Types Of Volcanoes

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Types Of Volcanoes

  1. 1. Types of volcanoes
  2. 2.  Tom Abbott, Biddulph High School and made available through www.sln.org.uk/geography and only for non commercial use in schools Composite Cone The volcano is built up of alternate layers of LAVA and ASH. They can explode with great violence. Mt St Helens - USA Conelet Dyke Crater Ash layer Lava Layer Flank eruption Lava Flow Crust Pyroclastic Flow
  3. 3. Formation of Composite Volcanoes <ul><li>These are found on destructive plate margins </li></ul><ul><li>Composite volcanoes have very steep sides and a narrow base. </li></ul><ul><li>Lava builds up in a magma chamber underneath the volcano. This can be added to as more oceanic crust melts at the plate margin </li></ul><ul><li>The lava is very thick (acid) and so clogs up the main vent of the volcano, causing a ‘plug’ effect </li></ul><ul><li>The pressure build-up causes an explosion which blows out ash, gas and lava </li></ul><ul><li>The neck of the volcano is then cleared which allows the lava to flow out of the crater. </li></ul><ul><li>The layers of lava become the sides </li></ul><ul><li>Alternate layers of ash and lava (ash is 1st in an eruption) </li></ul><ul><li>Eruptions don’t happen very often, but are explosive </li></ul><ul><li>e.g Mt St Helens, Washington USA </li></ul>
  4. 4. Shield ( Basic) Cone Shield Volcanoes are enormous features built up only from layers of lava. They produce lots of lava but they tend not to erupt violently. Mauna Loa Hawaii -USA 250 miles 10,000m Composite volcano to scale Layers of Lava Lava Flow
  5. 5. Formation of Shield Volcanoes <ul><li>These are found on constructive plate margins </li></ul><ul><li>They also occur at hot spots under the Earth’s surface (not on plate boundaries, but forming a chain of volcanic islands e.g. Mauna Loa, Hawaii USA </li></ul><ul><li>Shield volcanoes have gently sloping sides and are much wider than composite volcanoes </li></ul><ul><li>They erupt frequently – gentle eruptions </li></ul><ul><li>The lava is very runny – basic lava (basalt), with little ash. This spreads easily and cools to form the gentle sides </li></ul><ul><li>They usually occur on constructive margins where the sea floor is spreading at a mid-ocean trench </li></ul>
  6. 6. Differences between composite and shield explosive, Mauna Loa, narrow and steep, basaltic lava, destructive, wide and gentle slopes, non-violent, constructive, silica lava, Mt St Helens Composite Shield
  7. 7. Differences between composite and shield explosive, Mauna Loa, narrow and steep, basaltic lava, destructive, wide and gentle slopes, non-violent, constructive, silica lava, Mt St Helens basalt rhyolite rock Little ash, lots of runny lava Lots of ash Frequent eruptions Less frequent eruptions Wide base Narrow base gentle Steep sided Basic lava (runny) Acid lava (thick and sticky) Constructive plate boundaries Destructive plate boundaries effusive Explosive Composite Shield
  8. 8. Exam question <ul><li>Mauna Loa is a shield volcano. Explain the characteristics of shield volcanoes and how they are formed. (6 marks) </li></ul>
  9. 9. Mark scheme answer <ul><li>Level 1 (1-2 marks) </li></ul><ul><li>Basic - may be characteristics or formation, e.g. it has gentle sides, formed from runny lava, found at plate boundaries. </li></ul><ul><li>Level 2 (3-4 marks) </li></ul><ul><li>Clear, may still be imbalanced. Shield volcanoes have a wide base and gentle sides because they are formed of runny lava that flows long distances before cooling. </li></ul><ul><li>Level 3 (5-6 marks) </li></ul><ul><li>Detailed, needs to have both characteristics and formation but not necessarily both at Level 3. As for Level 2, plus more detail on their formation - usually at constructive boundaries, reference to basalt, hot spots all acceptable. </li></ul><ul><li>Credit good use of terminology. 6 marks </li></ul>

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