Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Chapter 8 Volcanoes Preview Section 1 Why Volcanoes Form Section 2 Types of Volcanoes Section 3 Effects of Volcanic Eruptions Concept Map < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Bellringer List observations of volcanoes you have seen in person or on television. Write your observations in your Science Journal. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form What You Will Learn • Most volcanoes are located at or near tectonic plate boundaries. • Volcanoes form at divergent boundaries, convergent boundaries, and hot spots. • The temperature, pressure, and fluid content of rock play roles in the formation of magma. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form • A volcano is a vent or fissure in Earth’s surface through which melted rock and gases pass. • An estimated 1,500 volcanoes have been active above sea level during the past 10,000 years. • Many more volcanoes have been active beneath the ocean. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • The map in the next slide shows the locations of some of the world’s most active volcanoes. • The map also shows the boundaries between tectonic plates. • A large number of volcanoes lie directly on tectonic plate boundaries. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • The plate boundaries that surround the Pacific Ocean have so many volcanoes that the area is called the Ring of Fire. • At tectonic plate boundaries, several processes cause rock to melt at lower-crustal or upper-mantle depths. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • Molten rock is called magma. • Because magma is less dense than the solid rock surrounding it, magma travels up toward the surface. • When magma reaches the surface, it erupts to form a volcano. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • Volcanoes can form at divergent boundaries, convergent boundaries, or hot spots. Divergent Boundaries • As tectonic plates pull away from each other at a divergent boundary, a set of deep vertical fractures called fissures form. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • Molten rock flows through these fissures onto the ocean floor. • The molten rock also forms submarine volcanoes. • Underwater mountain chains known as mid-ocean ridges are common at divergent boundaries. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • Most volcanic activity on Earth happens at mid-ocean ridges. • Most divergent boundaries are underwater. • However, Iceland is an island that is being pulled apart by a mid-ocean ridge. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued Convergent Boundaries • At a convergent boundary, two plates collide. • The denser plate slides under the other plate. • As the denser plate bends, a deep depression known as a trench forms. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • At a trench, one plate moves downward into the mantle. • The process in which one plate moves beneath another is called subduction < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • As the plate moves farther downward into Earth’s mantle, the rock is subjected to greater heat and pressure. • As a result, the plate releases fluids, which causes surrounding rock to melt. • Magma then moves upward through cracks in the Earth to form a volcano. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued Hot Spots • Hot spots are volcanically active places that are not located at tectonic plate boundaries. • Hot spots lie directly above columns of hot rock that rise through Earth’s mantle. • These columns are called mantle plumes. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form Where Volcanoes Form, continued • Mantle plumes are stationary. • As a tectonic plate moves over a mantle plume, rising magma causes a chain of volcanic islands to form. • The Hawaiian Islands formed as the Pacific Plate passed over a mantle plume. < Back Next > Preview Main Chapter 8 Volcanoes Hot Spots and Mantle Plumes < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms • Magma forms in the deeper parts of Earth’s crust and in the uppermost parts of the mantle. • In these locations, temperature and pressure are very high. • Changes in temperature and pressure cause magma to form. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form • Rock melts when its temperature increases or when the pressure on the rock decreases. • Water can lower the melting temperature of rock and cause the rock to melt. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms, continued Increasing Temperature • As a tectonic plate moves downward into the mantle, the plate is exposed to greater temperatures at depth. • This increase in temperature may cause minerals in the rock to melt. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms, continued • Not all minerals in the rock melt at the same time. • Different minerals have different melting temperatures. • Minerals that have low melting temperatures melt before minerals that have high melting temperatures. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms, continued Decreasing Pressure • Magma can form when pressure on rock decreases. • In Earth’s mantle, the pressure on rock is so great that the rock cannot expand. • Expansion is important in the formation of magma, because magma takes up more space than solid rock does. