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How Fossils Are Formed

Fossilization is the process that preserves the remains or traces of ancient life in rock. It is an exceptionally rare event, as most organisms decompose quickly after death. For fossilization to occur, a dead organism must be protected from decay, which usually means it needs to be buried rapidly by sediment such as sand, mud, or volcanic ash. This quick burial shields the remains from scavengers, bacteria, and weathering. The hard parts of an organism, like bones, teeth, and shells, are much more likely to become fossils than soft tissues like skin or muscles.

Once buried, the remains undergo changes over millions of years as the surrounding sediment slowly turns into rock. The most common type of fossilization is permineralization. In this process, groundwater rich in dissolved minerals seeps into the porous parts of the buried remains, such as the hollow spaces in bones. These minerals gradually crystallize out of the water, filling the empty spaces and eventually replacing the original organic material. The result is a stone copy of the original object, preserving its shape in remarkable detail.

However, permineralization is not the only way fossils can form. An entire organism, including its soft tissues, can be preserved under specific conditions. For example, insects can be trapped in amber, which is hardened tree resin. Larger animals, such as mammoths, have been found frozen in ice in Siberia. Another type of fossil is a mold or cast. A mold is an impression of an organism left in sediment, while a cast is created when that impression is filled in with other minerals. Each of these fossil types offers unique insights into Earth's ancient history.