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Section B · Natural systems 6 By the end of this chapter you should be able to: Concept map Internal forces: plate tectonics ✓ define crustal plates ✓ name and locate the Caribbean and adjacent plates ✓ distinguish between convergent, divergent and transform plate margins. earth earth’s crust internal convection plate movements divergent convergent transform volcanic activity fold mountains earthquakes Tectonics: the general name given to any major movement of the earth’s crust. continental drift ▸ The theory of plate tectonics Over a hundred years ago scholars commented on the way in which the continents seemed like parts of a jigsaw puzzle, and how, if they could be moved, they could be fitted together into one large land mass. Later on there was even a name given to this idea – continental drift. However, no one could come up with an explanation of how the continents could possibly move, and few people took it seriously. This changed in the 1960s, when scientists showed beyond doubt that the continents had indeed moved, and were still moving. Later on a mechanism 54 CSEC Geography SEC B prf4 CS4.indd 54 14/12/2009 2:40 PM 6 · Plate tectonics tic -A tla n Pacific Plate M id North American Plate Ri dg e Eurasian Plate Cocos Plate Philippine Plate African Plate Nazca Plate South American Plate Indo-Australian Plate ian Rid In c- ge i nt la At e dg Ri Ind n a di dMi Mid-Atlantic Ridge Pacific Plate Arabian Plate Caribbean Plate Antarctic Plate Earthquake zone Volcano Major earthquake Plate boundary South east India n Rid ge Plate movement Figure 6.1 Crustal plates, with the seven major plates highlighted. Locate the Caribbean Plate and note the other plates in this size range. convection currents ▸ plate tectonics ▸ ITQ 1 Name two facing continents that look like they could fit together. Crustal plates: the component portions of the earth’s crust that move over the surface of the earth in response to internal forces. for the movements was discovered: convection currents – the slow movement of semi-molten rock within the earth’s interior. The movement of the continents and the rest of the earth’s crust is today known as plate tectonics. The plates are the portions of the earth’s crust that actually move. Some of them have continents on them while others, like the Caribbean Plate, do not. In fact the continents are not the complete plate; they are simply the exposed portions of crustal plates. Crustal plates There are seven very large plates (table 6.1) and numerous small ones, and together these make up the entire crust of the earth (figure 6.1). Table 6.1 The largest crustal plates Name Eurasian Plate – Europe African Plate Indo-Australian Plate Pacific Plate Nazca Plate North American Plate South American Plate Antarctic Plate Size (km2) 67 800 61 300 19 100 103 300 15 600 75 900 43 600 60 900 55 CSEC Geography SEC B prf4 CS4.indd 55 14/12/2009 2:40 PM Section B · Natural systems The Caribbean Plate 0 0 0 0 NORTH AMERICAN PLATE NORTH AMERICAN PLATE 200 400 600 200 400 600 100 200 300 400 100 200 300 400 800 1000 km 800 1000 km 500 600 miles 500 600 miles N N Santiago 1946 Santiago 1946 El Chichon El Chichon 1982 1982 Guatemala Guatemala 1976 Guatemala 1902 Guatemala 1976 Santa Maria1902 Santa Maria Guatemala City 1918 1902 Guatemala City 19181986 1902 El Salvador El Salvador 1986 Area of high seismicity Area of high seismicity Direction of plate Direction of plate movement movement Recent seismic activity Recent seismic activity Major earthquake Major earthquake Major volcanic eruption Major volcanic eruption Active volcano Active volcano Other volcano with Other recentvolcano seismicwith activity recent seismic activity Kingston, Jamaica, 1907 Kingston, Jamaica, 1907 CARIBBEAN CARIBBEAN Nicaragua 1972 Nicaragua 1972 Aguadilla Saba Aguadilla Saba Mt Liamuige, St Kitts Mayagüez Mt Liamuige, St Kitts Mayagüez Soufrière Hills, Montserrat, 1997 Soufrière Hills, Montserrat, Soufrière Guadeloupe (1903, 1956,1997 1977) Soufrière Guadeloupe (1903, 1956, 1977) Morne Trois Pitons, Dominica Morne TroisPatates, Pitons, Dominica Morne Morne Patates, Dominica Mt Pelée, Martinique, 1902 Mt Pelée, 1902 GrosMartinique, Piton, St Lucia PLATE Piton, St Lucia PLATE La Soufrière, St Gros Vincent, 1902, 1974 La Soufrière, St