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PLATE TECTONICS
The theory is that the earth’s surface is covered
by a number of relatively thin plates which move
over the material below.
Plate Tectonics Theory
PLATE
It is a part of the Earth’s surface that behaves as
a single rigid unit. Plates are about 100 to 150
km thick. They may be made up of continental
crust or oceanic crust or both, on top of a layer
of the upper mantle. Plates move in relation to
the Earth’s axis and to each other.
There are seven large plates: the African,
Eurasian, Indo-Australian, Pacific, North
American, South American, and Antarctic plates,
and several smaller ones ( Cocos, Nazca,
Caribbean, Arabian, Philippine, Juan de Fuca,
Somali plates).
Plate Margin
• Is the edge of a plate. It is at the plate margins that most seismic, volcanic and tectonic activity is found. There are three types: (a) constructive margins, at which new crust is being formed; (b) destructive margins, at which one plate is moving down below another; (c) conservative (transform) margins, at which plates simply move past each other.
• Three differing types of plate margin can be distinguished:
1. Constructive or ocean ridge margins. These are the plate margins adjacent to the great mid ocean floor ridges with their extensive rifts of fissures through which basalt magma is poured out. As the plates move apart and as the magma solidifies along their margins so they become enlarged. Example- the mid Atlantic margins of the American and African plates.
• 2 Destructive margins: When two plates are converging it is believed that the leading edge of one plunges or subducts beneath the other. Such destructive boundaries between converging plates can be divided into three types:
• Type A. Ocean plate – Continental plate boundaries: The oceanic plate which is of higher density is forced beneath the continental plate. An example is where the Nazca (S.E. Pacific) plate collides with the South American plate. As the Nazca plate plunges at the subduction zone, the friction generated caused it to fracture and heat up. Marked by the occurrence of earthquakes and the generation of volcanic eruption. Example the Andean region
Type B. Continental plate – Continental plate boundaries.
Two fragments of continental crust may drift towards
one another as the ocean floor between them is consumed
at the subduction zone. This happens when the Indian
sub continent moved towards and collided with the
Eurasia plate. The accumulated sediment on the
continental margins are squeezed and uplifted to form the
Himalayan system of mountains.
Type C. Oceanic plate – Oceanic plate boundaries.
There is convergence between two oceanic plates and
one is subducted beneath the other. Such a boundary is
marked at the surface by the formation of ocean
trenches and associated chain of volcanic islands (island
arcs). Examples are the West Indies, Tonga, the
Aleutians, and the Marianas.
• 3. Conservative margins. These are the margins where plates slide past each other. The plates neither gain nor lose material. Example the great San Andreas fault, along the western margin of North America.
Island Arcs
A curved chain of islands with the convex (outer) side
of the curve facing the open ocean. There is a deep
oceanic trench on the convex side of the arc and deep
sea on the opposite side. Island arcs are regions where
deep-focus earthquakes occur. The islands may also
show volcanic activity.
Caribbean Plate
The Caribbean plate forms an oval shape around the
Pan-Caribbean i.e. all islands and countries that the
Caribbean sea touches. On the North it is bordered by
the Cayman Trench and the Puerto Rico Trench, on the
east is the island arc of the West Indies, and on the
south border is the South American plate and the
Acapulco Deep.
At the plate margins, countries experience
violent earthquakes, fault ruptures, tsunamis, seiches,
subsidence, submarine slides, active volcanic activities,
formation of deep sea trenches, formation of fold
mountains, formation of mid oceanic ridges. It is as a result of these events that human beings are
displaced.
Social Displacement
• 1. Displacement of population• 2. Destruction of crops• 3. Destruction of livestock• 4. Disruption to tourism• 5. Change in weather patterns• 6. Landslides• 7. Environmental pollution
• 8. Serious and uncontrolled fires• 9. Flooding• 10. Disruption of communication• 11.Destruction of settlements• 12. Disruption of communications• 13. Destruction of infrastructure• 14. Loss of farmland and forests• 15. Release of hazardous materials• 16. Spread of chronic illness\• 17. Breakage of sewage disposal system