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L_0065 | identification of minerals | T_0665 | Cleavage is the tendency of a mineral to break along certain planes. When a mineral breaks along a plane it makes a smooth surface. Minerals with different crystal structures will break or cleave in different ways, as in Figure 3.14. Halite tends to form cubes with smooth surfaces. Mica tends to form sheets. Fluorite can form octahedrons. Minerals can form various shapes. Polygons are shown in Figure 3.15. The shapes form as the minerals are broken along their cleavage planes. Cleavage planes determine how the crystals can be cut to make smooth surfaces. People who cut gemstones follow cleavage planes. Diamonds and emeralds can be cut to make beautiful gemstones. | text | null |
L_0065 | identification of minerals | T_0665 | Cleavage is the tendency of a mineral to break along certain planes. When a mineral breaks along a plane it makes a smooth surface. Minerals with different crystal structures will break or cleave in different ways, as in Figure 3.14. Halite tends to form cubes with smooth surfaces. Mica tends to form sheets. Fluorite can form octahedrons. Minerals can form various shapes. Polygons are shown in Figure 3.15. The shapes form as the minerals are broken along their cleavage planes. Cleavage planes determine how the crystals can be cut to make smooth surfaces. People who cut gemstones follow cleavage planes. Diamonds and emeralds can be cut to make beautiful gemstones. | text | null |
L_0065 | identification of minerals | T_0666 | Fracture describes how a mineral breaks without any pattern. A fracture is uneven. The surface is not smooth and flat. You can learn about a mineral from the way it fractures. If a mineral splinters like wood, it may be fibrous. Some minerals, such as quartz, fracture to form smooth, curved surfaces. A mineral that broke forming a smooth, curved surface is shown in Figure 3.16. | text | null |
L_0065 | identification of minerals | T_0667 | Minerals have other properties that can be used for identification. For example, a minerals shape may indicate its crystal structure. Sometimes crystals are too small to see. Then a mineralogist may use a special instrument to find the crystal structure. Some minerals have unique properties. These can be used to the minerals. Some of these properties are listed in Table 3.3. An example of a mineral that has each property is also listed. Property Fluorescence Magnetism Radioactivity Reactivity Smell Description Mineral glows under ultraviolet light Mineral is attracted to a magnet Mineral gives off radiation that can be measured with Geiger counter Bubbles form when mineral is ex- posed to a weak acid Some minerals have a distinctive smell Example of Mineral Fluorite Magnetite Uraninite Calcite Sulfur (smells like rotten eggs) | text | null |
L_0066 | formation of minerals | T_0668 | You are on vacation at the beach. You take your flip-flops off so you can go swimming. The sand is so hot it hurts your feet. You have to run to the water. Now imagine if it were hot enough for the sand to melt. Some places inside Earth are so hot that rock melts. Melted rock inside the Earth is called magma. Magma can be hotter than 1,000C. When magma erupts onto Earths surface, it is known as lava, as Figure 3.17 shows. Minerals form when magma and lava cool. | text | null |
L_0066 | formation of minerals | T_0669 | Most water on Earth, like the water in the oceans, contains elements. The elements are mixed evenly through the water. Water plus other substances makes a solution. The particles are so small that they will not come out when you filter the water. But the elements in water can form solid mineral deposits. | text | null |
L_0066 | formation of minerals | T_0670 | Fresh water contains a small amount of dissolved elements. Salt water contains a lot more dissolved elements. Water can only hold a certain amount of dissolved substances. When the water evaporates, it leaves behind a solid layer of minerals, as Figure 3.18 shows. At this time, the particles come together to form minerals. These solids sink to the bottom. The amount of mineral formed is the same as the amount dissolved in the water. Seawater is salty enough for minerals to precipitate as solids. Some lakes, such as Mono Lake in California, or Utahs Great Salt Lake, can also precipitate salts. Salt easily precipitates out of water, as does calcite, as Figure 3.19 shows. The limestone towers in the figure are made mostly of the mineral calcite. The calcite was deposited in the salty and alkaline water of Mono Lake, in California. Calcium-rich spring water enters the bottom of the lake. The water bubbles up into the alkaline lake. The | text | null |
L_0066 | formation of minerals | T_0670 | Fresh water contains a small amount of dissolved elements. Salt water contains a lot more dissolved elements. Water can only hold a certain amount of dissolved substances. When the water evaporates, it leaves behind a solid layer of minerals, as Figure 3.18 shows. At this time, the particles come together to form minerals. These solids sink to the bottom. The amount of mineral formed is the same as the amount dissolved in the water. Seawater is salty enough for minerals to precipitate as solids. Some lakes, such as Mono Lake in California, or Utahs Great Salt Lake, can also precipitate salts. Salt easily precipitates out of water, as does calcite, as Figure 3.19 shows. The limestone towers in the figure are made mostly of the mineral calcite. The calcite was deposited in the salty and alkaline water of Mono Lake, in California. Calcium-rich spring water enters the bottom of the lake. The water bubbles up into the alkaline lake. The | text | null |
L_0066 | formation of minerals | T_0671 | Underground water can be heated by magma. The hot water moves through cracks below Earths surface. Hot water can hold more dissolved particles than cold water. The hot, salty solution has chemical reactions with the rocks around it. The water picks up more dissolved particles. As it flows through open spaces in rocks, the water deposits solid minerals. When a mineral fills cracks in rocks, the deposits are called veins. Figure 3.20 shows a white quartz vein. When the minerals are deposited in open spaces, large crystals grow. These rocks are called geodes. Figure 3.20 shows a geode that was formed when amethyst crystals grew in an open space in a rock. | text | null |
L_0067 | mining and using minerals | T_0672 | A mineral deposit that contains enough minerals to be mined for profit is called an ore. Ores are rocks that contain concentrations of valuable minerals. The bauxite shown in the Figure 3.21 is a rock that contains minerals that are used to make aluminum. | text | null |
L_0067 | mining and using minerals | T_0673 | Ores have high concentrations of valuable minerals. Certain places on Earth are more likely to have certain ores. Geologists search for the places that might have ore deposits. Some of the valuable deposits may be hidden underground. To find an ore deposit, geologists will go to a likely spot. They then test the physical and chemical properties of soil and rocks. Ore deposits contain valuable minerals. They may also contain other chemical elements that indicate an ore deposit is nearby. After a mineral deposit is found, geologists determine how big it is. They outline the deposit and the surrounding geology on a map. The miners calculate the amount of valuable minerals they think they will get from the deposit. The minerals will only be mined if it is profitable. If it is profitable, they must then decide on the way it should be mined. The two main methods of mining are surface mining and underground mining. Placers are a type of surface deposit. | text | null |
