Lesson 18 - Module E. Planetary Engineering: Mesozoic...

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Module E. Planetary Engineering: Mesozoic Tectonics Lesson 18: Plate Motion through the Last 200 Million Years Introduction It is now universally accepted that lithospheric plates are currently moving around on the surface of the Earth, and there is every reason to think that this has happened throughout Earth's history. Before we look at how plates have interacted through time, we have to examine the question of why plates move at all. Mantle Convection Model Most of us have seen the convection cells that develop within a pot of boiling water, and this is essentially the process that many early plate tectonics researchers envisaged as the main driving force for plate tectonics. It was thought that heat from deep in the Earth (possibly the core) caused large scale convection to occur within the mantle, and this underlying convection caused the individual plates, comprising rigid crust and upper mantle, to be dragged passively along on top of the convection cells. Figure E-20. Mantle Convection Model In this model, plates are carried passively on the tops of large convection cells in the mantle. Ridge Push Model An alternative model for the driving force for plate motion involves the mid-ocean spreading ridges. At such ridges, thin, young, newly-formed oceanic crust is thermally uplifted because there is a large amount of hot mantle material upwelling along the ridges. The idea is that the oceanic plates essentially slide off this uplifted zone under the simple force of gravity (i.e. they "slide downhill"). The oceanic plate is therefore being pushed from the back, and ultimately is pushed down a subduction zone. Figure E-21. Models for Plate Motion
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Pictured above are the three models explaining plate motion represented by the forces (coloured arrows) driving slab motion. Mantle Convection Model, grey arrows; Ridge Push Model (green arrows); and Slab Pull Model (blue arrows). Slab Pull Model The third possible driving force, and the one that is now thought to be the most important, is the Slab Pull Model. It is well known that when oceanic lithosphere is subducted to considerable depth (> 400 km) at a subduction zone, the elevated heat and pressure converts the rocks that make up normal oceanic crust. Oceanic crust material ( basalt ) is less dense than the underlying mantle, and therefore floats on the mantle. These are transformed into a rock called eclogite , which has the same elemental composition as basalt but is much denser than the surrounding mantle. This eclogitic material therefore forms a huge weight on the lower edge of the subducting plate, and as the eclogite sinks down into the mantle, it pulls the rest of the plate behind it . What Drives the Movement of Plates? Three hypotheses (models) have been proposed to explain plate motion:
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This note was uploaded on 11/18/2011 for the course EOSC 116 taught by Professor Randell during the Winter '09 term at The University of British Columbia.

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Lesson 18 - Module E. Planetary Engineering: Mesozoic...

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