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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. SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 1 PLATE TECTONICS 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. SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 2 PLATE TECTONICS 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. SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 3 PLATE TECTONICS 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. SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 4 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 5 PLATE TECTONICS 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 6 PLATE TECTONICS 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 7 PLATE TECTONICS PLATE TECTONICS: SUMMARY SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 8 PLATE TECTONICS 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG [3] Page 9 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] SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 10 PLATE TECTONICS (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] SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 11 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 12 PLATE TECTONICS 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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 13 PLATE TECTONICS (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 SEKOLAH MENENGAH MUDA HASHIM TUTONG Page 14