Download Week 2 background reading

Plate Margins:
As previously discussed the plates are moving slowly and irregularly in relation to each other, typically at rates of
a few centimetres a year.
 In some locations – plates are moving away from each other – divergent
 In some locations – plates are moving towards each other – convergent margin
 In a few places - plates move past each other, either in opposite directions or in the same direction but at
different speeds – conservative margin
See website: http://education.sdsc.edu/optiputer/teachers/platetectonics.html
and click on presentation 4.
Divergent Plate Margins: the plates are moving apart and new crust is forming, therefore sometimes they
are referred to as Constructive Plate Margins.
 This may be a divergence of two plates of oceanic crust, or two plates of continental crust.
 It is due to the divergence of two convection cells, which brings magma from the asthenosphere towards
the surface.
 The pressure from the rising magma leads to a doming up of the Earth’s surface & the formation of a ridge.
 As the plates move apart, tensional faults occur into which the rising magma can enter.
 The magma cools and solidifies, producing new crust either within the existing crust or on the surface
following volcanic eruptions.
 Earthquake Activity:
o Shallow focus
o Low magnitude
o High frequency
o Due to the movements are at or near the surface and the pressure is easily released as the plates diverge.
Constructive margin – OCEANIC
CASE STUDY: Eurasian Plate & North American Plate
The divergence of the Eurasian plate and the North American plate in the midAtlantic is a good example of an oceanic constructive margin. A ridge and rift
system extends along the mid-Atlantic for about 10000km. This was created
Constructive margin - CONTINENTAL
CASE STUDY: Eastern African plate & African plate
The Eastern African Rift valley is an example of a constructive margin in an
area of continental crust. Eastern Africa is moving in a north-easterly direction,
diverging from the main Africa plate, which is heading north.
about 60 million years ago as Greenland & northwest Scotland separated to
form the Atlantic Ocean.
A series of underwater volcanoes exists along the margin, occasionally
appearing above sea-level as volcanic islands. An example of such islands is
Iceland, much of which is a lava plateau up to 200m above sea-level, with none
of its rocks more than 3 million years old.
The valley, which actually consists of two broadly parallel rifts, extending for
4000km from Mozambique to the Red Sea. Inward-facing scarp slopes reach
heights of 600m above the valley floor, while its width varies between 10 and 50
km. Rifts form when sections of crust between parallel fault lines subside.
http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21402