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Kaustubh J. Sane
HJD Institute of Technical
Education & Research
Theory of plate tectonics
Types of plates
Causes of plate motion
Importance of plate tectonics
Wegener’s continental drift hypothesis stated that the
continents had once been joined to form a single
Wegener proposed that the supercontinent, Pangaea,
began to break apart 200 million years ago and form the
According to the plate tectonics theory, the uppermost
mantle, along with the overlying crust, behaves as a strong,
rigid layer. This layer is known as the lithosphere.
There are two types of lithosphere
Thick (10-50 km)
Old (>200 m.y. and up to 3.5 b.y.)
Iron Poor (<1%) /
Silica Rich (>70%)
Less Dense (~ 2.5 g/cm3)
(mostly above see level)
Formed at Convergent Plate
– Thin (<10 km)
– Young (<200 my)
– Iron Rich (~5%) /
Silica Poor (~50%)
– Dense (s.g. ~3 x H2O)
– Low lying (5-11 km deep)
– Formed at Divergent Plate
Types of plate boundaries
Depending on the motion of the plate they are grouped
into 3 major types viz,
1. Divergent plate (constructive plate boundary)
2. Convergent plate ( destructive plate boundary)
3. Transform plate
Click here for a hyperlink to an animation of convection
Be sure to click the play button!
o Plates are moving away from each other
o Midocean ridges are created and new ocean
floor plates are created.
o The plate gives birth to new earth surface
hence it is also called as constructive plate
o As new material comes on surface mostly in
sea and increases the sea floor hence it is
called as sea floor spreading.
• Oceanic ridges are continuous elevated zones on the floor of
all major ocean basins. The rifts at the crest of ridges represent
divergent plate boundaries.
• Rift valleys are deep faulted structures found along the axes of
divergent plate boundaries. They can develop on the seafloor or
The place where two plates move
apart or diverge is called a divergent
This is a model of sea floor spreading at a divergent boundary called
a mid ocean ridge.
Did you know that the Earth’s longest mountain range is underwater and
is called the mid-ocean ridge?
The Mid-Ocean Ridge system, shown above snaking its way
between the continents, is more than 56,000 kilometers (35,000 mi)
long. It circles the earth like the stitching on a baseball!
A subduction zone occurs when one oceanic plate is
forced down into the mantle beneath a second plate
• Denser oceanic slab sinks into the asthenosphere.
• Pockets of magma develop and rise.
• Continental volcanic arcs form in part by volcanic activity
caused by the subduction of oceanic lithosphere beneath a
• Examples include the Andes, Cascades, and the Sierra
• Two oceanic slabs converge and one descends
beneath the other.
• This kind of boundary often forms volcanoes on the
• Volcanic island arcs form as volcanoes emerge
from the sea.
• Examples include the Aleutian, Mariana, and
• When subducting plates contain continental
material, two continents collide.
• This kind of boundary can produce new mountain
ranges, such as the Himalayas.
Continental crust to continental crust
At a transform fault boundary, plates grind past
each other without destroying the lithosphere.
• Most join two segments of a mid-ocean ridge.
• At the time of formation, they roughly parallel the
direction of plate movement.
• They aid the movement of oceanic crustal material.
A transform boundary is a place where two plates slip
past each other, moving in opposite directions.
The plates may move in opposite directions
or in the same directions but at different rates
and frequent earthquakes are created
(example: San Andreas Fault)