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CARIBBEAN STUDIES

The Impact of Geographical Phenomenon

PLATE TECTONICS

Mrs. Murphy

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Plate tectonics theorists:

1912 - German geologist, Alfred Wegener observed that

the shapes of the continents, fit together like a jigsaw

puzzle. He developed the theory of continental drift -

the continents were once joined in one large landmass (

supercontinent), over a long period of time they broke up

and drifted apart.

Canadian geophysicist J. Tuzo Wilson, stated that the

earth’s crust is a dynamic assembly of moving plates

whose interactions explain most geological phenomena,

including volcanoes and earthquakes.

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What is plate tectonics? • The theory that the earth’s crust is

made up of a series of rigid plates which float on a soft layer of the mantle and are moved by convection currents in the earth’s interior.

• The study of the movement of the plates on the earth’s surface and the resultant landforms.

• 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.

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• 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 (where they meet).

Plate boundaries are marked by seismic

activity and volcanic activity.

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• 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.

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• The theory of plate tectonics just outlined

suggests that the earth’s surface is divided up

into a number of segments of varying size which

are all slowly moving. There are some segments

usually of large size, that are relatively free from

earthquakes disturbances, and such segments or

plates are termed aseismic plates

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Why plate tectonics is useful

• Plate tectonics helps to explain many

geological events, such as earthquakes and

volcanic eruptions as well as mountain building

and the formation of the oceans and

continents.

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World tectonic plate boundaries

•

•

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Size of plates • Largest plates: the Pacific plate, the North

American/Atlantic plate, the Eurasian plate, the

Antarctic plate, and the African plate.

• Smaller plates: the Cocos plate, the Nazca plate,

the Caribbean plate, and the Gorda plate.

• Plate sizes vary: The Cocos plate is 2000 km (1400

mi) wide; the Pacific plate is almost 14,000 km

(nearly 9000 mi) wide.

• The Pacific plate is the largest approx. 100,000,000

km2

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Rate of plate movement

• Tectonic plates move at an average

speed of 2 – 3 inches (4 – 7 cm) per

year.

• Movement is slow and continuous and

has been taking place for thousands of

years

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Internal structure of the earth

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Components of the layers

• The inner core is made up of dense iron; the outer

core is made up of liquid iron.

• The lower mantle is made up of molten rock

surrounded by partially molten rock in the

asthenosphere.

• Part of the upper mantle and crust (lithosphere) is

made up of solid rock. It is a rigid layer.

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Why do plates move? • The upper mantle and crust (lithosphere) lie on

the lower mantle which is fluid. The rocks in

the lower mantle (asthenosphere) move in a

fluid manner because of the high temperatures

and pressures in it.

• Currents in the lower mantle form convection

cells which cause the plates to float.

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Types of plate boundaries/movements

1. Divergent plate boundaries (Zone of Divergence)

• Occur where two plates are moving apart from each other. In the oceanic crust, this process is called seafloor spreading. Magma, or molten rock material, rises to the sea floor surface along the rupture and forms new oceanic crust.

• On land, divergent plate boundaries create rift valleys—deep valley depressions formed as the land slowly splits apart.

• Eg. the Mid Atlantic Ridge

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Divergent plate boundary (sea floor

spreading) – rift valley

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2. Transform plate boundary.

(Zone of shearing)

• The plates slide past or slip alongside

each other.

• A transform boundary is located where

the Caribbean plate and South

American plates meet

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Transform and divergent plate boundaries.

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3. Convergent boundary (Also known as destructive boundary or

subduction zone)

• Plates move towards one another; the plates are consumed, or recycled back into the earth’s mantle.

• Earthquakes, volcanoes, oceanic ridges, deep sea trenches, island arcs, (e.g the West Indies, the Japanese islands), and fold mountains occur at convergent boundaries

Different type of plate margins

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.

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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:

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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. The line of descent is thus marked by the occurrence of earthquakes and the generation of volcanic action.

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.

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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).

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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.

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Formation of the Caribbean

• The eastern Caribbean islands lie on a convergent

plate boundary.

• As the Atlantic/America plate sinks beneath the

Caribbean plate, magma rises to the surface, it may

erupt to form a volcano, resulting in the formation of

the islands of the eastern Caribbean, (about 140

million years ago).

