As we understand them now, igneous rocks are formed
when rocks are melted and then cooled. Magma can cool slowly inside the
crust and upper mantle of the Earth, forming rocks like granite. Granite
has large minerals that can be seen with the naked eye. These are called
plutonic rocks. Quick cooling magmas are generally erupted onto
the Earth’s surface, called volcanic rocks.
Igneous rocks are found where plates diverge,
as lava rises and fills the gap between the plates. Igneous rocks also
form where plates converge. The subducting plate melts as
it sinks into the crust of the Earth, and the melt rises into the
overriding plate forming volcanoes.