Transcript (radioactive decay).

RADIOACTIVE
DATING
CH 10-3
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
•
Radioactivity was first
discovered by Henri Becquerel
in 1896 and Polish-French
chemist Marie Curie discovered
that radioactivity produced
new elements (radioactive
decay).
•
Ernest Rutherford first
formulated the law of
radioactive decay and was the
first person to determine the
age of a rock using radioactive
decay methods.
Marie Curie
Ernest Rutherford
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
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The number of protons (the atomic number) is fixed for any element and is
unique for each element but the number of neutrons in atoms of different
elements can vary. Atoms of an element having different numbers of
neutrons are referred to as the isotopes (of that element).
M&W4 Fig. 3.3; M&W5 Fig. 3.4
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
Radioactive decay occurs when an isotope of one element is transformed
into a different element by changes in the nucleus. There are three
different decay mechanisms:
“Parent”
“Daughter”
M&W4 Fig. 17.18; M&W5 Fig. 17.18
Radioactive Dating
When the atoms of one element break
down to form atoms of another element.
How can we tell age based on
the number of parent isotopes?
Radioactive isotopes “decay” at a particular rate. We
express this rate as the “HALF-LIFE”, which is the time
it takes for HALF of the parent isotopes to decay.
Half Life
The amount of time it takes for half of the
radioactive atoms to decay.
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
•
For radioactivity dating we use igneous rocks and minerals. The clock
starts when radioactive atoms that are present in the magma get
incorporated in the crystalline structure of certain minerals in the rocks.
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The crystals containing the parent atoms form and so we then have a
“container” with parents that can begin decaying to form daughters.
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We can then use measure the parent-daughter ratio. This is our “atomic
clock” that records the time since the rock crystallized.
M&W4 Fig. 17.21; M&W5 Fig. 17.21
GEOLOGIC DATING: ABSOLUTE AGE DETERMINATION
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To the oldest materials ever dated by the radioactive method are
found in the Jack Hills of western Australia and are tiny zircon grains
contained in sandstones and conglomerates. The zircons are 4.4
billion years old.
Scanning
electron
microscope
image of a
Jack Hills
zircon. Scale
bar is 0.1 mm
The very remote
“outback” of western
Australia--the Jack
Hills
Examples of Radioactive
Changes in Rock
Carbon 14
- 5,730 years changes to Nitrogen
 Potassium 40
– 1,3 billion years changes to Argon
 Uranium 238
– 5.5 billion years changes to Lead

Carbon 14 Dating
C14 is an isotope of carbon that
forms from Nitrogen in the
atmosphere. Living things
consume this radioactive
carbon.
Once dead, no new carbon is
absorbed, and C14 turns back
into Nitrogen.
The Half-Life of C14 is 5,730
years.
This method works best for
fossils younger than 50,000
years. Why?
(end)
M&W4 Fig. 17.24; M&W5 Fig. 17.24
The End!