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms, continued • At divergent boundaries and hot spots, hot mantle rock rises. • At a shallower depth, the pressure on the rock decreases. • The decrease in pressure allows the hot rock to expand and melt. < Back Next > Preview Main Chapter 8 Section 1 Why Volcanoes Form How Magma Forms, continued Adding Fluids • Oceanic lithosphere is composed of sediments and volcanic rocks that contain water and other fluids. • When oceanic lithosphere moves downward into the mantle, the fluids contact surrounding rock. • When fluids enter the hot mantle rock, the melting temperature of the rock decreases. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Bellringer Look through the section. Then, write definitions for each of the following words: composite volcano shield volcano cinder cone volcano Write your answers in your Science Journal. Review and, if necessary, revise your definitions after reading this section. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes What You Will Learn • Nonexplosive eruptions of basaltic magma occur at divergent boundaries. • Shield volcanoes that form from enormous volumes of basaltic magma occur at hot spots. • Explosive eruptions of silica-rich magma occur at convergent boundaries. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Types of Volcanoes • The process of magma formation is different at each type of plate boundary. • Therefore, the composition of magma differs in each tectonic setting. • Tectonic settings determine the types of volcanoes that form and the types of eruptions that take place. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries • At a divergent boundary, the lithosphere becomes thinner as two plates pull away from each other. • A set of deep cracks form in an area called a rift zone. • Hot mantle rock rises to fill these cracks. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • As the rock rises, a decrease in pressure causes hot mantle rock to melt and form magma. • The magma that reaches Earth’s surface is called lava. • Lava that flows at divergent boundaries forms from melted mantle rock. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • This lava is rich in the elements iron and magnesium. It is relatively poor in silica. • Because of its composition, lava from mantle rock cools to form dark-colored rock. • The term mafic describes magma, lava, and rocks that are rich in iron and magnesium. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • Because it is low in silica, mafic lava is runny and not sticky. • This type of lava generally produces nonexplosive eruptions. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • Mid-ocean ridges are underwater mountain chains that form where two tectonic plates are moving apart. • As the plates move apart, magma from the mantle rises to fill cracks that form in the crust. • Some of the magma erupts as basaltic lava on the ocean floor. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • The magma and lava cool to become part of the oceanic lithosphere. • As the plates continue to move, older oceanic lithosphere moves away from the mid-ocean ridge. • New cracks form, and new lithosphere forms in the rift zone. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • The process in which new sea floor forms as older sea floor is pulled apart is called sea-floor spreading. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • The mid-ocean ridge called the Mid-Atlantic Ridge is unusually active. • This activity has built part of the ridge into a large island known as Iceland. • Long linear cracks called fissures have formed where the Atlantic and Eurasian plates are moving apart. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Divergent Boundaries, continued • Basaltic magma rises to Earth’s surface through these fissures and erupts nonexplosively. • Icelandic volcanoes, such as Krafla, are often associated with large, connected fissure systems. • Lava erupts frequently through these fissures. As a result, Iceland is continually getting bigger. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots • A hot spot forms in a tectonic plate over a mantle plume. • Mantle plumes are columns of hot, solid rock that rise through the mantle by convection. • Plumes are thought to originate at the boundary between the mantle and the outer core. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • When the top of a mantle plume reaches the base of the lithosphere, the mantle rock spreads out and “pools” under the lithosphere. • Because pressure on the rock is low at this shallow depth, the rock melts. • Large volumes of magma are released onto the ocean floor. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • Continuous eruptions may produce a volcanic cone. • As the plate continues to move over the mantle plume, a chain of volcanoes may form. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • Because lava at hot spots comes from the mantle, it is mafic and fluid. • Most eruptions at hot spots are nonexplosive. • The type of rock that forms from this lava depends on the temperature, gas content, flow rate, and slope of the lava flow. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued Shield Volcanoes • Shield volcanoes usually form at hot spots. • Shield volcanoes form from layers of lava left by many nonexplosive eruptions. • The lava is very runny, so it spreads out over a wide area. