Name
Notes-Dating Rocks with Fossils and Radioactivity
Fossils are an important tool for interpreting the geologic past. _________________ are the remains or traces of prehistoric life. They are important components of sediment and sedimentary rocks.
The type of fossil that is formed is determined by the conditions under which an organism ________ and was buried. Some fossils remain unaltered where others can be changed. Some fossils get replaced by other minerals such as ______________________ wood. _______________ and casts can also reflect some of the structure of an organism. Sometimes a fossil can be preserved through carbonization or trapped in ____________ residues from ancient trees. Indirect fossil evidence can also be used to help determine the past. Animal tracks, worm
____________ and gastroliths can be found in the rock record. Two conditions are important for preservation: rapid burial and the possession of hard parts.
In the late 18 th
century an English engineer named William Smith found that fossils weren’t just randomly distributed throughout rock layers, but that each layer contained
______________________ fossils that didn’t occur in layers above and below them.
With Smith’s observation scientists came up with one of the most important principles in historical geology. The principle of fossil succession states that fossil organisms succeed one another in a definite and determinable order. Therefore, any time period can be recognized by its fossil content.
Once fossils were recognized as time indicators, they became the most useful means of correlating rocks of similar age.
________________ ______________are key to identifying ages. Index fossils are widespread geographically, are limited to a short span of geologic time, and occur in large numbers.
Fossils can also be used to interpret and describe ancient environments.
Scientists can tell if there was a shallow sea, or the temperature of the water based on the organisms that were living there.
While fossils are a good way to correlate or ___________ existing rock formation around a region or around the world there is only one reliable way to get numerical dates of the past, using _______________________. When nuclei of atoms are unstable, they spontaneously break apart, or decay, in a process called radioactivity.
All radioactive elements have a _____________________. A half-life is the amount of

time necessary for one half of the nuclei in a sample to decay to its stable isotope.
One of the most important results of the discovery of radioactivity is that it provides a way to calculate the ______ of rocks and minerals that contain certain radioactive isotopes. This procedure is called ___________________ ________________. Each radioactive isotope has been decaying at a constant rate since the formation of the rocks in which it occurs.
Let’s take Uranium for example. When magma cools and
Uranium exists in this magma once the rock solidifies the uranium starts to decay. As the uranium ___________, atoms of the daughter product are formed, and measurable amounts of ________ eventually accumulate. Other radioactive elements are also used to date rocks. Uranium-238, 235, Thorium-232, Rubidium-87, _________________-40 are also used. An accurate radiometric date can be obtained only if the mineral remained in a closed system during the entire period since its formation. To date recent events carbon-14 is used in ____________________ _________________.
Carbon-14 is in all living organisms. The stable isotope carbon-12 vs. the amount of carbon-14 can determine the age of the item being dated. Since carbon-12 has a half-life of 5730 years, geologic events up to about _________ years ago can be dated.
Radiometric dating methods have produced thousands of dates for events in Earth’s history. Rocks formed on Earth have been dated to be as much as ___ billion years old.
Meterorites have been dated at ____ billion years old. Radiometric dating has supported scientists like ________________ and Darwin who inferred that geologic time must be immense. Modern ______________ methods have proved that there has been enough time for the processes we observe to have accomplished tremendous tasks.