#### Transcript Radioactive Decay

```SECTION 10:3
Decay
 During
decay, the atoms
of one element
break down to form
atoms of another
element.

The half-life of a radioactive element is the amount
of time it takes for half of the radioactive atoms to
decay.
DETERMINING ABSOLUTE AGES

Geologists use radioactive dating to
determine the absolute ages of rocks.
PERCENTAGES

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%
PERCENTAGES

Practice Problem

What percent of a radioactive element will remain after five
half-lives?

3.125%
DETERMINING ABSOLUTE AGES

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.
IDENTIFYING MAIN IDEAS

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.
Main Idea
Using radioactive dating, scientists can determine…
Detail
Detail
Detail
the absolute ages of
the most ancient
rocks using
potassium-40.
the absolute ages
of fossils up to
ago using carbon14.
the ages of
sedimentary rocks
by dating the
igneous intrusions
and extrusions near
the sedimentary
rock.
SECTION 10:4:
DETERMINING
ABSOLUTE AGES
- The Geologic Time Scale
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.
- The Geologic Time Scale
SEQUENCING

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 Scale
Precambrian Time
Paleozoic Era: Permian
Paleozoic Era: Cambrian Period
Mesozoic Era: Triassic
Paleozoic Era: Ordovician Period
Mesozoic Era: Jurassic
Paleozoic Era: Silurian
Mesozoic Era: Cretaceous
Paleozoic Era: Devonian
Cenozoic Era: Tertiary
Paleozoic Era: Carboniferous
Cenozoic Era: Quaternary
```