Plate Margins

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Plate Margins

• We must now look at the 3 main types of plate
  margin
• Constructive.
• Destructive.
• Conservative.
• Each type of plate margin has different rocks and
  types of hazard

Credit Hawaii Natural History Association
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Plate Margins
Credit U.S. Geological Survey
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Destructive Plate Margins

• May also be called
• convergent plate margins.
• Why?
• What makes them
• converge?
• There are 3 types of destructive plate margins
  depending on what types of plate are involved
• Oceanic v oceanic
• Oceanic v continental
• Continental v continental
• Each will produce different hazards and rocks.

Credit Hawaii Natural History Association
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Golden Rule 1

• Why do plates/rocks melt
• Because the temperature has increased.
• Because the pressure is decreased.
• Because it is saturated in water.
• If one or more of these situations exists then
  the plate/rock will melt.

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Golden Rule 2

• What kind of magma is produced when a plate/rock
  melts?
• Remember how we classify igneous rocks.
• Acid gt 66 quartz rich
• Intermediate 52 66
• Basic 44 52 quartz poor
• Ultrabasic lt 44
• Quartz is one of the first minerals to melt, so
• When the initial magma melts the one it produces
  will be richer in SiO2 and so will move towards
  the acid end of the scale.
• If you melt an ultrabasic rock it will produce a
  basic magma.
• If you melt a basic rock the magma will be more
  intermediate.

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Golden Rule 3

• The more SiO2 in a magma the more explosive will
  be the volcanic eruptions.
• Why?
• Because the magmas are more viscous/sticky.
• Gasses come out of solution when a magma reaches
  the surface (P drops) but they cannot escape
  easily.
• They therefore expand putting pressure on the
  magma chamber.
• Eventually the P is so great that the top/plug is
  blown off or the side slips and a violent
  eruption occurs.

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Oceanic v Oceanic collision
Credit U.S. Geological Survey
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Oceanic v Oceanic collision

• The features/landforms
• Subduction zone describes the whole area where
• subduction is taking place.
• Ocean Trench is the deep valley formed in the
  ocean floor as the subducted plate bends.
• Benioff zone zone of earthquakes set off by the
  solid lithospheric plate forcing its way through
  the mantle.
• Island Arc An arc of islands running parallel to
  the trench/subduction zone created by volcanic
  activity fed by magma from the melting subducted
  plate.

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Oceanic v Oceanic 2

• The processes
• Subduction
• Occurs where a piece of oceanic lithosphere bends
  and sinks beneath another plate.
• Why do oceanic plates subduct?
• Because oceanic crust is denser (3.0).

Credit U.S Geological Survey
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Oceanic v Oceanic 3

• Seismic activity
• As the cold/solid oceanic plate sinks through the
  mantle there is frictional resistance.
• When the resistance is overcome and the plate
  moves the energy is released as seismic waves
  earthquake.
• This is the Benioff zone.

Credit U.S. Geological Survey
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Oceanic v Oceanic 4

• Melting
• As mentioned earlier there are 3 reasons why
  crust may melt.
• Which is occurring in this situation?
• Increased T as the plate sinks deeper.
• Also the plate is saturated in water.
• The melting oceanic crust is basic so what kind
  of magma will form when it melts?
• More SiO2 rich and so will move towards being
  intermediate.
• This magma will move upwards through the
  overlying mantle and thin crust to the surface.

Credit U.S. Geological Survey
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Oceanic v Oceanic 5

• Explosive volcanic activity.
• Basic/intermediate magma will be fairly violent.
• Enough volcanic activity will occur to build
  volcanic islands above sea level in an arc
  parallel to the plate margin (island arc).

Credit U.S. Geological Survey
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Oceanic v Oceanic examples
Credit U.S. Geological Survey

• Montserrat/Caribbean arc Soufriere Hills.
• Aleutian islands.
• Indonesia Krakatoa
• Philippines Pinatubo
• Kamchatka.

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Oceanic v Continental Crust collision

• The features/landforms
• Many are the same
• Trench.
• Subduction zone.
• Benioff zone
• Continent based volcanoes.
• Fold mountains.

