What are volcanoes 
    and how are they formed

To put it simply, a volcano is a place in the earth where hot magma for the earth's mantle
reaches the surface of the earth and becomes lava. As I stated earlier, tetonic plates covering
the earth shift around. At a convergent boundry, an ocean plate slide under a plate supporting
land mass. As the ocean plate gets subducted, it goes into the earths mantle and turns into
magma. The pressure of the plate can also push magma already in the mantle to the earths surface,
thats how volcanos are formed.