L_0067 | mining and using minerals | T_0674 | Surface mining is used to obtain mineral ores that are near the surface. Blasting breaks up the soil and rocks that contain the ore. Enormous trucks haul the broken rocks to locations where the ores can be removed. Surface mining includes open-pit mining, quarrying, and strip mining. As the name suggests, open-pit mining creates a big pit from which the ore is mined. Figure 3.22 shows an open-pit diamond mine in Russia. The size of the pit grows as long as the miners can make a profit. Strip mines are similar to open-pit mines, but the ore is removed in large strips. A quarry is a type of open-pit mine that produces rocks and minerals that are used to make buildings and roads. | text | null |
L_0067 | mining and using minerals | T_0675 | Placer minerals collect in stream gravels. They can be found in modern rivers or ancient riverbeds. California was nicknamed the Golden State. This can be traced back to the discovery of placer gold in 1848. The amount of placer gold brought in miners from around the world. The gold formed in rocks in the Sierra Nevada Mountains. The rocks also contained other valuable minerals. The gold weathered out of the hard rock. It washed downstream and then settled in gravel deposits along the river. Currently, California has active gold and silver mines. California also has mines for non-metal minerals. For example, sand and gravel are mined for construction. | text | null |
L_0067 | mining and using minerals | T_0676 | If an ore is deep below Earths surface it may be too expensive to remove all the rock above it. These deposits are taken by underground mining. Underground mines can be very deep. The deepest gold mine in South Africa is more than 3,700 m deep (that is more than 2 miles)! There are various methods of underground mining. Underground mining is more expensive than surface mining. Tunnels must be blasted into the rock so that miners and equipment can get to the ore. Underground mining is dangerous work. Fresh air and lights must be brought in to the tunnels for the miners. The miners breathe in lots of particles and dust while they are underground. The ore is drilled, blasted, or cut away from the surrounding rock and taken out of the tunnels. Sometimes there are explosions as ore is being drilled or blasted. This can lead to a mine collapse. Miners may be hurt or killed in a mining accident. | text | null |
L_0067 | mining and using minerals | T_0677 | Most minerals are a combination of metal and other elements. The rocks that are taken from a mine are full of valuable minerals plus rock that isnt valuable. This is called waste rock. The valuable minerals must be separated from the waste rock. One way to do this is with a chemical reaction. Chemicals are added to the ores at very high temperatures. For example, getting aluminum from waste rock uses a lot of energy. This is because temperatures greater than 900o C are needed to separate out the aluminum. It also takes a huge amount of electricity. If you recycle just 40 aluminum cans, you will save the energy in one gallon of gasoline. We use over 80 billion cans each year. If all of these cans were recycled, we would save the energy in 2 billion gallons of gasoline! | text | null |
L_0067 | mining and using minerals | T_0678 | We rely on metals, such as aluminum, copper, iron, and gold. Look around the room. How many objects have metal parts? Metals are used in the tiny parts inside your computer, in the wires of anything that uses electricity, and to make the structure of a large building, such as the one shown in the Figure 3.23. | text | null |
L_0067 | mining and using minerals | T_0679 | Some minerals are valuable simply because they are beautiful. Jade has been used for thousands of years in China. Native Americans have been decorating items with turquoise since ancient times. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. A gemstone is a material that is cut and polished to use in jewelry. Many gemstones, such as those shown in Figure 3.24, are minerals. Gemstones are beautiful, rare, and do not break or scratch easily. Generally, rarer gems are more valuable. If a gem Gemstones also have other uses. Most diamonds are actually not used as gemstones. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, which makes the gems ruby and sapphire, is used in products like sandpaper. Synthetic rubies and sapphires are also used in lasers. | text | null |
L_0067 | mining and using minerals | T_0679 | Some minerals are valuable simply because they are beautiful. Jade has been used for thousands of years in China. Native Americans have been decorating items with turquoise since ancient times. Minerals like jade, turquoise, diamonds, and emeralds are gemstones. A gemstone is a material that is cut and polished to use in jewelry. Many gemstones, such as those shown in Figure 3.24, are minerals. Gemstones are beautiful, rare, and do not break or scratch easily. Generally, rarer gems are more valuable. If a gem Gemstones also have other uses. Most diamonds are actually not used as gemstones. Diamonds are used to cut and polish other materials, such as glass and metals, because they are so hard. The mineral corundum, which makes the gems ruby and sapphire, is used in products like sandpaper. Synthetic rubies and sapphires are also used in lasers. | text | null |
L_0067 | mining and using minerals | T_0680 | Metals and gemstones are often shiny, so they catch your eye. Many minerals that we use everyday are not so noticeable. For example, the buildings on your block could not have been built without minerals. The walls in your home might use the mineral gypsum for the sheetrock. The glass in your windows is made from sand, which is mostly the mineral quartz. Talc was once commonly used to make baby powder. The mineral halite is mined for rock salt. Diamond is commonly used in drill bits and saw blades to improve their cutting ability. Copper is used in electrical wiring, and the ore bauxite is the source for the aluminum in your soda can. | text | null |
L_0067 | mining and using minerals | T_0681 | Mining provides people with many resources they need, but mining can be hazardous to people and the environment. Miners should restore the mined region to its natural state. It is also important to use mineral resources wisely. Most ores are non-renewable resources. | text | null |
L_0067 | mining and using minerals | T_0682 | After the mining is finished, the land is greatly disturbed. The area around the mine needs to be restored to its natural state. This process of restoring the area is called reclamation. Native plants are planted. Pit mines may be refilled or reshaped so that they can become natural areas again. The mining company may be allowed to fill the pit with water to create a lake. The pits may be turned into landfills. Underground mines may be sealed off or left open as homes for bats. | text | null |
