火山灰
Volcanic ash is particularly suitable for preservation of wood, because large quantities of silica are released as the ash weathers. The presence of petrified wood in a sedimentary bed is often an indication of the presence of weathered volcanic ash. Petrified wood can also form in arkosic sediments, rich in feldspar and other minerals that release silica as they break down. The warm supermonsoon climates of the Carboniferous through Permian periods seem to have favored this process. Preservation of petrified forests in volcanic ash beds is less affected by climate and preserves a greater diversity of species.