PLATE TECTONICS

Joseph Prosper

Monday 22nd October 2007

Antigua State College

PLATE TECTONICS

•Theory of Plate Tectonics

•Location of the Caribbean plate

•Social displacement caused by earthquakes and volcanoes

• The theory of plate tectonics advances the idea that the earth’s outer crust is divided up into a number of rigid, shifting plates of varying size – six major ones which are of continental proportions and a number of others which are quite small – and that as these plates slide pat one another, converge or move apart, continental drift, mountains are formed, and new crust comes into being.

• Plate margin is the edge of the plate. It is at the plate margins that most seismic, volcanic and tectonic activity is found.

• Plate boundary is the line between two plates that touches each other. Plate boundaries are marked by seismic activity and volcanic activity.

• A plate is part of the earth’s surface that behaves as a single rigid unit. Plates are about 100-150 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 (African, Eurasian, Indo Australian, Pacific, North American, South American, and Antarctic and several smaller ones (Nazca, Cocos, Caribbean).

• Plate movements cause continents to drift at about 2.5 km a year. They are carried by convection currents in the magma.

• Zone of Divergence (Pull – Apart Zone) – a constructive margin, a region where two plates are moving away form each other, for example, the Mid Atlantic Ridge

• Zone of Convergence – (Subduction Zone) - a region in which the lithospheric plates is force down or subducted, into the asthenosphere and mesophere. The movement of the lithoshperic plates is though to be the cause of the earthquakes that occur in island arc regions. As it moves down into the mantle, the plate is heated and at a depth of between 100-300 km it is partly melted. At 700 km it breaks up completely.

• Zone of Shearing (Transform fault) - It is a fault along which two plates move past each other without lithosphere being formed or destroyed. A typical transform fault is a strike-slip fault that cuts across a Mid Oceanic Ridge. There is seismic activity at the transform fault between the two points where it meet the Mid Oceanic Ridge. There are continental mid oceanic ridge, for example, San Andreas Fault in California and the North Anatolian transform fault south of the Black Sea,

• 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 they become enlarged. Examples are the Mid Atlantic margins of the American and African plates.

2. Destructive margins:

Just as new ocean floor is being created in some places, in others, it is being destroyed. 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:

A. Ocean plate – Continental Plate Boundaries:

Here it is believed that the oceanic plate which is of higher density is forced beneath the continental plate. An example is where the Nazca (S.E. Pacific) plate is moving eastwards and collides with the South American plate. As the Nazca plate plunges at the subduction zone, so the friction generated caused it to fracture and heat up. The line of descent (the Benoiff zone) is thus marked by the occurrence of earthquakes and the generation of volcanic action so common in the Andean region

B. Continental Plate – Continental Plate Boundaries

In some cases 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 continental moves towards and collides with the Eurasian plate. The accumulated sediments on the continental margins are squeezed and uplifted to form the Himalayan mountains.

–C. Oceanic Plate – Oceanic Plate Boundaries.

Here 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 Tonga, the Aleutians, and the Marianas, the West Indies

3. Neutral Margins

These are the margins where plates slide past each other and where there is, as it were, neutral activity: the plates neither gain nor lose material. It is believed that the great San Andreas fault, along the western margin of North America, which has long been recognised as a line of major seismic activity, marks a sliding zone or line of plate contact.

Distribution of Earthquakes

• Along the Pacific coast – e.g. Japan, Hawaii, Mexico, Nicaragua, East Indies

• Within the West Indies e.g. The Greater and Lesser Antilles.

• From the Mediterranean to East Africa. E.g. Turkey, Iran, Italy

Distribution of Volcanoes

• Close to continental coastlines e.g. along the western coasts of the Americas

• Along the mid oceanic submarine ridges e.g. the volcanoes of the mid Atlantic Ocean ridge.

• In regions of faulting and earthquake disturbance e.g. the Middle East and the East African Great Rift Valley

• In zones of recent mountain building e.g. the Andes and the fold mountains of South east Asia

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.