Tectonic Hazards Watch
The main reason is because the plates move against each other causing friction causing a tectonic hazard. The gaps that are left in within the plates allows the mantle to arise.
Have a look at Bitesize, they have quizzes you can complete and answers to your questions:
It's says must include the following words: convection currents, tectonic plates, plate boundaries, collision and energy
Where two plates converge (convergent boundary), one of three scenarios could take place. If two continental plates converge, one will subduct and create a rift valley where rift volcanoes can be found. Where two oceanic plates converge, one plate subducts and a volcanic island arc forms. Where one oceanic plate and continental plate converge, the oceanic will subduct as it is denser and this in turn produces a volcano over time. Young fold mountains are commonly seen at convergent boundaries and are formed via orogeny. Earthquakes are very powerful here due to energy released from the friction created during subduction, and magma is formed as subduction melts the plates.
Conservative boundaries are where the tectonic plates slide and jolt past one another laterally, with neither subducting. Incredibly powerful earthquakes form here due to sheer amounts of energy released via friction. At collision boundaries, the plates collide into each other forming fold mountains and this is also accompanied with violent earthquakes.