Plate Tectonics and Movements (IGNOU) PDF

BGYET – 147 Indira Gandhi National Open University GEOMORPHOLOGY AND School of Sciences GEOTECTONICS Block 4 PLATE TECTONICS AND MOVEMENTS Unit 14 Plate Tectonics 87 Unit 15 Plate Bou...

BGYET – 147 Indira Gandhi National Open University GEOMORPHOLOGY AND School of Sciences GEOTECTONICS Block 4 PLATE TECTONICS AND MOVEMENTS Unit 14 Plate Tectonics 87 Unit 15 Plate Boundary Processes 103 Unit 16 Movement of the Indian Plate 123 Unit 17 Major Tectonic Features of Peninsular India 141 Glossary 159 83 Course Design Committee Prof. Vijayshri Dr. A.K. Biyani Prof. L. S. Chamyal Prof. K. R. Hari Former Director Department of Geology Department of Geology School of Studies in Geology School of Sciences Govt. DBS (PG) College M.S.University of Baroda & Water Resources IGNOU, New Delhi Dehradun Vadodara, Gujarat Management Prof. V. K. Verma (Retd.) Prof. Pankaj Srivastava Prof. V. Srivastava Pt. Ravishankar Shukla Department of Geology Centre of Advanced Centre of Advanced Study University, Raipur, Chhattisgarh University of Delhi, Study in Geology in Geology Dr. K. Anbarasu Delhi University of Delhi, Delhi Banaras Hindu University Department of Geology Prof. Pramendra Dev Prof. M. A. Malik Varanasi, UP National College (Retd.) Department of Geology Prof. R.K. Ganjoo Tiruchirapalli, Tamilnadu School of Studies in Earth University of Jammu Department of Geology Faculty of Geology Sciences, Vikram Jammu, J & K University of Jammu Discipline University, Ujjain, MP Prof. D. C. Srivastava Jammu School of Sciences, IGNOU Prof. P. Madhusudhana Department of Earth Prof. (Mrs.) Madhumita Das Prof. Meenal Mishra Reddy (Retd.) Science Department of Geology Department of Geology Indian Institute of Utkal University Prof. Benidhar Deshmukh Dr. B.R. Ambedkar Open Technology Roorkee Bhubaneshwar, Odisha Dr. Omkar Verma University Roorkee, Uttarkhand Dr. R.A. Singh Dr. M. Prashanth Hyderabad Department of Geology Dr. Kakoli Gogoi LSM Govt. PG College, Pithoragarh Block Preparation Team Course Contributors Prof. Vaibhava Srivastava (Unit 14) Prof. Sreepat Jain (Unit 15 and 17) Dr. Omkar Verma (Unit 16 and 17) Centre of Advanced Study in Department of Applied Geology, Adama School of Sciences, IGNOU, Geology, Banaras Hindu University, Science and Technology University, New Delhi Varanasi, U P Ethiopia Content and Language Editors Dr. A.K. Biyani Department of Geology Govt. DBS (PG) College Dehradun Course Coordinators: Dr. Omkar Verma and Prof. Benidhar Deshmukh Audio Visual Materials Dr. Amitosh Dubey Prof. Meenal Mishra Producer, EMPC, IGNOU Content Coordinator Production Mr. Rajiv Girdhar Mr. Hemant Kumar A.R. (P), MPDD, IGNOU S.O. (P), MPDD, IGNOU Acknowledgement: Ms. Savita Sharma for assistance in preparation of CRC and some of the figures. March, 2022 © Indira Gandhi National Open University, 2022 ISBN: Disclaimer: Any material adapted from web-based resources or any other sources in this block are being used only for educational purposes only and not for commercial purposes and their copyrights rest with the original authors. All rights reserved. No part of this work may be reproduced in any form, by mimeograph or any other means, without permission in writing from the Indira Gandhi National Open University. Further information on the Indira Gandhi National Open University courses may be obtained from the University’s office at Maidan Garhi, New Delhi-110 068 or the official website of IGNOU at www.ignou.ac.in. Printed and published on behalf of Indira Gandhi National Open University, New Delhi by the Registrar, MPDD, IGNOU. Printed by : 84  BLOCK 4: PLATE TECTONICS AND MOVEMENTS The concepts of continental drift, sea-floor spreading and palaeomagnetism provide numerous indications that Earth is not a stable entity. Indeed, it is a dynamic planet consisting of numerous fragments of lithosphere. As listed above, concepts of continental drift, sea-floor spreading and palaeomagnetism gave rise a popular theory known as theory of plate tectonics. It is considered as unifying theory in geology because it explains the ways in which Earth works. The theory of plate tectonics states that the rigid lithosphere of the Earth is composed of seven major plates and numerous smaller plates, all of which are in motion in different directions. The movements of the plates result in many tectonic activities, which we often see in the form of volcanisms, earthquakes or tsunamis. These tectonic activities are also responsible for formation of beautiful sculpture of mountains, oceans and landforms as a by-product in due course of time. This block has four units. Unit 14: Plate Tectonics deals with the development of theory of plate tectonics, lithosphere and asthenosphere. It also describes types of plate movements, associated tectonics features and mechanisms of plate tectonics. Unit 15: Plate Boundary Processes covers plate boundaries processes take place at convergent, divergent and transform faults boundaries. It briefly describes ophiolites and their emplacement. Unit 16: Movement of the Indian Plate introduces separation of the Indian plate from Gondwana supercontinent, northward drift of the plate and its ultimate docking with the Asian plate. It also presents geotectonic features of the Himalaya and the Indian Ocean. The last unit, i.e., Unit 17: Major Tectonic Features of Peninsular India focuses on the main Precambrian geotectonic features such as cratons, mobile belts, suture zones and rifts of the Peninsular India. Expected Learning Outcomes After studying this block, you should be able to:  define theory of plate tectonics and concept of asthenosphere and lithosphere;  describe historical background for the development of the theory of plate tectonics;  discuss lithospheric plates and their mechanism and movement types;  describe plate boundaries processes that take place at convergent, divergent and transform faults.  discuss chronology of separation of the Indian plate from rest of Gondwanan continents;  describe northward drift of the plate and its collision with the Asian plate;  explain tectonic features of the Himalaya and the Indian Ocean; and  describe Archaean cratonic blocks and Proterozoic mobile belts and suture zones of peninsular India. We hope that after studying this block you will be able to get understand various aspects of plate tectonics and its applications in geology. Wishing you success in this endeavour! 85 86  UNIT 14 PLATE TECTONICS Structure____________________________________________________________________ 14.1 Introduction 14.7 Triple Junctions and their Present-Day Examples Expected Learning Outcomes 14.8 Activity 14.2 Plate Tectonics 14.9 Summary 14.3 Lithosphere and Asthenosphere 14.10 Terminal Questions 14.4 Plates and their Types 14.11 References 14.5 Rates of the Plate Movement 14.12 Further/Suggested Readings 14.6 Mechanisms of the Plate Tectonics 14.13 Answers 14.1 INTRODUCTION Earthquakes, volcanic activities, sea-floor spreading, continental drift, convection currents and other similar activities combinedly point to dynamic nature of the Earth. Its dynamism is manifested through visible and invisible activities and these are very slow (few cm/year) rate; spreading of ocean floor) to very fast (volcanic eruptions, earthquakes) in operations. A large variety of dynamic activities are interlinked and movements of the rigid lithospheric plates play controlling roles. The drift of continents, sea-floor spreading and palaeomagnetism provide solid evidences that Earth is made-up of the rigid plates that are in continuous motion. The resulting tectonic activities are also responsible for formation of beautiful major sculptures like mountains, oceans and other landforms as by-products in due course of time. Plate tectonics is a unifying theory for geology because this logically explains the formation of tectonics features on the Earth’s surface. While studying the earlier units, you are now well aware about the major geotectonic features found on the Earth. In this unit, we will discuss development of plate tectonics theory, lithosphere and asthenosphere. We also discuss three types of plate movements; associated tectonics features and mechanisms of plate tectonics. Block 4 Plate Tectonics and Movements.................................................................................................................................................................. Expected Learning Outcomes___________________________ After reading this unit, you will be able to:  define theory of plate tectonics;  describe historical background for the development of the theory of plate tectonics;  write about concept of asthenosphere and lithosphere;  discuss lithospheric plates and their movement types;  describe driving forces acting behind the plate movements; and  explain mantle plume, triple junctions and hotspots. 