How Plate Tectonics Works Figure 6 and the text below explains the causes of crustal plate movements. It therefore explains how plate tectonics works.
Moving coastal plates Oceans
Dividing line betwen lithosphere and ‘slushy’ part of upper mantle
Plates separate
Plates collide E
B Mantle C
Hot spot
A D
Plates collide F
Core 5 The causes of crustal plate movements
l Parts of the earth’s upper mantle are so hot that rock there takes the form of semi- molten or ‘slushy’ magma. But some parts of the upper mantle are even hotter than other parts. These extra hot parts (such as the place labelled A in Figure 5) are sometimes referred to as hot spots. The magma in these hot spots is less dense and so tends to rise slowly towards the lithosphere.
l When the rising slushy magma approaches the solid lithosphere it is forced to spread out horizontally. As it does so it creates enough tension to eventually tear the lithosphere apart. This causes the earth’s crust to break up into plates that diverge (separate from each other), for example at the place labelled B.
l Temperatures near the lithosphere are cooler than they are deeper down in the mantle. The horizontally moving magma near the lithosphere must therefore eventually cool down. When it does so, it becomes denser and sinks down into the mantle (at C and D) and eventually returns to its original position. In this way, a circular convection current of magma is completed.
4
l Different convection currents move towards each other beneath some parts of the lithosphere. Where this happens, plates converge (move together) and collide with each other. This happens at the places labelled E and F.
Lithosphere
Convection current
Convection current
Convection current
Convection current
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