Explain the process of Absolute Dating?

Question

Also,

Describe how fossils form in sedimentary rocks?

Answer 1

Absolute dating relies on radioactive decay of elements in a rock. When an igneous rock is formed it may contain elements that are a mixture of radioactive and non-radioactive atoms. The radioactive isotopes decay in some manner that results in either a new isotope or a new element altogether. The ratio (now) of the original radioactive isotope and the daughter product is determined chemically. When the ratio is known then the half-life curve for that isotope is used to determine the absolute age of the specimen. This procedure also works for previously-living but uncontaminated specimens containing carbon.

Fossils form in several ways. One way is by a track, trail, or impression. An animal runs (or slithers, or whatever) through mud, the mud is buried by sand or more mud, compacted, and cemented. Then it is raised back up (by further tectonic or other forces) to be exposed either naturally or artificially by man. This type of fossil is called a trace fossil. Other trace fossils are tunnels, borings (into bone or shell), tooth marks, and so on.

Shells and bone can be buried, and when the bone or shell decomposes a hole (called a mold) may remain. If the surrounding soft sediment becomes hard rock the remaining mold is another type of fossil. If a shell (such as a clam) is filled with sediment and then the shell itself is dissolve, leaving the material inside, it is called a cast. An impression of the cast in the sediment is an internal mold.

Shell, wood, or bone can be replaced by other, more resistant material, forming a replacement type fossil. This is a chemical replacement. Wood material replaced in this way is generally called petrified wood.

Shell material can undergo a physicochemical recrystallization process, wherein the original mineral making up the shell is still there, but the crystals making up the shell have grown larger. This usually results in some "blurring" of the shell features.

Finally, small animals may be trapped in amber or tar. I guess you would call such fossils entrapment type fossils.

So, there are several types of fossilization.