Answer:
whether they were formed by cooling above or below the earth's surface.
Step-by-step explanation:
Given that Igneous Rock is a form of rock which is characterized by its formation from the cooling and solidification of magma or lava. Again, Igneous rock can either be Extrusive or Intrusive.
However, igneous rocks are considered as extrusive, when basically they are formed from lava on the surface of the earth.
On the other hand, igneous rocks are considered as intrusive, when basically they are formed from magm below the surface of the earth.
Hence, Igneous rock is considered extrusive or intrusive based on "whether they were formed by cooling above or below the earth's surface."