The term fossil describes a wide range of natural artifacts. Generally speaking, a fossil is any evidence of past plant or animal life that is preserved in the material of the Earth's crust. But when most people talk about fossils, they mean a specific subsection of this group -- fossils in which the shape of the animal or plant has been preserved, while the actual organic matter of its body is gone. These amazing remnants, which date to prehistoric times, were formed very slowly by dynamic geological processes.
In most cases, the fossilization process began when a plant or animal died and was quickly covered with sediments, usually at the bottom of a body of water. The loose sediments protected the bodily remains from the elements, bacteria and other forces that cause weathering and decay. This slowed the decaying process down so that some of the remains (in most cases, only hard material like bone or shell) were preserved for thousands of years. During this time, sediment layers continued to collect above the bone. Eventually, these sediment layers became hard, solid rock.
Sometime after this hard rock layer formed, water percolated down through the rock and washed the preserved remains away. Since the rock above was hard and rigid, it didn't fall down into the empty space where the remains used to be. This empty space formed a natural mold of the animal, perfectly preserving the shape of the original remains.
In some cases, percolating water carried minerals into the mold. These minerals hardened to make a natural cast of the form, just as an artist might make a sculpture cast by filling a mold with plaster. All the original organic material disappeared, but nature left a precise mineral reproduction of the plant or animal remains. In cases where minerals did not fill the mold, paleontologists may fill it themselves, creating an artificial cast.
This is just one scenario of fossil creation, of course -- there are all sorts of other ways nature might form a fossil. A lot of prehistoric insects, for example, have been fossilized in amber. This sort of fossilization occurred when the insect was enveloped in the liquid sap from a tree. Just like the sediments at the bottom of a body of water, the sap material protected the insect from decay and eventually hardened. Animal fossils are also found in tar pits, bogs, quicksand and volcanic ash.
Another interesting fossil type is petrified wood. Petrified wood generally forms when trees fall into a river, where they become saturated and then buried in mud, ash, silt and other materials. Minerals, such as the silica in volcanic ash, seep into the tree and fill in tiny pores in wood's cells. This changes the overall composition of the wood, turning it into stone material, while preserving its original structure. The variety of minerals in petrified wood creates striking vivid colors.
In addition to fossilized plant and animal bodily remains, paleontologists study fossilized animal footprints and trails, and even fossilized animal dung (called coprolite). These fossils are enlightening because they reveal something about how prehistoric animals moved and what they ate.
The fossil record, the total collection of fossils in the world, is extraordinarily important to our understanding of the Earth's history. Fossils tell us which plants and animals existed in prehistoric times, and where they lived. They also tell us something about when they lived. Based on the position of fossils in the layers of the Earth's crust, paleontologists can determine which animals predate other animals and which animals lived at the same time.
Using carbon dating, paleontologists can sometimes estimate the age of fossils. This provides the age of the rock layer where the fossil was found, which helps scientists date all the other material at that level. Without fossils, we would have a much more incomplete picture of Earth's early history.