L_0067 | mining and using minerals | T_0683 | Mining can cause pollution. Chemicals released from mining can contaminate nearby water sources. Figure 3.26 shows water that is contaminated from a nearby mine. The United States government has mining standards to protect water quality. | text | null |
L_0067 | mining and using minerals | T_0684 | 5. What are some disadvantages of underground mining? 6. What is the bottom line when it comes to deciding how what and how to mine? 7. How is land reclaimed after mining? Is it ever fully recovered? 8. How might the history of the Golden State been different if placers had not been found in its rivers? | text | null |
L_0075 | inside earth | T_0748 | If someone told you to figure out what is inside Earth, what would you do? How could you figure out what is inside our planet? How do scientists figure it out? | text | null |
L_0075 | inside earth | T_0749 | Geologists study earthquake waves to see Earths interior. Waves of energy radiate out from an earthquakes focus. These are called seismic waves (Figure 6.1). Seismic waves change speed as they move through different materials. This causes them to bend. Some seismic waves do not travel through liquids or gases. Scientists use all of this information to understand what makes up the Earths interior. | text | null |
L_0075 | inside earth | T_0750 | Scientists study meteorites to learn about Earths interior. Meteorites formed in the early solar system. These objects represent early solar system materials. Some meteorites are made of iron and nickel. They are thought to be very similar to Earths core (Figure 6.2). An iron meteorite is the closest thing to a sample of the core that scientists can hold in their hands! | text | null |
L_0075 | inside earth | T_0751 | Crust, mantle, and core differ from each other in chemical composition. Its understandable that scientists know the most about the crust, and less about deeper layers (Figure 6.3). Earths crust is a thin, brittle outer shell. The crust is made of rock. This layer is thinner under the oceans and much thicker in mountain ranges. | text | null |
L_0075 | inside earth | T_0752 | There are two kinds of crust. Oceanic crust is made of basalt lavas that flow onto the seafloor. It is relatively thin, between 5 to 12 kilometers thick (3 - 8 miles). The rocks of the oceanic crust are denser (3.0 g/cm3 ) than the rocks that make up the continents. Thick layers of mud cover much of the ocean floor. | text | null |
L_0075 | inside earth | T_0753 | Continental crust is much thicker than oceanic crust. It is 35 kilometers (22 miles) thick on average, but it varies a lot. Continental crust is made up of many different rocks. All three major rock types igneous, metamorphic, and sedimentary are found in the crust. On average, continental crust is much less dense (2.7 g/cm3) than oceanic crust. Since it is less dense, it rises higher above the mantle than oceanic crust. | text | null |
L_0075 | inside earth | T_0754 | Beneath the crust is the mantle. The mantle is made of hot, solid rock. Through the process of conduction, heat flows from warmer objects to cooler objects (Figure 6.4). The lower mantle is heated directly by conduction from the core. Hot lower mantle material rises upwards (Figure 6.5). As it rises, it cools. At the top of the mantle it moves horizontally. Over time it becomes cool and dense enough that it sinks. Back at the bottom of the mantle, it travels horizontally. Eventually the material gets to the location where warm mantle material is rising. The rising and sinking of warm and cooler material is convection. The motion described creates a convection cell. | text | null |
L_0075 | inside earth | T_0755 | The dense, iron core forms the center of the Earth. Scientists know that the core is metal from studying metallic meteorites and the Earths density. Seismic waves show that the outer core is liquid, while the inner core is solid. Movement within Earths outer liquid iron core creates Earths magnetic field. These convection currents form in the outer core because the base of the outer core is heated by the even hotter inner core. | text | null |
L_0075 | inside earth | T_0756 | Lithosphere and asthenosphere are layers based on physical properties. The outermost layer is the lithosphere. The lithosphere is the crust and the uppermost mantle. In terms of physical properties, this layer is rigid, solid, and brittle. It is easily cracked or broken. Below the lithosphere is the asthenosphere. The asthenosphere is also in the upper mantle. This layer is solid, but it can flow and bend. A solid that can flow is like silly putty. | text | null |
L_0077 | seafloor spreading | T_0764 | Before World War II, people thought the seafloor was completely flat and featureless. There was no reason to think otherwise. | text | null |
L_0077 | seafloor spreading | T_0765 | But during the war, battleships and submarines carried echo sounders. Their goal was to locate enemy submarines (Figure 6.9). Echo sounders produce sound waves that travel outward in all directions. The sound waves bounce off the nearest object, and then return to the ship. Scientists know the speed of sound in seawater. They then can calculate the distance to the object that the sound wave hit. Most of these sound waves did not hit submarines. They instead were used to map the ocean floor. | text | null |
L_0077 | seafloor spreading | T_0766 | Scientists were surprised to find huge mountains and deep trenches when they mapped the seafloor. The mid-ocean ridges form majestic mountain ranges through the deep oceans (Figure 6.10). Deep sea trenches are found near chains of active volcanoes. These volcanoes can be at the edges of continents or in the oceans. Trenches are the deepest places on Earth. The deepest trench is the Mariana Trench in the southwestern Pacific Ocean. This trench plunges about 11 kilometers (35,840 feet) beneath sea level. The ocean floor does have lots of flat areas. These abyssal plains are like the scientists had predicted. | text | null |
L_0077 | seafloor spreading | T_0766 | Scientists were surprised to find huge mountains and deep trenches when they mapped the seafloor. The mid-ocean ridges form majestic mountain ranges through the deep oceans (Figure 6.10). Deep sea trenches are found near chains of active volcanoes. These volcanoes can be at the edges of continents or in the oceans. Trenches are the deepest places on Earth. The deepest trench is the Mariana Trench in the southwestern Pacific Ocean. This trench plunges about 11 kilometers (35,840 feet) beneath sea level. The ocean floor does have lots of flat areas. These abyssal plains are like the scientists had predicted. | text | null |
L_0077 | seafloor spreading | T_0767 | Warships also carried magnetometers. They were also used to search for submarines. The magnetometers also revealed a lot about the magnetic properties of the seafloor. | text | null |
L_0077 | seafloor spreading | T_0768 | Indeed, scientists discovered something astonishing. Many times in Earths history, the magnetic poles have switched positions. North becomes south and south becomes north! When the north and south poles are aligned as they are now, geologists say it is normal polarity. When they are in the opposite position, they say that it is reversed polarity. | text | null |
