As the percent of iron in an igneous rock increases, its density will increase. Igneous rocks are rocks with a formed by the solidification of molten lava or magma. Magma is molten rock which is formed inside the earth's core. There is a difference between igneous rocks that have formed under the earth's crust or as a result of volcanic activity. Igneous rocks that contain a high percentage of iron and magnesium are dark in colour. An example of an igneous rock is Basalt.