Fossils are most commonly preserved in which type of rock?

• A. Igneous
• B. Metamorphic
• C. Sedimentary
• D. Glassy volcanic rock

Answer: (C)

Fossils are most likely to be preserved when they are buried and protected by layers of sediment that settle out of water. Sedimentary rock forms from these accumulations, so it captures and encases organisms as the sediments lithify into rock. This environment is gentle enough to allow fossilization processes to occur—minerals can slowly replace organic material (per min er alization), leaves a carbon imprint (carbonization), or create molds and casts of shells and bones—so the details can be preserved over long timescales.

Igneous rocks come from molten material, and the intense heat and melting that accompany solidification usually destroy organic remains. Metamorphic rocks form under high heat and pressure that rearrange minerals and often obliterate any original fossil structures. Glassy volcanic rock is a fast-cooling form of igneous rock, sharing the same issue of high temperatures and rapid changes that typically prevent fossil preservation. So, while fossils can be found in some settings, sedimentary rocks are by far the most common and reliable setting for fossil preservation.