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  1. 1. Tectonic processes Volcanic hazards
  2. 2. Syllabus… <ul><li>Relate the distribution and nature of volcanic activity to plate margins and hot spots. </li></ul><ul><li>Identify the types of eruption and volcanic forms as basic, acid, fissure and composite. </li></ul><ul><li>Identify primary hazards (such as ash, lava, directed blast, nuée ardente and pyroclastic flows) </li></ul><ul><li>Identify secondary hazards (such as landslides, lahars (mudflows) and atmospheric impacts). </li></ul>
  3. 3. References <ul><li>Bishop pg 59 – 83 </li></ul><ul><li>Case study – primary Mt Pelee </li></ul><ul><li>Case study </li></ul><ul><li>Secondary – Nevada del Ruiz 1985 pg 69 </li></ul><ul><li>Ross pg38 – 41 </li></ul><ul><li>Case study Montserrat pg 42 </li></ul>
  4. 4. Volcanic activity – What you already know <ul><li>Working in pairs, produce an A3 mind map illustrating what you know about volcanoes. </li></ul><ul><li>Include terminology, distribution, case studies etc. </li></ul>Animated guide: Volcanoes
  5. 5. Syllabus details: Relate the distribution and nature of volcanic activity to plate margins and hot spots. Identify the types of eruption and volcanic forms as basic, acid, fissure and composite. Identify primary hazards (such as ash, lava, directed blast, nuée ardente and pyroclastic flows) and secondary hazards (such as landslides, lahars (mudflows) and atmospheric impacts).
  6. 6. terminology <ul><li>Tephra – rock debris </li></ul><ul><li>Lapilli > 2mm </li></ul><ul><li>Ash <2mm </li></ul><ul><li>Lahar </li></ul><ul><li>Pyroclastic flow </li></ul><ul><li>Basalt – basic rock </li></ul><ul><li>Andesite - intermediate </li></ul><ul><li>Rhyolite – acid rocks </li></ul><ul><li>Vent – central pipe of a volcano through which magma passes </li></ul><ul><li>Fissure – crack in the earth’s surface through which magma emerges </li></ul><ul><li>Hot spot – plume of hot magma within the upper mantle </li></ul><ul><li>Subduction </li></ul><ul><li>Constructive </li></ul><ul><li>Destructive </li></ul><ul><li>Island arc </li></ul>
  7. 7. Relate the distribution and nature of volcanic activity to plate margins and hot spots
  8. 8. <ul><li>When molten magma escapes to the surface it forms a volcano. </li></ul><ul><li>If magma escapes through a single hole or vent these are central vent volcanoes. </li></ul><ul><li>If the magma escapes along a crack a line of volcanoes called fissure volcanoes is formed. </li></ul>
  9. 9. The shape of a volcano depends on the: <ul><li>chemical composition of the magma </li></ul><ul><li>the explosiveness of the eruption. </li></ul><ul><li>The amount of change since the eruption </li></ul>
  10. 10. <ul><li>Lava produced by the upward movement of material from the mantle is basaltic and tends to be located along mid-ocean ridges, over hot spots and alongside rift valleys. Shield volcanoes are associated with constructive plate margins and hot spots. </li></ul>Constructive plate boundaries 73% of magma which reaches the earth’s surface occurs along these boundaries. The main areas are the mid ocean ridges where melting of the upper mantle produces basaltic magma. The eruptions tend to be non-violent (=effusive) and as most occur on the sea floor they do not represent a major hazard to people except where portions of ocean ridge emerge above sea level to form inhabited islands such as Iceland Continental constructive boundaries occur where continental crust is being moved apart e.g. the East African rift valley system.
  11. 12. <ul><li>Lava that results form the process of subduction is andesitic and occurs as island arcs or at destructive boundaries where oceanic crust is being destroyed </li></ul>Destructive plate boundaries 80% of the world’s active volcanoes occur along destructive boundaries. As the oceanic plate descends into the mantle and melts basic magma rises upwards and mixes with continental crust to produce magma with a higher silica content than along the oceanic ridges (Mt Cotopaxi – Ecuador). These andesitic and rhyolitic magmas result in violent activity . Volcanic island arcs (Japan – Mt Fujiyama, Mt Unzen) form where two oceanic plates are colliding the resulting volcanic activity is of andestitic magmas.
