the sediment it was carrying. The sediment may build up to form a delta. Several other features of streams and rivers are also shown in Figure 13.5. Small streams often flow into bigger streams or rivers. The small streams are called tributaries. A river and all its tributaries make up a river system. At certain times of year, a stream or river may overflow its banks. The area of land that is flooded is called the floodplain. The floodplain may be very wide where the river flows over a nearly flat surface. A river flowing over a floodplain may wear away broad curves. These curves are called meanders. | text | null |
L_0015 | surface water | T_0139 | All of the land drained by a river system is called its basin, or watershed. One river systems basin is separated from another river systems basin by a divide. The divide is created by the highest points between the two river basins. Precipitation that falls within a river basin always flows toward that river. Precipitation that falls on the other side of the divide flows toward a different river. Figure 13.6 shows the major river basins in the U.S. You can watch an animation of water flowing through a river basin at this link: http://trashfree.org/btw/graphics/watershed_anim.gif | text | null |
L_0015 | surface water | T_0139 | All of the land drained by a river system is called its basin, or watershed. One river systems basin is separated from another river systems basin by a divide. The divide is created by the highest points between the two river basins. Precipitation that falls within a river basin always flows toward that river. Precipitation that falls on the other side of the divide flows toward a different river. Figure 13.6 shows the major river basins in the U.S. You can watch an animation of water flowing through a river basin at this link: http://trashfree.org/btw/graphics/watershed_anim.gif | text | null |
L_0015 | surface water | T_0140 | After a heavy rain, you may find puddles of water standing in low spots. The same principle explains why water collects in ponds and lakes. Water travels downhill, so a depression in the ground fills with standing water. A pond is a small body of standing water. A lake is a large body of standing water. Most lakes have freshwater, but a few are salty. The Great Salt Lake in Utah is an example of a saltwater lake. The water in a large lake may be so deep that sunlight cannot penetrate all the way to the bottom. Without sunlight, water plants and algae cannot live on the bottom of the lake. Thats because plants need sunlight for photosynthesis. The largest lakes in the world are the Great Lakes. They lie between the U.S. and Canada, as shown in Figure 13.7. How great are they? They hold 22 percent of all the worlds fresh surface water! | text | null |
L_0015 | surface water | T_0141 | Ponds and lakes may get their water from several sources. Some falls directly into them as precipitation. Some enters as runoff and some from streams and rivers. Water leaves ponds and lakes through evaporation and also as outflow. | text | null |
L_0015 | surface water | T_0142 | The depression that allows water to collect to form a lake may come about in a variety of ways. The Great Lakes, for example, are glacial lakes. A glacial lake forms when a glacier scrapes a large hole in the ground. When the glacier melts, the water fills the hole and forms a lake. Over time, water enters the lake from the sources mentioned above as well. Other lakes are crater lakes or rift lakes, which are pictured in Figure 13.8. Crater lakes form when volcanic eruptions create craters that fill with water. Rift lakes form when movements of tectonic plates create low places that fill with water. | text | null |
L_0015 | surface water | T_0143 | Some of Earths freshwater is found in wetlands. A wetland is an area that is covered with water, or at least has very soggy soil, during all or part of the year. Certain species of plants thrive in wetlands, and they are rich ecosystems. Freshwater wetlands are usually found at the edges of steams, rivers, ponds, or lakes. Wetlands can also be found at the edges of seas. | text | null |
L_0015 | surface water | T_0144 | Not all wetlands are alike, as you can see from Figure 13.9. Wetlands vary in how wet they are and how much of the year they are soaked. Wetlands also vary in the kinds of plants that live in them. This depends mostly on the climate where the wetland is found. Types of wetlands include marshes, swamps, and bogs. A marsh is a wetland that is usually under water. It has grassy plants, such as cattails. A swamp is a wetland that may or may not be covered with water but is always soggy. It has shrubs or trees. A bog is a wetland that has soggy soil. It is generally covered with mosses. | text | null |
