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Plate tectonics The Structure of the Earth A thin crust 10-100km thick mantle – has the properties of a solid but it can also flow. 1000 – 2500 o C core – made of molten nickel and iron. Outer part is liquid and inner part is solid. 4000 – 4700 o C The average density of the Earth is much higher than the crust, so the inner core must be very dense Tectonic theory People once thought that the oceans and the continents were formed by shrinkage from when the Earth cooled down after being formed. Alfred Wegener proposed something different. Consider Africa and South America: These continents look like they “fit” together. They also have similar rock patterns and fossil records. These two pieces of evidence led me to believe that there was once a single land mass. This is my TECTONIC THEORY. The plates Continental Drift What causes the plates to move? The lithosphere describes the crust and the upper mantle. These both move like plastic (even though they are solid). The move because of convection currents. As heat is produced from the core, it heats rock in the mantle. This rock rises as it is less dense When it reaches near the top of the mantle the rock starts to cool and will eventually become denser and start to sink. The lithosphere is moved by these convection currents Movements of the crust 1) Plates move past each other, causing earthquakes. These are called transform plate boundary 2) Plates move away from each other – a “constructive plate boundary”. The gap is filled with magma which cools to form basalt. This is called sea floor spreading. 3) Plates move towards each other – a “destructive plate boundary”. The less dense one slides underneath (“subduction”) and partially melts. This causes volcanoes and earthquakes. Transform plate boundaries The relative motion of the plates is horizontal. They can occur underwater or on land, and crust is neither destroyed nor created. Because of friction, the plates cannot simply glide past each other. Rather, stress builds up in both plates and when it exceeds the threshold of the rocks, the energy is released – causing earthquakes. Conservative Plate Boundary Why do most volcanoes occur around plate boundaries? Plates colliding (destructive) New Zealand examples Destructive Plate Boundary Forming mountains Where continental crust meets continental crust Both continental crusts are too light to subduct so a continent-continent collision occurs, creating especially large mountain ranges. The most spectacular example of this is the Himalayas. This is where India is now This is where India was millions of years ago The intense heat and pressure from this process causes the rocks to change structure into metamorphic rocks. Mountain formation Or… Plates moving apart (constructive) The space created can also fill with new crustal material sourced from molten magma that forms below. Divergent boundaries can form within continents but will eventually open up and become ocean basins. i. On land Divergent boundaries within continents initially produce rifts, which produce rift valleys. ii. Under the sea The most active divergent plate boundaries are between oceanic plates and are often called midoceanic ridges. Constructive Plate Boundary New Zealand examples