In historical terms the difference between neptunism and vulcanism

is that neptunism is a discredited scientific theory of geology, held by many scientists around the late 18th century and early 19th century, that rocks were formed from the crystallisation of minerals in the early Earth's oceans while vulcanism is the belief, held mostly in the 18th century, that rocks were formed in fire. This theory was a rival theory to Neptunism.

As a noun vulcanism is

an alternative spelling of lang=en.