L_0077 | seafloor spreading | T_0769 | Scientists were also surprised to discover a pattern of stripes of normal and reversed polarity. These stripes surround the mid-ocean ridges. There is one long stripe with normal magnetism at the top of the ridge. Next to that stripe are two long stripes with reversed magnetism. One is on either side of the normal stripe. Next come two normal stripes and then two reversed stripes, and so on across the ocean floor. The magnetic stripes end abruptly at the edges of continents. Sometimes the stripes end at a deep sea trench (Figure 6.11). | text | null |
L_0077 | seafloor spreading | T_0770 | The scientists used geologic dating techniques on seafloor rocks. They found that the youngest rocks on the seafloor were at the mid-ocean ridges. The rocks get older with distance from the ridge crest. The scientists were surprised to find that the oldest seafloor is less than 180 million years old. This may seem old, but the oldest continental crust is around 4 billion years old. Scientists also discovered that the mid-ocean ridge crest is nearly sediment free. The crust is also very thin there. With distance from the ridge crest, the sediments and crust get thicker. This also supports the idea that the youngest rocks are on the ridge axis and that the rocks get older with distance away from the ridge (Figure 6.12). Something causes the seafloor to be created at the ridge crest. The seafloor is also destroyed in a relatively short time. | text | null |
L_0077 | seafloor spreading | T_0771 | The seafloor spreading hypothesis brought all of these observations together in the early 1960s. Hot mantle material rises up at mid-ocean ridges. The hot magma erupts as lava. The lava cools to form new seafloor. Later, more lava erupts at the ridge. The new lava pushes the seafloor that is at the ridge horizontally away from ridge axis. The seafloor moves! In some places, the oceanic crust comes up to a continent. The moving crust pushes that continent away from the ridge axis as well. If the moving oceanic crust reaches a deep sea trench, the crust sinks into the mantle. The creation and destruction of oceanic crust is the reason that continents move. Seafloor spreading is the mechanism that Wegener was looking for! | text | null |
L_0078 | theory of plate tectonics | T_0772 | The Cold War helped scientists to learn more about our planet. They set up seismograph networks during the 1950s and early 1960s. The purpose was to see if other nations were testing atomic bombs. Of course, at the same time, the seismographs were recording earthquakes. | text | null |
L_0078 | theory of plate tectonics | T_0773 | The scientists realized that the earthquakes were most common in certain areas. In the oceans, they were found along mid-ocean ridges and deep sea trenches. Earthquakes and volcanoes were common all around the Pacific Ocean. They named this region the Pacific Ring of Fire (Figure 6.13). Earthquakes are also common in the worlds highest mountains, the Himalaya Mountains of Asia. The Mediterranean Sea also has many earthquakes. | text | null |
L_0078 | theory of plate tectonics | T_0774 | Earthquakes are used to identify plate boundaries (Figure 6.14). When earthquake locations are put on a map, they outline the plates. The movements of the plates are called plate tectonics. The lithosphere is divided into a dozen major and several minor plates. Each plate is named for the continent or ocean basin it contains. Some plates are made of all oceanic lithosphere. A few are all continental lithosphere. But | text | null |
L_0078 | theory of plate tectonics | T_0774 | Earthquakes are used to identify plate boundaries (Figure 6.14). When earthquake locations are put on a map, they outline the plates. The movements of the plates are called plate tectonics. The lithosphere is divided into a dozen major and several minor plates. Each plate is named for the continent or ocean basin it contains. Some plates are made of all oceanic lithosphere. A few are all continental lithosphere. But | text | null |
L_0078 | theory of plate tectonics | T_0775 | Convection within the Earths mantle causes the plates to move. Mantle material is heated above the core. The hot mantle rises up towards the surface (Figure 6.16). As the mantle rises it cools. At the surface the material moves horizontally away from a mid-ocean ridge crest. The material continues to cool. It sinks back down into the mantle at a deep sea trench. The material sinks back down to the core. It moves horizontally again, completing a convection cell. | text | null |
L_0078 | theory of plate tectonics | T_0776 | Plate boundaries are where two plates meet. Most geologic activity takes place at plate boundaries. This activity includes volcanoes, earthquakes, and mountain building. The activity occurs as plates interact. How can plates interact? Plates can move away from each other. They can move toward each other. Finally, they can slide past each other. These are the three types of plate boundaries: Divergent plate boundaries: the two plates move away from each other. Convergent plate boundaries: the two plates move towards each other. Transform plate boundaries: the two plates slip past each other. The features that form at a plate boundary are determined by the direction of plate motion and by the type of crust at the boundary. | text | null |
L_0078 | theory of plate tectonics | T_0777 | Plates move apart at divergent plate boundaries. This can occur in the oceans or on land. | text | null |
L_0078 | theory of plate tectonics | T_0778 | Plates move apart at mid-ocean ridges. Lava rises upward, erupts, and cools. Later, more lava erupts and pushes the original seafloor outward. This is seafloor spreading. Seafloor spreading forms new oceanic crust. The rising magma causes earthquakes. Most mid-ocean ridges are located deep below the sea. The island of Iceland sits right on the Mid-Atlantic ridge (Figure 6.17). | text | null |
L_0078 | theory of plate tectonics | T_0779 | A divergent plate boundary can also occur within a continent. This is called continental rifting (Figure 6.18). Magma rises beneath the continent. The crust thins, breaks, and then splits apart. This first produces a rift valley. The East African Rift is a rift valley. Eastern Africa is splitting away from the African continent. Eventually, as the continental crust breaks apart, oceanic crust will form. This is how the Atlantic Ocean formed when Pangaea broke up. | text | null |
L_0078 | theory of plate tectonics | T_0780 | A convergent plate boundary forms where two plates collide. That collision can happen between a continent and oceanic crust, between two oceanic plates, or between two continents. Oceanic crust is always destroyed in these collisions. | text | null |
L_0078 | theory of plate tectonics | T_0781 | Oceanic crust may collide with a continent. The oceanic plate is denser, so it undergoes subduction. This means that the oceanic plate sinks beneath the continent. This occurs at an ocean trench (Figure 6.19). Subduction zones are where subduction takes place. As you would expect, where plates collide there are lots of intense earthquakes and volcanic eruptions. The subducting oceanic plate melts as it reenters the mantle. The magma rises and erupts. This creates a volcanic mountain range near the coast of the continent. This range is called a volcanic arc. The Andes Mountains, along the western edge of South America, are a volcanic arc (Figure 6.20). | text | null |
L_0078 | theory of plate tectonics | T_0782 | Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate. As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes, known as an island arc (Figure 6.21). Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arcs (Figure 6.22). | text | null |