14.2 PLATE TECTONICS The theory of plate tectonics states that the relatively thin rigid lithosphere of the Earth is composed of seven major plates and numerous smaller plates, all of which are in motion in different directions over the asthenosphere. It is a well accepted theory in geology because it explains almost all large-scale geological structures and processes operating on the Earth (McConnell and Abe, 2015). Therefore, plate tectonics theory is known as unifying theory or principle of geology. Historical Background It took more than fifty years for evolving the of concept of plate tectonics on the foundation of continental drift which was gradually reinforced by a series of astonishing developments of new ideas like of sea-floor spreading and gathering of evidences by tens of scientists and groups. Therefore, the credit of development of the theory of plate tectonics does not go to a single person, rather it developed through a chain incorporation of observations, discoveries, data and discussions. The initial focus of geoscientists was on identification and description of rocks, minerals, fossils and they used to think Earth as a whole possesses limited mobility, which is well marked in rocks in the form of folds and faults and continents were believed to be stationary. This idea was commonly known as Fixist hypothesis (Moores and Twiss, 1995). According to this hypothesis, the vertical motions were predominantly the accepted movements and isostasy was chiefly the driving force behind the vertical motion. In 1909, the Yugoslav seismologist, A. Mohorovicic, discovered the sharp increase in primary wave velocity at the base of the crust. Later, the German seismologist, B. Gutenberg in 1914, first recognised the core-mantle structure of the Earth. In meantime, French geophysicist, B. Brunhes, discovered reversely magnetised basalt in 1909 and Japanese geophysicist, M. Matuyama in 1928, proposed that the Earth’s magnetic field was in reversed in polarity during the major part of the Quaternary and only 7 lakh years ago reverse polarity was replaced or changed into normal polarity. These studies though later proved to be very important and revolutionary, but there was no unifying model of the existence in the early twentieth century. 88 Unit 14 Plate Tectonics................................................................................................................................................................. The direct challenge to the fixist model came in 1912 and 1915 with Wegener’s continental drift hypothesis. Wegener’s hypothesis created a great deal of interest worldwide. The controversy came for discordance in 1926, where in a meeting of American Association of Petroleum Geologists in New York, where Wegener could not convincingly explain the driving force and mechanism of continental drift. After the New York meeting, the continental drift hypothesis was out of scene for long time, but a South African geologist A. Du Toit strongly supported the idea in his book ‘Our Wandering Continents’ which got published in 1937, but the hypothesis’ breath was revived in 1950’s when it got support from the palaeomagnetic studies. In early 1930s, the Dutch geophysicist F.A. Veining-Meinesz and his team made a surprising discovery that deep sea trenches in the Caribbean and in Indonesia were see associated with negative gravity anomaly and seemed to violate the laws of isostasy. The British geologist, Arthurs Holmes, proposed the idea that because of down going convection currents the crust is down buckling into the mantle. At about the same time in 1928, the Japanese seismologist, K. Wadati, recognised earthquake sources beneath Japan are located along an inclined planar zone near the trenches. Vening-Meinesz took with him an American geologist, Harry Hess, in his marine expeditions. Later, Hess published the compilation and synthesis of the detailed topographic characteristics of the western Pacific Ocean including trenches off Japan, the Marianas and the Philippines. In the late 1940s and early 1950s, the American seismologist, Hugo Benioff, recognised the presence of inclined seismic zone in the Pacific dipping into the mantle, thus, confirming the early discovery of Wadati. In the late 1950s, Gutenberg discovered a zone of low seismic velocity approximately 100 km depth in the mantle. The pace of new discoveries and problems accelerated with the identification of thousands of kilometers long fracture zones in the oceans in late 1950s and early 1960s. During this time period, palaeomagnetism ended the isolated developments when it was recognised that Europe and North America had different polar wander paths. The simplest explanation was that the continents had moved relative to one another. This was a blow to the fixist and support to the continental drift hypothesis. In 1958, the Australian tectonist, S.W. Carey, tried to fit the continents on a spherical table and accepted the drift theory with drifting away of continents from the mid-oceanic ridges. He also proposed many other tectonic relationships that were subsequently confirmed. He also propounded his idea that the Earth is expanding, which is the root cause of continental drifting, though this idea of Earth’s expansion could not gain popularity. In 1963, important synthesis came independently from L.W. Morley in Canada and from F.J. Vine and D.H. Mathews in Cambridge, England. It is popularly known as Vine-Mathews-Morley hypothesis proposes that there were several polarity reversals in the Earth’s magnetic field, which the ocean-floors have preserved in strips like a tape recorder on the both sides of the Mid-oceanic ridges symmetrically. The Canadian geophysicist, J.T. Wilson in 1965 proposed the idea of transform fault, which disrupts the mid-oceanic ridges. Based on seismicity maps of 1960 to 1968, the American seismologists, M. Barazangi and L. Dorman in 1969, showed a pronounced concentration of seismicity in the 89 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. narrow zones along the mid-oceanic ridges and in planar zone dipping beneath the island arcs to the depth up to 700 km. This map made it clear how tectonically active the Earth is and also that the tectonics of the Earth is characterised by rigid-body motion of large plates on a sphere. It also made clear that the deformation and seismic activities are concentrated along the boundaries of the plates, where they interact with one another. A global conference in December 1969 was organised by William R. Dickinson in California that explored the application of plate tectonics to continental geology and orogeny and also for ancient continental geology. Now it is clear that continental drift was not a single episode of Earth’s history, but an inherent and continuous process. 14.3 LITHOSPHERE AND ASTHENOSPHERE We are familiar with a commonly followed three layers structure namely, the core, mantle and crust of the Earth’s interior, which are largely based on chemical composition and density of Earth’s interior obtained by indirect means e.g. by seismic waves. The Earth is also classified as core, lower mantle, mesosphere, asthenosphere and lithosphere on the basis of different sets of physical properties and behaviour of the rock material present inside the Earth. After awareness of the core and lower mantle layers, now, let us get acquainted with the term asthenosphere and lithosphere for proper understanding of plate tectonics. Lithosphere Lithosphere (lithos, meaning rocks) is the outermost layer of the Earth and lies above the asthenosphere. It is composed of Earth’s crust (both oceanic and continental) and rigid and relatively cool part of the upper mantle (Fig. 14.1). Lithosphere is physically considered as strong and rigid region that deforms in elastic way (Kearey and others, 2009). The thickness of lithosphere above the astheonsphere is not uniform and varies from palce to place on the Earth. The average the thickness of lithosphere is 100 km and may go upto 300 km below the orogenic mountains. The thickness of lithosphere is less than 50 km below the oceanic crust. The lithosphere is not a single shell, but consists of many different large segments or blocks of lithosphere, which are called as lithospheric plates, tectonic plates or simply plates. These plates are considered rigid bodies floating over the asthenosphere and tectonic deformations generally take place at the boundaries of the plates because of the interactions one plate with other plate(s). The boundaries of present-day plates are chiefly drawn along densely concentrated, linear shaped earthquakes foci area and loci of active and dormant volcanoes. The lithospheric plates move horizontally over the asthenosphere. Asthenosphere Asthenosphere (asthenes, meaning without strength) is the part of upper mantle (Fig. 14.1). It is also known as Low Velocity Zone (LVZ) because the velocity of seismic waves decreases in this zone. Asthenosphere lies below the lithosphere at an average depth of 100 km and extending to a depth of 350 to 90 Unit 14 Plate Tectonics................................................................................................................................................................. 