About 34 million years ago, a redwood forest grew in an ancient valley alongside streams and meadows where Eocene mammals roamed. A complex of volcanoes located near the present-day town of Guffy, Colorado, periodically erupted, blanketing the landscape in ash and sending volcanic mudflows (lahars) down the slopes.
Fossilization (Unit 3) - Permineralization at Florissant
One day, one or more of the volcanoes near Guffey violently erupted, melting snow from its flanks and sending ash into the air and lahars down into the redwood forest of the ancient Florissant valley. The lahars were thick and moved at a great enough speed to uproot or tear down some trees and bury others.
The buried stumps were saturated with groundwater saturated with minerals from the ashfall.
Ash consists of thousands of microscopic particles of volcanic glass and minerals. Since volcanic glass is unstable at the Earth's surface, it slowly dissolves and enters the groundwater, which surrounds and penetrates the stumps. As time passed, the mineral-laden water eventually became supersaturated with silica, and opal and chalcedony began to form.
Opal and chalcedony began to precipitate along the cell walls. Sometimes this first generation of crystals may have continued to grow, completely filling the cell.
In many cases, multiple generations of crystals grew within the cell. The difference between each generation of crystals can often be seen under the microscope. Differences in the types of minerals and their impurities will also produce different colors that can be seen without a microscope.
A final generation of crystals sometimes completely filled each cell. If the process of crystallization completely encases the original cellulose, the cell wall may be preserved.
If the cellulose rotted away, all that was behind is the minerals. The voids were sometimes filled by another generation of minerals. Carbon remnants of the original cellulose are often present in petrified wood.