1. Geography Notes
Chapter 1 : Plate Tectonics
Causes of Pate Movement
Crustal plates resting on mantle are constantly in
motion due to movement of magma . Heated Magma
expands and pushes plates away from each
other .(Plate divergence) Cooled magma contracts and sinks pulling plates together .
Oceanic-Oceanic plate divergence
Explanation:
1. When Rising Convection currents push two plates away from each other .
2. Fractures and cracks develop at plate boundaries
3. Constant flow of lava( formerly magma) causes formation of new crusts while pushing old
crusts further apart. SEAFLOOR SPREADING and is considered constructive as new crusts are
made
4. Oceanic Ridges are formed.
Examples of oceanic ridges
Mid Atlantic Ridge
Formed by Eurasian plate and north American plate
Formed ICELAND ( volcanic Island)
Continental-continental plate divergence
Examples
African plate & Eurasian plate
When two continental crusts diverge, forming a depression of a rift valley .
Ex. East African Rift Valley

2. Convergent Plate Movement
When forces of compression move plates towards each other
Denser Plate subducts less denser plate is pushed up
Type 1. Oceanic – Continental Convergence
1. The less denser plate is pushed up while the
Denser plate subducts into the molten mantle
And is destroyed.
2. Development of long narrow and deep oceanic trench
3. Less dense plate rises and fold to form fold mountains .
Example
Oceanic Nazca Plate & continental South American Plate ( Diagram Above)
1. Nazca Plate pushed against the South American Plate.
2. Denser American Plate is subducted beneath the less dense south American Plate
3. Subducted Nazca Plate melts and forms Magma .
4. Magma from mantle escapes through vents and erupts
5. Erupted Materials such as lava and pyroclastics bulid up over time and form volcanoes
6. Overlying continental South American Plate is pushed up to form mountains called the
ANDES MOUNTAINs
7. Active volcanoes are present at convergent plate boundaries.
Type 2.Oceanic –Oceanic Plate Convergence
Pacific plate & Philippines plate
1. Oceanic trenches are developed
2. Chain of subduction volcanoes called
The MARIANAS ISLANDS
Type3. Continental – Continental Plate convergence
Both crusts have equal density thus no subduction happening
So area between crust is highly contorted and warped
Examples,
Eurasian & Indian Plate
Fold mountains formed : THE HIMALAYAS

3. Explanation of Divergent Plate Movement being CONSTRUCTIVE
Development of liner features along the plate boundary . Due to the eruption of lava when the
plates diverge, new crust is formed and the old crust are pushed away , further from each other thus
as new crusts are formed , divergent plate movement Is considered as constructive.
Explanation of Convergent plate movement being DESTRUCTIVE
When two plates move towards each other , either subduction or plate collision happens . For
subduction, the denser plate is destroyed in the mantle . For plate collision , due to the intense
pressure from the plates puhing against each other EARTHQUAKES , EXPLOSIVE VOLCANOES are
common at plate boundaries , thus the plates are deformed and destroyed at convergent plate
boundaries thus it is considered as DESTRUCTIVE
Transform Plate Boundry
Explanation: lateral movement of plates as they slide pass each other .
Examples: San Andreas Fault .
North American Plate Pushing South Eastwards & Pacific Plate pushing North Westwards.
Effects :
1. Tremendous stress build up at one of contact
2. Fracture Zones
3. Distorted Alignment of land and infranstructure
4. Devasating Earthquakes
5. Collaspse & buring of properties and lives
6. 1906 San Francisco Earthquake took >3000 lives and
caused widespread damage.