other facts

• continental crust


  less dense than oceanic, much thicker with a mean of 35km, mainly made of granite, silicon and alluminum
• oceanic crust


  denser at around 3kg / m³, less thick at around 5-10km, mainly made of basalt, silicon and magnesium
• mantle


  rich in magnesium and iron, made of the asthenosphere (slowly flowing and over 100km deep) and lithosphere (rigid)
• cores


  liquid outer core and solid inner core
• mohorovicic disontinuity


  the boundary between the mantle and the crust
• what is the evidence for the theories of plate tectonics

  continental fit, paleomagnetism, fossils, geological sequences and glacial evidence
• fossils


  there are evidence of seeds and animals such as glosspetris that could not have survived travelling across oceans
• glacial evidence


  there are materials and animals found near the equator where it would have been too warm for them to form, also there are striations in the bedrock
• continental fit


  the world used to fit like a puzzle, called pangea, for example South America and Africa
• paleomagnetism


  the earth has a magnetic field, as fresh magma rises it pushes older rock away causing sea-floor spreading (so its clear that sea floors spread at mid ocean ridges and subduct at ocean trenches). as magma cools it preserves the polarity of the previous layer and iron-bearing minerals become aligned
• geological sequences


  there are matching rock sequences for example of glacial, marine and non-marine rock found in South America, Antarctica and Australia or on mountain chains in Canada and Scotland. coal can only be made in tropical areas yet is found elsewhere
• what are the three types of plate boundaries

  convergent (destructive)

  divergent (constructive)

  conservative
• divergent plate boundaries


  plates move away from eachother, often below the ocean.

  • long chains of mountains
  • broken up by transform faults (no volcanoes along faults)
  • earthquakes and volcanoes occu
• what features are at divergent plate boundaries

  effusive eruptions (shield volcanoes), pillow lavas, graben, black smokers and rift valleys
• transform faults 


  large faults at right angles to a ridge, when they slip earthquakes often occur (found at divergent boundaries)
• pillow lavas


  found at divergent plate boundaries, are mounds formed as magma cools rapidly on sea bed
• rift valley


  found at divergent plate boundaries, magma rises and puts pressure on overlying rocks
• graben


  found at divergent plate boundaries, rift zones on land when the continental crust has thinned and stretched forming a sunken valley
• black smoker


  found at divergent plate boundaries, mid ocean ridges have super heated jets of water containing metal sulphides
• example of divergent plate boundaries

  fast spreading at 16cm/yr = Pacific and Nazca plates

  slow spreading at 3cm/yr = North America and Eurasia plates
• conservative plate boundaries


  plates slide past eachother

  • frictional resistance causes a build up of pressure causing rocks to fracture and earthquakes to occur
  • no volcanoes
  • could modify river drainage or create giant tears
• example of conservative plate boundaries


  san andreas fault system between North America and Pacific
• convergent plate boundaries


  plates move towards eachother

  • oceanic to oceanic
  • continental to continental
  • oceanic to continental
• oceanic to oceanic (convergent)

  the slightly older plate will subduct, forming a trench. the descending plate melts and magma rises creating island arcs

  • Aleutian Islands, Pacific 'rim of fire'
• continetal to continental (convergent)

  little to any subduction occurs, collision creates fold mountains

  • the Alps, Africa and Eurasia plates
• oceanic to continental (convergent)

  the denser oceanic plate subducts at a 30-70⁰ angle, creating a trench. intense pressure in the benioff zone at 45⁰ causes faulting, fracturing and eatthquakes. mountain chains may forms as plates are folded and uplifted, subduction may also cause plates to melt and volcanoes to form as the less dense magma rises

  • the Andes, South America
• features of explosive eruptions

  at convergent plate boundaries

  • highly viscous lava made of rhyolite, flows slowly (causes plugs of solidified magma that increase pressure)
  • explosive (tops of cones shatter)
  • not frequent
  • erupt gas, ash, lava bombs and tephra
  • steep sided composite volcanoes and calderas
• caldera


  at least 2km wide volcanic craters that form as an explosive eruption blows the top off a volcano, the magma chamber is emptied and the lack of support causes the volcano to collapse

  • yellowstone 75km wide
• composite volcano


  found at explosive eruptions and convergent plate boundaries. made up of layers. dykes cut across layers, sills run between layers.

  • Indonesia has 130 active volcanoes
• what other types of eruptions are there

  icelandic, hawaiin, strombolian, vulcanian, vesuvian and plinian
• features of effusive eruptions

  form at divergent plate boundaries

  • basaltic, low viscosity lava so very runny
  • limited explosive force
  • frequent
  • shield volcanoes and lava plateaux
  • erupts gas and lava
• lava plateaux


  st divergent plate boundaries, magma erupting from multiple fissures covers vast areas in runny lava called flood basalts. millions of years of denundation has created more varied

  • deccan plateau, India over 500,000km²
• shield volcanoes


  at divergent plate boundaries eruptions are often underwater at mid-ocean ridges, iceland being an exception. the basaltic lava creates sloping sides, common in hawaii
• super volcanoes


  a volcano that erupts over 1000km³ if material in a single event, having major effects on humans and plants

  • exist as giant calderas such as yellowstone super volcano (75km wide)
• eruptions at hot spots

  a mantle plume in the asthenosphere is a hotter area of mantle that rises and partially melts the lithosphere creating intense activity, as the plate moves the volcano above becomes inactive.
• how is volcanic activity measured

  the magnitude (amount erupted) and the intensity (speed) are used with the Volcanic Explosivity Index (0-8) with a ten fold increase each time. less useful for effusive eruptions. height, volume, duration and descriptions need to be acknowledged.

  • yellow stone = 8
  • krakatoa = 6
• what hazards are created by volcanoes

  tephra, lahars, floods and tsunamis, pyroclastic flow, lava flows and gases
• lava flows


  basaltic lava (effusive eruptions) can run for long distances, acidic doesnt run far. there are little to no casualties but everything in path is burned or destroyed
• tephra


  any material that was ejected into the air by the volcano, such as pumice or lava bombs. this disrupts transport, creates breathing difficulties, and may cause buildings to collapse from ash
• gas (volcanoes)


  carbon monoxide, carbon dioxide and sulphur dioxide are silent and deadly. when sulphur dioxide combines with water it forms acid rain