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DISTRIBUTION OF OCEANS AND CONTINENTS 33 (v) Caroline plate : Between the Philippine Transform Boundaries and Indian plate (North of New Guinea) Where the crust is neither produced nor These plates have been constantly moving over the globe throughout the history of the destroyed as the plates slide horizontally past each other. Transform faults are the planes of earth. It is not the continent that moves as separation generally perpendicular to the mid- believed by Wegener. Continents are part of a oceanic ridges. As the eruptions do not take plate and what moves is the plate. Moreover, it allalongtheentirecrestatthesametime,there may be noted that all the plates, without is a differential movement of a portion of the exception, have moved in the geological past, plate away from the axis of the earth. Also, the andshallcontinuetomoveinthefutureaswell. rotation of the earth has its effect on the Wegener had thought of all the continents to separated blocks of the plate portions. have initially existed as a super continent in theformofPangaea.However, laterdiscoveries How do you think the rate of plate reveal that the continental masses, resting on movement is determined? theplates,havebeenwanderingallthroughthe geological period, and Pangaea was a result of Rates of Plate Movement convergingofdifferentcontinentalmassesthat The strips of normal and reverse magnetic field were parts of one or the other plates. Scientists that parallel the mid-oceanic ridges help usingthepalaeomagneticdatahavedetermined scientists determine the rates of plate the positions held by each of the present movement. These rates vary considerably. The continental landmass in different geological Arctic Ridge has the slowest rate (less than 2.5 periods (Fig 4.4). Position of the Indian sub- cm/yr), and the East Pacific Rise near Easter continent (mostly Peninsular India) is traced Island, in the South Pacific about 3,400 km with the help of the rocks analysed from the west of Chile, has the fastest rate (more than Nagpur area. 15 cm/yr). There are three types of plate boundaries: Force for the Plate Movement Divergent Boundaries At the time that Wegener proposed his theory Wherenewcrustisgeneratedastheplatespull of continental drift, most scientists believed away from each other. The sites where the that the earth was a solid, motionless body. plates move away from each other are called However, concepts of sea floor spreading and spreading sites. The best-known example of the unified theory of plate tectonics have divergentboundariesistheMid-AtlanticRidge. emphasised that both the surface of the earth At this, the American Plate(s) is/are separated and the interior are not static and motionless from the Eurasian and African Plates. but are dynamic. The fact that the plates move is now a well-accepted fact. The mobile rock Convergent Boundaries beneath the rigid plates is believed to be Wherethecrustisdestroyedasoneplatedived moving in a circular manner. The heated under another. The location where sinking of material rises to the surface, spreads and a plate occurs is called a subduction zone. begins to cool, and then sinks back into deeper Therearethreewaysinwhichconvergencecan depths. This cycle is repeated over and over to occur. These are: (i) between an oceanic and generate what scientists call a convection cell continental plate; (ii) between two oceanic or convective flow. Heat within the earth comes plates; and (iii) between two continental from two main sources: radioactive decay and plates. residual heat. Arthur Holmes first considered Reprint 2026-27