Constructive
Two plates are moving apart, which allows magma to rise up from the mantle to the earths surafce which creates new crust. Eg mid-alantic ridge. The plate movement at these boundaries causes volcanic eruptions and can also casue earthquakes due to friction of the plates as they move over the mantle. Crust on either side tneds to have cracks/faults in it caused by pressure of moving plates.
Destructive
Oceanic and continental plate collide, the oceanic plate is denser so it sinks beneath the continental plate (subduction). As it sinks it takes sea water with it which melts to form magma less dense than the mantle. It will rise up through the continental crust and explodes at the surface as a volcano. The trapped sea water turns to steam making the volcano very explosive. There`s not much lava but a lot of steam, ash and gas. As the oceanic sinks beneath the continental it creates friction, which can cause earthquakes. As the continental moves towards the oceanic, the land on the edge of the plate gets crumpled to form Fold Mountains. Eg Andes.
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