Chapter 10notes


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Chapter 10notes

  1. 1. Chapter 10 Volcanoes and other Igneous Activity
  2. 2. <ul><li>Extra Credit – Inquiry Activity page 279…. Internet research and map plotting….. </li></ul>
  3. 3. The Nature of Volcanic Eruptions <ul><li>Review the Mt. St. Helens eruption on May 18, 1980. </li></ul>
  4. 5. Mt. St. Helens <ul><li>Nearly a cubic kilometer of ash and rock debris ejected </li></ul><ul><li>Yakima, Washington, 130 kilometers to the east, so dark at noon it appeared to be midnight. </li></ul>
  5. 6. Factors Affecting Eruptions <ul><li>The primary factors that determines whether a volcano erupts violently or quietly include: </li></ul><ul><li>1. magma composition </li></ul><ul><li>2. magma temperature </li></ul><ul><li>3. dissolved gases in the magma </li></ul>
  6. 7. Viscosity <ul><li>is a substance’s resistance to flow. </li></ul><ul><li>Example: Maple syrup is more viscous than water. When you heat the syrup is becomes more fluid and less viscous. </li></ul><ul><li>The mobility of lava is strongly affected by temperature . As the lava cools its viscosity increases, its mobility decreases and the lava comes to a halt. </li></ul>
  7. 8. <ul><li>The viscosity of magma is directly related to its silica content . </li></ul><ul><li>Generally, the higher the silica content the higher the viscosity. </li></ul><ul><li>Because of the silica content, rhyolitic lavas are very viscous and don’t flow easily. </li></ul><ul><li>Basaltic lavas , which contain less silica, tend to be more fluid . </li></ul>
  8. 9. Dissolved Gases <ul><li>During explosive eruptions, the gases trapped in magma provide the force to eject molten rock from the vent, or opening to the surface. </li></ul>
  9. 10. Volcanic Gases <ul><li>1. Water vapor </li></ul><ul><li>2. CO 2 </li></ul><ul><li>3. SO 2 </li></ul><ul><li>4. H 2 </li></ul>Magma that with lots of dissolved gases tends to be more violent or explosive 5. CO 6. H 2 S 7. HCl 8. HF
  10. 11. Air Pollution through a SO 2 Cloud
  11. 12. <ul><li>As magma rises its pressure is reduced allowing the dissolved gases to be released suddenly. </li></ul><ul><li>Very fluid basaltic magmas allow expanding gases to bubble upward and escape relatively easily. </li></ul><ul><li>Therefore, eruptions of fluid basaltic lavas, as in Hawaii, are relatively quiet. </li></ul>
  12. 13. <ul><li>High viscous lava magmas slow the upward movement of expanding gases. </li></ul><ul><li>The gases collect in bubbles and pockets that increase in size until they explosively eject the molten rock from the volcano. </li></ul><ul><li>Example: Mt. St. Helens </li></ul>
  13. 14. Volcanic Material <ul><li>Lava Flows </li></ul><ul><li>Pahoehoe – (looks like rope) hot basaltic lavas are usually very fluid because of their low silica content. Flow rates of 10 – 300 meters per hour are common. When basaltic lavas cool they form a relatively smooth skin with wrinkles as the still molten subsurface lava continues to flow. </li></ul><ul><li>  </li></ul><ul><li>A-A – (block-like) its surface is rough, jagged blocks with dangerously sharp edges. </li></ul>
  14. 15. Gases <ul><li>The actually percentage of gases in magma is about 1-6 percent of total weight. </li></ul><ul><li>The actual quantity of gases emitted during an eruption can exceed thousands of tons each day. </li></ul>
  15. 16. <ul><li>Gases & amounts </li></ul><ul><li>Water vapor – 70% </li></ul><ul><li>CO 2 – 15% </li></ul><ul><li>Nitrogen – 5% </li></ul><ul><li>Sulfur – 1% </li></ul><ul><li>Chlorine </li></ul><ul><li>Hydrogen </li></ul><ul><li>Argon </li></ul>
  16. 17. <ul><li>Pyroclastic Materials </li></ul><ul><li>Ejected materials propelled to great heights by dissolved gases. </li></ul><ul><li>The fragments ejected during eruptions range in size from very fine dust and volcanic ash to pieces that weigh several tons. </li></ul>
  17. 18. <ul><li>Size </li></ul><ul><li>Small bead to walnuts (cinders) are called lapilli </li></ul><ul><li>Larger than 64 millimeter in diameter are called blocks </li></ul><ul><li>Ejected glowing lava are called bombs </li></ul>
  18. 19. Fragments of tephra ejected from volcanoes <ul><li>1. Dust </li></ul><ul><li>2. Ash </li></ul><ul><li>3. Cinders </li></ul><ul><li>4. Bombs </li></ul>
  19. 21. Cinders
  20. 22. Bombs
  21. 23. Types of Volcanoes <ul><li>Three main types of volcanoes </li></ul><ul><li>1. Shield volcanoes </li></ul><ul><li>2. Cinder cones </li></ul><ul><li>3. Composite Cones </li></ul>
  22. 24. Anatomy of a Volcano
  23. 25. <ul><li>Shield Volcanoes – formed by the accumulation of fluid basaltic lavas. </li></ul><ul><li>These volcanoes are very broad and slightly domed. Most shield volcanoes have grown up from the ocean floor and formed islands. </li></ul><ul><li>Examples: Hawaii & Iceland </li></ul>
  24. 28. Mauna Loa Belknap, Oregon
  25. 29. Kilauea
  26. 31. <ul><li>Cinder Cones – Ejected lava fragments the size of cinders, which harden in the air, build a cinder cone. </li></ul><ul><li>These volcanoes result from gas-rich basaltic magma. They sometimes extrude lava. </li></ul>
  27. 32. <ul><li>Cinder cones have a steep sided shape. They are usually the product of a single eruption that sometimes lasts only a few weeks and rarely more than a few years. </li></ul><ul><li>Once the eruption ends, the magma in the pipe connecting to the magma chamber solidifies, and the volcano never erupts again. </li></ul><ul><li>Their cone is usually small from 30 – 300 meters in height. </li></ul>
  28. 33. <ul><li>There are large numbers of cinder cones around the Earth. </li></ul><ul><li>Many exist in volcano fields like Flagstaff, Arizona, which consists of 600 cones. </li></ul><ul><li>Others form on sides of larger volcanoes, for example, Mt. Etna has dozens of cinder cones dotting its flanks. </li></ul>
  29. 35. Intermediate – Cinder Cones
  30. 36. <ul><li>Composite Cones – or stratovolcanoes – most are located around the Pacific Ocean in an area called the Ring of Fire . </li></ul><ul><li>Examples: Andes of South America, Cascade Range in the western U.S. The Cascade Range includes Mt. St. Helens, Mt. Rainier, and Mt. Garibaldi. </li></ul>
  31. 37. <ul><li>The most active regions on the Ring of Fire are located long curved belts of volcanic islands next to deep ocean trenches. </li></ul><ul><li>Examples of these include the Aleutian Islands, Japan, the Philippines, and New Zealand. </li></ul>
  32. 38. <ul><li>A composite cone is a large, nearly symmetrical structure composed of layers of both lava and Pyroclastic deposits. They generally produce very viscous lavas that are gas-rich. </li></ul><ul><li>Composite cones may generate the most explosive eruptions that eject huge amounts of pyroclastic material. </li></ul><ul><li>Examples of the classic composite cone include Fujiyama in Japan, Mt. Shasta in California. </li></ul><ul><li>About 50 composite volcanoes have erupted in the U.S. in the last 200 years. </li></ul>
  33. 40. Costa Rica Mt. Rainier Mt. Pelee
  34. 41. Dangers from Composite Cones <ul><li>Pyroclastic flows – they consist of hot gases, glowing ash, and larger rock fragments. They can race down the sides of the cone at nearly 200 mph. </li></ul><ul><li>Lahars – mudflows, many result when large amount of snow and ice melts during an eruption. In other cases, rain saturates the ground resulting in a mudflow. </li></ul>
  35. 42. Pyroclastic flows are super heated volcanic materials forming a cloud. This cloud can include volcanic glass
  36. 43. Landslides <ul><li>Landslides are produced: </li></ul><ul><ul><li>During the eruption by the explosion </li></ul></ul><ul><ul><li>By producing enough heat to melt the snow on the volcanic peak </li></ul></ul>
  37. 44. Lahars <ul><li>Occur when snow melts quickly creating mud flows and landslides </li></ul><ul><li>Move a tremendous amount of debris </li></ul><ul><li>Can clog streams and rivers </li></ul><ul><li>Like other volcanic events, they are very dangerous </li></ul>
  38. 45. Other Volcanic Features <ul><li>Caldera – is a large depression in a volcano. </li></ul><ul><li>They may form one of two ways: </li></ul><ul><li>1. collapse of the top of the composite volcano after an explosive eruption. </li></ul><ul><li>2. the collapse of the top of the composite volcano after the magma chamber is drained. </li></ul><ul><li>  </li></ul><ul><li>Example: Crater Lake, Oregon, which formed about 7,000 years ago when Mt. Mazama violently erupted and collapsed. </li></ul>
  39. 48. <ul><li>Necks and Pipes – most volcanoes are fed magma through conduits, called pipes , connecting a magma chamber to the surface. </li></ul><ul><li>When cinder cones erode leaving the pipe exposed, the resistant rock about the surface is called a volcanic neck . </li></ul><ul><li>Example: The best-known volcanic pipes are the diamond-bearing pipes of South Africa. The rocks filling these pipes formed at depths of at least 150 km. Where the pressure is great enough to form diamonds. </li></ul><ul><li>Ship Rock in New Mexico and Devil’s Tower </li></ul>
  40. 49. Extinct Volcano Conduit or volcano throat
  41. 50. <ul><li>Lava Plateaus – Material extruded from fissures in great volume over a large area. </li></ul><ul><li>Example: Columbia Plateau in northwestern U.S. Here large numbers of fissures extruded lava in flows some of which were 50 meters thick and buried the landscape under a plateau nearly 1.6 km. thick. </li></ul>
  42. 52. Fissures or Fissure Eruptions
  43. 53. Intrusive Igneous Activity <ul><li>Plutons </li></ul><ul><li>Structures that result from the cooling and hardening of magma at depth are called plutons. </li></ul><ul><li>  </li></ul><ul><li>What can these plutons that are formed at depth be exposed at the surface? </li></ul><ul><li>Erosion   </li></ul><ul><li>Intrusive igneous bodies, or pluton, are classified according to their shape, size, and relationship to the surrounding rock layers. </li></ul>
  44. 54. Sills and Laccoliths <ul><li>Sills and laccoliths are plutons that form when magma is intruded close to the surface. </li></ul><ul><li>  </li></ul><ul><li>Sills – form when magma is injected along sedimentary bedding surfaces, parallel to the bedding planes. Overlying sedimentary rock must be lifted to a height equal to the thickness of the sill. </li></ul>
  45. 55. <ul><li>Laccoliths – are similar to sills when magma is intruded between sedimentary layers close to the surface. The magma that forms laccoliths is more viscous and collects in a lens shape pushing the overlying strata upward. </li></ul>
  46. 56. Laccolith
  47. 57. <ul><li>Dikes – some plutons form when magma is injected into fractures, cutting across preexisting rock layers. Dikes are vertical features that can form when magma from a large magma chamber invades fractures in the surrounding rocks. </li></ul>
  48. 58. <ul><li>Batholiths – are the largest of the intrusive features. Batholiths are very thick. The Idaho batholith covers 40,000 sq. km. A batholith must have a surface exposure greater than 100 sq. km. to be considered. </li></ul>
  49. 59. Batholiths <ul><li>A batholith is a mass of rock formed when a large body of magma cools inside the crust. </li></ul><ul><li>Several large batholiths form the core of mountain ranges in western North America. Half Dome in Yosemite National Park, California, is part of the Sierra Nevada batholith. </li></ul>