650 km. The asthenosphere is hot, soft, semi-viscous in nature and consists of very little molten rock material around mineral grains. This zone allows the lithospheric plate to float and move over it. The upper boundary of the asthenosphere may lie about 60 km below the oceanic crust while the base of the asthenosphere is can be as deep as 700 km (Kearey and others, 2009). The average thickness of asthenosphere however, lies between 180 to 220 km. The asthenosphere is thought to play critical role in movements of the plates on the Earth’s surface. Fig. 14.1: Internal structure of the Earth showing lithosphere and asthenosphere. (Source: https://igs.indiana.edu/Geothermal/) 14.4 PLATES AND THEIR TYPES As mentioned above, lithospheric plates are portions, blocks or segments of the lithosphere. The demarcation of boundary of a single plate helps to find whether it is oceanic, continental or continent-oceanic plate and whether it is a small or larger plate. Composition-wise plates are of the following three types: Oceanic plate: It is comprised entirely of an oceanic crust. Continental plate: It is wholly composed of continental crust. Continent-oceanic plate: It is comprised partially of oceanic and partially of continental crust. It may be noted that the lithosphere of the Earth is made up of a mosaic of discrete relatively rigid plates. There are seven major plates and several minor plates (Fig. 14.2). The names of seven major plates are: 1) Indo-Australian plate 2) Eurasian plate 3) North American plate 4) South American plate 5) African plate 91 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. 6) Pacific plate 7) Antarctic plate The Nazca plate, Cocos plate, Scotia plate, Philippines Sea plate, Iran plate, Chinese plate, Arabian plate, Nubian plate and Somali plate are the names of a few minor plates. Fig. 14.2: Major plates of the world. (Source: http://eqseis.geosc.psu.edu/~cammon) Nature of the Plates: The thickness of the plates is controlled by 1400oC isotherm. This isotherm represents the temperature of partial melting that transforms the mantle into quasi plastic medium. Thus lithospheric plates are made up of both crust and upper mantle. The thickness of oceanic plate increases as we go away from the spreading ridges. The average thickness of oceanic plate is about 60 km and for continental plate, the average thickness is about 100 km. As a result of plate tectonics, the total area of ocean plate has gradually decreased and that of the continental plate has increased. In the current configuration, the seven major plates exceed an area of about 107 km2. Six intermediate sized plates namely Arabia, Caribbean, Cocos, Nazca, Philippines and Scotia currently range in area from 106 to 107 km2. Numerous plates exist that are smaller than 105 km2 such as Indonesia, Fiji, Bismarck etc. such plates have been called as micro-plates. As per the concepts of the plate tectonics, the plates move apart from a divergent boundary and get converge along a convergent boundary. New crust is generated along the accreting of constructive plate margins and destroyed in mantle along subduction zone. The phenomenon of plate tectonics is able to explain many problems of earthquakes, volcanoes, island arcs, trenches, mountains etc. Plate Boundary/ Plate Margin: Plate boundary is the surface trace of the zone of motion between two plates. Plate boundary is rarely a sharp plane; normally it is several km wide regions or zones. Two plate margins meet at a common plate boundary. Plate margin is marginal part of a particular plate. The plate margins or plate boundaries are of the following four types (Fig. 14.3): 92 Unit 14 Plate Tectonics................................................................................................................................................................. Fig. 14.3: Schematic showing three types of plate boundaries. (Source: https://g105lab.sitehost.iu.edu/1425chap13.htm) a) Constructive or divergent or accreting plate margin: Constructive plate boundary represents zone of divergence where there is continuous upwelling of molten rocks (i.e. lava) and thus, new (oceanic) crust is formed. This type of boundary occurs at the mid oceanic ridges. b) Destructive or convergent or consuming plate margin: Here two plates converge towards each other, one plate which is heavier in density buckles down or subducts (subducting plate) below the lighter plate (over-riding plate or stationary plate) and is consumed or destroyed in the mantle. The zone where subduction occurs is called as subduction or Benioff Zone. c) Conservative or shear or transform fault margin: Here two plates neither converge or diverge and only slide or shear past each other along transform faults. d) Recently, a fourth type of plate margins have been recognised as ‘Plate Boundary Zone’ where the plate margin is not a single line, but rather a zone where many microplates in between two major plates, play their role in the tectonics. For example, the zone between African and Eurasian plates. Before proceeding further, let us have a short break to check your progress. SAQ 1 a) Name seven major plates of the globe. b) What is asthenosphere? c) Which plate boundary is called conservative type? 14.5 RATES OF THE PLATE MOVEMENTS We often become curious that if the tectonic plates are moving continuously why don’t we experience this motion? Or do we experience the movement at the time of earthquakes only? Or how do geologists find the rate of movement 93 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. of the tectonic plates? Well, our curiosity may find the answer when we go through the followings. The current plate movements can be tracked directly by means of ground- based or space-based geodetic measurements. However, because plate motions are global in scale, they are best measured by satellite-based methods. Global Positioning System (GPS) has been the most useful for studying the Earth’s crustal movements. By repeatedly measuring distances between specific points, geologists can determine if there has been active movement along faults or between plates. Evidence of the past rates of the plate movement also can be obtained from geological mapping of markers. If a rock formation of known age (marker rock) mapped on one side of a plate boundary can be matched with the same formation on the other side of the boundary, then measuring the distance between offset across the structure or distance between the rock formation and the axis, can give an estimate of the average rate of plate motion. This is a simple and effective technique, which is used to determine the rates of plate motion at divergent boundaries, for example, the Mid-Atlantic Ridge and transform boundaries such as the San Andreas Fault. We know that the ocean-floor have records of the Earth’s magnetic normal and reversed polarity events in the form of linear strips. Therefore, knowing the approximate duration of the reversal, we can also calculate the average rate of plate movement during a given time span. These average rates of plate separations can range widely. The Arctic Ridge has the slowest rate (less than 2.5 cm/year). The East Pacific Rise near Easter Island in the South Pacific about 3,400 km west of Chile has the fastest rate (more than 15 cm/year). 