1. The tree becomes soft and porous.
2. The water in the soil contains small amounts of minerals. Usually silica oxide is present. This is the building block of sand. Water with silica will trickle into the tree.
3. The grain, vessels and cell walls of the tree create a mould. Silica crystallizes and a thin layer of crystal is formed on the mould.
4. The plant material falls apart, but the silica stays behind and forms a stone replica.
The above step-by-step plan seems simple, but it is a delicate process. The combination of temperature, acidity and pressure has to be perfect so that crystallization occurs smoothly. Also, there has to be sufficient silica in the water.
How long does it take a tree to fossilize? That depends. Non-fossilized trees believed to be more than 10,000 years old have been dug up, while some trees have been known to fossilize after just 80 years. It can go even faster if you create optimal conditions. Recently, Japanese researchers hung poles in a hot, acid, silica lake near the Tate-yama volcano in Honshu. They pulled up the pieces of wood once a year. Amazingly, the wood started to fossilize after only seven years. But this is nothing compared to American scientists of the Pacific North West Laboratory in Richland (USA).
They soaked pieces of wood for two days in hydrochloric acid and another two days in a silica solution. They then baked the wood for two hours at a temperature of 1,400 degrees Celsius in an acid-free oven. The fossilized wood was ready in a few days.