EARTH: What are fossils?

Fossils are the preserved remains of animals, plants, and other organisms that were once alive. The process of fossilization may vary according to external conditions and tissue type. Permineralization is a process of fossilization that occurs when an organism is buried. Water from the ground, lakes, or oceans seeps into the pores of organic tissue and forms a crystal
cast with deposited minerals.
In some cases the original remains of the organism completely dissolve or are otherwise destroyed. The remaining organism shaped hole in the rock is called an external mold. If this
hole is later filled with other minerals, it is a cast.
Replacement occurs when the shell, bone or other tissue is replaced with another mineral.
A compression fossil is a fossil preserved in sedimentary rock that has undergone physical compression.
Geochronology is the science of determining the age of rocks, fossils, and sediments. A variety of dating methods is used by geologists to achieve this.