14.6 MECHANISMS OF THE PLATE TECTONICS The general theory of plate tectonics takes the following assumptions: a) Sea-floor spreading occurs and new oceanic crust is continuously generated at irregular line source (i.e. along the active Mid-oceanic ridges). b) The Earth is of constant surface area or if not, the changes at a rate, which is small by comparison with the rate of generation of new surface area by spreading. c) Once formed, new crust forms part of a rigid plate which may or may not incorporate continental material. An important question is that why do plates move? Or which cause plates to move? It is important to note that the driving forces for plate tectonics are still not accurately identified. However, many hypotheses have been proposed for explaining the mechanism of the motion of the plates. Some of them are described below: Convection current hypothesis: According to this hypothesis, the convection currents operating in mantle cause movement of plates along them. 94 Unit 14 Plate Tectonics................................................................................................................................................................. The plates drag the mantle: This hypothesis argues that because asthenosphere cannot be separated by lithosphere, hence, it is the plate movement which drags the mantle. Gravity slide: According to this hypothesis, it is the Earth’s gravity that causes plates to slide towards asthenosphere; hence, the lateral movements of the plates are generated. Mantle plumes: It states that mantle plumes arising from lower mantle and at core-mantle boundary create hotspots and drive the plate in radial direction. In recent times, the mantle plumes and hotspots model have gained much popularity for the driving forces for plate tectonics. Lets us discuss this model in details. Mantle Plumes and Hotspots The mantle plume model was originally proposed by an American Geophysicist, W.J. Morgan in 1971. This model assumes a ‘plume’ to be important source of heat transfer from lower to upper mantle. A mantle plume is a rising column of hot material a few hundred km in diameter that comes upward from lower mantle or from the core-lower mantle boundary into asthenosphere and spreads out like thunderhead beneath the lithospheric plates (Fig. 14.4). The complementary return flow, due to mantle convection current, would involve a uniform sinking of entire mantle below the asthenosphere, which is in addition to the down flow associated with subduction zones. The lateral spreading of plume material in the asthenosphere produces radial shear stress on the base of the overlying lithosphere. If a number of plumes are aligned in a line, then flow in the asthenosphere would be laterally away from the line of plumes, and the resulting shear stresses would act to pull the lithosphere apart. Thus, a spreading center is created along the line of plume. Fig. 14.4: Mantle plume: a) Convection currents in mantle; and b) Plume rise in lower mantle creating hot spot on Earth’s surface. Lava outpouring on hot spots form active volcanic mounds which in due course of time due to movement of plate become aseismic ridge. (Source: redrawn after Moores and Twiss, 1995) The mantle plumes are upwelling from the lower mantle and are immobile in nature. They complete their life cycle in more than a hundred million years and 95 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. their surface manifestations are known as hotspots. The hotspots are stationary, intra-plate and long-lived volcanic regions on the Earth’s surface that are fed from the by underlying mantle. These places are anomalously hot compared with the surroundings and give rise to localised regions of volcanic activity. Hotspot volcanoes are considered to have a fundamentally different origin from island arc volcanoes because the later form over subduction zones at converging plate boundaries. The hotspots also differ from the volcanism over the divergent boundary along Mid-Oceanic Ridge (MOR) because the MOR have source of magma at shallower depth. Hotspots may occur over a single plate and such type of setting is known as intra-plate sitting. The Deccan traps of peninsular of India is a result of massive volcanic eruption that took place about 66 million years ago, when the Indian plate crossed oven the present Reunion hotspot situated in the African plate. Kerguelen is another hotspot located in the Indian Ocean and its volcanic eruption some 118-112 million years ago led to the formation of the Rajmahal traps of the eastern India. Hotspots are also used as reference frames to analyse the finite plate motions. Most of the hotspots over the Earth’s surface have been located in the oceanic regions (Fig. 14.5), though hotspots have also been observed over the continents. Fig. 14.5: Map of hotspots. Note that the hotspot volcanism is not related to the Mid-Oceanic ridges or island arc volcanisms. (Source: redrawn after Moores and Twiss, 1995) The hotspots are often found associated with the aseismic ridges, which are formed due to outpouring of magma on a plate surface over a considerable time period (Fig.14.5). Aseismic ridges are long, linear and mountainous topographic elevations found in the oceans and usually have anomalously thick oceanic crust. Earthquakes do not occur within aseismic ridges, and it is this feature that distinguishes them from oceanic spreading centers. Two ridges namely the Ninety Degree East and Eighty Five East ridges are present in Bay of Bengal and extending into Indian Ocean. 96 Unit 14 Plate Tectonics................................................................................................................................................................. SAQ 2 a) Name the most popular model for mechanism of plate movements. b) What is hotspot? Name one example of hotspot in the Pacific Ocean c) Why ridges created by hotspots are called aseismic ridges? 14.7 TRIPLE JUNCTIONS AND THEIR PRESENT- DAY EXAMPLES The lithosphere is a mosaic of inter-locking plates and most of the tectonic activities are found along the boundary of the two interacting plates, however, there are several places where three different plates come together. The place where three plates meet is known as triple junction. In other words, the contact region of three plates represents a triple junction. You are already aware that plate boundaries are a junction where two plates meet together. You are also aware that ridges usually associated with divergent plate margins, trenches with convergent plate margins and strike-slip fault with transform faults. Based on the combinations of these three basic types, the triple junctions broadly classified into three types, as given below (Fig. 14.6): Ridge-Ridge-Ridge (RRR) triple junction: It a place where three ocean ridges meet together (Fig. 14.6a). Common example of RRR triple junctions is in the South Atlantic Ocean where ridges of three plates namely, South American, Antarctic and African meet. Trench-Trench-Trench (TTT) triple junction: It is the triple junction between three oceanic trenches (Fig. 14.6b). It occurs in central Japan. Ridge-Ridge-Fault (RRF) triple junction: It is a triple junction between two oceanic ridges and one transform fault (Fig. 14.6c). (a) (b) (c) Fig. 14.6: Types of triple junctions: a) RRR triple junction; b) TTT triple junction; and c) RRF triple junction. (Source: https://www.open.edu/openlearn/ science-maths-technology/science/geology/plate-tectonics/content-section- 3.9#) Other types of triple junction are Trench-Trench-Fault (TTF), Trench-Trench- Ridge (TTR), Fault-Fault-Ridge (FFR), Fault-Fault-Trench (FFT) and Ridge- Trench-Fault (RTF). A triple junction is stable or unstable depends on geometry of the plates, relative velocities of the plates and orientations of the plate boundaries (Moores and Twiss, 1995). In stable triple junction, the configuration of junction does not change with time. But, in case of unstable triple junction the configuration of junction evolves with time. RRR triple junction is an example of stable triple 97 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. junction. The TTT triple junction is an unstable triple junction. Some of the common present-day triple junctions with their types are listed in Table 14.1. Table 14.1: Present-day examples of triple junctions. R stands for Ridge, F for Fault and T for Trench. Triple Name of Locatio Plates involved Ridge/Fault/Trench junction triple n Involved type junction RRR Afar triple Ethiopia, Nubian, Somalian Red Sea Rift, Aden junction Africa and Arabian Ridge and East African plates Rift RRR Azores triple Atlantic North American, Mid-Atlantic Ridge and junction Ocean Eurasian and Terceira Rift African plates RRR Bouvet triple Atlantic South American, Mid-Atlantic Ridge, junction Ocean African and Southwest Indian Ridge, Antarctic plates and South American- Antarctic Ridge RRR Indian Indian African, Central Indian Ridge, Ocean Ocean Indo-Australian Southwest Indian Ridge (Rodrigues) and Antarctic and Southeast Indian triple plates Ridge junction RRR Galapagos Pacific Pacific, Cocos East Pacific Rise and triple Ocean andNazca plates Galapagos Rise junction FFF Karliova Near Arabian, Eurasian North Anatolian Fault triple Turkey and Anatolian and East Anatolian junction (Turkish) plates Fault TTT Boso triple Pacific North American Sagami, Izu-Bonin and junction Ocean Pacific and Japan Trenches Philippines Sea plates TTR Chile triple Pacific S America, Nazca Chile Rise, South junction Ocean and Antarctic American Plate and plates Peru-Chile Trench FFT Mendocino Pacific North American, Gorda and North triple Ocean Pacific and Gorda American plates and junction plate Mendocino and San Andreas Faults RRF Macquarie Pacific Indo-Australian, Macquarie Ridge triple Ocean Pacific and Complex and numerous junction Antarctic plates other fractures zones 98 Unit 14 Plate Tectonics................................................................................................................................................................. 