Vincent, 1902, 1974 Kick-em-Jenny Kick-em-Jenny Arenal 1968 Arenal 1968 SOUTH AMERICAN PLATE SOUTH AMERICAN PLATE COCOS COCOS PLATE PLATE NAZCA PLATE NAZCA PLATE St Lucia St Lucia Caribbean Caribbean Sea Sea Grenada Grenada CARIBBEAN PLATE CARIBBEAN PLATE St Vincent St Vincent Barbados Barbados Atlantic Atlantic Ocean Ocean SOUTH AMERICAN PLATE SOUTH AMERICAN PLATE Figure 6.2 The Caribbean Plate, showing the volcanoes along its western and eastern margins. The section below shows how these volcanoes are formed at the margin. ITQ 2 Name the plate you would be on if you lived in (a) The Bahamas, (b) Jamaica, (c) Guyana. The Caribbean Plate (figure 6.2) is not one of the seven large plates, but is still quite big and similar in size to the Philippine Plate and the Arabian Plate. It measures about 3300 sq km. It is bounded by Jamaica and Hispaniola to the north, the Eastern Caribbean islands to the east, the coast of South America to the south and Central America in the west. Most of it is covered by the Caribbean Sea. Beyond the boundaries lie the much larger North American and South American plates, but in the west the smaller Cocos Plate separates it from the huge Pacific Plate. The Nazca Plate south of Panama has the smallest boundary with the Caribbean Plate, and also separates it, and most of South America, from the Pacific Plate. CSEC Geography Fig 6.2Geography 56CSEC same Fig 6.2size same size TechType illustrator CS2 TechType CSEC Geography SEC B prf4 CS4.indd illustrator CS2 56 14/12/2009 2:40 PM 6 · Plate tectonics Plate margins plate margins ▸ The boundaries of the plates, known as the plate margins, are the most active parts of the plates themselves. A crustal plate can be considered as a raft of a portion of the earth’s crust moving over the earth’s surface. On the plate itself not much is happening, but along its edges there is an abundance of activity, depending on which edge we are considering. It is usual to consider a plate as having three possible types of margins (figure 6.4): •a divergent or constructive margin where plates are formed •a convergent or destructive margin where plates are destroyed •a transform margin where plates pass alongside each other. Figure 6.3 The active crater of a modern volcano in the Galapagos Islands, which are located on the boundary between the Cocos and Nazca plates. The black rocks are fresh basalt lava and the red colouring comes from sulphurous deposits as gas continues to escape from the vent. Divergent margins divergent margins ▸ magma ▸ lava ▸ constructive margins ▸ ITQ 3 Name two other mid-oceanic ridges. Zone of divergence Divergent margins are where plates are formed and are therefore separating. Thermal currents drive magma (molten rock) beneath the earth’s crust to the surface and pour out huge quantities of lava onto the sea floor. This lava forms great ranges of mountains beneath the oceans (and sometimes on land), and as it continues to flow, the newly formed volcanic rock is pushed away from the margin. This both creates new plate material and also drives the plates away from this margin. For this reason it is also known as the constructive margin, as it is here that plates are constructed. A major example is the Mid-Atlantic Ridge, which is creating the North American Plate and the Eurasian Plate in the North Atlantic, and the South American Plate and the African Plate in the South Atlantic. Zone of convergence Mid-oceanic ridge Sea level Back arc ridge Island arc Transform fault (plan view) Fore arc ridge Mid-oceanic ridge Trench Transform fault Continental crust Oceanic crust Oceanic crust Mantle Upwelling currents Mantle Earthquake zone Figure 6.4 The three kinds of plate margins, showing the features created by the plate movements. 