In the case of North America and for want of a better example lets discus here the great petrified forests of Arizona. These once great tropical forests experienced a cataclysmic event. Huge trees fossils like the great sequoias of North America today eighty to sixty metres tall were felled and carried off by a great flood. The flood swept the trees away for a great distance, maybe 150-100km eventually depositing thousands of large branchless trunks to a place which is now a desert region. In prehistoric times this was a wetland. The tree trunks had been pretty much stripped of all branches in the rolling action of the flood.
The next process and at some point after now partially decayed palaeontologists theorise massive volcanic eruptions took place spewing millions of tons of ash into the atmosphere and down on top of the recumbent forest. The ash layer was several metres deep, as much as eight or nine metres, smoothing the the tree trunks, this action took the oxygen out of the environment, wether at that time a shallow lagoon or wetland. This anaerobic environment, or in water an anoxic state, without or with little oxygen restricts micro organisms that normally colonise and break down organic matter. These cannot survive without oxygen and therefore do not get to work to eat away and decay the soft rotting timber.
Now the process of petrification can really begin in earnest, the absorption of minerals. Some common silica minerals involved in petrifaction include quartz, calcite, siderite (iron carbonate), and apatite (calcium phosphate), silica’s are amongst the most common minerals in the earth crust. The trees tissues are impregnated by the heavy mineralised water solution, cell by cell the water passing through deposits minerals which eventually crystallise in the tissues of the tree.
The final stage is when the cells at the molecular level change and metamorphosize, the remaining organic material, the wood tissues chemically change, the wood ossifies, petrification is taking place, eventually the petrification of the wood is complete and hardens into stone.This is fossilisation and creates from once soft tissues of living organisms a cast in stone, which displays varying colours which denote the types of mineral content, for instance red colouration in quartz typically means a concentration of Irons.
The image below from our online catalogue illustrates how amazing the transformation of living organic material to petrified stone can be. This amazing tree trunk from Madagascar displays the once living bark, the many pores and branchlet points, along with the natural creases of the bark clearly seen in their original three dimesional form. The green colouration of the bark is due to the mineralisation process and may well denote a heavy consentration of copper, chromium or cobolt.