L_0015 | surface water | T_0145 | People used to think that wetlands were useless. Many wetlands were filled in with rocks and soil to create lands that were then developed with roads, golf courses, and buildings. Now we know that wetlands are very important. Laws have been passed to help protect them. Why are wetlands so important? Wetlands have great biodiversity. They provide homes or breeding sites to a huge variety of species. Because so much wetland area has been lost, many of these species are endangered. Wetlands purify water. They filter sediments and toxins from runoff before it enters rivers, lakes, and oceans. Wetlands slow rushing water. During hurricanes and other extreme weather, wetlands reduce the risk of floods. Although the rate has slowed, wetlands are still being destroyed today. | text | null |
L_0015 | surface water | T_0146 | A flood occurs when so much water enters a stream or river that it overflows its banks. Flood waters from a river are shown in Figure 13.10. Like this flood, many floods are caused by very heavy rains. Floods may also occur when deep snow melts quickly in the spring. Floods are a natural part of the water cycle, but they can cause a lot of damage. Farms and homes may be lost, and people may die. In 1939, millions of people died in a flood in China. Although freshwater is needed to grow crops and just to live, too much freshwater in the same place at once can be deadly. | text | null |
L_0016 | groundwater | T_0147 | Freshwater below Earths surface is called groundwater. The water infiltrates, or seeps down into, the ground from the surface. How does this happen? And where does the water go? | text | null |
L_0016 | groundwater | T_0148 | Water infiltrates the ground because soil and rock are porous. Between the grains are pores, or tiny holes. Since water can move through this rock it is permeable. Eventually, the water reaches a layer of rock that is not porous and so is impermeable. Water stops moving downward when it reaches this layer of rock. Look at the diagram in Figure 13.11. It shows two layers of porous rock. The top layer is not saturated; it is not full of water. The next layer is saturated. The water in this layer has nowhere else to go. It cannot seep any deeper into the ground because the rock below it is impermeable. | text | null |
L_0016 | groundwater | T_0149 | The top of the saturated rock layer in Figure 13.11 is called the water table. The water table isnt like a real table. It doesnt remain firmly in one place. Instead, it rises or falls, depending on how much water seeps down from the surface. The water table is higher when there is a lot of rain and lower when the weather is dry. | text | null |
L_0016 | groundwater | T_0150 | An underground layer of rock that is saturated with groundwater is called an aquifer. A diagram of an aquifer is shown in Figure 13.12. Aquifers are generally found in porous rock, such as sandstone. Water infiltrates the aquifer from the surface. The water that enters the aquifer is called recharge. | text | null |
L_0016 | groundwater | T_0151 | Most land areas have aquifers beneath them. Many aquifers are used by people for freshwater. The closer to the surface an aquifer is, the easier it is to get the water. However, an aquifer close to the surface is also more likely to become polluted. Pollutants can seep down through porous rock in recharge water. An aquifer that is used by people may not be recharged as quickly as its water is removed. The water table may lower and the aquifer may even run dry. If this happens, the ground above the aquifer may sink. This is likely to damage any homes or other structures built above the aquifer. | text | null |
L_0016 | groundwater | T_0152 | One of the biggest aquifers in the world is the Ogallala aquifer. As you can see from Figure 13.13, this aquifer lies beneath parts of eight U.S. states. It covers a total area of 451,000 square kilometers (174,000 square miles). In some places, it is less than a meter deep. In other places, it is hundreds of meters deep. The Ogallala aquifer is an important source of freshwater in the American Midwest. This is a major farming area, and much of the water is used to irrigate crops. The water in this aquifer is being used up ten times faster than it is recharged. If this continues, what might happen to the Ogallala aquifer? | text | null |
L_0016 | groundwater | T_0153 | The top of an aquifer may be high enough in some places to meet the surface of the ground. This often happens on a slope. The water flows out of the ground and creates a spring. A spring may be just a tiny trickle, or it may be a big gush of water. One of the largest springs in the world is Big Spring in Missouri, seen in Figure 13.14. Water flowing out of the ground at a spring may flow downhill and enter a stream. Thats what happens to the water that flows out of Big Spring in Missouri. If the water from a spring cant flow downhill, it may spread out to form a pond or lake instead. Lake George in New York State, which is pictured in Figure 13.15, is a spring-fed lake. The lake basin was carved by a glacier. | text | null |