14.8 ACTIVITY Given below is the global tectonic map (Fig. 14.7). Plot the location of the following: a) Reunion, Kerguelen, Hawaii, Macdonald, Iceland and Afar hotspots. b) Bouvet and Azores triple junctions. Fig. 14.7: Map showing main tectonic features. 14.9 SUMMARY Let us summarise what we have read in this unit: The evolution of the theory of plate tectonics has been a journey passing through different hypotheses like continental drift, sea-floor spreading, palaeomagnetism and convection currents proposed in the early part of 20th century. The theory of plate tectonics in fact developed through the unifications of theories, observations and contribution coming from geological, palaeontological, seismological, oceanographic, palaeomagnetism gravitational, meteorological and many other studies carried out over decades. The tectonic plates are a segment of lithosphere that float and move over the asthenosphere. The boundaries of these rigid plates have been marked by focused concentration of earthquakes and volcanic activities because of interaction of one plate with the other. A tectonic plate may comprise totally oceanic, totally continental or partially continental and partially oceanic segment of the lithosphere. 99 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. New crusts are created where two plates diverge from each other and where the two plates converge in motion the crust get destroyed by subducting under another plate. Along the transform fault the crust is neither created nor destroyed, therefore this plate boundary is known as conservative plate boundary. Convection currents in the mantle are probably the chief forces that drive the plates. Hotspots are the places on the Earth’s surface where a linear stream of magma called plume comes from lower mantle or core-mantle boundary, outpours lava without any significant seismic activity. These volcanoes form linear topographic ridges called aseismic ridges. Mantle plume is the most popular model to explain the movements of the plates. Many geotectonic features such as mountains, trenches island arcs, aseismic ridges and activities like earthquakes and volcanoes can be satisfactorily explained by the theory of plate tectonics. Triple junctions are the places where three plates come together. 14.10 TERMINAL QUESTIONS 1. With near diagrams, explain the constitution of lithosphere and asthenosphere. Also discuss their role in plate tectonics. 2. What is mantle plume? Explain its contribution in volcanism, plate movements and earthquakes. 3. What do you understand by triple junction in plate tectonics? 4. On the world map given below (Fig. 14.8) show the locations of the following: African plate, Indo-Australian plate, Nazca plate, South American plate, Antarctic plates, Mid Atlantic Ridge and Hawaii-Emperor Aseismic Ridge. Fig. 14.8: Blank map of the world. 100 Unit 14 Plate Tectonics................................................................................................................................................................. 14.11 REFERENCES Kearey, P., Klepeis, K.A. and Vine, F.J. (2009) Global Tectonics, Wiley India Pvt. Ltd, New Delhi. McConnell, R.L. and Abel, D.C. (2015) Environmental Geology Today, Jones and Bartlett Learning, Burlington. Moores, E.M. and Twiss, R.J. (1995) Tectonics, W.H. Freeman and Company, New York. 14.12 FURTHER/SUGGESTED READINGS Condie, K.C. (2003) Plate Tectonics and Crustal Evolution, Butterworth- Heinemann, Oxford. Foulger, G.R. (2010) Plates vs Plumes, Wiley-Blackwell, Sussex. Gass, I.G., Smith, P.J. and Wilson, R.C.L. (1979) Understanding the Earth, The Artemis Press Limited, Sussex. Kearey, P. and Vine, F.J. (1996) Global Tectonics (2nd Edition), Blackwell Science, Oxford. 14.13 ANSWERS Self Assessment Question 1 a) The seven major plates of the globe are Eurasian plate, North American plate, South American plate, African plate, Indo-Australian plate, Pacific plate and Antarctic plate. b) Asthenosphere is the part of upper mantle that lies below the lithosphere at an average depth of 100 km and extending to a depth of 350 to 650 km. The asthenosphere is hot, semi-viscous in nature and consists of partially molten rock. This zone allows the lithospheric plate to float and move over it. The asthenosphere is thought to play critical role in movements of the plates on the Earth’s surface. c) The transform fault is called conservative plate boundary because along the transform fault the crust is neither created nor destroyed. Self Assessment Question 2 a) Mantle plume is the most popular model for mechanism of plate movements. b) Hotspots are the surface manifestations of mantle plumes. The hotspots are volcanic regions on the Earth’s surface that are thought to be fed by underlying mantle. Hawaii is a most common hotspot of the Pacific Ocean and it has formed a volcanic island known as Hawaiian island. c) The ridges created by hotspots are long, linear and mountainous topographic elevations found in the oceans. They usually have anomalously thick oceanic crust and earthquakes do not occur in these ridges. Hence, these ridges are known as aseismic ridges. 101 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. Terminal Questions 1. Please refer to section 14.3. 2. Please refer to section 14.6 3. Please refer to section 14.7. 4. Consult Fig. 14.2. 102  UNIT 15 PLATE BOUNDARY PROCESSES Structure____________________________________________________________________ 15.1 Introduction 15.4 Processes at Divergent Plate Boundaries Expected Learning Outcomes 15.5 Processes at Transform Faults 15.2 Types of Plate Boundaries 15.6 Ophiolites and their Emplacement 15.3 Processes at Convergent Plate Boundaries 15.7 Activity Island Arcs and Volcanic Arcs 15.8 Summary Fore-Arc 15.9 Terminal Questions Trenches 15.10 References Marginal Basins 15.11 Further/Suggested Readings Benioff zone or Wadatti-Benioff zone 15.12 Answers Collision 15.13 Magmatism 15.1 INTRODUCTION You have read that theory of plate tectonics considered as a unifying theory for understanding the dynamics of Earth. Earlier, it was believed that continents and ocean basins were fixed and immobile features of the Earth. However, limited horizontal displacement in the form of strike-slip faults, recumbent folds, nappe and thrust/overthrust in crustal deformation during orogenic movements were permissible and the concept of continental drift was just a ridiculous idea. Later, the concepts of sea-floor spreading, palaeomagentism, convection currents and mantle plumes clearly indicate that continents are not stationary. In fact, lithosphere of the Earth is comprised many moving lithospheric plates, which are of entirely continental, oceanic or both in nature. These plates are in motion and moving in different direction with different velocity. Theory of plate tectonics satisfactorily explains the origin and evolution of many geotectonic features such as mountains, trenches, island arcs, and aseismic ridges. It also helps us to understand phenomena like earthquakes and volcanoes. Block 4 Plate Tectonics and Movements.................................................................................................................................................................. The convergent, divergent and transform faults are three types of plate boundaries where many geological processes are operating. In this unit, we will discuss the plate boundaries processes take place at convergent, divergent and transform faults. We will also briefly describe ophiolites and their emplacement at the end of the unit. Expected Learning Outcomes___________________________ After reading this unit, you will be able to:  list three types of plate boundaries;  describe ocean ridge, generation of oceanic lithosphere, rift valley and magmatism at divergent plate boundaries;  discuss arcs, trenches, marginal basins and collision at convergent plate boundaries;  explain oceanic and continental transform faults; and  write about Benioff zone and ophiolites. 