How Fossilization Occurs: From Bone to Stone

Marisa
4 days ago
2 min read

Fossilization, the transformative journey from living organism to rock is incredibly rare. Fossilization requires a perfect storm of conditions from the moment a living organism dies or is buried to encasing it or preserving it in rock.

Where fossils are formed

Fossil-formation begins when an organism dies, ideally in an environment conducive to preservation, like a riverbed or lake. Fossils usually form where dead organisms are quickly buried in sediment, such as on river floodplains, in lakes, deltas, shallow seas and even by volcanic ash. This does two things: rapid burial protects the organisms from scavengers and decay. And, environments such as muddy sea floors, swamps and still lake bottoms are low in oxygen. This slows decomposition and makes it more likely for fossils to be preserved.

How do bone and organic material turn into rock?

When organic remains are buried in mud, sand or ash for millions of years, minerals such as silica, calcite or apatite seep into the sediment and then into the bone and organic material. These minerals infiltrate the porous spaces within bones and shells and harden to form rock.

In the simplest terms, fossils get their different colors from the kinds of minerals that were in the ground where they formed. In iron-rich areas, the fossils can turn red, brown, or yellow. Phosphate often produces black fossils, while limestone results in grey-colored fossils.

Other complex geological, organic and chemical reactions also play a part. Material density, pressure, and pH can affect fossil color over the millennia. And, minerals seep into an out of fossils over millions of years which can also cause color changes.

One of the rarest and most beautiful types of fossils are those that have been preserved in opal. Dinosaur and plants fossils from Lightning Ridge, Australia are formed by silica to create extraordinarily beautiful opalized fossil casts as seen below.