Best Answer: Igneous rocks form from a melt of molten rock (magma, or lava) and can be Plutonic (intrusive) or Volcanic (extrusive). Plutonic rocks: * Generally form/solidify underground.Most commonly within magma tubes. * Have relatively large crystals.xtrusive rocks form from lava at or above the ground, and intrusive rocks form from magma below the ground. Granite is intrusive, pumice is extrusive or this: Igneous Rock, rock formed when molten or partially molten material, called magma, cools and solidifies.