•

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The Caribbean & Atlantic Plate Boundaries

•

-

--

--

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A convergent boundary

• Molten material comes to the surface and

forms islands

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A convergent boundary

• Formation of volcanoes or fold mountains

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Effects of plate tectonics on the Caribbean

• 1) Volcanoes.

• A volcano is a vent or opening in the earth’s crust

through which hot molten rock (magma) and gases

from the interior of the earth are released.

• Volcanoes occur as the oceanic crust sinks under

the continental crust. Magma escapes and rises to

the surface and forms volcanoes.

• Eighty percent of the world’s active volcanoes occur

at convergent zones.

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Live volcanoes in the Eastern Caribbean

There are 19 ‘live’ volcanoes in the Eastern Caribbean

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Major volcanic eruptions in the Eastern

Caribbean

Year Volcano Effects

1718 Mt. Soufriere,

St. Vincent

Unknown number of

casualties among Caribs

1812 Mt. Soufriere,

St. Vincent

80 killed; major damage to

sugar industry

1902

Mt Pele`,

Martinique

30 000 killed; St Pierre

destroyed; economic cost US$1

000 000 000

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Major volcanic eruptions in the Eastern

Caribbean Year Volcano Effects

1902 Mt. Soufriere,

St. Vincent

1600 killed; major damage to the

sugar industry; economic cost:

US$200 000 000

1976 -

77 Soufriere,

Guadeloupe

No casualties. Economic cost:

US$100 000 000

1979 Mt. Soufriere,

St. Vincent

No casualties. Massive ash fall.

Economic cost: US$100 000 000

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Pyroclastic flow and surge

• A pyroclastic flow : a hot (100 – 6000C), fast

moving mixture of ash, rock fragments and gas. It usually travels down valleys and cause total destruction of the area over which it flows. This has been the main cause of destruction and loss of life in Montserrat

• A pyroclastic surge is a dilute turbulent cloud of gases and rock debris that moves over the ground at great speeds. It is formed in a similar way to a pyroclastic flow, but the effect is more widespread.

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Major volcanic eruptions in the region

Year

Volcano

Effects

1995 -

1997

Souffriere

Hills,

Montserrat

19 deaths; destruction of

capital, Plymouth; southern

portion of island evacuated;

relocation of population; mass

migration; population reduction

from 12, 771 (1996) to about

4000; economy devastated;

economic cost: US$500 000 000

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Positive effects of volcanoes

Feature Location Benefits

Volcanic (igneous)

rocks

Antigua,

(Greencastle) St.

Lucia etc

Road and building

construction

Crater Lakes Hot

springs, sulphur

springs, fumeroles

Grenada,

Dominica, Nevis,

St Lucia

Tourist attractions

High mountains and

fertile soil

Windward Islands,

Bendals, Old Road

etc.

Relief rainfall -

agriculture – food

production.

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Effects of plate tectonics on the Caribbean

• 2) Earthquakes

• An earthquake results from slow build up of

pressure within the crust at convergent

boundaries. If pressure is suddenly released,

then the plates jerk past each other resulting in

vibrations in the crust.

• Over 600 earthquakes occur in the Caribbean

annually.

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Earthquake terminology • A violent shaking of the Earth’s crust that may cause

destruction to buildings and installations and results from the sudden release of tectonic stress.

• Focus or hypocenter: the point within the earth where an earthquake originates

• Epicenter: the point on the earth’s surface directly above the focus

• Earthquakes also contain surface waves that travel out from the epicenter along the surface of the earth. Two types of these surface waves occur: Rayleigh waves, named after British physicist Lord Rayleigh, and Love waves, named after British geophysicist A. E. H. Love. Surface waves also cause damage to structures, as they shake the ground underneath the foundations of buildings and other structures.

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Earthquakes for October 17th – 24th 2005

Red 30 km deep; purple 30 – 70 km deep; blue 70 -100 km

deep; black > 100 km deep

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The Richter Scale

•

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General effects of earthquakes

• loss of life

• loss of property

• fires from broken gas, or power lines

• disruption of transport and other services

• exposure due to lack of shelter

• shortage of food

• shortage of clean water

• disease from polluted water supplies.

