Volcano is a rupture in the crust of a planetary mass object such as earth 
That sounds technical. I know that magma spews out.
Hot volcanic as and gases escape to the air
They can cause a lot of natural pollution, that's for sure.
Volcanoes are found where tectonic plates are diverging
That's where there are so many in the Pacific!
That is true. They can be found tectonic plates are converging under water
They have a massive impact on the atmosphere, as well.
That is true. the gas and ash are environmental pollutants 
Yes they are, but they can be okay if in low enough concentrations.