15.2 TYPES OF PLATE BOUNDARIES Plate tectonics has enabled us to predict geological events and explains almost all aspects of what the Earth experiences. The theory explains where and why mountains, earthquakes and volcanoes occur. It also provides the ages of deformational events, the ages and shapes of continents and ocean basins, as well as other aspects of the Earth. Plate tectonics is a holistic theory and explains crustal movements. According to this theory, the Earth is made up of number of lithospheric plates (~100 km thick) those float on the hot and ductile asthenosphere. The plates behave as rigid bodies with some ability to flex, but deformation occurs mainly along plate boundaries. The majority of geotectonic processes are operating at plate boundaries or margins; therefore, it is important to know about various types of plate boundaries, before discussing the processes. The plates move in three ways—toward each other, away from each other, and sliding past each other. These are accordingly classified into three types (Fig. 15.1): convergent, divergent and transform. Fig. 15.1: Three types of plate tectonic boundaries: convergent, divergent and transform. (Source: modified from https://www.geologycafe.com/; https://gotbooks.miracosta.edu/earth_science/images/plate_boundaries.jpg) Let us briefly discuss these plate boundaries. 104 Unit 15 Plate Boundary Processes................................................................................................................................................................. i) Convergent Plate Boundaries These occur where lithosphere of one plate is pushed into the mantle. These are marked by oceanic trenches and subduction zones. Following are the three types of convergent plate boundaries. Oceanic-oceanic convergence: When two oceanic lithospheric plates collide, one oceanic plate subducts beneath the other resulting in the formation of an oceanic trench on the sea-floor side (Fig. 15.2a). Oceanic-continent convergence: When oceanic lithospheric plate collides with a plate of continental lithospheric characters and the oceanic plate subducts beneath the continental plate due to its higher density. It is also called the continental convergent margin (Fig. 15.2b). Continent-continent convergence: It is also called as the continental collision margin (Fig. 15.2c). Both plates are continental lithospheric in nature and one plate subducts beneath another continental lithospheric plate. (a) (b) 105 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. (c) Fig. 15.2: Types of convergent plate boundaries: a) Oceanic-oceanic convergence; b) Oceanic-continent convergence; and c) Continent- continent convergence. (Source: modified after Pidwirny, 2013; https://pubs.usgs.gov/gip/dynamic/understanding.html) ii) Divergent Plate Boundaries At these boundaries, the plates move away from each other. This can occur in the middle of a continent or in the middle of the ocean (Fig. 15.3). Fig. 15.3: Divergent plate boundaries; continental (above) and oceanic (below). (Source: modified from https://geology.com/nsta/) iii) Transform Plate Boundaries Transform boundaries occur where two plates horizontally slide past one another (Fig. 15.1). 15.3 PROCESSES AT CONVERGENT PLATE BOUNDARIES As you are aware that at convergent plate boundaries, two plates converge towards each other, one plate which is heavier in density goes down or 106 Unit 15 Plate Boundary Processes................................................................................................................................................................. subducts also known as subducting plate below the lighter plate also known as overriding or stationary plate and consumed or destroyed in the mantle (Fig. 15.2b). During the convergence, various tectonic processes take place around this boundary and will result in development of many tectonics features. Let us discuss processes operate at convergent plate boundaries. 15.3.1 Island Arcs and Volcanic Arcs A volcanic island arc is formed when two oceanic plates converge and form a subduction zone (Fig. 15.4a). The magma of basaltic composition is produced by fractional heating and partial melting. A continental volcanic arc is formed by the subduction of an oceanic plate beneath a continental plate (Fig. 15.4b). The magma produced is more silica-rich than that formed at a volcanic island arc. Thus, both types are a result of subduction, but island arcs are oceanic-oceanic interactions, while volcanic arcs are continent-oceanic interactions. (a) (b) Fig. 15.4: Volcanic arcs: a) Volcanic island arc (Source: modified https://www.clipart. email/clipart/island-arc-clipart-93534.html); and b) Continental volcanic arc. (Source: modified from https://www.swellnet.com/sites/default/files/inline- images/sumatra-subduction.jpg; https://commons.wikimedia.org/wiki/). Island Arcs: These are long belts of active volcanoes with strong and frequent seismic activities found along convergent plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone (Fig. 15.5). Island arcs can either be active or inactive. 107 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. The active island arcs are ridges of recent volcanoes with an associated deep seismic zone. They also possess a distinct curved form, a chain of active or recently extinct volcanoes and a deep-sea trench. The inactive island arcs are a chain of islands that contain older volcanic and volcaniclastic rocks. Most modern island arcs are found near the continental margins (Fig. 15.5). The Aleutian, Kuril, Japanese, Ryukyu, Izu-Bonin, Philippine, Mariana, Solomon, Tonga-Kermadec and Andaman-Nicobar are common examples of island arcs. In the Indian Ocean, a tangle of arcs commonly described as the Indonesian Archipelago. In the Atlantic Ocean, these include Caribbean and South Sandwich island arcs. Broadly, there three series of volcanic rocks occur in island arcs namely, tholeiitic (basaltic andesites and andesites), calc-alkaline (andesites) and alkaline (alkaline basalts and the rare, very high potassium-bearing lavas). This volcanic series is related to the age and depth of the subduction zone. The tholeiitic magma series is well represented above young subduction zones formed by magma from relative shallow depths. The calc-alkaline and alkaline series are found in mature subduction zones and are related to magma of greater depths. Andesite and basaltic andesite are the most abundant volcanic rocks in island arc and are indicative of calc-alkaline magmas. Fig. 15.5: Island arcs. (Source: modified from https://www.slideserve.com/sydney/ volcanic-arcs-chapters-16-and-17; http://www.nature.nps.gov/geology/ education/images/GRAPHICS/figure_island-arc-setting-WEB.jpg). Volcanic Arcs: When the downward-moving slab reaches to a depth of about 100 km (60 miles), the slab becomes hot enough to drive off its most volatile components, thereby initiating partial melting of mantle in the plate above the subduction zone (also known as the mantle wedge) of the subducting plate. Melting in the mantle wedge produces magma, which is predominantly basaltic in composition. This magma rises to the surface and gives birth to a line of volcanoes in the over-riding plate, known as a volcanic arc. It is typically located a few hundred kilometres behind the oceanic trench. A basin may form within this region or between the trench and volcanic islands, which is known as a fore-arc basin and may be filled with sediments derived from the volcanic arc (Fig. 15.4b) or with remains of oceanic crust (Fig. 15.5). 108 Unit 15 Plate Boundary Processes................................................................................................................................................................. The island arcs system usually comprises fore-arc, trenches, marginal basins (also known as back-arc basin) and the Benioff zone or the Wadatti-Benioff zone. The components of this system are discussed below. 15.3.2 Fore-Arc A fore-arc is the region between an oceanic trench and the associated volcanic arc (Figs. 15.4b and 15.5). Fore-arc regions are found at convergent margins and include an accretionary wedge (also termed as accretionary prism formed by deposition of sediments) and fore-arc basin. Due to tectonic stresses as one tectonic plate rides over another, the fore-arc region is a source for great thrust earthquakes. The Mariana, Central Andean and Banda are common examples of fore-arc regions of the globe. 