Fossil wood designates the remains of plants from older geological periods, which after dying out found themselves quickly enough in a suitable environment, where their decomposition – rotting – was prevented. Such conditions subsisted in the distant past and still subsist today, for example in marshes, mud, silt, sand, volcanic sand and ash, resin, earthwax, tar …
The long-lasting physico-chemical process in which a dead organism transforms into a fossil is called fossilization.
In regard to the course and the final shape of the fossil, the following methods of fossilization are distinguished:
- carbonization or charring
- petrification
- impregnation
- incrustation
- mummification or desiccation
- conservation
Reconstruction of the Upper Carboniferous landscape on the basis of finds of fossil plants excavated at Grajski hrib (Castle Hill) in Ljubljana and in the hills of Posavsko hribovje. Author: Barbara Jurkovšek
Carbonization or charring
Carbonization or charring occurs in anaerobic conditions characterized by the absence of oxygen. Such conditions persist in swamps, silt, mud and pitch. Organic residues turn into carbon. Example: The remains of trees and other plants sink into mud. Diverse sediments accumulate on them. Long-time charring processes begin to take place at increased pressure and temperature, as well as in the absence of oxygen. From plant tissues, coal is formed.
In regard to the degree of charring, the following types of coal are distinguished:
- Lignite – its carbonization rate is low. It contains 60 to 70% carbon and 35 to 73% water. A type of lignite with preserved wood structure is called xylitol, which is suitable for processing.
- Brown coal – contains up to 77% carbon and up to 60% water.
- Black coal – the process of its charring is longer than in brown coal. It contains from 77 to 93% carbon and only up to 15% water. The original form of the plant and its internal structure is usually modified a great deal or even destroyed.
- Anthracite – coal with the highest degree of charring. It contains over 93% carbon and less than 4% water. It is crumbly and non-porous with generally indistinct structure of the original plant.
Petrification and impregnation
In these two related ways of fossilization, wood or a similar substance is impregnated by mineral solutions. Through time, minerals partially or completely replace the organic material of plant residues. The “wood” becomes harder and explicitly heavier – it petrifies. In spite of it, its structure and form may remain unchanged. The substitute- secondary minerals are usually non-metallic. These are most often calcite and quartz. Less often, metallic minerals occur as substitutes: pyrite, limonite, hematite, marcasite, etc.
Incrustation
Incrustation is a relatively rare but fast form of fossilization, which can be observed in nature. In the course of incrustation, plants or only their remains are most often laden with mineral substances in the form of a crust. This is usually calcite, which is excreted from liquid water at a temperature lower than 30˚C. In the resulting coating, also called calcite flowstone, mostly the surface structures of plants are preserved. Aragonite and quartz are deposited in this way less frequently, some other minerals even less than the above two minerals.
The resulting formation is called tuft, less often travertine or tuff. It can contain the remains of organisms or, in the event of organism going to ruin, retains only its external form, which is called a pseudofossil.
The thickness of the mineral deposit varies a great deal. In some cases, the amounts of biochemical sedimentary rock formed in this way are large enough even for industrial exploitation. Sufficiently compact tufa is fractured and as such used as a building or decorative stone.
Mummification or desiccation and conservation
Mummification or desiccation and preservation are less prevalent methods of fossilization. The first entails complete drying of dead organisms, while the second has to do with the preservation of organisms in an agent that prevented their disintegration. Conservation is enabled by: earth wax, bitumen, phosphorite, ice and amber. The latter is a product – the resin of ancient, predominantly Tertiary conifers and at the same time an agent in which many different organisms from past geological periods have survived.
