If a plant dies and falls into a body of water or other place that is poor in oxygen, it falls to the bottom where it becomes buried by sediment. Over tens of thousands of years, more layers of sediment build up over the bottom adding more weight over the plant which raises the temperatures felt by the plant. All elements in the plant decay, except for carbon. This results in carbon film with little resemblance to the original plant except for the carbon material. Also known as carbonization or distillation, it is the most common manner in which plants become fossils.
Petrified wood occurs in one of two ways: perimineralization or replacement. Both processes share many similarities with the only difference being the amount of original matter left in the preserved product. Water seeps into microscopic holes in the structure of the wood of a plant. This water carries with it minerals which become deposited into the wood. Soft portions of the wood wash away with the water. Replacement dissolves most of the hard portions in the wood, and perimineralization leaves more of the wood intact while simply filling in the gaps with minerals from the water. Since more of the original piece of wood survives, more details of the structure of wood petrified through perimineralization are visible than wood petrified through replacement.
Impresses occur through authegenic preservation. When a plant dies, it falls into a soft material, the plant itself will decay, but an impression of the leaf or plant remains behind in the soft sand, ash or limestone. Over time, the soft material holding the impression can change to become stone. Most impressions are found in shale, ash and sandstone.
- Photo Credit Wikicommons: Woudloper: public domain, Wikicommons: CC License 3.0 SA BY: EncycloPetey, Wikicommons: CC License 3.0 SA BY: Yakudza, Wikicommons: public domain: Tillman
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