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Selected Caribbean earthquakes

Year Island Effects

1692 Port Royal,

Jamaica

2000 killed, a part of the city was

submerged, buildings destroyed.

1842 Haiti More than 700 killed. In addition

200 killed in tsunami in Port-de-

Paix.

1843

Antigua,

Nevis,

Montserrat

Major structural damage; St

John’s Cathedral damaged.

A tsunami also affected Antigua

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Selected Caribbean earthquakes

Year Island Effects

1867 Virgin

Islands

20 killed

1907 Kingston,

Jamaica

Major destruction by fire and

tidal wave

1946

Dominican

Republic

100 killed by a tsunami in the

town of Mantanzas

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Selected Caribbean earthquakes

Year Island Effects

1974 Antigua Structural damage

2004 North of

Dominica

(6.3

magnitude)

Structural damage to churches

in Portsmouth and Vielle Case.

One death in Guadeloupe. Also

felt in Antigua, St Maarten,

Nevis, Montserrat and St.

Vincent

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Some earthquakes in 2005

Date Location Magnitude

Feb 7th North east of Dominica 4.8

Feb 14th North of Dominica

March 22nd Near Trinidad 4.6

March 23rd Northwest of Trinidad 3.9

1st May Northeast of St. Lucia 3.7

June 23rd 9 quakes N. of Dominica 3.6 – 4.5

Aug 4th Northwest of Trinidad 4.5

Aug 30th Northeast of Martinique 5.1

Oct 24th Northwest of Trinidad 4.9

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Implications for construction & disaster

preparedness

• What disaster mitigation methods are in place?

• Construction of earthquake resistant buildings?

• Restrictions on the height of buildings?

• Emergency exits in buildings?

• Adequate medical facilities to handle disasters?

• Regular public education/awareness?

• Earthquake drills in schools?

• ARE WE PREPARED TO HANDLE MAJOR

DISASTERS?

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Some landforms formed by plate tectonics

• Landforms: a natural physical feature of

the earth’s surface, for example, a valley,

mountain, or plain, beaches, marshes,

caves, waterfalls.

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Landforms and their use

Landform Territory Benefits

Waterfall Dominica,

Jamaica, Guyana

Recreation, tourist

attraction

Boiling Lake, Dominica Tourist attraction,

Sulphur

springs

Dominica, Nevis Therapeutic

Mountains Windward Islands,

Jamaica

Rainfall,

agriculture,

forestry,

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Landforms and their use Landform Territory Benefits

Marshes

Mangroves

All islands Prevents coastal erosion; a

spawning area for fish;

protection for young marine

life

Lagoons Barbuda Fishing, coastal protection

Plains Trinidad

etc

Agriculture, settlement

Beaches

Leeward

Islands

Recreation, fishing, tourism,

sand mining

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Location of Kick ‘em Jenny

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Kick `em Jenny

• A shallow submarine volcano

• Located 8km ( 5 miles ) north of Grenada

• The summit is 180m (590 ft) below sea level

• The only ‘live’ submarine volcano in the Caribbean

• Erupted twelve times since 1939

• It is actively degassing

• Most closely monitored volcano in the Caribbean

• It is not growing towards the surface

• It is a serious threat to shipping and there is a 1.5 km

(approx 1 mile) exclusion zone around it.

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Trenches and Troughs • Trenches/Troughs: Bartlett Trough, between

Cuba and Jamaica, over 20 000 feet deep.

• Brownson Trough, north of Puerto Rico and

the Virgin Islands, 30, 000 feet deep

• Anegada Trough, between the Caribbean Sea

and the Atlantic Ocean?

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Tsunamis in the Caribbean • An ocean wave caused by large scale disturbance of the ocean floor or

surface that displaces a large mass of water.

• Waves can reach 15m high (50 ft) and travel 600 kph (372 mph)

• Caused by earthquakes, volcanic eruptions and landslides into the sea.

• Ten earthquake generated tsunamis in the past 500 years, only 450 people have been killed by tsunamis

• Volcanoes, earthquakes and hurricanes have killed 30 000, 15 000, and 15000 people respectively

• In 2003 a major dome collapse of Soufriere Hills Volcano, caused a tsunami 4m high in Montserrat and 1 m high in Guadeloupe.

• Scientists believe that there is a low probability of a major tsunami in the region