L_0016 | groundwater | T_0153 | The top of an aquifer may be high enough in some places to meet the surface of the ground. This often happens on a slope. The water flows out of the ground and creates a spring. A spring may be just a tiny trickle, or it may be a big gush of water. One of the largest springs in the world is Big Spring in Missouri, seen in Figure 13.14. Water flowing out of the ground at a spring may flow downhill and enter a stream. Thats what happens to the water that flows out of Big Spring in Missouri. If the water from a spring cant flow downhill, it may spread out to form a pond or lake instead. Lake George in New York State, which is pictured in Figure 13.15, is a spring-fed lake. The lake basin was carved by a glacier. | text | null |
L_0016 | groundwater | T_0154 | Some springs have water that contains minerals. Groundwater dissolves minerals out of the rock as it seeps through the pores. The water in some springs is hot because it is heated by hot magma. Many hot springs are also mineral springs. Thats because hot water can dissolve more minerals than cold water. Grand Prismatic Spring, shown in Figure 13.16, is a hot mineral spring. Dissolved minerals give its water a bright blue color. The edge of the spring is covered with thick orange mats of bacteria. The bacteria use the minerals in the hot water to make food. | text | null |
L_0016 | groundwater | T_0155 | Heated groundwater may become trapped in spaces within rocks. Pressure builds up as more water seeps into the spaces. When the pressure becomes great enough, the water bursts out of the ground at a crack or weak spot. This is called a geyser. When the water erupts from the ground, the pressure is released. Then more water collects and the pressure builds up again. This leads to another eruption. Old Faithful is the best-known geyser in the world. You can see a picture of it in Figure 13.17. The geyser erupts faithfully every 90 minutes, day after day. During each eruption, it may release as much as 30,000 liters of water! | text | null |
L_0016 | groundwater | T_0156 | Most groundwater does not flow out of an aquifer as a spring or geyser. So to use the water thats stored in an aquifer people must go after it. How? They dig a well. A well is a hole that is dug or drilled through the ground down to an aquifer. This is illustrated in Figure 13.18. People have depended on water from wells for thousands of years. To bring water to the surface takes energy because the force of gravity must be overcome. Today, many wells use electricity to pump water to the surface. However, in some places, water is still brought to the surface the old-fashioned way with human labor. The well pictured in Figure 13.19 is an example of this type of well. A hand-cranked pulley is used to lift the bucket of water to the surface. | text | null |
L_0017 | introduction to the oceans | T_0157 | When Earth formed 4.6 billion years ago, it would not have been called the water planet. There were no oceans then. In fact, there was no liquid water at all. Early Earth was too hot for liquid water to exist. Earths early years were spent as molten rock and metal. | text | null |
L_0017 | introduction to the oceans | T_0158 | Over time, Earth cooled. The surface hardened to become solid rock. Volcanic eruptions, like the one in Figure 14.1, brought lava and gases to the surface. One of the gases was water vapor. More water vapor came from asteroids and comets that crashed into Earth. As Earth cooled still more, the water vapor condensed to make Earths first liquid water. At last, the oceans could start to form. | text | null |
L_0017 | introduction to the oceans | T_0159 | Earths crust consists of many tectonic plates that move over time. Due to plate tectonics, the continents changed their shapes and positions during Earth history. As the continents changed, so did the oceans. About 250 million years ago, there was one huge land mass known as Pangaea. There was also one huge ocean called Panthalassa. You can see it in Figure 14.2. By 180 million years ago, Pangaea began to break up. The continents started to drift apart. They slowly moved to where they are today. The movement of the continents caused Panthalassa to break into smaller oceans. These oceans are now known as the Pacific, Atlantic, Indian, and Arctic Oceans. The waters of all the oceans are connected. | text | null |