15.3.3 Trenches On the subducting side of the island arc, there is a deep and narrow oceanic trench, which is the trace at the Earth’s surface of the boundary between the down-going and over-riding plates (Figs. 15.4b and 15.5). This trench is created by the downward gravitational pull of the relatively dense subducting plate on the leading edge of the plate (Fig. 15.4b). They are formed by flexing of the oceanic lithosphere, developing on the ocean side of island arcs. Earthquakes frequently occur along this subduction boundary with the seismic hypocenters located at increasing depth under the island arc. These quakes define the Benioff zone (Fig. 15.6). These are the deepest features of ocean basins, for example, the Mariana trench is the one of the deepest trenches of the world (~ 10,885 m or 36,000 feet deep). Around 4.2 km deep Andaman trench is present in Bay of Bengal in the Indian Ocean. The Indus Suture Zone of the Ladakh, northern India is an example filled-up trench. 15.3.4 Marginal Basins A basin developed between the continental margin and the island arcs on the concave side of the arc is known as marginal basin (Figs. 15.4b and 15.5). These basins have a crust which is either oceanic or intermediate between the oceanic (Fig. 15.5) and continental crust (Fig. 15.4b). They are also referred to as marginal seas and back-arc basin. They are formed in the inner concave side of island arcs bounded by back-arc ridges (Figs. 15.4b and15.5). Marginal basins are typically very long (several hundreds to thousands of kilometers) and relatively narrow (a few hundred kilometers). They develop in response to tensional tectonics due to rifting of an existing island arc. These basins are formed behind an island arc and are characterised by significant hydrothermal activity with deep-sea vents. These basins are typically found along the western margin of the Pacific Ocean near the convergence of two tectonic plates. Examples include the Sea of Japan, the Mariana Trough in the Philippine Sea and the South Fiji Basin. 15.3.5 Benioff zone or Wadatti-Benioff zone Benioff zone or Wadatti-Benioff zone term was named after seismologists H. Benioff and K Wadatti, who independently identified these zones. This is a planar zone that dips under the over-riding plate and characterised by the intense seismic and volcanic activities. The dip of Benioff zones ranges from 109 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. 10° to near vertical. It is defined by the location of seismic events below the arc. Earthquakes occur from near surface to a depth of about 700 km below the Earth’s surface (Fig. 15.6). These earthquakes originate beneath volcanic island arcs and continental margins above active subduction zones (Figs. 15.4b and 15.5). They can be produced by slip along the subduction thrust fault or slip on faults within the down going plate, as a result of bending and extension as the plate is pulled into the mantle (Fig. 15.6). Fig. 15.6: Benioff zone or Wadatti-Benioff zone. (Source: modified after https://www.hexrpg.com/f/30/352888?page=2). 15.3.6 Collision When two plates collide, the colliding plates create three tectonic settings: oceanic-oceanic, oceanic-continental and continental-continental (Fig. 15.2). Earthquakes are common whenever any large slab of Earth comes into contact with each other and convergent boundaries are no exception. In fact, majority of the Earth’s most powerful quakes have occurred at or near these boundaries. Collision takes place at continental-continental tectonic setting, when large slabs of crust collide against each other (Fig. 15.2c). This results in very little subduction, as most of the rocks are too light to be carried very far down into the dense mantle. Instead, the continental crust at these convergent boundaries gets folded, faulted and thickened, forming great mountain chains of uplifted rock. The magma cannot penetrate this thick crust; instead, it cools intrusively and forms granite. High grade metamorphosed rock, like gneiss, is also common. The Himalayas and the Tibetan Plateau are excellent examples, 110 Unit 15 Plate Boundary Processes................................................................................................................................................................. characterised by fold-thrust mountain belts that develop around the zone of collision. 15.3.7 Magmatism Convergent plate boundaries are known for intense magmatic activities. The sinking or down going lithospheric plate forms a subduction zone. In subduction zone, the down going oceanic plate descends into the mantle and presence of water within oceanic sediments induces melting of down going slab (Fig. 15.7). If both plates are oceanic, volcanic flows of basaltic composition occur. Under this condition, an Island arc is produced where subduction occurs (Fig. 15.4a). The volcanic rocks found above subduction zones are mainly of the andesite series, when an oceanic crust subducts beneath the continental crust (15.4b). These include lavas with composition ranging from basalt through basaltic andesite to dacite and rhyolite. Virtually, all andesites are associated with a subduction zone where oceanic crust under thrusts in a continental plate or has done so in the past. A gradual change is observed in chemical composition of andesite, which is turning more potassic as well as richer in Cs, Rb, Ba, U and Th when moving towards inland from the subduction zone owing to increasing thickness of crust (Fig. 15.7). The igneous activities around convergent margin over Earth history are responsible for forming most continental crust and economic ore deposits. Fig. 15.7: Relationship between magmatism and tectonic plates. (Source: modified after Jain, 2014 and http://www.geologyin.com/2015/01/the-relationship- between-igneous-rocks.html). SAQ 1 a) List three types of plate boundaries. b) What types of the plates are involved in the formation of volcanic island arc? c) Which types of plates are involved in collision process? d) Deep focus earthquakes occur in ……………… zone. 111 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. 15.4 PROCESSES AT DIVERGENT PLATE BOUNDARIES Constructive plate boundary represents zone of divergence where there is continuous upwelling of molten rocks (i.e. lava) and thus, allowing new lithosphere to form from the newly came out magma (Fig. 15.8). This either occurs at the Mid-Ocean Ridges (the so-called centres of sea-floor spreading lie in the middle of the ocean as shown in Fig. 15.8) or at rifted continental margins, which represent rifts in a continent (Fig. 15.9). Fig. 15.8: Image of the Mid-Ocean Ridge along the divergent plate boundary in the Atlantic Ocean. It also shows transform faults and fracture systems associated with the Mid-Atlantic Ridge. (source; modified from Atlas des Mers et des Oceans, image, cinabrio over-blog.es, 2008, http://i66.serving.com/u/f66/11/66/76/31/a_1410.jpg). Fig.15.9: Continental rift along the divergent plate boundary. (Source: modified after Santosh and Omori, 2008b). UHT stands for ultra-high temperature. 112 Unit 15 Plate Boundary Processes................................................................................................................................................................. Let us discuss processes operate at divergent plate boundary. 15.4.1 Ocean Ridge Topography You are aware that divergent plate boundaries mainly occur on the Mid- Oceanic Ridges, which are having aggregate length over 65,000 km. Through these ridges plates move away in opposite directions from the ridge axes, thereby creating new oceanic lithosphere by exuding basaltic magma (Fig. 15.7). The new oceanic crust pushes aside in opposite directions (Fig. 15.10). Hence, the age of this diverging oceanic crust becomes progressively older in both directions away from the ridge. Thus, the morphology of ridges is controlled by the rate of separation. Spreading rates at different points around the Mid-Ocean Ridge system vary widely. For example, the spreading rate of the Southwest Indian Ocean Ridge is less than 20 mm/year. It is not surprising therefore that many of the essential characteristics of the ridges, such as topography, structure, and rock types, vary as a function of spreading rate (Fig. 15.10). (a) 113 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. (b) Fig. 15.10: Ocean ridge topography: a) The Mid-Atlantic Ridge showing the youngest crust at spreading margin and the oldest near the continents (Source: https://commons.wikimedia.org/wiki/File:Atlantic_bathymetry.jpg and https://www.ngdc.noaa.gov/mgg/image/2minrelief.html); and (b) showing the control of sea-floor spreading rate in the development of morphology of the ocean-floor. (Source: modified after Olive and others, 2016). Ridges form long and linear uplifted features of the Earth’s ocean basins and are regions of shallow focus earthquakes (Kearey and others, 2009). The Mid- Oceanic Ridges represent elongated belt of undersea global mountains, which are 2 to 3 km high above the ocean-floor. Topographically, they are from 1000 to 4000 km wide, but width is considerably more in East Pacific Rise. Their topography is quite rugged and orients parallel to the crests. The axes of ridges generally lie parallel to the spreading margins. It is to be noted that spreading margins in ocean basin covers a length of about 55,000 km. Therefore, they form a continuous chain of underwater mountains and occur in all oceans. For example, the Mid-Atlantic Ridge is ridge of the Atlantic Ocean, Pacific-Antarctic Ridge and East-Pacific Rise are ridges of the Pacific Ocean, and Central Indian Ridge, Carlsberg Ridge, Southeast Indian Ridge and Southwest Indian Ridge are ridges of the Indian Ocean. 