http://science.nationalgeographic.com/science/earth/the-dynamic-earth/plate-tectonics-article.html
Sec 3 IHE GE 2014/Chan ML
Convection Currents
Rising, spreading convection currents
= pull plates apart
Sinking convection currents = drag
plates together
Divergent Plates
(Constructive Boundaries)
Cause:
Plates move in opposite direction due to
the convection movement.
Effects/Landforms
• new crust is created (oceanic ridges)
• Oceans are born and grow wider
• When a diverging boundary occurs on
land a 'rift', or separation will arise and
over time that mass of land will break
apart into distinct land masses and
the surrounding water will fill the
space between them.
• Earthquakes
• Volcanoes
http://www.platetectonics.com/book/images/Divergent1.gif
Oceanic-Oceanic
The divergent Mid-Atlantic
Ridge rises above sea level
at Thingvellir, with the North
American plate to the west
and the Eurasian plate to
the east.
Formation of Oceanic Ridge (answer)
• As the N. American and Eurasian plates move apart (divergent
movement) in a constructive boundary due to convection currents
• Tensional stresses cause fractures in the lithosphere to occur
• The zone where the plates separate is the spreading centre,
• Allowing basaltic magma to well up, cool and solidify; thus becoming
/ forming new sea floor (this process is called sea floor spreading)
• As the plates continue to move apart, more basaltic magma piles up,
cool and solidify; and new mountains are formed
• The mountains closest to the spreading zone are the youngest
• The rows of mountains rise from 2,000 to 4,000m from the ocean
floor – these are the mid-oceanic ridges
• Eg: Mid-Atlantic Ridge
Continental-Continental
• As the Nubian and Somalian plates move
apart,
• They are stretched and fractures are created
• The land in between sinks, creating a linear
depression, known as a rift valley
• As a result, earthquakes are experienced in
the region
• Magma may also rise to the surface creating
volcanoes
• Eg: the East African Rift Valley
Convergent Plates
(Destructive Boundaries)
3 types:
• Oceanic – oceanic
• Oceanic – continent
• Continent - continent
http://www.scienceclarified.com/images/uesc_08_img0459.jpg
Oceanic-Oceanic
The Japanese Islands are examples of volcanic island arcs formed by the
convergence of two oceanic plates.
http://www.lee.edu/~cguldenzopf/Images/Historical/Historicalmw45.jpg
Cause:
• 2 oceanic plates collide
• One plate sinks beneath the other
• Subduction zone
Effects/Landforms:
• Trench
• Volcanoes
• Earthquake
Formation of Trench and volcanoes
(answer)
Trench in a convergence between Oceanic and Continental
Plate (destructive boundary)
Name of plates
• When a thinner and denser oceanic plate converges with
a thicker and lighter continental plate
• The oceanic plate descends beneath the continental
• This descent is called subduction
• The old oceanic lithosphere is destroyed and as it dives
to the mantle, it contributes to the formation of more
magma
• A long narrow and deep oceanic trench is formed where
the plate dips into the mantle
• Eg: Peru-Chile Trench, Java Trench
• The movement of the plates cause faults to
occur
• Rocks break and are displaced relative to
each other,
• the melting of the subducting oceanic plate
produces silica-rich magma
• Being less dense than the mantle, the
magma moves up any break or fractures
• When it escapes through vents to the land
surface, it forms subduction volcanoes
• Eg: Mt. St. Helens, Mt Pinatubo
Oceanic-Continental
Cause:
• Oceanic plate collides with thinner & lighter
continental plate
• Subduction occurs
Effects/Landforms
• Descending plate is destroyed
• Trenches are formed
• Volcanoes
• Earthquakes
http://www.revisionworld.com/files/oceaniccont.jpg
Formation of Fold Mountains
(answer)
• When two continental masses collide, one is pushed under the other
for a short distance
Name of plates
• When a thinner and denser oceanic plate converges with a thicker
and lighter continental plate
• The oceanic plate descends beneath the continental
• This descent is called subduction
• The old oceanic lithosphere is destroyed and as it dives to the
mantle, it contributes to the formation of more magma
• The edges of these plates are also contorted and buckled up,
creating a great uplift, forming fold mountains
• Since sediments from the ocean floor are contorted and uplifted,
marine fossils were found at the summit of fold mountains!!!
• Eg: Rocky Mountains (N. America), Andes Mountains (South
America)
Continental-Continental
Cause:
• 2 continental plates collide
• No subduction
Effects/Landforms
• Great uplift
• Rocks are contorted
• Earthquakes
Earthquakes are shown as yellow squares.
http://www.visionlearning.com/library/module_viewer.php?mid=66
Formation of Fold Mountains
(answer)
• When two continental masses collide, the Indian plate is
pushed under the Eurasian plate for a short distance
• However, there is NO subduction
• Because both are light and buoyant
• This process produces a great uplift as the rocks at the
edges of these two plates are contorted and buckled up
• Due to the absence of magma supply, there are no
volcanoes in this area
• Eg: the Himalayan Mountains
Transform Boundaries
• occur in a few places to accommodate lateral
motion, in which plates slide past one another.
• very rare on continents, but they are dramatic
where they do occur. Example: the San
Andreas Fault (USA) and The Alpine Fault (New
Zealand).
• Most transform
boundaries occur in short
segments along
mid-oceanic ridges
http://www.visionlearning.com/library/module_viewer.php?mid=66
Sit back and relax… well… can’t be too relaxed…
Here’s a summary… don’t fall asleep
https://www.youtube.com/watch?v
=0mWQs1_L3fA
References
• Chong, Marianne (2001) Aspects of
Physical Geography
• Waugh, David (2009) Geography: An
Integrated approach