Download Plate Tectonics Constructive Plate Margins

Plate Tectonics
Constructive Plate Margins
Constructive plate margins, this is
where there are two plates moving
away from each other causing new
oceanic crust to be formed and
mid-ocean ridges are created by the
build up of molten rock on the sea
floor due to the mantle building up.
This new crust pushes the old crust
aside and forcing the plates apart at
a very slow rate (10kmper 1million
years) volcanoes or volcanic
islands are usually associated with
the Mid-Atlantic Ridges.
The Mid-Atlantic Ridge
The Mid-Atlantic Ridge runs all the
way up the Atlantic ocean and was
created by the Eurasian and North
American plate moving away from
each other and the separation of the
African and South American Plate.
This ridge runs straight through the
middle of Iceland and this has
caused a lot of problems for
Iceland and its inhabitants such as:
• Earthquakes and
• Volcanic eruptions.
Iceland Case Study
On November 14th, 1963 it was
reported that there was an
explosion under the sea which was
followed by smoke steam and
emissions of pumice stone and left
a 130m cone of ash from the sea
bed and later formed the isle of
Surtsey. Five months later on the
4th of April, 1964 lava flowed from
the sea and covered the ash and this
made sure that the Island was to
survive.
Iceland Case Study (Continued)
Nine years later on the 23rd January 1973, about 2`0’clock in the morning there
was an earth tremor that stopped the clocked at Iceland’s main fishing port of
Heimaey. The tremor was caused by the separation of the Eurasian and North
American plate and fisherman saw this happen and say that “the earths crust
was broke open and lava was just pouring out of this 2kms long fissure.
Once the volcano had calmed down the inhabitants of Heimaey were evacuated
and when the volcano had finally stopped erupting six months later the
inhabitants were only to find their houses burnt or covered in a layer of ash up
to five metres deep and the entrance to their harbour blocked.
Rift Valleys
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As the crust is being pulled apart
fractures appear.
The fractures break up the crust.
As the crust pulls away the pieces
of land fall and slot downwards.
As the land has been pulled apart
and has fallen down it has formed a
rift valley.
On the left is an air photo of the
Koko rift section of O’ahu.
Destructive Plate Margins
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Destructive margins occur when
Continental crust and Oceanic crust
converge.
At one point the Oceanic crust will
dip down and form a subduction
zone.
In this zone high friction breaks up
the plates causing earthquakes.
As the plate descends the increase
in pressure can trigger major
earthquakes.
Fold mountains and Island Arc are
also formed.
Examples are: The Nazca and
American Plate,
Also Peru-Chile deep sea trench.
Collision Plate Margins
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This is Continental crust against
Continental crust.
Two plates move together and
collide, the land moves upwards
and its opposite reaction is to move
back down.
If the land was at the bottom of the
sea and it moved up it would still
have shells on it.
Earthquakes give off 3 types of
shock waves: Primary, Secondary,
Longitudinal.
E.g. Earthquakes, Taiwan 1999
Sept,
E.g. Volcanoes, Mount St Helens
1980, May.
Conclusion
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In conclusion we can see that constructive plate margins create more land. As
the plates split apart and magma rises up from beneath and creates mid-ocean
ridges. The Mid-Atlantic Ridge being the most famous. Rift valleys are also
features of constructive plate margins.
Destructive plate margins, however, destroy the land. As one plate sinks
beneath the other the disappearing land is turned into magma, features include
subduction zones and deep sea trenches.
Collision plate margins don’t destroy or create land. They collide and move
together, this causes earthquakes and volcanos.
Links
Books: Geography in Place 1, Michael Raw & Sue Shaw
Sites: http://pubs.USGS.gov/publications/text/undertsanding.html
GCSE Bitesize
http://volcano.und.nodak.edu/vwdocs/site.index.html