How is a volcano formed? There are four types of plate boundaries – ___________ (when two plates move away from each other), ___________ (when an oceanic and continental plate move towards each other), ___________ (when two plates slide past each other) and ___________ (when two continental plates crash into each other). Although ___________ happen at all of the plate boundaries, volcanoes only happen at destructive and constructive plate boundaries. Destructive plate boundaries Firstly, the continental crust and __________ crust move towards each other. As the oceanic crust is _______ _______or SUBDUCTED beneath the continental crust the pressure is increased. As well as pressure increasing, friction also increases. This means that the ___________ of the rock goes up. As the crust continues down it heats up even more and begins to _________. The molten rock collects in a _________ chamber. As the pressure increases the molten rock begins to force its way upwards through the _______ ___________. Eventually it breaks through the crust and a volcano __________ at the surface. As time goes on, more eruptions happen and the volcano builds up __________ of rock. This is repeated for millions of years and so a volcano is __________. Constructive plate boundary Volcanoes happen at this type of plate boundary because two plates move ___________ from each other. As they separate, molten rock (magma) rises from the __________ to fill the gap between the two plates. This forms a mid-___________ ridge. A good example of a constructive plate boundary can be found where the North American plate is moving away from the Eurasian plate. This has caused the Mid-___________ ridge to form and has created Iceland through volcanic activity. These volcanoes tend to be more _____________ eruptions than those that happen at the destructive plate boundaries. What are the different types of volcano? A volcano is called __________ if it has not erupted for thousands of years, and its shape has been worn away by the wind and rain. If it has not erupted for years, or even centuries, but still seeps gas sometimes, it is known as a ___________ (sleeping) volcano. However, the ones we need to worry about are the __________ volcanoes because these have erupted recently and are likely to do so again soon.