  12. 14. Hot spots Hotspots are small areas of the crust with an unusually high heat flow and are found away from plate boundaries . Slowly rising mantle rocks create volcanic activity on the surface. The movement of the lithospheric plates over the hot spot produces a chain of what are mainly now extinct volcanoes . Examples of hot spots are Hawaiian Islands , Maldives, Azores. Eruptions are normally of low viscosity basaltic magmas, effusive eruptions
  13. 15. Identify the types of eruption and volcanic forms as basic, acid, fissure and composite <ul><li>Ref Waugh pg 24, 25 </li></ul>
  14. 16. How can volcanoes be classified: 1. Shape <ul><li>1. Fissure eruptions </li></ul><ul><li>When two plates move apart lava may be ejected through a fissure </li></ul><ul><li>e.g. Heimaey eruption of 1973 </li></ul><ul><li>The basalt may form large plateas, filling in hollows rather than build up into cone shaped peaks </li></ul>
  15. 17. <ul><li>2. Basic or shield volcanoes </li></ul><ul><li>e.g. Mauna Loa, Hawaii </li></ul><ul><li>Lava flows out of a central vent and can spread over wide areas before solidifying </li></ul><ul><li>The cone has long gentle sides made up of many layers of lava from repeated flows </li></ul><ul><li>3. Acid or dome volcanoes </li></ul><ul><li>Acid lava quickly solidifies on exposure to the air. This produces a steep sided convex cone. Viscous lava, often rhyolite </li></ul><ul><li>e.g. Mt Pelee, </li></ul><ul><li>Puy region of central France </li></ul>
  16. 18. <ul><li>4 . Ash and cinder cones </li></ul><ul><li>e.g. Paricutin </li></ul><ul><li>Formed from ash, cinders and volcanic bombs ejected from the crater. The sides are steep and symmetrical </li></ul><ul><li>5. Composite cones </li></ul><ul><li>Many of larger volcanoes result from alternating types of eruption in which first ash and then lava (usually andesitic) are ejected e.g. Etna </li></ul>
  17. 19. The nature of the eruption <ul><li>The classification of volcanoes is based on the degree of violence of the explosion. </li></ul><ul><li>This in turn reflects the pressure and amount of gas in the magma </li></ul>
  18. 20. Why volcanoes explode
  19. 21. <ul><li>Icelandic – lava flows gently from a fissure </li></ul><ul><li>Hawaiian – lava is emitted gently but from a vent </li></ul><ul><li>Strombolian – small but frequent eruptions </li></ul><ul><li>Vesuvian – violent explosion after a long period of inactivity </li></ul><ul><li>Krakatoan – exceptionally violent explosion that may remove much of the original cone </li></ul><ul><li>Pelean – violent eruption is accompanied by pyroclastic flows that may include a nuee ardente </li></ul><ul><li>Plinian where large amounts of lava and pyroclastic materoia; are ejected </li></ul>Ref Waugh pg 28
  20. 22. Summary table:
  21. 23. Tasks: <ul><li>What is the difference between central vent volcanoes and fissure volcanoes? </li></ul><ul><li>How do the relative amounts of silica and volatiles affect the nature of volcanic eruptions? </li></ul><ul><li>With the aid of diagrams, compare composite and shield volcanoes. Refer to the shape of the volcanoes, the nature of their eruptions and their relationship with plate margins. </li></ul>
  22. 24. Identify primary hazards and secondary hazards <ul><li>Primary Volcanic hazards: </li></ul><ul><li>Ref Bishop page 65 - 68 </li></ul><ul><li>Lava flows </li></ul><ul><li>Pyroclastic flows and surges = nuees ardentes </li></ul><ul><li>Ash and tephra </li></ul><ul><li>Volcanic gases </li></ul>
  23. 25. Lava flows <ul><li>Lava flows pose a threat more to property in their path than to human life. </li></ul><ul><li>The most dangerous are fissure eruptions of basaltic magma which reach speeds of 50 km per hour on steep slopes and can spread 10s of km from their source. </li></ul><ul><li>Andesitic and rhyolitic lavas flow more slowly rarely reaching more than 8km from their source. </li></ul><ul><li>Lava flows are at their most hazardous when large quantities are released quickly </li></ul><ul><li>Lavas destroy everything in their path and are likely to result in high but localised economic loses such as agricultural land, buildings and roads. </li></ul><ul><li>Kilauea in Hawaii has been erupting lava since 1983, covering 78km2 and destroying 180 houses but with ne deaths. The eruptions added 120km2 of new land and are a major tourist attraction. </li></ul>
  24. 26. Pyroclastic flows and surges = nuees ardentes <ul><li>Pyroclastic flows are mixtures of hot rock fragments, lava particles and ash buoyed up by hot gases . </li></ul><ul><li>Deadly flows , commonly associated with subduction-zone volcanoes. The flow moves from the vent at high speeds and can extend up to 40km from its source. </li></ul><ul><li>Pyroclastic material is denser than the surrounding air and moves close to the ground following areas of lower relief such as river valleys. </li></ul><ul><li>Pyroclasitc surges develop from flows as they move downhill. </li></ul><ul><li>Pyroclastic flows develop from a collapsing eruption column, lava dome collapse or they simply boul out of the crater. The most dangerous pyroclastic flows are those directed sideways by a lateral eruption of a volcano where the summit crater is blocked by viscous magma and the gases build up pressure which is released explosively through a weak point in the volcano’s flanks. These events occur with andesitic or rhyolitic explosive eruptions. </li></ul><ul><li>The force and full volume of the pyroclastic material is projected down the volcano rather than into the atmosphere. The high speed and temperatures of the flow destroy everything in their path. </li></ul>
  25. 28. Ash and tephra fall <ul><li>Volcanic ash (<2mm diameter) and tephra (>2mm diameter) forms when magma is fragmented by explosions or when solid rocks are broken up by groundwater turning explosively into steam. </li></ul><ul><li>The finest ash can be carried high into the atmosphere and is carried by winds over large areas . This can affect world weather patterns and present a hazard to aircraft . </li></ul><ul><li>Ash falls can cause breathing difficulties . Large accumulations of ash may cause building collapse </li></ul><ul><li>Agricultural production may be reduced buy large ash falls – Mt St Helens ash deposits 30kg / m2 on crops in Eastern Washington State </li></ul>
  26. 29. The largest fragment is about 5 mm in diameter. Tephra between 2 and 64 mm in diamter is called lapilli; tephra <2 mm in diamter is called ash.
