Fossils form when the remains or traces of plants and animals are preserved in rocks over millions of years through a series of physical and chemical processes.

Main Steps of Fossil Formation

• The process usually begins when an organism dies and its remains are quickly buried by sediment such as mud, sand, or volcanic ash. Rapid burial reduces the chances of decomposition and scavenging.

• Over time, more sediment builds up and compresses, eventually hardening into sedimentary rock. This protects the remains from further damage or decay.

• The soft tissues of most organisms rot away, leaving behind only hard parts like bones, teeth, shells, or wood.

• Minerals carried by groundwater can then penetrate the buried remains. These minerals crystallize, filling the cellular spaces and replacing the original material through a process called permineralization or petrification. Alternatively, the original hard parts may dissolve, leaving a cavity (mold); minerals later fill this cavity, forming a cast.

• In rare cases, organisms are preserved whole by being trapped in substances like amber, tar, or ice.

• After millions of years, geological processes like erosion, mountain building, or earthquakes can expose the fossil to the Earth's surface for discovery.

Types of Fossilization

• Permineralization: Minerals fill spaces within the remains, turning them to stone.

• Mold and Cast Formation: Hard parts dissolve, leaving a mold; minerals fill the mold, creating a cast.

• Replacement: Minerals replace the original materials cell by cell.

• Carbonization: Only a thin residue of carbon remains, leaving a detailed impression.

• Amber, Ice, or Tar Preservation: Organisms can be trapped in tree resin, ice, or natural tar, preserving their tissues in great detail.

• Pyritization and Silicification: Minerals like pyrite or silica replace parts of the organism, preserving its form.

Key Factors for Fossil Formation

• Rapid burial and protection from decay and scavengers are crucial for fossilization.
• Hard parts (bones, shells, teeth) are much more likely to fossilize than soft tissues.

• Most fossils are found in sedimentary rocks, which are ideal for preservation due to their formation by gradual sediment layering.

Fossil formation is a rare event, which is why only a small proportion of ancient life is preserved for scientists to study today.