IGNEOUS ROCK is molten rock that flows from volcanoes as lava.  It is also formed from the molten MAGMA in the MANTLE of the earth.  The mantle is the layer beneath the crust of the earth's surface.  As the magma moves upward into the  crust, it cools forming rock and crystals of various types.  In other words, igneous rock is formed by crystallization or solidification of magma.  Igneous rocks make up most of the rock on the earth's crust, about four-fifths of it.

Igneous Rock may be Intrusive or Extrusive.  Intrusive igneous rocks form by the solidification of magma below the Earth's surface. Extrusive igneous rocks form by the solidification of magma on the Earth's surface.