Plate Margins - PowerPoint PPT Presentation

1 / 30
About This Presentation

Plate Margins


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 ... – PowerPoint PPT presentation

Number of Views:87
Avg rating:3.0/5.0
Slides: 31
Provided by: A110


Transcript and Presenter's Notes

Title: Plate Margins

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

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

Golden Rule 2
  • What kind of magma is produced when a plate/rock
  • 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

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
  • 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.

Oceanic v Oceanic collision
Credit U.S. Geological Survey
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

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
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
  • This is the Benioff zone.

Credit U.S. Geological Survey
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
  • This magma will move upwards through the
    overlying mantle and thin crust to the surface.

Credit U.S. Geological Survey
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
Oceanic v Oceanic examples
Credit U.S. Geological Survey
  • Montserrat/Caribbean arc Soufriere Hills.
  • Aleutian islands.
  • Indonesia Krakatoa
  • Philippines Pinatubo
  • Kamchatka.

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
Oceanic v Continental Crust 2
  • The processes.
  • Again very similar to ocean v ocean with a subtle
  • Subduction (but only the oceanic crust will
  • Seismic activity.
  • Melting
  • Mountain building (orogenesis)

Credit Hawaii Natural History Association
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
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
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
Continental v Continental Crust
Credit U.S. Geological Survey
Credit U.S. Geological Survey
Continental v Continental Crust
  • The features/landforms.
  • Mountain ranges
  • The processes
  • Mountain building (orogenesis).
  • No subduction
  • because continental
  • crust will not subduct.

Credit U.S. Geological Survey
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
Continental v Continental Crust
  • Will there be earthquakes?
  • Orogenesis is still taking place.
  • All the energy is being put into deforming the
  • So YES there will be many large earthquakes.

Conservative Plate Margins
  • This is where 2 plates are sliding past each
    other horizontally but are not being created or
  • 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
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
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.
Constructive Plate Margin
Credit U.S. Geological Survey
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
Along conservative margins there is neither a
loss nor a gain and the plates slip past each
other, these margins are marked by transform
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

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
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
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).
Write a Comment
User Comments (0)