There have been many speculations about our planet earth and its origins and composition. The age of the earth has been a question that has only recently been estimated as accurately as possible. Unlike the recent history of the earth, the very distant past is not on written record. So how do geologists know that the earth is approximately 4.5 billion old or more? How do they know the age of shells and other particles found in the earth’s crust? The secret mysteries of the earth were unlocked by the discovery of fossils, radioactive material dating of rocks and the discovery that the earth does move. (1)

The lore of fossils is attributed to its link to dinosaurs. However, fossils are much older than the age of the dinosaurs. Dinosaurs were extinct 65 million years ago and they ruled the earth for 100 million years before that. (2) Remember earth is estimated to be well over 4.5 billion years old. There is a lot of time for fossil growth and deposition before the age of the dinosaurs. Fossils that have an estimated age of 3.5 billion years have been found on all of the continents of the earth.

There are a few things about fossils that must be understood to be aware of what they mean in terms of geologic succession of the earth. Fossils are the remains of previously living organisms. Most of the fossils found that are related to the early history of the earth are fossils of extinct organisms. This ties into the Darwin theory on succession. Scientists have made detailed graphs or webs of the succession of many species that inhabit the earth today and traced them back to the organisms found in the bedrock of the earth. These webs show the theory of "survival of the fittest" as they observed that certain traits disappear along the line of succession that proved to aid the species in its livelihood. Another important fact to comprehend about fossils is that different fossils appear in rocks of differing ages because life on earth changed as time moved on. (2)

Fossils determine many different factors of the rocks and region they are found. The law of fossil succession states, "The kinds of animals and plants found as fossils change through time. When we find the same kinds of fossils in rocks from different places, we know that the rocks are the same age." (2) This leads us to look at index fossils. Index fossils are guides to certain geologic time periods. The index fossil is a form of life that only existed during that time period thus they are used to determine the age of the rocks that they are found in. (2) This method of dating rocks was actually discovered in the eighteenth and nineteenth centuries by the English geologist and engineer William Smith and the French paleontologists Georges Cuvier and Alexandre Brongniart. They discovered that rocks of the same age could be found miles apart with the same fossils in them. By carefully cataloging their observations of which fossils were found in which rocks, these men and other geologists were able to confer that rocks of the same age could be found on either side of the English Channel.

"Smith and others knew that the succession of life forms preserved as fossils is useful for understanding how and when the rocks formed. Only later did scientists develop a theory to explain that succession". (2)

Fossils are an integral part of the earth equation. Without them, we would not have a good representation or historical record of the great past of our planet. Although, they are not the only indicator used, radioactive dating of rocks and minerals are opening doors that were previously thought to be outside of the human reach.

The radiometric scale is a more recent development borrowed from the physical sciences and applied to geologic problems. The ability to use radioactive elements to date the materials that exist in the composition of rocks is due to the discovery of the natural radioactive decay of uranium in 1896 by Henry Becquerel, the French physicist. Followed shortly thereafter by Professor Boltwood in 1907, a radiochemist of Yale University, who applied the idea of using radioactivity to approximate the ages of ancient particles, rocks, and fossils. Professor Boltwood published the first list of geologic ages based on radioactivity. They have since been revised but his accomplishments were on the correct track.

Radioactive decay is a spontaneous process in which an isotope (the parent) loses particles from its nucleus to form an isotope of a new element (the daughter). An isotope is an atom of the same element with a differing atomic weight from the stable form of the element. The rate of decay is expressed in terms of an isotope's half-life, or the time it takes for one-half of a particular radioactive isotope to decay. Most radioactive isotopes have short half-lives and lose their radioactivity within a few days or years. There are some that have longer half-lives; they are used to determine the age of materials on the geologic scale. The parent isotopes and corresponding daughter products most commonly used to determine the ages of ancient rocks are carbon to nitrogen, potassium to argon, rubidium to strontium and uranium to lead. (1)

When using the method of radioactive dating, scientists must be extremely accurate. A small error in measurement can result in an estimated age of an organism or rock that could be many years or millions of years off. An example is if the measurements have an error of one percent, the estimated figure could be off by a million years too much or too little. Usually two or more methods of analysis are used on the same specimen of rock to confirm the results. (1)

Another interesting way to determine how the history of the earth panned out is the discovery that the earth is in constant motion. A German scientist names Alfred Wegner came up with the idea of continental drift while studying the similarities between the coastlines of South America and Africa. He found evidence of reptiles and ferns on different continents, glaciers that occurred where tropical forests prevailed currently and the same type of rocks on close continents that are separated by an ocean.(3) This discovery led to further examination and the discovery that the earth’s crust is composed of lithospheric plates. The theory of continental drift refers to the movement of 20 plates worldwide, of which nine are major plates.(3) This theory helps explain why some organisms occurred where they did in history and the possible formation of the earth and its composition.

The history of the earth can be thought of as a book. The fossils and rocks contained on the earth are the pages. Some are intact and others are still missing. The methods of index fossils, radioactive dating and continental drift are the chapters that help to explain this great mystery.

Sources

1. Geologic Time. http://pubs.usgs.gov/gip.geotime/contents.html July 1997.

2. Fossils, Rocks and Time. http://pubs.usgs.gov/gip/fossils/contents.html August 1997.

3. Plate Techtonics. http:// www.hcrhs.hunterdon.k12.nj.us/science/ptech.html

             January 2001