plate margins and earth structure

• what are the layers of earth

  crust

  mantle

  outer core

  inner core
• what is the lithosphere

  the solid outer layer of the earth

  - the crust and the top portion of the upper mantle

  - lithosphere is what is split into tectonic plates
• what is the crust

  the surface of the earth

  there are two types
• what are the two types of crust

  continental and oceanic
• what is continental crust


  the relatively thick part of the earth's crust that forms the large landmasses

  -made mostly of silicate material

  -low density igneous rock

  -avg of 30-50km thick

  -thicker than oceanic but less dense
• what is oceanic crust

  the relatively thin part of the earth's crust that underlies the ocean basins

  -made mostly of basaltic material

  -high density igenous rock

  -avg 6-8km thick

  -thinner and denser than continental
• what is the asthenosphere


  the partly molten and partly solid rock layer that the solid lithosphere (and therefore tectonic plates) floats on
• how thick is the mantle

  2900km
• what are the two theories of tectonic movement

  ridge push

  slab pull
• what is ridge push/how does it work

  sea floor spreading at a ridge due to an eruption pushing a plate at a boundary into the mantle (forming a trench)
• what is slab pull


  when the denser plate at a trench is pulled down into the mantle
• what is a subductive plate


  the denser plate at a trench which gets pulled down/subducted
• what are the types of plate margins

  constructive/divergent

  destructive/convergent

  conservative/passivetransformative
• what is a constructive/divergent plate margin

  occurs when plates are moving away from one another, causes spreading of a plate

  
  

  has two types
• how does a constructive plate ''happen''?


  rising convection currents in the mantle push up and move along the bottom of the lithosphere so that it flows sideways beneath it

  - causes the lithosphere plate to be dragged along in the direction of the flow
• what is found at a constructive plate boundary

  some of the youngest rocks are found on the earths surface at a constructive plate margin

  - due to new eruptions forming new rock as the gap created by the spreading plates is filled with rising magma from the asthenosphere

  - the cooled solidified magma forms new dense basaltic rock

  
  

  volcanoes

  
  

  earthquakes
• what are the two types of divergence

  sea floor spreading

  stretching and collapsing
• what is sea floor spreading

  in oceanic areas, sea floor spreading occurs on either side of a mid ocean ridge eg the mid atlantic ridge 7km in length

  - the process by which molten material adds new oceanic crust to the ocean floor the formation of fresh areas of oceanic crust which occurs through the upwelling of magma at mid-ocean ridges and its subsequent outward movement on either side.
• what is stretching and collapsing

  in continental areas, stretching and collapsing of the crust creates rift valleys eg the great african rift valley
• what is a mid ocean ridge


  they are the longest continuous uplifted feature on the surface of the planet

  it is an underwater chain of mountains

  
  

  can have volcanic eruptions along the ridges

  - this can build submarine volcanoes which overtime may grow the rise above the sea level which creates a volcanic island such as surtsey (south of iceland)
• what is a rift valley

  A deep valley that forms where two plates move apart, like a mid ocean trench except on land
• what are the 3 different types of convergent situations

  1) an oceanic plate moves to a continental plate, eg the collision of Nazca plate and South american plate forming the andes

  
  

  2)oceanic on oceanic convergence

  
  

  3) continental on continental convergence
• what happens at a oceanic->continental convergent boundary


  where they meet, the denser oceanic plate is forced under the lighter continental one - this is subduction

  - the plate melts in the benioff zone

  
  

  the downward warping of the oceanic plate forms a very deep part of the ocean known as a trench
• what is the benioff zone

  -as the plate descends, the sloping plane creates deep focus earthquakes

  
  

  - the further the rock descends the hotter the surroundings become, this as well as heat from friction begins to make it melt into magma into the part of the subduction zone called the benioff zone

  
  

  -as this magma is less dense than the surrounding asthenosphere, the material begins to rise as plutons of magma as a volcanic eruption

  
  

  -causes an andesitic eurption with andesitic lava (viscous and flows less easily) which creates a complex and explosive volanoe at a destructive plate margin
• what is a deep focus earthquake

  focus of 70-700km below earths surface

  less likely to trigger an earthquake than shallow focus
• what is andesitic lava


  Lava produced from subduction
• what can happen as a plate desends

  the increase in pressure can trigger major eathquakes along the line of the subducting plates which can be shallow, intermediate or deep focus
• what is an oceanic -> oceanic convergent boundary

  where two pieces of oceanic crust on different plates collide and one is subducted below the other

  
  

  - can form a volcanic island arc
• what is a volcanic island arc

  a chain or group of islands that forms from volcanic activity along a subduction zone
• example of a volcanic island arc

  the caribbean islands

  
  

  the volcanoes of the west indies which result from the subduction of the oceanic lithosphere created at the mid atlantic ridge- beneath the east edge of the caribbean plate
• what formation is also associated with an island arc

  deep ocean trenches, such as the Marianas trench in the pacific which is over 10km deep
• what is a continental -> continental convergence boundary

  -due to the continental crusts lower buoyancy they will not be subducted where they meet

  
  

  -instead they collide with each other, their edges and sediments (created by friction) being forced into fold mountains
• what forms at a continental/continental convergence boundary

  fold mountains
• example of fold mountains


  the collision of the indo-austrailian plate with the eurasian plate to form the Himalayas

  
  

  -there are no volcanoes here but the whole area is seismically active, causing devastating earthquakes such as pakistan in 2005 and sichuan 2008
• what has to happen for volcanoes to be present

  subduction
• what is a conservative/passive/transformative plate boundary


  where plates slide past each other

  
  

  there is no creation or destruction of the crust

  
  

  subduction does not occur and there is no volcanic activity

  
  

  earthquakes are shallow focus
• example of a conservative boundary


  the san andreas fault

  
  

  it has seismically active sections where small earthquakes are frequent and locked sections where there is no known recent movement

  
  

  the strain builds up in the locked sections over many years until there is a sudden 'break'

  - the last big earhquake on a 'locked' section was in san francisco

  check slides for more