L_0078 | theory of plate tectonics | T_0782 | Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate. As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes, known as an island arc (Figure 6.21). Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arcs (Figure 6.22). | text | null |
L_0078 | theory of plate tectonics | T_0783 | Continental lithosphere is low in density and very thick. Continental lithosphere cannot subduct. So when two continental plates collide, they just smash together, just like if you put your hands on two sides of a sheet of paper and bring your hands together. The material has nowhere to go but up (Figure 6.23)! Earthquakes and metamorphic rocks result from the tremendous forces of the collision. But the crust is too thick for magma to get through, so there are no volcanoes. | text | null |
L_0078 | theory of plate tectonics | T_0783 | Continental lithosphere is low in density and very thick. Continental lithosphere cannot subduct. So when two continental plates collide, they just smash together, just like if you put your hands on two sides of a sheet of paper and bring your hands together. The material has nowhere to go but up (Figure 6.23)! Earthquakes and metamorphic rocks result from the tremendous forces of the collision. But the crust is too thick for magma to get through, so there are no volcanoes. | text | null |
L_0078 | theory of plate tectonics | T_0784 | Continent-continent convergence creates some of the worlds largest mountains ranges. The Himalayas (Figure are the remnants of a larger mountain range. This range formed from continent-continent collisions in the time of Pangaea. | text | null |
L_0078 | theory of plate tectonics | T_0784 | Continent-continent convergence creates some of the worlds largest mountains ranges. The Himalayas (Figure are the remnants of a larger mountain range. This range formed from continent-continent collisions in the time of Pangaea. | text | null |
L_0078 | theory of plate tectonics | T_0785 | Two plates may slide past each other in opposite directions. This is called a transform plate boundary. These plate boundaries experience massive earthquakes. The worlds best known transform fault is the San Andreas Fault in California (Figure 6.25). At this fault, the Pacific and North American plates grind past each other. Transform plate boundaries are most common as offsets along mid-ocean ridges. Transform plate boundaries are different from the other two types. At divergent plate boundaries, new oceanic crust is formed. At convergent boundaries, old oceanic crust is destroyed. But at transform plate boundaries, crust is not created or destroyed. | text | null |
L_0078 | theory of plate tectonics | T_0786 | Knowing where plate boundaries are helps explain the locations of landforms and types of geologic activity. The activity can be current or old. | text | null |
L_0078 | theory of plate tectonics | T_0787 | Western North America has volcanoes and earthquakes. Mountains line the region. California, with its volcanoes and earthquakes, is an important part of the Pacific Ring of Fire. This is the boundary between the North American and Pacific Plates. | text | null |
L_0078 | theory of plate tectonics | T_0788 | Mountain ranges also line the eastern edge of North America. But there are no active volcanoes or earthquakes. Where did those mountains come from? These mountains formed at a convergent plate boundary when Pangaea came together. About 200 million years ago these mountains were similar to the Himalayas today (Figure 6.26)! There were also earthquakes. | text | null |
L_0078 | theory of plate tectonics | T_0789 | Scientists think that Pangaea was not the first supercontinent. There were others before it. The continents are now moving together. This is because of subduction around the Pacific Ocean. Eventually, the Pacific will disappear and a new supercontinent will form. This wont be for hundreds of millions of years. The creation and breakup of a supercontinent takes place about every 500 million years. | text | null |
L_0078 | theory of plate tectonics | T_0790 | Most geological activity takes place at plate boundaries. But some activity does not. Much of this intraplate activity is found at hot spots. Hotspot volcanoes form as plumes of hot magma rise from deep in the mantle. | text | null |
L_0078 | theory of plate tectonics | T_0791 | A chain of volcanoes forms as an oceanic plate moves over a hot spot. This is how it happens. A volcano forms over the hotspot. Since the plate is moving, the volcano moves off of the hotspot. When the hotspot erupts again, a new volcano forms over it. This volcano is in line with the first. Over time, there is a line of volcanoes. The youngest is directly above the hot spot. The oldest is the furthest away (Figure 6.27). The Hawaii-Emperor chain of volcanoes formed over the Hawaiian Hotspot. The Hawaiian Islands formed most | text | null |
L_0078 | theory of plate tectonics | T_0792 | Hot spots are also found under the continental crust. Since it is more difficult for magma to make it through the thick crust, they are much less common. One exception is the Yellowstone hotspot (Figure 6.28). This hotspot is very active. In the past, the hotspot produced enormous volcanic eruptions. Now its activity is best seen in the regions famous geysers. | text | null |
L_0080 | nature of earthquakes | T_0803 | Almost all earthquakes occur at plate boundaries. All types of plate boundaries have earthquakes. Convection within the Earth causes the plates to move. As the plates move, stresses build. When the stresses build too much, the rocks break. The break releases the energy that was stored in the rocks. The sudden release of energy creates an earthquake. During an earthquake the rocks usually move several centimeters or rarely as much as a few meters. Elastic rebound theory describes how earthquakes occur (Figure 7.21). | text | null |
L_0080 | nature of earthquakes | T_0804 | Where an earthquake takes place is described by its focus and epicenter. | text | null |
L_0080 | nature of earthquakes | T_0805 | The point where the rock ruptures is the earthquakes focus. The focus is below the Earths surface. A shallow earthquake has a focus less than 70 kilometers (45 miles). An intermediate-focus earthquake has a focus between 70 and 300 kilometers (45 to 200 miles). A deep-focus earthquake is greater than 300 kilometers (200 miles). About 75% of earthquakes have a focus in the top 10 to 15 kilometers (6 to 9 miles) of the crust. Shallow earthquakes cause the most damage. This is because the focus is near the Earths surface, where people live. | text | null |
L_0080 | nature of earthquakes | T_0806 | The area just above the focus, on the land surface, is the earthquakes epicenter (Figure 7.22). The towns or cities near the epicenter will be strongly affected by the earthquake. | text | null |