L_0017 | introduction to the oceans | T_0160 | Oceans cover more than 70 percent of Earths surface and hold 97 percent of its surface water. Its no surprise that the oceans have a big influence on the planet. The oceans affect the atmosphere, climate, and living things. | text | null |
L_0017 | introduction to the oceans | T_0161 | Oceans are the major source of water vapor in the atmosphere. Sunlight heats water near the sea surface, as shown in Figure 14.3. As the water warms, some of it evaporates. The water vapor rises into the air, where it may form clouds and precipitation. Precipitation provides the freshwater needed by plants and other living things. Ocean water also absorbs gases from the atmosphere. The most important are oxygen and carbon dioxide. Oxygen is needed by living things in the oceans. Much of the carbon dioxide sinks to the bottom of the seas. Carbon dioxide is a major cause of global warming. By absorbing carbon dioxide, the oceans help control global warming. | text | null |
L_0017 | introduction to the oceans | T_0162 | Coastal areas have a milder climate than inland areas. They are warmer in the winter and cooler in the summer. Thats because land near an ocean is influenced by the temperature of the oceans. The temperature of ocean water is moderate and stable. Why? There are two major reasons: 1. Water is much slower to warm up and cool down than land. As a result, oceans never get as hot or as cold as land. 2. Water flows through all the worlds oceans. Warm water from the equator mixes with cold water from the poles. The mixing of warm and cold water makes the water temperature moderate. Even inland temperatures are milder because of oceans. Without oceans, there would be much bigger temperature swings all over Earth. Temperatures might plunge hundreds of degrees below freezing in the winter. In the summer, lakes and seas might boil! Life as we know it could not exist on Earth without the oceans. | text | null |
L_0017 | introduction to the oceans | T_0163 | The oceans provide a home to many living things. In fact, a greater number of organisms lives in the oceans than on land. Coral reefs, like the one in Figure 14.4, have more diversity of life forms than almost anywhere else on Earth. | text | null |
L_0017 | introduction to the oceans | T_0164 | You know that ocean water is salty. But do you know why? How salty is it? | text | null |
L_0017 | introduction to the oceans | T_0165 | Ocean water is salty because water dissolves minerals out of rocks. This happens whenever water flows over or through rocks. Much of this water and its minerals flow in rivers that end up in the oceans. Minerals dissolved in water form salts. When the water evaporates, it leaves the salts behind. As a result, ocean water is much saltier than other water on Earth. | text | null |
L_0017 | introduction to the oceans | T_0166 | Have you ever gone swimming in the ocean? If you have, then you probably tasted the salts in the water. By mass, salts make up about 3.5 percent of ocean water. Figure 14.5 shows the most common minerals in ocean water. The main components are sodium and chloride. Together they form the salt known as sodium chloride. You may know the compound as table salt or the mineral halite. The amount of salts in ocean water varies from place to place. For example, near the mouth of a river, ocean water may be less salty. Thats because river water contains less salt than ocean water. Where the ocean is warm, the water may be more salty. Can you explain why? (Hint: More water evaporates when the water is warm.) | text | null |
L_0017 | introduction to the oceans | T_0167 | In addition to the amount of salts, other conditions in ocean water vary from place to place. One is the amount of nutrients in the water. Another is the amount of sunlight that reaches the water. These conditions depend mainly on two factors: distance from shore and depth of water. Oceans are divided into zones based on these two factors. The ocean floor makes up another zone. Figure 14.6 shows all the ocean zones. | text | null |
L_0017 | introduction to the oceans | T_0168 | There are three main ocean zones based on distance from shore. They are the intertidal zone, neritic zone, and oceanic zone. Distance from shore influences how many nutrients are in the water. Why? Most nutrients are washed into ocean water from land. Therefore, water closer to shore tends to have more nutrients. Living things need nutrients. So distance from shore also influences how many organisms live in the water. | text | null |
