What is a Fossil? - Page 2 of 2
By Ed Strauss, Washington
The most common example of the cast type of fossil is petrified wood; trunks, limbs, or roots. They are created in a similar way as one would make a lead statue from a mold. First the original is encased in investment. Wood is encased in a pyroclastic flow or sediment. The model statue is encased in plaster of Paris. The wood rots or burns up leaving a hollow cavity just as cooking the plaster of Paris burns up the wax model inside leaving a cavity. Then the hollow cavity is filled. The wood mold is filled with silica and/or other minerals. The statue mold is filled with lead. Once the encasing material is removed we are left with a replica of the original.
By comparing fossils from different ages it is clear that different types of plants and animals change at different rates. Sharks are a good example of an animal that has not changed much over millions of years, while the horse is an example of one that has.
Dating fossils is usually done by dating the rock that surrounds them. Geologists have several methods of dating minerals or fossils. The most commonly known is carbon 14 dating. Carbon 14 is a radioactive form of carbon that is present in all living things and undergoes a relatively uniform change over time after the plant or animal has died. This change can be measured and thus the fossil can be dated. Since carbon 14 decays fairly rapidly (half of it decays every 5730 years), this method is only useful for dating relatively young fossils. Geologists can also date older volcanic rocks with more complicated methods that measure the changes in amounts of certain trace radioactive elements in those rocks. Again these changes are relatively uniform over time and can be used to measure the time since the rocks were formed.
What should I do if I find a fossil?
If it is on Federal land it is best to leave it where it is and report your discovery to the paleontology department of the closest University. If it is on private land and you have permission to collect there you should carefully record the location of your find and label your specimen so you will be able to tell others where you found it. This is because you might have found something very important to scientific research and you will want to be able to assist in its study. If the fossils you find are encased in rock it is much better to first find out if they are significant to science before attempting to remove them. The process of removing fossils from rocks is generally very difficult and time consuming. It would be a great loss to have an important fossil ruined by impatient, damaging extraction.
Fossils represent the living things of the past. It is how we study the history of changes that have occurred in the plant and animal kingdoms. Paleontology, the study of ancient life, is just as fascinating and important as the study of the history of mankind.
(Text copyright© 2001 by Ed Strauss. Petrified Wood From Western Washington All rights reserved.)
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