Radioactive elements decay at a certain constant rate and this is the basis of radiometric dating. But, the decay elements need to be set, much like you would re-set a stop watch for a runner, to ensure an accurate measurement. When minerals get subducted into the Earth and come back as volcanic magmas or ash, this essential re-sets the radiometric clock back to zero and therefore a reliable age date is possible. Sedimentary rocks may have radioactive elements in them, but they have been re-worked from other rocks, so essentially, there radiometric clock has not been re-set back to zero.
Geologic Age Dating Explained
Absolute Ages of Rocks | Earth Science
Key concepts Absolute Dating The problem: By the mid 19th century it was obvious that Earth was much older than years, but how old? This problem attracted the attention of capable scholars but ultimately depended on serendipitous discoveries. Initially, three lines of evidence were pursued: Hutton attempted to estimate age based on the application of observed rates of sedimentation to the known thickness of the sedimentary rock column, achieving an approximation of 36 million years. This invoked three assumptions: Constant rates of sedimentation over time Thickness of newly deposited sediments similar to that of resulting sedimentary rocks There are no gaps or missing intervals in the rock record.
Why is it difficult to date sedimentary rocks using radiometric dating techniques?
Problems in Dating Sedimentary Rocks Sidebar 3 Stratified sedimentary rocks and their included fossils confront us with important concepts relating to the history of the earth. Are sedimentary rocks and fossils millions of years old? Radioisotope dating of sedimentary rocks might be supposed to provide the answer. However, the radioisotope dates that have been obtained by standard geochronological practice have been recognized to be doubtful because: 1. For example, radiogenic parent 40K is often uncommon in sedimentary rocks, or the radiogenic daughter 40Ar is lost from open mineral structures.
You need special minerals that do not add or loss elements easily during the different processes that a rock may unde … r go. A carbon-based life form acquires carbon during its lifetime. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition.