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  2. 2. What is a Volcano?A volcano is a vent, or opening in the crust from which pour molten rock, rock debris, gases and steamThe name comes from the Roman god of fire Vulcan.When magma penetrates the surface it is known as lava. Prepared by: A Smith 2
  3. 3. What is a Volcano? Prepared by: A Smith 3
  4. 4. Prepared by: A Smith 4
  5. 5. Global distribution of volcanoes Prepared by: A Smith 5
  6. 6. Global distribution of volcanoesMost volcanoes are found along convergent and divergent plate boundaries.At these points there are molten rocks and magma to supply the volcanoes.There are about 1300 potentially active volcanoes in the world.A small number of volcanoes are along way from plate boundaries known as hot spots. Prepared by: A Smith 6
  7. 7. Global distribution of volcanoesAt these points the temperature at the boundary of the mantle ad crust is unusually high and there are lines of weakness in the crust which magma can follow to reach the surface.An example of an hot spot is the Hawaiian IslandsSome hot spots can also be found beneath continents such as Yellowstone Basin in the USA Prepared by: A Smith 7
  8. 8. Hot spotsPrepared by: A Smith 8
  9. 9. Volcanic Eruption?The sudden occurrence of a violent discharge of steam and volcanic material.An eruption begins when pressure on a magma chamber forces magma up through the conduit and out the volcanos vents.When the magma chamber has been completely filled, the type of eruption partly depends on the amount of gases and silica in the magma.The amount of silica determines how sticky (level of viscosity) the magma is and water provides the explosive potential of steam. Prepared by: A Smith 9
  10. 10. Stages of a volcanoActive- when eruptions occur at frequent intervalsDormant-when eruptions are infrequent and one has not occurred for some timeExtinct- when it is thought a volcano will never erupt again. Prepared by: A Smith 10
  11. 11. Ways volcano affect humansLava flowsTephraPyroclastic flowsVolcanic gasesLaharsTsunamis Prepared by: A Smith 11
  12. 12. Lava flowsLava flows are streams of molten rock that pour or ooze from an erupting vent.Lava is erupted during either nonexplosive activity or explosive lava fountains.Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. Prepared by: A Smith 12
  13. 13. Lava flowsThe speed at which lava moves across the ground depends on several factors, including : (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube;  (4) rate of lava production at the vent. Prepared by: A Smith 13
  14. 14. Lava flowsPrepared by: A Smith 14
  15. 15. TephraWhen a volcano erupts it will sometimes eject material such as rock fragments into the atmosphere. This material is known as tephra.The largest pieces of tephra (greater than 64 mm) are called blocks and bombs. Blocks and bombs are normally shot ballistically from the volcano (refer to the gas thrust zone described in the direct blast section). Prepared by: A Smith 15
  16. 16. TephraBecause these fragments are so large they fall out near their source. Blocks and bombs as large as 8-30 tons have fallen as far away as 1 km from their source (Bryant, 1991).Small blocks and bombs have been known to travel as far away as 20-80 km !Some of these blocks and bombs can have velocities of 75-200 m/s Prepared by: A Smith 16
  17. 17. TephraPrepared by: A Smith 17
  18. 18. Pyroclastic flowsPyroclastic flows are fluidized masses of rock fragments and gases that move rapidly in response to gravity. Pyroclastic flows can form in several different ways. They can form when an eruption column collapses, or as the result of gravitational collapse or explosion on a lava dome or lava flow.These flows are more dense than pyroclastic surges and can contain as much as 80 % unconsolidated material. Prepared by: A Smith 18
  19. 19. Pyroclastic flowsThe flow is fluidized because it contains water and gas from the eruption, water vapor from melted snow and ice, and air from the flow overriding air as it moves downslope. Prepared by: A Smith 19
  20. 20. Pyroclastic flowPrepared by: A Smith 20
  21. 21. Volcanic gasesMagma contains dissolved gases that are released into the atmosphere during eruptions.Gases are also released from magma that either remains below ground (for example, as an intrusion) or is rising toward the surface.In such cases, gases may escape continuously into the atmosphere from the soil, volcanic vents, fumaroles, and hydrothermal systems. Prepared by: A Smith 21
  22. 22. Volcanic gasesAt high pressures deep beneath the earths surface, volcanic gases are dissolved in molten rock.But as magma rises toward the surface where the pressure is lower, gases held in the melt begin to form tiny bubbles.The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma to continue its upward journey. Prepared by: A Smith 22
  23. 23. Volcanic gasesPrepared by: A Smith 23
  24. 24. LaharsLahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter. Prepared by: A Smith 24
  25. 25. LaharsLahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun. Prepared by: A Smith 25
  26. 26. LaharsPrepared by: A Smith 26
  27. 27. TsunamisA series of water waves caused by the displacement of a large volume of a body of water, typically an ocean or a large lake.Earthquakes, volcanic eruptions and other underwater explosions, landslides, glacier calvings, meteorite impacts and other disturbances above or below water all have the potential to generate a tsunami. Prepared by: A Smith 27
  28. 28. TsunamisPrepared by: A Smith 28
  29. 29. Intrusive volcanic featuresDykeSillLaccolithBatholithPlug Prepared by: A Smith 29
  30. 30. DykeWhen a mass of magma cuts across bedding planes, it forms a wall-like feature called a dyke.Sometimes the rocks on either side of a dyke are more resistant to erosion.When this happens, the dyke forms a depression. Dykes sometimes occur in swarms, as in Arran and parts of western Scotland and northern Ireland. Prepared by: A Smith 30
  31. 31. DykePrepared by: A Smith 31
  32. 32. SillWhen a sheet of magma lies along a bedding plane it forms a structure called a sill. Some sills form ridge-like escarpments when exposed by erosion. Prepared by: A Smith 32
  33. 33. LaccolithA sheet intrusion that has been injected between two layers of sedimentary rock. The pressure of the magma is high enough that the overlying strata are forced upward, giving the laccolith a dome or mushroom-like form with a generally planar base.Laccoliths tend to form at relatively shallow depths and are typically formed by relatively viscous magmas, such as those that crystallize to diorite, granodiorite, and granite. Prepared by: A Smith 33
  34. 34. LaccolithPrepared by: A Smith 34
  35. 35. BatholithThis is a very large mass of magma which accumulates in the crust. Sometimes it forms the root or core of a mountain.Batholiths are made of granite and they form surface features only after they have been exposed by denudation.Batholits are exposed at the suface in south-west England where they form Dartmoor, Bodmin Moor, Lands End and the Scilly Isles. These are the cores of an ancient mountain chain the tops of which have long since been removed by erosion. Prepared by: A Smith 35
  36. 36. BatholithPrepared by: A Smith 36
  37. 37. Extrusive volcanic featuresAcid lavaBasic lavaDome volcanoesAsh and cinder conesComposite conesShield volcanoesCalderasBasalt plateaus Prepared by: A Smith 37
  38. 38. Extrusive volcanic featuresMagma sometimes reaches the surface through a vent (hole), or a fissure (crack) in the surface rocks. When magma emerges at the surface it is called lava.If lava emerges via a vent, it usually builds up a volcano, which is a cone-shaped mound.If it emerges from a fissure, it may build up a lava plain, or a lava plateau. Volcanic eruptions also take place on some ocean floors. Prepared by: A Smith 38
  39. 39. Acid lavaAcid lava comes from composite cone volcanoes, is slow moving and viscousAcid lava is produced when there is subduction at destructive plate boundaries.This lava is silica-rich and has temperatures of about 800°C. It cools and solidifies quickly and produces steep sided volcanoes. Prepared by: A Smith 39
  40. 40. Acid lavaAs it solidifies quickly, it may solidify in the central pipe, blocking the passageway and causing a buildup of pressure, which produces violent eruptions.The solidified lava forms sheets of rough, jagged rocks called aá. Flows of boulders and rubble called block rubble and mounds of lava called domes are also formed. Prepared by: A Smith 40
  41. 41. Acid lavaPrepared by: A Smith 41
  42. 42. Basic lavaBasic lava comes from shield volcanoes, is runny and flows faster.Basic lava is produced at constructive plate boundaries.This lava is rich in iron and magnesium and is rather fluid. It has a temperature of about 1200°C.It flows for a distance before solidifying and forms gently sloping volcanoes. The lava solidifies into smooth, folded sheets of rock called pahoehoe. Prepared by: A Smith 42
  43. 43. Basic lavaPrepared by: A Smith 43
  44. 44. Dome VolcanoesA dome volcano is a round shaped mountain made of viscous lava.The lava has a high silica content that prevents the lava from flowing very far from its vent. Most domes are formed by dacite and rhyolite lavas. Prepared by: A Smith 44
  45. 45. Dome VolcanoesPrepared by: A Smith 45
  46. 46. Dome VolcanoesPrepared by: A Smith 46
  47. 47. Ash and cinder conesLava is blown to great heights when it is violently ejected, and it breaks into small fragments which fall back to earth and build up a cone.Good examples of ash and cinder cones are Volcano De Fuego (Guatemala) and Paricutin (Mexico). Prepared by: A Smith 47
  48. 48. Composite conesThis type of cone is formed of alternate layers of lava and ash.The volcano begins each eruption with great violence forming a layer of ash.As the eruption proceeds, the violence ceases and lava pours out forming a layer on top of the ash. Lava often escapes from the sides of the cone where it builds up small conelets Prepared by: A Smith 48
  49. 49. Composite conePrepared by: A Smith 49
  50. 50. Shield volcanoesA shield volcano is a type of volcano usually built almost entirely of fluid lava flows.They are named for their large size and low profile, resembling a warriors shield.This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes.This results in the steady accumulation of broad sheets of lava, building up the shield volcanos distinctive form. Shield volcanoes contain low viscosity magma making it have flowing mafic lava. Prepared by: A Smith 50
  51. 51. Shield Volcanoes Prepared by: A Smith 51
  52. 52. Shield VolcanoPrepared by: A Smith 52
  53. 53. CalderasThese are ones where the diameter of the circular to oval crater exceeds 1 mile.These form when so much lava is erupted (blown out) so rapidly it partially empties the underlying magma chamber.When this happens the summit of the volcanic structure collapses into the emptied magma chamber.Typically the erupted material occurs as airfall or pyroclastic flows. Prepared by: A Smith 53
  54. 54. CalderasPrepared by: A Smith 54
  55. 55. CalderasPrepared by: A Smith 55
  56. 56. Basalt PlateauOne or a succession of high-temperature basaltic lava flows from fissure eruptions which accumulate to form a plateau. Also known as flood basalt.Example the Indian Deccan Prepared by: A Smith 56
  57. 57. Basalt plateauPrepared by: A Smith 57