L_0080 | nature of earthquakes | T_0807 | Nearly 95% of all earthquakes take place along one of the three types of plate boundaries. As you learned in the Plate Tectonics chapter, scientists use the location of earthquakes to draw plate boundaries. The region around the Pacific Ocean is called the Pacific Ring of Fire. This is due to the volcanoes that line the region. The area also has the most earthquakes. About 80% of all earthquakes strike this area. The Pacific Ring of Fire is caused by the convergent and transform plate boundaries that line the Pacific Ocean basin. About 15% of all earthquakes take place in the Mediterranean-Asiatic belt. The convergent plate boundaries in the region are shrinking the Mediterranean Sea. The convergence is also causing the Himalayas to grow. The remaining 5% of earthquakes are scattered around the other plate boundaries. A few earthquakes take place in the middle of a plate, away from plate boundaries. | text | null |
L_0080 | nature of earthquakes | T_0808 | Transform plate boundaries produce enormous and deadly earthquakes. These quakes at transform faults have shallow focus. This is because the plates slide past each other without moving up or down. The largest earthquake on the San Andreas Fault occurred in 1906 in San Francisco. Other significant earthquakes in California include the 1989 Loma Prieta earthquake near Santa Cruz (Figure 7.23) and the 1994 Northridge earthquake near Los Angeles. There are many other faults spreading off the San Andreas, which produce around 10,000 earthquakes a year (Figure | text | null |
L_0080 | nature of earthquakes | T_0808 | Transform plate boundaries produce enormous and deadly earthquakes. These quakes at transform faults have shallow focus. This is because the plates slide past each other without moving up or down. The largest earthquake on the San Andreas Fault occurred in 1906 in San Francisco. Other significant earthquakes in California include the 1989 Loma Prieta earthquake near Santa Cruz (Figure 7.23) and the 1994 Northridge earthquake near Los Angeles. There are many other faults spreading off the San Andreas, which produce around 10,000 earthquakes a year (Figure | text | null |
L_0080 | nature of earthquakes | T_0809 | Convergent plate boundaries also produce strong, deadly earthquakes. Earthquakes mark the motions of colliding plates and the locations where plates plunge into the mantle. These earthquakes can be shallow, intermediate or deep focus. The Philippine plate and the Pacific plate subduct beneath Japan, creating as many as 1,500 earthquakes every year. In March 2011, the 9.0 magnitude Tohoku earthquake struck off of northeastern Japan. Damage from the quake was severe. More severe was the damage from the tsunami generated by the quake (Figure 7.25). In all, 25,000 people were known dead or missing. The Cascades Volcanoes line the Pacific Northwest of the United States. Here, the Juan de Fuca plate subducts beneath the North American plate. The Cascades volcanoes are active and include Mount Saint Helens. Major earthquakes occur here approximately every 300 to 600 years. The last was in 1700. Its magnitude was between 8.7 and 9.2. It has now been more than 300 years since that earthquake. The next massive earthquake could strike the Pacific Northwest at any time. | text | null |
L_0080 | nature of earthquakes | T_0810 | The collision of two continents also creates massive earthquakes. Many earthquakes happen in the region in and around the Himalayan Mountains. The 2001 Gujarat, India earthquake is responsible for about 20,000 deaths, with many more people injured or made homeless. | text | null |
L_0080 | nature of earthquakes | T_0811 | Earthquakes also occur at divergent plate boundaries. At mid-ocean ridges, these earthquakes tend to be small and shallow focus because the plates are thin, young, and hot. Earthquakes in the oceans are usually far from land, so they have little effect on peoples lives. On land, where continents are rifting apart, earthquakes are larger and stronger. | text | null |
L_0080 | nature of earthquakes | T_0812 | About 5% of earthquakes take place within a plate, away from plate boundaries. These intraplate earthquakes are caused by stresses within a plate. The plate moves over a spherical surface, creating zones of weakness. Intraplate earthquakes happen along these zones of weakness. A large intraplate earthquake occurred in 1812. A magnitude 7.5 earthquake struck near New Madrid, Missouri. This is a region not usually known for earthquakes. Because very few people lived here at the time, only 20 people died. The New Madrid Seismic Zone continues to be active (Figure 7.26). Many more people live here today. | text | null |
L_0080 | nature of earthquakes | T_0813 | Seismic waves are the energy from earthquakes. Seismic waves move outward in all directions away from their source. Each type of seismic wave travels at different speeds in different materials. All seismic waves travel through rock, but not all travel through liquid or gas. Geologists study seismic waves to learn about earthquakes and the Earths interior. | text | null |
L_0080 | nature of earthquakes | T_0814 | Seismic waves are just one type of wave. Sound and light also travel in waves. Every wave has a high point called a crest and a low point called a trough. The height of a wave from the center line to its crest is its amplitude. The horizontal distance between waves from crest to crest (or trough to trough) is its wavelength (Figure 7.27). | text | null |
L_0080 | nature of earthquakes | T_0815 | There are two major types of seismic waves. Body waves travel through the Earths interior. Surface waves travel along the ground surface. In an earthquake, body waves are responsible for sharp jolts. Surface waves are responsible for rolling motions that do most of the damage in an earthquake. | text | null |
L_0080 | nature of earthquakes | T_0816 | Primary waves (P-waves) and secondary waves (S-waves) are the two types of body waves (Figure 7.28). Body waves move at different speeds through different materials. P-waves are faster. They travel at about 6 to 7 kilometers (about 4 miles) per second. Primary waves are so named because they are the first waves to reach a seismometer. P-waves squeeze and release rocks as they travel. The material returns to its original size and shape after the P-wave goes by. For this reason, P-waves are not the most damaging earthquake waves. P-waves travel through solids, liquids and gases. S-waves are slower than P-waves. They are the second waves to reach a seismometer. S-waves move up and down. They change the rocks shape as they travel. S-waves are about half as fast as P-waves, at about 3.5 km (2 miles) per second. S-waves can only move through solids. This is because liquids and gases dont resist changing shape. | text | null |
L_0080 | nature of earthquakes | T_0817 | Surface waves travel along the ground outward from an earthquakes epicenter. Surface waves are the slowest of all seismic waves. They travel at 2.5 km (1.5 miles) per second. There are two types of surface waves. Love waves move side-to-side, much like a snake. Rayleigh waves produce a rolling motion as they move up and backwards (Figure 7.29). Surface waves cause objects to fall and rise, while they are also swaying back and forth. These | text | null |
L_0080 | nature of earthquakes | T_0818 | Earthquakes can cause tsunami. These deadly ocean waves may result from any shock to ocean water. A shock could be a meteorite impact, landslide, or a nuclear explosion. An underwater earthquake creates a tsunami this way: The movement of the crust displaces water. The displacement forms a set of waves. The waves travel at jet speed through the ocean. Since the waves have low amplitudes and long wavelengths, they are unnoticed in deep water. As the waves reach shore they compress. They are also pushed upward by the shore. For these reasons, tsunami can grow to enormous wave heights. Tsunami waves can cause tremendous destruction and loss of life. Fortunately, few undersea earthquakes generate tsunami. | text | null |