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • Over time, the layers of lava create a volcanic mountain that has gently sloping sides. • The sides of shield volcanoes are not very steep, but the volcanoes can be very large. • The Hawaiian shield volcano, Mauna Kea, is the tallest mountain on Earth. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued Parts of a Volcano • Most volcanoes share a specific set of features. • The magma that feeds the eruptions pools deep underground in a structure called a magma chamber. • At Earth’s surface, lava is released through openings called vents. < Back Next > Preview Main Chapter 8 Volcanoes Magma and Vents < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • Before erupting as lava, magma rises from the magma chamber to Earth’s surface through cracks in the crust. • This movement of magma causes small earthquakes that can be used to predict an eruption. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • Lava may erupt from a central summit crater of a shield volcano. • Lava may also erupt from fissures along the sides of the shield volcano. • After erupting from a vent, the fluid lavas move downslope in lava flows. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Hot Spots, continued • A lava flow is a long river of molten rock. • Often the flow will cool and solidify on top while lava in the interior continues to flow. • Flowing lava in the interior travels through long, pipelike structures known as lava tubes. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries • At a convergent boundary, a plate that contains oceanic lithosphere may descend into the mantle beneath another plate. • The descending lithosphere contains water. • As the lithosphere descends into the mantle, temperature and pressure increase. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • The subducting lithosphere releases water into the surrounding mantle and overlying crust. • The water lowers the temperature of the rock, and the rock melts. • The magma that forms rises through the crust and erupts. These eruptions form a chain of volcanoes parallel to the plate boundary. < Back Next > Preview Main Chapter 8 Volcanoes Volcano Formation at Convergent Boundaries < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • Magmas at convergent boundaries are composed of melted mantle rock and melted crustal rock. • Therefore, fluid mafic lava and lava rich in silica and feldspar form at these boundaries. • Lavas rich in silica and feldspar cool to form lightcolored rocks. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • The term felsic is used to describe magma, lava, and rocks that are rich in silica and feldspars. • Silica-rich magma tends to trap water and gas bubbles. • This causes enormous gas pressure to develop within the magma. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • As the gas-filled magma rises to Earth’s surface, pressure is rapidly released. • This change in pressure causes a powerful explosive eruption. • Pyroclastic materials are released during the eruption. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • Pyroclastic material forms when magma explodes from a volcano and solidifies in the air. • Pyroclastic material also forms when powerful eruptions shatter existing rock. • Four types of pyroclastic material include volcanic bombs, lapilli, volcanic ash, and volcanic blocks. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • Pyroclastic flows are produced when a volcano ejects enormous amounts of hot ash, dust, and toxic gases. • This glowing cloud of pyroclastic material can race down the slope of a volcano at speeds of more than 200 km/h. • This speed is faster than the speed of most hurricane-force winds. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • The temperature at the center of a pyroclastic flow can exceed 700°C. • At this high temperature, a pyroclastic flow burns everything in its path. • Extreme winds and temperatures make pyroclastic flows the most dangerous of all volcanic phenomena. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued Cinder Cone Volcanoes • Cinder cone volcanoes are the smallest type of volcano. • They generally reach heights of no more than 300 m. • Cinder cone volcanoes are made of pyroclastic material. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • Cinder cone volcanoes most often form from moderately explosive eruptions. • They have steep sides and a wide summit crater. • Unlike other types of volcanoes, cinder cone volcanoes usually erupt only once in their lifetime. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued Composite Volcanoes • Composite volcanoes are also called stratovolcanoes. • They form from both explosive eruptions of pyroclastic material and quieter flows of lava. • This combination of eruptions forms alternating layers of pyroclastic material and lava. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued • Composite volcanoes have a broad base and sides that get steeper toward the crater. • These volcanoes may generate many eruptions. However, eruptions may occur at intervals of hundreds of years or more. • Mount Fuji in Japan is a famous composite volcano. < Back Next > Preview Main Chapter 8 Section 2 Types of Volcanoes Volcanoes at Convergent Boundaries, continued < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Bellringer Create a labeled drawing based on what you think happens when a volcano erupts. Then, write a hypothesis about how the volcanic eruption may affect the surrounding areas. Write your answers in your Science Journal. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions What You Will Learn • Volcanic eruptions can cause the loss of human life and the devastation of wildlife habitats. • Volcanic eruptions can cause the average global temperature of Earth to decrease. • Volcanic eruptions provide benefits to humans and to the environment. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions • Volcanic explosions can have local and global effects. • In April 1815, Tambora volcano in Indonesia erupted explosively. • The pyroclastic flows and falling debris killed about 10,000 people in the area. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued • High in the atmosphere, ash and gas spread around Earth. • The average global temperature decreased by as much as 3°C for one to two years. • The lower temperature caused crop failures and starvation, particularly in New England and Europe. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued • The effects of lower temperatures led to the deaths of about 82,000 people. • Therefore, an estimated total of 92,000 people around the world lost their lives as a result of the Tambora eruption. < Back Next > Preview Main Chapter 8 Volcanoes Effects of Volcanoes on Earth < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued Local Effects of Volcanic Eruptions • Volcanic eruptions can cause loss of human life and loss of wildlife habitat. • The blast from an explosive eruption can knock down trees, destroy buildings, and kill humans and animals. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued • Pyroclastic flows can burn everything in their path. • Hot volcanic materials can melt the snowcap on a mountain, causing devastating floods. • Volcanic ash can mix with water to form fast-moving mudflows called lahars. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued • The weight of falling ash can collapse structures, bury crops, and damage engines. • Volcanic ash can also cause respiratory problems in humans. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued Global Effects of Volcanic Eruptions • Large volcanic eruptions can affect Earth’s climate for several years. • During large eruptions, ash and sulfur-rich gases can be pushed into the stratosphere. • As the ash and gases spread around the planet, they absorb and scatter sunlight. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Negative Effects of Volcanic Eruptions, continued • Enough sunlight may be absorbed or scattered to lower the average global temperature of Earth. • In 1991, after the eruption of Mount Pinatubo in the Philippines, the amount of sunlight to reach Earth’s surface decreased by 2-4%. • This decrease caused the average global temperature of Earth to decrease by several tenths of a degree for several years. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions Volcanic Soils • Volcanic soils are some of the most fertile soils on Earth. • Volcanic rocks are made of minerals that contain elements important to plant growth. • When volcanic rocks break down, they form soils that contain many nutrients that plants can use. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued Geothermal Energy • Magma heats the rocks that surround it. • These rocks often hold water that also becomes heated. • This heated water, called geothermal water, may reach temperatures of hundreds of degrees Celcius. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • Geothermal water contains large amounts of heat energy. • This energy can be tapped by drilling wells to reach the hot water. • Water can also be pumped through heated rocks to obtain energy. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • Geothermal water can be used to drive turbines that generate electricity. • It can also be used to heat homes, grow crops, or keep roads free of ice. • In Reykjavik, Iceland, 85% of all homes are heated with geothermal water. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • Volcanic rocks are often used in construction. • As early as 300 BCE, Romans made concrete from volcanic ash and lime. • This material was used to build the Colosseum in Rome in 80 CE. The strength of this material has allowed the Colosseum to stand for nearly two thousand years. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • As recently as the 20th century, volcanic ash was used to make concrete for dams in the United States. • Today, basalt and pumice are often used in the construction of roads and bridges, and the production of concrete. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • Volcanic rocks have many other uses. • Volcanic ash absorbs moisture, so it is used in cat litter. • Because pumice is abrasive, it is used in facial scrubs, soaps, cleaners, and polishes. < Back Next > Preview Main Chapter 8 Section 3 Effects of Volcanic Eruptions Benefits of Volcanic Eruptions, continued • Pumice is added to soil to allow air and water to circulate more easily through the soil. • Because metals in pumice are not water soluble, pumice is used alone or with silica sand to filter drinking water. < Back Next > Preview Main Chapter 8 Volcanoes Concept Map Use the terms below to complete the concept map on the next slide. eruptions shield volcanoes cinder cone volcanoes composite volcanoes lava < Back Next > Preview Main Chapter 8 Volcanoes Concept Map < Back Next > Preview Main Chapter 8 Volcanoes Concept Map < Back Next > Preview Main