57 CSEC Geography SEC B prf4 CS4.indd 57 14/12/2009 2:40 PM Section B · Natural systems Convergent margins convergent margins ▸ destructive margins ▸ island arc ▸ Because the plates are being created and are spreading there must be a place where they stop moving; this is the convergent margin. In this zone the advancing edge of a plate meets another plate and is driven under it, often with violent consequences, creating volcanoes and earthquakes (and possibly tsunamis). For this reason they are often known as destructive margins. The boundary where the Caribbean Plate meets the Cocos Plate is a fine example of a convergent and destructive margin. The result is many earthquakes and volcanoes. The lava from the volcanoes creates land if it reaches the surface. Much of the mountainous spine of Central America was formed in this way. In other areas, where the destruction is less violent, small islands may form along the margin, which is usually curved and known as an island arc. The mountainous and volcanic Eastern Caribbean islands from Saba to Grenada were formed in this way (figure 6.5). ITQ 4 Name two more island arcs. Anegada Fault Virgin Is. Puerto Rico Anguilla St Martin St Croix Saba Quill Mt Liamuiga Eustatius Nevis St Kitts Antigua Nevis Peak Montserrat Caribbean Sea Soufriere Hills Guadeloupe Atlantic Ocean Mt Soufriere Dominica Soufriere Mt Pelee Active arc Martinique Inactive arc Centres of volcanic activity St Lucia St Vincent Gros Piton La Soufriere Barbados Grenadines South Caribbean Fault Grenada Kick ‘em Jenny Tobago Trinidad 0 0 50 100 50 150 100 km miles Figure 6.5 The Eastern Caribbean island arc. Note how the volcanoes follow the line of the arc, and the two faults that mark the northern and southern edges of the Caribbean Plate. Transform margins transform margins ▸ ITQ 5 What country lies on the southern transform margin of the Caribbean Plate? The third type of plate boundaries are transform margins, where the plates slide past each other without being constructive or destructive. Nevertheless, this is not an inactive area, as the plates themselves are vast sections of the earth’s crust, and as they drag past each other the result is often massive earthquakes. The northern boundary of the Caribbean Plate is a transform margin. This is where it slides past the North American Plate, roughly through Hispaniola and Jamaica. This explains why earthquakes are common along this line. 58 CSEC Geography SEC B prf4 CS4.indd 58 14/12/2009 2:40 PM 6 · Plate tectonics Summary ▸ Tectonic activity refers to movements in the earth’s crust. ▸ The earth’s crust consists of a number of crustal plates of various sizes; the Caribbean Plate is one of the smaller ones. ▸ Plate tectonics refers to the movement of crustal plates in response to convection currents deep inside the earth. ▸ Most tectonic activity occurs at plate margins. ▸ Divergent (constructive) margins are zones of mountain building and volcanic activity as new plate material is brought to the surface; this often takes place beneath the oceans. ▸ At convergent (destructive) margins, one plate is forced under another, giving rise to volcanoes and earthquakes. ▸ At transform margins plates slide past one another and earthquakes are common. ITQ1 ITQ2 ITQ3 ITQ4 South America and Africa. (a) North American Plate, (b) Caribbean Plate, (c) South American Plate. Atlantic–Indian Ridge, Mid-Indian Ridge, South-east Indian Ridge. Falkland Islands; Aleutian Islands; Indonesia (Java/Sumatra); Japanese Islands; Philippines; Mariana Islands. ITQ5 Venezuela. Answers to ITQs Examination-style questions Examine the diagram of the Caribbean Plate and answer the following questions: 0 0 NORTH AMERICAN PLATE 200 400 600 800 1000 km N 100 200 300 400 500 600 miles Santiago 1946 El Chichon 1982 Kingston, Jamaica, 1907 Guatemala Guatemala 1976 1902 Santa Maria Guatemala City 1918 1902 El Salvador 1986 CARIBBEAN Aguadilla Saba Mt Liamuige, St Kitts Mayagüez Soufrière Hills, Montserrat, 1997 Soufrière Guadeloupe (1903, 1956, 1977) Morne Trois Pitons, Dominica Morne Patates, Dominica Mt Pelée, Martinique, 1902 Gros Piton, St Lucia PLATE La Soufrière, St Vincent, 1902, 1974 Kick-em-Jenny Nicaragua 1972 Area of high seismicity Direction of plate movement Recent seismic activity Major earthquake Arenal 1968 SOUTH AMERICAN PLATE Major volcanic eruption Active volcano Other volcano with recent seismic activity COCOS PLATE NAZCA PLATE 1 Name an earthquake in the Greater Antilles. Lucia 2 Name a volcanic St eruption in the Eastern Caribbean. 3 Suggest reasons why Trinidad has not had a volcanic eruption but could Barbados Vincent have anStearthquake. Caribbean through the plate margin in the Eastern Sea4 Sketch and label a cross-section Atlantic Ocean Caribbean. Grenada 59 CARIBBEAN PLATE CSEC Geography SEC B prf4 CS4.indd 59 SOUTH AMERICAN PLATE 14/12/2009 2:40 PM