L_0080 | nature of earthquakes | T_0819 | The Boxing Day Tsunami struck on December 26, 2004. This tsunami was by far the deadliest of all time (Figure registered magnitude 9.1. The quake struck near Sumatra, Indonesia, where the Indian plate is subducting beneath the Burma plate. It released about 550 million times the energy of the atomic bomb dropped on Hiroshima. Several tsunami waves were created. The tsunami struck eight countries around the Indian Ocean (Figure 7.31). About 230,000 people died. More than 1.2 million people lost their homes. Many more lost their way of making a living. Fishermen lost their boats, and businesspeople lost their restaurants and shops. Many marine animals washed onshore, including dolphins, turtles, and sharks. | text | null |
L_0080 | nature of earthquakes | T_0820 | Like other waves, a tsunami wave has a crest and a trough. When the wave hits the beach, the crest or the trough may come ashore first. When the trough comes in first, water is sucked out to sea. The seafloor just offshore from the beach is exposed. Curious people often walk out onto the beach to see the unusual sight. They drown when the wave crest hits. One amazing story from the Indian Ocean tsunami is that of Tilly Smith. Tilly was a 10-year-old British girl who was visiting Maikhao Beach in Thailand with her parents. Tilly had learned about tsunami in school two weeks before the earthquake. She knew that the receding water and the frothy bubbles at the sea surface meant a tsunami was coming. Tilly told her parents, who told other tourists and the staff at their hotel. The beach was evacuated and no one on Maikhao Beach died. Tilly is credited with saving nearly 100 people! | text | null |
L_0080 | nature of earthquakes | T_0821 | Most of the Indian Ocean tragedy could have been avoided if a warning system had been in place(Figure 7.32). As of June 2006, the Indian Ocean now has a warning system. Since tsunami are much more common in the Pacific, communities around the Pacific have had a tsunami warning system since 1948. Warning systems arent always helpful. People in communities very close to the earthquake do not have enough time to move inland or uphill. Farther away from the quake, evacuation of low-lying areas saves lives. | text | null |
L_0081 | measuring and predicting earthquakes | T_0822 | Seismic waves are measured on a seismograph. Seismographs contain a lot of information, and not just about earthquakes. | text | null |
L_0081 | measuring and predicting earthquakes | T_0823 | A seismograph is a machine that records seismic waves. In the past, seismographs produced a seismogram. A seismogram is a paper record of the seismic waves the seismograph received. Seismographs have a weighted pen suspended from a stationary frame. A drum of paper is attached to the ground. As the ground shakes in an earthquake, the pen remains stationary but the drum moves beneath it. This creates the squiggly lines that make up a seismogram (Figure 7.33). Modern seismographs record ground motions using electronic motion detectors. The data are recorded digitally on a computer. | text | null |
L_0081 | measuring and predicting earthquakes | T_0824 | Seismograms contain a lot of information about an earthquake: its strength, length and distance. Wave height used to determine the magnitude of the earthquake. The seismogram shows the different arrival times of the seismic waves (Figure 7.34). The first waves are P-waves since they are the fastest. S-waves come in next and are usually larger than P-waves. The surface waves arrive just after the S-waves. If the earthquake has a shallow focus, the surface waves are the largest ones recorded. A seismogram may record P-waves and surface waves, but not S-waves. This means that it was located more than halfway around the Earth from the earthquake. The reason is that Earths outer core is liquid. S-waves cannot travel | text | null |
L_0081 | measuring and predicting earthquakes | T_0825 | One seismogram indicates the distance to the epicenter. This is determined by the P-and S-wave arrival times. If a quake is near the seismograph, the S-waves arrive shortly after the P-waves. If a quake is far from the seismograph, the P-waves arrive long before the S-waves. The longer the time is between the P-and S-wave arrivals, the further away the earthquake was from the seismograph. First, seismologists calculate the arrival time difference. Then they know the distance to the epicenter from that seismograph. Next, the seismologists try to determine the location of the earthquake epicenter. To do this they need the distances to the epicenter from at least three seismographs. Lets say that they know that an earthquakes epicenter is 50 kilometers from Kansas City. They draw a circle with a 50 km radius around that seismic station. They do this twice more around two different seismic stations. The three circles intersect at a single point. This is the earthquakes epicenter (Figure 7.35). | text | null |
L_0081 | measuring and predicting earthquakes | T_0826 | The ways seismologists measure an earthquake have changed over the decades. Initially, they could only measure what people felt and saw, the intensity. Now they can measure the energy released during the quake, the magnitude. Early in the 20th century, earthquakes were described in terms of what people felt and the damage that was done to buildings. The Mercalli Intensity Scale describes earthquake intensity. There are many problems with the Mercalli scale. The damage from an earthquake is affected by many things. Different people experience an earthquake differently. Using this scale, comparisons between earthquakes were difficult to make. A new scale was needed. | text | null |
L_0081 | measuring and predicting earthquakes | T_0827 | Charles Richter developed the Richter magnitude scale in 1935. The Richter scale measures the magnitude of an earthquakes largest jolt of energy. This is determined by using the height of the waves recorded on a seismograph. Richter scale magnitudes jump from one level to the next. The height of the largest wave increases 10 times with each level. So the height of the largest seismic wave of a magnitude 5 quake is 10 times that of a magnitude 4 quake. A magnitude 5 is 100 times that of a magnitude 3 quake. With each level, thirty times more energy is released. A difference of two levels on the Richter scale equals 900 times more released energy. The Richter scale has limitations. A single sharp jolt measures higher on the Richter scale than a very long intense earthquake. Yet this is misleading because the longer quake releases more energy. Earthquakes that release more energy are likely to do more damage. As a result, another scale was needed. | text | null |
L_0081 | measuring and predicting earthquakes | T_0828 | The moment magnitude scale is the favored method of measuring earthquake magnitudes. It measures the total energy released by an earthquake. Moment magnitude is calculated by two things. One is the length of the fault break. The other is the distance the ground moves along the fault. | text | null |