  50. 60. Origin of Magma <ul><li>Geologists conclude the magma originates when essentially solid rock, located in the crust and upper mantle, partially melts. </li></ul><ul><li>The most obvious way to generate magma from solid rock is to raise the temperature above the level at which the rock begins to melt. </li></ul>
  51. 61. Role of Heat <ul><li>The change of temperature with depth is known as the geothermal gradient . </li></ul><ul><li>On average, temperature raises between 20-30 0 C per kilometer in the crust. </li></ul><ul><li>100 kilometers of depth temperatures range between 1400 - 1600 0 C. This is near but not quite at the melting point of rocks. </li></ul>
  52. 62. <ul><li>To reach melting point additional heat must be generated. There are several ways: </li></ul><ul><li>1. subduction zones – heat is generated by friction </li></ul><ul><li>2. Hotter mantle rocks can rise intruding crustal rocks </li></ul><ul><li>These two processes can create small amount of magma . </li></ul><ul><li>Large amount of magma can be created without additional heat. </li></ul>
  53. 63. Role of Pressure <ul><li>Pressure also increases with depth. </li></ul><ul><li>Melting causes an increase in volume, occurs at higher temperatures at depth because of greater confining pressure. </li></ul><ul><li>An increase in confining pressure causes an increase in confining pressure causes in increase in rock’s melting pressure. </li></ul><ul><li>Reducing the confining pressure lowers a rock’s melting pressure . </li></ul><ul><li>This is triggers decompression melting. </li></ul><ul><li>This process generates magma beneath Hawaii where plumes of hot rock melt as they rise toward the surface. </li></ul>
  54. 64. Role of Water <ul><li>Another important factor is water content. </li></ul><ul><li>Water causes rock to melt at lower temperatures . </li></ul>
  55. 65. Plate Tectonics and Igneous Activity <ul><li>Convergent Plate Boundaries </li></ul><ul><li>The connection between plate tectonics and volcanism is that plate tectonics provide the mechanisms by which mantle rocks melt to generate magma. </li></ul>
  56. 66. <ul><li>Ocean-Ocean </li></ul><ul><li>Results in a chain of volcanoes on the ocean floor. </li></ul><ul><li>Eventually, as these structures grow, island arcs are formed. </li></ul>
  57. 67. <ul><li>Ocean – Continent </li></ul><ul><li>Results in magma rising beyond the subduction zone creating a volcanic continental arc . </li></ul><ul><li>The only difference is the magma is silica rich as it pushes its way through the continental crust. </li></ul>
  58. 68. <ul><li>Divergent Plate Boundaries </li></ul><ul><li>As material rises from the mantle its confining pressure decreases . </li></ul><ul><li>The rocks undergo decompression melting; producing lots of magma that is less dense than the mantle material it originally came from. </li></ul><ul><li>This new basalt is now floating on top of the mantle and is an actual part of the ocean crust. </li></ul>
  59. 69. <ul><li>Intraplate Igneous Activity </li></ul><ul><li>Intraplate volcanism occurs within the plate, not at its margin. </li></ul><ul><li>Examples: Hawaiian Islands and Yellowstone National Park </li></ul>
  60. 70. <ul><li>Most intraplate volcanism occurs where a mass of hotter than normal magma material called a mantle plume rises toward the surface. </li></ul><ul><li>Mantle plumes form at the core-mantle boundary. As these plumes rise they undergo decompression melting forming basaltic magma. </li></ul><ul><li>The result is a small volcanic region a few hundred kilometers across called a hot spot . </li></ul>
  61. 71. <ul><li>40 hot spots have been identified. Volcanic activity on Hawaii is the result of a hot spot. </li></ul><ul><li>Another example is the Columbia Plateau in the northwestern U.S. </li></ul>
  62. 72. Landforms From Lava and Ash