Fossils form in what type of rock

K-5 GeoSource. PDF version. Learn More. What is a fossil? How do fossils form? What effect does sediment size have on fossils? What kinds of fossils are there? Under what conditions do fossils form? The heat and pressure required to change, or metamorphose, rocks usually destroys any fossils. However, special circumstances do happen.

For example, fossilized shells and bacteria have been found in marble, which is metamorphosed limestone. The initial heat of igneous rocks would seem an impossible environment for fossil formation. But when ash from volcanic eruptions buries the surrounding area, the ash sometimes encapsulates organisms. Fossils of trees and shelled organisms like brachiopods sometimes occur in ash layers.

Cathryn Whitehead graduated from the University of Michigan in She has published numerous articles for various websites. Her poems have been published in several anthologies and on Poetry. Whitehead has done extensive research on health conditions and has a background in education, household management, music and child development. Related Articles The 5 Types of Fossils.

Kinds of Fossil Rocks. The Four Types of Fossils. What Is a Frozen Fossil? What Is a Petrified Fossil? Organic Sedimentary vs. Chemical Sedimentary Rock. Fossil Facts for Children. Since rapid burial in sediment is important for the formation of fossils, most fossils form in marine environments, where sediments are more likely to accumulate.

Fossils come in many types. Those that consist of an actual part of an organism, such as a bone, shell, or leaf, are known as body fossils ; those that record the actions of organisms, such as footprints and burrows, are called trace fossils. Body fossils may be preserved in a number of ways. These include preservation of the original mineral skeleton of an organism, mineral replacement chemical replacement of the material making up a shell by a more stable mineral , recrystallization replacement by a different crystal form of the same chemical compound , permineralization filling of empty spaces in a bone or shell by minerals , and molds and casts, which show impressions of the exterior or interior of a shell.

Chemical fossils are chemicals produced by an organism that leave behind an identifiable trace in the geologic record. Chemical fossils provide some of the oldest evidence for life on Earth.

Index fossils are used to determine the age of many deposits that cannot be dated radiometrically. An ideal index fossil lived during a short period of time, was geographically and environ- mentally widespread, and is easy to identify.

Some of the most useful index fossils are hard-shelled organisms that were once part of the marine plankton. Paleontologists use fossils as a record of the history of life. Fossils are also extremely useful for understanding the ancient environment that existed in an area when they were alive. The study of the relationships of fossil organisms to one another and their environment is called paleoecology.

Fossils are also the most important tool for dating the rocks in which they are preserved. Because species only exist for a certain amount of time before going extinct , their fossils only occur in rocks of a certain age. The relative age of such fossils is determined by their order in the stacks of layered rocks that make up the stratigraphic record older rocks are on the bottom and younger rocks on the top—a principle called the Law of Superposition.

Such fossils are known as index fossils. The most useful index fossils are abundant, widely distributed, easy to recognize, and occur only during a narrow time span. Exceptions to this rule occur when conditions favor rapid burial and mineralization or very slow decay.

The absence of oxygen and limited disruption of the sediment by burrowing are both important for limiting decay in those deposits where soft tissues are preserved. This fossil assemblage includes the remains of many animals with hard mineralized skeletons, such as echinoderms and brachiopods, but many of these fossils also preserve non-mineralized parts, such as tentacles, gut tracts, and soft appendages. See Chapter 1: Geologic History to learn more about accreted terranes.