L_0081 | measuring and predicting earthquakes | T_0829 | Each year, more than 900,000 earthquakes are recorded. 150,000 of them are strong enough to be felt by people. About 18 each year are major, with a Richter magnitude of 7.0 to 7.9. Usually there is one earthquake with a magnitude of 8 to 8.9 each year. Earthquakes with a magnitude in the 9 range are rare. The United States Geological Survey lists five such earthquakes on the moment magnitude scale since 1900 (see Figure 7.36). All but one, the Great Indian Ocean Earthquake of 2004, occurred somewhere around the Pacific Ring of Fire. | text | null |
L_0081 | measuring and predicting earthquakes | T_0830 | Scientists are not able to predict earthquakes. Since nearly all earthquakes take place at plate boundaries, scientists can predict where an earthquake will occur (Figure 7.37). This information helps communities to prepare for an earthquake. For example, they can require that structures are built to be earthquake safe. Predicting when an earthquake will occur is much more difficult. Scientists can look at how often earthquakes have struck in the past. This does not allow an accurate prediction for the future. Small tremors, called foreshocks, often happen a short time before a major quake. The ground may also tilt as stress builds up in the rocks. Water levels in wells also change as groundwater moves through rock fractures. These do not usually allow accurate predictions. Folklore tells of animals behaving strangely just before an earthquake. Most people tell stories of these behaviors after the earthquake. Chinese scientists actively study the behavior of animals before earthquakes to see if there is a connection. So far nothing concrete has come of these studies. Once an earthquake has started, many actions must take place. Seismometers can detect P-waves a few seconds before more damaging S-waves and surface waves arrive. Although a few seconds is not much, computers can shut down gas mains and electrical transmission lines. They can initiate protective measures in chemical plants, nuclear power plants, mass transit systems, airports, and roadways. | text | null |
L_0081 | measuring and predicting earthquakes | T_0830 | Scientists are not able to predict earthquakes. Since nearly all earthquakes take place at plate boundaries, scientists can predict where an earthquake will occur (Figure 7.37). This information helps communities to prepare for an earthquake. For example, they can require that structures are built to be earthquake safe. Predicting when an earthquake will occur is much more difficult. Scientists can look at how often earthquakes have struck in the past. This does not allow an accurate prediction for the future. Small tremors, called foreshocks, often happen a short time before a major quake. The ground may also tilt as stress builds up in the rocks. Water levels in wells also change as groundwater moves through rock fractures. These do not usually allow accurate predictions. Folklore tells of animals behaving strangely just before an earthquake. Most people tell stories of these behaviors after the earthquake. Chinese scientists actively study the behavior of animals before earthquakes to see if there is a connection. So far nothing concrete has come of these studies. Once an earthquake has started, many actions must take place. Seismometers can detect P-waves a few seconds before more damaging S-waves and surface waves arrive. Although a few seconds is not much, computers can shut down gas mains and electrical transmission lines. They can initiate protective measures in chemical plants, nuclear power plants, mass transit systems, airports, and roadways. | text | null |
L_0082 | staying safe in earthquakes | T_0831 | Earthquake magnitude affects how much damage is done in an earthquake. A larger earthquake damages more buildings and kills more people than a smaller earthquake. But thats not the only factor that determines earthquake damage. The location of an earthquake relative to a large city is important. More damage is done if the ground shakes for a long time. The amount of damage also depends on the geology of the region. Strong, solid bedrock shakes less than soft or wet soils. Wet soils liquefy during an earthquake and become like quicksand. Soil on a hillside that is shaken loose can become a landslide. Hazard maps help city planners choose the best locations for buildings (Figure 7.38). For example, when faced with two possible locations for a new hospital, planners must build on bedrock rather than silt and clay. | text | null |
L_0082 | staying safe in earthquakes | T_0832 | The 1985 Mexico City earthquake measured magnitude 8.1. The earthquake killed at least 9,000 people, injured 30,000 more, and left 100,000 people homeless. It destroyed 416 buildings, and seriously damaged 3,000 other buildings. The intense destruction was due to the soft ground the city is built on. Silt and clay fill a basin made of solid rock. In an earthquake, seismic waves bounce back-and-forth off the sides and bottom of the rock basin. This amplifies the shaking. The wet clay converts to quicksand (Figure 7.39). Many buildings were not anchored to bedrock. They settled into the muck. This caused enormous damage. Water, sewer, and electrical systems were destroyed, resulting in fires. Acapulco was much closer to the epicenter, but since the city is built on bedrock it suffered little damage. | text | null |
L_0082 | staying safe in earthquakes | T_0833 | The amount of damage depends on the amount of development in the region. The 1964 Great Alaska Earthquake, near Anchorage, was the largest earthquake ever recorded in North America. The gigantic quake had a magnitude of 9.2. The earthquake lasted for several minutes and the ground slipped up to 11.5 meters (38 feet). An area of 100,000 square miles (250,000 square km) was affected. The ground liquefied, causing landslides (Figure 7.40). The earthquake occurred at a subduction zone, and large tsunami up to 70 meters (20 feet) high were created. Despite the intensity of the earthquake, only 131 people died. Most deaths were due to the tsunami. Property damage was just over $300 million ($1.8 billion in 2007 U.S. dollars). The reason there was such a small amount of damage is that very few people lived in the area (Alaska had only been a state for five years!). A similar earthquake today would affect many more people. | text | null |
L_0082 | staying safe in earthquakes | T_0833 | The amount of damage depends on the amount of development in the region. The 1964 Great Alaska Earthquake, near Anchorage, was the largest earthquake ever recorded in North America. The gigantic quake had a magnitude of 9.2. The earthquake lasted for several minutes and the ground slipped up to 11.5 meters (38 feet). An area of 100,000 square miles (250,000 square km) was affected. The ground liquefied, causing landslides (Figure 7.40). The earthquake occurred at a subduction zone, and large tsunami up to 70 meters (20 feet) high were created. Despite the intensity of the earthquake, only 131 people died. Most deaths were due to the tsunami. Property damage was just over $300 million ($1.8 billion in 2007 U.S. dollars). The reason there was such a small amount of damage is that very few people lived in the area (Alaska had only been a state for five years!). A similar earthquake today would affect many more people. | text | null |
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