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