Download PLATE TECTONICS

PLATE TECTONICS
What is a Plate?
 A plate is a broken piece of earth’s crust that floats on top of a mantle. (Fig. 1)
Fig. 1
Fig. 2 - The structure of the earth
 Figure 2 shows the Earth’s structure which consists of crust, mantle and core.
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Why Plates move?
 The force that brings about the motion of the crustal plates comes from the
tremendous heat found in the Earth’s interior.
 This heat causes the rocks in the mantle to melt and become molten rock called
magma. When magma is heated, it expands, rises and generates convection
currents which push the plates away from each other.
 When it cools, the magma sinks and brings the plates towards each other. This
constant rising and sinking of the magma results in the movement of the plates
over the Earth’s surface.
How do plates move?
The plates move in three (3) different ways. There are;
i.
Plate split and move apart in some areas
ii. Plate collide or one plate may slip beneath another
iii. Plate scrape and slide past one another
What are plates boundaries?
 Plate boundaries are the boundaries where the plates move. Plate boundaries
are also known as plate margins. They are found at the edge of the lithospheric
plates and are of three types. The boundaries are characterized by their distinct
motion.
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Types of plate boundaries
 Convergent plate boundaries (move towards each other)
 Divergent plate boundaries (move away from each other)
 Transform plate boundaries (slide past each other)
Convergent Plate boundary (Destructive boundary)
 These are plate margin where two plates move towards each other or converge
(Fig.3). As the plates collide, some crust is destroyed due to the impact. When a
continental plate and oceanic plate collide, the oceanic plate sinks under the
continental plate since the oceanic plate is denser than continental plate. This
process is called subduction. The area where subduction occurs is called
subduction zone.
 When two continental plates converge, one plate will be forced only slightly under
the other, but no subduction will take place. Thus, the pressing together of two
plates will fold the crust and forms what we known as Fold Mountains.
 Similarly, when one oceanic plate converge, the other oceanic plate may subduct
beneath the other, results in an oceanic trench. However, there is no head-on
collision for two oceanic plates. Magma will then rise up to form volcanoes.
 Convergent movement is considered destructive because part of the Earth’s
crust is destroyed and melted in the process.
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Fig. 3 - Collision between the Nazca Plate and the South American Plate
Divergent Plate boundary (Constructive boundary)
 At a divergent boundary or spreading centre, two plates move away from one
another (Fig.4). As the two moves apart, magma from the mantle underneath the
crust is forced to rise up to the surface, cools and solidifies at the plate boundary.
This divergent boundary is considered constructive since new crust is formed
and in some places produces new volcanic islands.
 Divergent takes place at the boundary of the oceanic plates and forms new sea
floor. This process is called sea-floor spreading.
 As magma rises up to the surface, it piles up and solidifies, slowly forming a long
chain of mountains on the ocean floor, called Oceanic ridge.
 The most famous divergent plate movement takes place along the plate
boundaries between the North American and Eurasian and South American and
African Plates. The boundaries run from south to north at the centre of the
Atlantic Ocean floor.
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PLATE TECTONICS
 The divergent plate movement widens the Atlantic Ocean by about 3cm a year.
On the floor of the Atlantic Ocean, a line of undersea ridges is formed along the
plate boundaries. The ridges are known collectively as the Mid-Atlantic Ridge.
Fig 4 – Iceland and the Mid-Atlantic Ridge
Transform Plate boundary (Conservative boundary)
 It is called conservative because plate material is neither created nor destroyed
at these boundaries, but rather plates slide past each other (Fig.5). When plates
slide past each other, it creates a transform Fault. Faulting occurs and
earthquakes often take place along the plate boundary but little or no volcanic
activity take place.
 The movement is not smooth due to friction between the rocks of the two plates.
Sometimes the two plates would get ‘stuck’ and lock together. When there is
sufficient build up of pressure, rocks in the plates break and get jerked apart.
This results in earthquakes. An example of this is found along the boundary
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where the North American Plate meets the Pacific Plate. This is the location of
the famous San Andreas Fault.
Fig. 5 – Plates slide past each other
Distribution of Plates
Fig. 6 – Distribution Of Plates
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What are the major earth’s plates?
▪ African plate
▪ Antarctic plate
▪ Arabian plate
▪ Australian plate
▪ Cocos plate
▪ Eurasian plate
▪ India plate
▪ Indo-Australian plate
▪ Nazca plate
▪ North American plate
▪ Pacific plate
▪ Philippines plate
▪ Scotia plate
▪ South American plate
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PLATE TECTONICS
PLATE TECTONICS: SUMMARY
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Classwork/Homework
1 (a) Study Fig. 1 which shows major plates.
Fig.1
(i)
What is meant by Plate?
[1]
(ii) Name the plates which formed Andes.
[2]
(iii) State the plate boundary which formed Himalayas.
[1]
(iv) The following processes take place at certain plate boundaries shown on Fig. 1.
For each process, name two plates which share a boundary along which it is
happening.
A
sea floor spreading
B
subduction
C
plates sliding past each other
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[3]
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PLATE TECTONICS
2 Fig. 2 is a map showing plates, plate margins and the directions and speeds of plate
movement.
Fig. 2
(a) Five places, 1 – 5, are marked on Fig. 4. For each question write one number. You
may use any of the numbers 1 – 5 once, more than once or not at all.
Which number on the map shows a:
(i) place where earthquakes occur;
[1]
(ii) place where no major earthquakes occur;
[1]
(iii) place where plates are moving towards each other;
[1]
(iv) fold mountain;
[1]
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(v) place where plates are sliding past each other;
[1]
(vi) subduction zone;
[1]
(vii) place where sea floor spreading is happening
[1]
(b) The South Atlantic and the North Atlantic Ocean are getting wider. Using information
from Fig. 2, calculate how much wider the ocean are getting each year. State the
correct units in your answer.
[2]
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PLATE TECTONICS
Answer For Question 1:
(i)
What is meant by Plate?
-
A Plate is a broken piece of earth’s crust that floats on top of a mantle.
(ii) Name the plates which formed Andes.
-
[2]
Nazca Plate and South American Plate
(iii) State the plate boundary which formed Himalayas.
-
[1]
[1]
Indo-Australian Plate and the Eurasian Plate convergent boundary
(iv) The following processes take place at certain plate boundaries shown on Fig. 1.
For each process, name two plates which share a boundary along which it is
happening.
A
sea floor spreading
-
North American and Eurasian
-
South American and African Plate
B
subduction
-
C
Nazca Plate and South American Plate
plates sliding past each other
-
[3]
North American Plate meets the Pacific Plate
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Answer for Question 2:
(a) Five places, 1 – 5, are marked on Fig. 2. For each question write one number. You
may use any of the numbers 1 – 5 once, more than once or not at all.
Which number on the map shows a:
(i) place where earthquakes occur;
-
1, 2, 4, 5
(ii) place where no major earthquakes occur;
-
[1]
5, 2
(vii) place where sea floor spreading is happening
-
[1]
4
(vi) subduction zone;
-
[1]
2
(v) place where plates are sliding past each other;
-
[1]
2, 5
(iv) fold mountain;
-
[1]
3
(iii) place where plates are moving towards each other;
-
[1]
[1]
1
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(b) The South Atlantic and the North Atlantic Ocean are getting wider. Using information
from Fig. 2, calculate how much wider the oceans are getting each year. State the
correct units in your answer.
[2]
-
South Atlanctic = 0.3cm
-
North Atlantic
= 0.5cm
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