  27. 30. Volcanic gases <ul><li>Active volcanoes produce large amounts of water vapour, carbon dioxide, sulphur dioxide, chlorine, hydrogen sulphide, hydrogen, helium, carbon monoxide and hydrogen chloride. </li></ul><ul><li>Carbon dioxide is the most dangerous because it is heavier than air, collects in depressions and can be high enough to cause suffocation. </li></ul>
  28. 32. Secondary volcanic hazards. <ul><li>Acidic rain </li></ul><ul><li>Flooding from melting ice caps and ice fields </li></ul><ul><li>Lahars </li></ul>Identify secondary volcanic hazards pg 68 - 70
  29. 33. Lahars <ul><li>Lahars are mudflows of volcanic material </li></ul><ul><li>They are a huge threat to people. </li></ul><ul><li>The amount of ash and debris produced by eruptions can mix with water and then move rapidly over long distances. </li></ul><ul><li>The source of the water may be: </li></ul><ul><ul><li>heavy rain </li></ul></ul><ul><ul><li>the melting of ice on a volcanic peak following an eruption e.g.1985 Nevado del Ruiz </li></ul></ul><ul><ul><li>Lahars may also be induced by the collapse of a crater lake </li></ul></ul><ul><li>1982 – 83 eruption of Galunggung Indonesia resulted in in thunderstorm generated lahars which made 35 000 homeless and wiped out the local agricultural economy by burying 94,000 ha of crops and fish farms. </li></ul>Early war n ing for volcanic mudslides Deadly mudslide at Ecuador volcano Mount Pinatubo: Lahars could devastate the surrounding country
  30. 34. Mud slide fears over Mexico volcano
  31. 35. landslides <ul><li>Gravity driven slides of masses of rock and loose volcanic material. </li></ul><ul><li>Can occur as a result of an eruption e.g. Mt St Helens </li></ul><ul><li>Can occur at any time as a result of heavy rainfall or earthquakes </li></ul><ul><li>Ground deformation of volcanic slopes by rising magma may also trigger slope instability </li></ul>
  32. 36. tsunami
  33. 37. Nevado del Ruiz Volcano, Colombia, 1985 USGS website
  34. 38. Tasks <ul><li>Produce a detailed fact file (word document) about one of the given volcanic eruptions, </li></ul><ul><li>Location, - geographical location. </li></ul><ul><li>name and direction of plates </li></ul><ul><li>geological situation – hot spot or type of plate boundary </li></ul><ul><li>Date of eruption </li></ul><ul><li>Nature of eruption – noting primary and secondary hazards </li></ul><ul><li>Social, economic and environmental consequences of the eruption </li></ul><ul><li>To be uploaded to wiki by Friday 16 th January </li></ul>
  35. 39. references <ul><li>Bishop pg 59 – 83 </li></ul><ul><li>Case study – primary Mt Pelee </li></ul><ul><li>Case study </li></ul><ul><li>Secondary – Nevada del Ruiz 1985 pg 69 </li></ul><ul><li>Ross pg38 – 41 </li></ul><ul><li>Case study Montserrat pg 42 </li></ul>
  36. 40. Case studies <ul><li>Mount St Helens 1985 (MEDC) </li></ul><ul><li>Nevado del Ruiz 1985 (LEDC secondary) </li></ul><ul><li>Mount Pelee Martinique 1902 </li></ul><ul><li>Montserrat (LEDC) </li></ul>