Igneous rocks rarely contain fossils because the formation of a fossil requires sedimentation. A fossil results when the remains of a plant or animal are covered by sediment that hardens, forming the mold that eventually fills with minerals. Igneous rocks form under heat and pressure that destroy organic remnants.Continue Reading
For a fossil to form, the remains of a living thing must be buried quickly enough to prevent scavengers from destroying the carcass, and the layers of sediment must harden quickly enough to form a mold around the remains before they decay. When this set of circumstances is achieved, the result is a void in the sedimentary rock that approximates the shape and arrangement of the original remains. Water flows through the rock and deposits minerals into this void, eventually creating a stone representation of the original remains called a fossil.
Igneous rocks form when lava or magma cools, crystallizing into various types of stone. If lava were to flow over the remains of a plant or animal, it would burn away the organic tissue well before it cooled enough to solidify around the remains and create a fossil. Certain types of igneous rocks form from cooler pyroclastic material such as mud and ash, but even these materials are much less likely to create fossils than layered sediment.Learn more about Geology