L_0017 | introduction to the oceans | T_0169 | Two main zones based on depth of water are the photic zone and aphotic zone. The photic zone is the top 200 meters of water. The aphotic zone is water deeper than 200 meters. The deeper you go, the darker the water gets. Thats because sunlight cannot penetrate very far under water. Sunlight is needed for photosynthesis. So the depth of water determines whether photosynthesis is possible. There is enough sunlight for photosynthesis only in the photic zone. Water also gets colder as you go deeper. The weight of the water pressing down from above increases as well. At great depths, life becomes very difficult. The pressure is so great that only specially adapted creatures can live there. | text | null |
L_0018 | ocean movements | T_0170 | Most ocean waves are caused by winds. A wave is the transfer of energy through matter. A wave that travels across miles of ocean is traveling energy, not water. Ocean waves transfer energy from wind through water. The energy of a wave may travel for thousands of miles. The water itself moves very little. Figure 14.9 shows how water molecules move when a wave goes by. | text | null |
L_0018 | ocean movements | T_0170 | Most ocean waves are caused by winds. A wave is the transfer of energy through matter. A wave that travels across miles of ocean is traveling energy, not water. Ocean waves transfer energy from wind through water. The energy of a wave may travel for thousands of miles. The water itself moves very little. Figure 14.9 shows how water molecules move when a wave goes by. | text | null |
L_0018 | ocean movements | T_0171 | Figure 14.9 also shows how the size of waves is measured. The highest point of a wave is the crest. The lowest point is the trough. The vertical distance between a crest and a trough is the height of the wave. Wave height is also called amplitude. The horizontal distance between two crests is the wavelength. Both amplitude and wavelength are measures of wave size. The size of an ocean wave depends on how fast, over how great a distance, and how long the wind blows. The greater each of these factors is, the bigger a wave will be. Some of the biggest waves occur with hurricanes. A hurricane is a storm that forms over the ocean. Its winds may blow more than 150 miles per hour! The winds also travel over long distances and may last for many days. | text | null |
L_0018 | ocean movements | T_0172 | Figure 14.10 shows what happens to waves near shore. As waves move into shallow water, they start to touch the bottom. The base of the waves drag and slow. Soon the waves slow down and pile up. They get steeper and unstable as the top moves faster than the base. When they reach the shore, the waves topple over and break. | text | null |
L_0018 | ocean movements | T_0173 | Not all waves are caused by winds. A shock to the ocean can also send waves through water. A tsunami is a wave or set of waves that is usually caused by an earthquake. As we have seen in recent years, the waves can be enormous and extremely destructive. Usually tsunami waves travel through the ocean unnoticed. But when they reach the shore they become enormous. Tsunami waves can flood entire regions. They destroy property and cause many deaths. Figure 14.11 shows the damage caused by a tsunami in the Indian Ocean in 2004. | text | null |
L_0018 | ocean movements | T_0174 | Tides are daily changes in the level of ocean water. They occur all around the globe. High tides occur when the water reaches its highest level in a day. Low tides occur when the water reaches its lowest level in a day. Tides keep cycling from high to low and back again. In most places the water level rises and falls twice a day. So there are two high tides and two low tides approximately every 24 hours. In Figure 14.12, you can see the difference between high and low tides. This is called the tidal range. | text | null |
L_0018 | ocean movements | T_0175 | Figure 14.13 shows why tides occur. The main cause of tides is the pull of the Moons gravity on Earth. The pull is greatest on whatever is closest to the Moon. Although the gravity pulls the land, only the water can move. As a result: Water on the side of Earth facing the Moon is pulled hardest by the Moons gravity. This causes a bulge of water on that side of Earth. That bulge is a high tide. Earth itself is pulled harder by the Moons gravity than is the ocean on the side of Earth opposite the Moon. As a result, there is bulge of water on the opposite side of Earth. This creates another high tide. With water bulging on two sides of Earth, theres less water left in between. This creates low tides on the other two sides of the planet. | text | null |
L_0018 | ocean movements | T_0176 | The Suns gravity also pulls on Earth and its oceans. Even though the Sun is much larger than the Moon, the pull of the Suns gravity is much less because the Sun is much farther away. The Suns gravity strengthens or weakens the Moons influence on tides. Figure 14.14 shows the position of the Moon relative to the Sun at different times during the month. The positions of the Moon and Sun relative to each other determines how the Sun affects tides. This creates spring tides or neap tides. Spring tides occur during the new moon and full moon. The Sun and Moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range. Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range. This animation shows the effect of the Moon and Sun on the tides: | text | null |
