Radioactive Decay

During radioactive decay, the atoms of one element break down to form atoms of another element.

- Radioactive Dating

- Radioactive Dating

The half-life of a radioactive element is the amount of time it takes for half of the radioactive atoms to decay.

- Radioactive Dating

Geologists use radioactive dating to determine the absolute ages of rocks.

What percentage of a radioactive element will be left after three half-lives? First multiply 1/2 three times to determine what fraction of the element will remain.

You can convert this fraction to a percentage by setting up a proportion:

To find the value of d, begin by cross-multiplying, as for any proportion:

1 X 100 = 8 X d d = d = 12.5%

- Radioactive Dating

Practice Problem

What percent of a radioactive element will remain after five half-lives?

3.125%

- Radioactive Dating

- Radioactive Dating

The age of a sedimentary rock layer can be determined relative to the absolute age of an igneous intrusion or extrusion near the sedimentary rock.

Main Idea

Detail Detail Detail

As you read the section “Determining Absolute Ages,” write the main idea in a graphic organizer like the one below. Then write three supporting details that further explain the main idea.

Using radioactive dating, scientists can determine…

the absolute ages of the most ancient rocks usingpotassium-40.

the absolute ages of fossils up to about 50,000 years ago using carbon-14.

the ages of sedimentary rocks by dating the igneous intrusions and extrusions near the sedimentary rock.

- Radioactive Dating

- The Geologic Time Scale

Because the time span of Earth’s past is so great, geologists use the geologic time scale to show Earth’s history.

As you read, make a flowchart like the one below that shows the eras and periods of geologic time. Write the name of each era and period in the flowchart in the order in which it occurs.Geologic Time ScalePrecambrian Time

Paleozoic Era: Cambrian Period

Paleozoic Era: Ordovician Period

Paleozoic Era: Silurian

Paleozoic Era: Devonian

Paleozoic Era: Carboniferous

Paleozoic Era: Permian

Mesozoic Era: Triassic

Mesozoic Era: Jurassic

Mesozoic Era: Cretaceous

Cenozoic Era: Tertiary

Cenozoic Era: Quaternary

- The Geologic Time Scale