Credit Hawaii Natural History Association
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Oceanic v Continental Crust 2

• The processes.
• Again very similar to ocean v ocean with a subtle
  difference.
• Subduction (but only the oceanic crust will
  subduct).
• Seismic activity.
• Melting
• Mountain building (orogenesis)

Credit Hawaii Natural History Association
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Oceanic v Continental Crust 3

• What happens to the melt as it enters the
  continental crust?
• Remember that the magma is now intermediate(ish).
• It travels up through the continental crust that
  has what composition?
• Acid.
• The hot magma melts the continental crust and
  adds SiO2 rich crust to the intermediate magma to
  make magma that is very intermediate or acid.

Credit U.S. Geological Survey
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Oceanic v Continental Crust 4

• Acid/intermediate magma will produce what kind of
  volcanic activity?
• Very explosive and dangerous.
• Mountain building (orogenesis).
• This involves crust being compressed and either
  folding or faulting.
• Fault movement sets off earthquakes.
• There will therefore be earthquakes linked to
  both the Benioff zone and orogenesis.

Credit U.S. Geological Survey
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Oceanic v Continental Examples

• Volcanoes along the Andes
• Popacatapetl (Mexico)
• Nevada del Ruiz (Columbia)
• The Rockies/Cordillera
• Mt. St. Helens

Eruptions of Mt. St. Helens,
Credit/ U.S. Geological Survey
Credit/ U.S. Geological Survey
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Continental v Continental Crust
Credit U.S. Geological Survey
Credit U.S. Geological Survey
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Continental v Continental Crust

• The features/landforms.
• Mountain ranges
• The processes
• Mountain building (orogenesis).
• No subduction
• because continental
• crust will not subduct.

Everest
Credit U.S. Geological Survey
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Continental v Continental Crust

• Will there be volcanoes?
• The crust is up to 90 km thick and so is hot
  enough at its base to melt.
• However, the melt produced is so viscous and the
  distance to the surface so far that magma will
  not make it to the surface.
• Therefore NO VOLCANOES.

Credit U.S. Geological Survey
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Continental v Continental Crust

• Will there be earthquakes?
• Orogenesis is still taking place.
• All the energy is being put into deforming the
  rocks.
• So YES there will be many large earthquakes.

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Conservative Plate Margins

• This is where 2 plates are sliding past each
  other horizontally but are not being created or
  destroyed.
• There is only one really good example in the
  world California, San Andreas Fault.

Credit U.S. Geological Survey (Photographer
Robert E Wallace
Credit U.S. Geological Survey
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Conservative Plate Margin

• What kind of volcanic activity will there be?
• None.
• Why not?
• Nothing is melting.
• Will there be any earthquakes?
• Yes and very large.

Credit U.S. Geological Survey
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Constructive Plate Margin
Credit U.S. Geological Survey
May also be called divergent plate
margins. Why? New crust is generated along ocean
ridges. Examples are The Mid Atlantic Ridge and
the East Pacific Rise.
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Constructive Plate Margin
Credit U.S. Geological Survey
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Constructive Plate Margin
Along conservative margins there is neither a
loss nor a gain and the plates slip past each
other, these margins are marked by transform
faults.
Along conservative margins there is neither a
loss nor a gain and the plates slip past each
other, these margins are marked by transform
faults.
Credit U.S. Geological Survey

• Mid ocean ridges are characterised by
• Active volcanoes (submarine volcanoes and in
  places emergent volcanic islands)
• Small earthquakes
• Lack of sediment
• High heat flow

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Constructive Plate Margin
The new crust formed at this plate margin, along
with a layer of the upper most part of the
mantle, moves away from the ridge and new
material is added to the trailing edge.
Credit U.S. Geological Survey
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Constructive Plate Margin

• Iceland forms example of Mid Ocean Ridge that has
  grown into a volcanic complex above sea level

Credit U.S. Geological Survey
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Passive Plate Margins
Credit Hawaii Natural History Association

• Nothing is happening.
• 2 pieces of crust are fused together.
• Like the N. American continental plate and the
  Atlantic oceanic plate.
• However, in time the margin may become active.
• It is already starting to subduct in the
  Caribbean arc region (Montserrat).