Fossils and Radiometric Dating

Fossil Formation

Fossils: Evidence of Past Life

Fossils: remains or traces of prehistoric life. Only found in sedimentary rocks.

The type of fossil that is formed is determined by the conditions under which an organism died and how it was buried.

Fossil Formation Unaltered Remains

• Some remains of organisms—such as teeth, bones, and shells—may not have been altered, or may have changed hardly at all over time.

Fossils: Evidence of Past Life

Fossil Formation

Fossils: Evidence of Past Life

Altered Remains• remains of an organism that changed over time.

• Types: petrified or turned to stone, molds, casts, Carbonization, or Amber

Fossil Formation

Fossils: Evidence of Past Life

Indirect Evidence• Trace fossils are indirect evidence of prehistoric

life. Like animal tracks, worm burroughs and gastroliths.

Conditions Favoring Preservation• Two conditions important for preservation: rapid

burial and the possession of hard parts.

In the late 18th century an English engineer named William Smith found that fossil weren’t just randomly distributed throughout rock layers, but that each layer contained specific 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.

Fossils and Correlation

Fossils: Evidence of Past Life

fossil succession: fossils succeed one another in a certain order. Any time period can be recognized by its fossils.

Index fossils: widespread geographically, are limited to a short span of geologic time, and occur in large numbers.

Overlapping Ranges of Fossils

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 link existing rock formations around a region or around the world there is only one reliable way to get numerical dates of the past, that’s using radioactivity.

Radioactivity

Dating with Radioactivity

Radioactivity: the spontaneous decay of unstable atomic nuclei.

Common Types of Radioactive Decay

Half-Life

Dating with Radioactivity

A half-life is the amount of time necessary for one-half of the nuclei in a sample to decay to a stable isotope.

Radiometric Dating

Dating with Radioactivity

radioactive isotopes decay at a constant rate since their formation

Radiometric dating: calculating the absolute ages of rocks and minerals that contain radioactive isotopes.

Radiometric dating uses radioactive elements in minerals of rocks.

Radiometric Dating As a radioactive isotope decays, atoms of

the daughter product are formed and accumulate.

Dating with Radioactivity

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 decays, atoms of the daughter product are formed, and measurable amounts of lead eventually accumulate.

Uranium-238

Lead-206

4.5 by 9 by 13.5 byFormation of rock 18 by 22.5 by

*Uranium-238 has a half live of 4.5 billion years.

Other radioactive elements are also used to date rocks. Uranium-238, 235, Thorium-232, Rubidium-87, Potassium-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.

Dating with Carbon-14 Radiocarbon dating: determining age by

comparing the amount of carbon-14 to the amount of carbon-12.

Dating with Radioactivity

After death, the amount of carbon-14 gradually decreases. By comparing the ratio of carbon-14 to carbon-12, radiocarbon dates can be determined.

Radiocarbon dating: determining age by comparing the amount of carbon-14 to the amount of carbon-12.

Since carbon-14 has a half-life of 5730 years, geologic events up to about 75,000 years ago can be dated using radiocarbon dating.

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 4 billion years old. Meteorites have been dated at 4.6 billion years old.

Using Radiometric Methods to Help Date Sedimentary Rocks

Radiometric dating has supported scientists like Hutton and Darwin who inferred that geologic time must be immense. Modern dating methods have proved that there has been enough time for the processes we observe to have accomplished tremendous tasks.