The diagram below shows how magma pushes up between the two plates:
The magma chamber is a collection of magma inside the Earth, below the volcano.
The main vent is the main outlet for the magma to escape.
Secondary vents are smaller outlets through which magma escapes.
The crater is created after an eruption blows the top off the volcano.
An eruption occurs when pressure in the magma chamber forces magma up the main vent, towards the crater at the top of the volcano. Some magma will also be forced out of the secondary vent at the side of the volcano.