Plate Tectonics

 Tectonic plates move or float on top of the upper mantle.

 However they do not float freely.  The plates are forced in specific directions by the flow of magma beneath.

Plate Tectonics – cont’d

 Plates move with the flow of magma.  The magma closer to the core rises heats and then towards the surface as its density decreases.

 Once the rising magma reaches the lithosphere it moves in opposite directions.  The magma forms convectional currents .

Plate Tectonics – Convection Currents

Divergent Boundaries



Tensional Forces

occur where two tectonic plates are pulled

apart

. The tension is created as the plates move away from each other. 

Ridge Zones

sometimes occur where two plates move apart. The

magma rises

the plates and forms a

ridge.

between  Again caused by

the magma convectional currents in

Tensional Forces – Ridge Zones This diagram above shows “Sea Floor Spreading”

Where’s the TENSION?

Convergent Boundaries



Compressional Forces

occur where two tectonic plates come together. They compress against each other.



Subduction Zones

sometimes occur where compressional forces result from two plates colliding and one plate

slips under

the other.

 Again caused by

convectional currents in the magma

Compressional Forces – Subduction

Where’s the Subduction / Compression?

More about Plate Tectonics



our text p. 10 - 12

 

watch it at: http://www.pbs.org/wgbh/aso/tryit/t ectonics/intro.html