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

supercontinent.

Wegener proposed that the supercontinent, Pangaea,

began to break apart 200 million years ago and form the

present landmasses.

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

Oceanic lithosphere

Continental lithosphere

Continental Crust 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)

High Rising (mostly above see level) Formed at Convergent Plate

Boundaries

Oceanic Crust

– 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 Boundaries

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 boundary.

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 on land.

Can you explain this diagram!?

The place where two plates move apart or diverge is called a divergent boundary.

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?

: www.ocean.udel.edu

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

Oceanic-Continental

• 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 continent.

• Examples include the Andes, Cascades, and the Sierra Nevadas.

• Two oceanic slabs converge and one descends beneath the other.

Oceanic-Oceanic

• This kind of boundary often forms volcanoes on the ocean floor.

• Volcanic island arcs form as volcanoes emerge from the sea.

• Examples include the Aleutian, Mariana, and Tonga islands.

• When subducting plates contain continental material, two continents collide.

Continental-Continental

• This kind of boundary can produce new mountain ranges, such as the Himalayas.

Also the way the Appalachian Mountains formed!

from: http://www.geo.lsa.umich.edu/~crlb/COURSES/270

Continental crust to continental crust

collision

India-Asia (Himalayas)

Before collision

After collision

At a transform fault boundary, plates grind past

each other without destroying the lithosphere.

Transform faults

• 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)