15.4.2 Generation of Oceanic Lithosphere New oceanic lithosphere is constantly being produced at the Mid-Ocean Ridges and is recycled back to the mantle at subduction zones. Therefore, oceanic lithosphere is much younger at the Mid-Ocean Ridges than its continental counterpart. As newly formed lithosphere moves away from an oceanic ridge, it gradually cools and heat flow decreases away from constructive plate boundaries (Figs. 15.10a and 15.11). Exuding lava at the Mid-Ocean Ridge axis cools and turns into hard rock and becomes permanently magnetised in the 114 Unit 15 Plate Boundary Processes................................................................................................................................................................. direction of the Earth’s magnetic field at the ridge axis (Fig. 15.11). With change in magnetic polarity of the Earth, the exuding material also records the change. This creates a symmetrical pattern of magnetic stripes of opposite polarity on either side of Mid-Oceanic Ridges. These patterns of stripes, which are studied by a palaeomagnetism, provide the history of sea-floor spreading. Fig. 15.11: Schematic showing magnetic polarity pattern along the Mid-Oceanic Ridge formed by upwelling of magma coming from the mantle. (Source: modified from https://divediscover.whoi.edu/mid-ocean-ridges/magnetics- polarity/) 15.4.3 Rift Valley The initiation of divergent plate boundary in a continental region is marked by the development of rifting, basaltic volcanic eruption and upliftment (Fig. 15.9). During rifting, the continental crust is stretched and thinned, thereby producing shallow-focus earthquakes on normal faults. A rift valley or graben is formed. These normal faults act as a pathway for basaltic magma to raise from the mantle and erupt on the surface as cinder cones and basalt flows. It is to be noted that during continental rifting, the thinned crust breaks into two parts separated by an oceanic ridge that finally, led to the formation of new ocean. A typical example of this includes the Red Sea rift zone located between East Africa and Arabian Peninsula, where the process of creating new ocean is in its early stage. 15.4.4 Magmatism The magmatic processes broadly operate at three places such as Mid-Oceanic Ridges, rift valley and intra-plate setting. In case of the Mid-Ocean Ridges magmatism, basalts are formed by the eruption of magma. Basalt is characterised by porphyritic grains of plagioclase with rarely occurrences of olivine phenocrysts. Eruption under water produces pillow lava interspersed with rare sheet flows. The basalt is relatively high in magnesian (5–11% MgO) and Al2O3 (12–18%), and low in concentrations of K2O. The rift valley magmatism provides evidences of the upwelling of hot mantle beneath the crust and the surface gets elevated by the thermal expansion of 115 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. the surface rock due to warming from base. This fact was proved in the opening of the Red Sea, where the crust was initially stretched and thinned (Fig. 15.9). Here, numerous normal faults broke the crust and the surface subsides into a central graben. Shallow earthquakes and basaltic eruptions occur for example in the African Rift valleys in eastern Africa. The intra-plate magmatic processes usually associated with hot-spot volcanisms that are feeded by the vertically rising mantle plumes (Fig. 15.7). Magmatic/mantle plumes are nearly liquid, very hot, thermal features form due to increase in temperature either in the lower mantle or at the core-mantle boundary. The change in state from solid state to partial liquid creates instabilities and mass is forced to move upward. The plumes are characterised by a mushroom-shaped head and a long and thin centrally located tail. When a mantle plume head rises upward in upper mantle huge quantities of basaltic magma is generated. At the same time this also alters the upper mantle connective cells pattern and creates divergent environment that led to the initiation of continental break-up, rifting, drifting, uplift, spreading and massive volcanic activity. Plumes are unrelated to the upper mantle convection currents and play an important role in Earth’s tectonics. Plumes are having an independent motion and are not related upper mantle convection. The immobile, long-lived lower mantle upwelling sites on the surface constitute the hotspots and if a continent is passing over hotspot area then time progressive volcanoes trail is preserved. The hot spots themselves are stationary in position and as any continental plate moves over any hotspot, the magma will create a linear chain of volcanoes. Thus, hot spots are very useful to trace the past movements of the tectonic plates (Fig. 15.7). The Rajmahal and Deccan traps of India were created by hot-spots, namely Kerguelen and Reunion, respectively, during Cretaceous. 15.5 PROCESSES AT TRANSFORM FAULTS Transform faults are a variety of strike-slip faults, in which two plates move parallel to each other along fault plane either in opposite directions or in the same direction, but at different rates (Figs. 15.1 and 15.12). Transform fault margins are also called conservative margins because lithosphere is neither created nor destroyed. Transform faults on the basis of their locations described into two types: oceanic and continental. The shallow to rarely deep focus earthquakes may occur along transform faults, which sometimes cause large amounts of damage. It is to be noted that the 2012 Sumatran earthquake of 8.6 magnitude was the largest ever recorded earthquake for a strike-slip fault. 15.5.1 Oceanic Transform Faults Most of the oceanic transform faults are small length faults on the sea-floor that run from one segment of spreading ridge to the other (Fig. 15.12). When diverging plates break apart across their length, the shape of the break is preserved in the spreading ridge. The ridge quickly adopts the stair-step configuration, minimising overall energy (Fig. 15.12). Thus, in oceanic transform faults, one oceanic plate slips horizontally by another; lithosphere is neither created nor destroyed, but it is conserved. With a few exceptions, transform 116 Unit 15 Plate Boundary Processes................................................................................................................................................................. plate boundaries do not exhibit volcanoes. The oceanic transform faults are also sites of mineralisation, particularly the sea-floor massive sulfide deposits. Fig. 15.12: Oceanic transform faults. (Source: modified after https://scienceterms.net/geology/transform-boundary/). 15.5.2 Continental Transform Faults These faults occur on land. Because of the thickness of the continental lithosphere and its variety of rocks, transform boundaries on continents are not simple cracks, but they are wide zones of deformation (Fig. 15.13). The continental transform faults are generally accompanied by numerous other faults of variable size and type. As compared to the oceanic transform faults, the earthquakes in continental transform faults are of shallow focus types. They are more complex than their short oceanic counterparts. The San Andreas Fault system is a typical example of a continental transform fault (Fig. 15.13). Fig. 15.13: Continental San Andreas transform faults. (Source: modified after https://www.pinterest.com/pin/334603447284812196/) 117 Block 4 Plate Tectonics and Movements.................................................................................................................................................................. 15.6 OPHIOLITES AND THEIR EMPLACEMENT Ophiolite term is used for a group of rocks consisting of mainly igneous rocks, deep water fine grained sedimentary and very low grade metabasalt intruded with dykes. The ophiolitic assemblage is consisting of peridotite, chromite, dunite, mafic and ultramafic cumulates, massive gabbro, mafic sheeted dike complex, deep-sea sediments and shallow water/terrestrial sediments. This kind of group represents an ocean-floor setting or different oceanic crustal layers. Due to obstruc