L_0018 | ocean movements | T_0176 | The Suns gravity also pulls on Earth and its oceans. Even though the Sun is much larger than the Moon, the pull of the Suns gravity is much less because the Sun is much farther away. The Suns gravity strengthens or weakens the Moons influence on tides. Figure 14.14 shows the position of the Moon relative to the Sun at different times during the month. The positions of the Moon and Sun relative to each other determines how the Sun affects tides. This creates spring tides or neap tides. Spring tides occur during the new moon and full moon. The Sun and Moon are in a straight line either on the same side of Earth or on opposite sides. Their gravitational pull combines to cause very high and very low tides. Spring tides have the greatest tidal range. Neap tides occur during the first and third quarters of the Moon. The Moon and Sun are at right angles to each other. Their gravity pulls on the oceans in different directions so the highs and lows are not as great. Neap tides have the smallest tidal range. This animation shows the effect of the Moon and Sun on the tides: | text | null |
L_0018 | ocean movements | T_0177 | Another way ocean water moves is in currents. A current is a stream of moving water that flows through the ocean. Surface currents are caused mainly by winds, but not the winds that blow and change each day. Surface currents are caused by the major wind belts that blow in the same direction all the time. The major surface currents are shown in Figure 14.15. They flow in a clockwise direction in the Northern Hemi- sphere. In the Southern Hemisphere, they flow in the opposite direction. | text | null |
L_0018 | ocean movements | T_0178 | Winds and surface currents tend to move from the hot equator north or south toward the much cooler poles. Thats because of differences in the temperature of air masses over Earths surface. But Earth is spinning on its axis underneath the wind and water as they move. The Earth rotates from west to east. As a result, winds and currents actually end up moving toward the northeast or southeast. This effect of Earths rotation on the direction of winds and currents is called the Coriolis effect. | text | null |
L_0018 | ocean movements | T_0179 | Large ocean currents can have a big impact on the climate of nearby coasts. The Gulf Stream, for example, carries warm water from near the equator up the eastern coast of North America. Look at the map in Figure 14.16. It shows how the Gulf Stream warms both the water and land along the coast. | text | null |
L_0018 | ocean movements | T_0180 | Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense? Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic. Downwelling can take place in other places where surface water becomes very dense (see Figure 14.17). When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents. | text | null |
L_0018 | ocean movements | T_0180 | Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense? Water becomes more dense when it is colder and when it has more salt. In the North Atlantic Ocean, cold winds chill the water at the surface. Sea ice grows in this cold water, but ice is created from fresh water. The salt is left behind in the seawater. This cold, salty water is very dense, so it sinks to the bottom of the North Atlantic. Downwelling can take place in other places where surface water becomes very dense (see Figure 14.17). When water sinks it pushes deep water along at the bottom of the ocean. This water circulates through all of the ocean basins in deep currents. | text | null |
L_0018 | ocean movements | T_0181 | Sometimes deep ocean water rises to the surface. This is called upwelling. Figure 14.18 shows why it happens. Strong winds blow surface water away from shore. This allows deeper water to flow to the surface and take its place. When water comes up from the deep, it brings a lot of nutrients with it. Why is deep water so full of nutrients? Over time, dead organisms and other organic matter settle to the bottom water and collect. The nutrient-rich water that comes to the surface by upwelling supports many living things. | text | null |
L_0019 | the ocean floor | T_0182 | Scientists study the ocean floor in various ways. Scientists or their devices may actually travel to the ocean floor. Or they may study the ocean floor from the surface. One way is with a tool called sonar. | text | null |
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