Transcript Document

Finding Absolute Age
Radioactive Decay
-Nuclei of some isotopes are unstable
-The parent material decays and
gives off particles (decay products)
http://www.physics.carleton.ca/~watson/1000_level/Recent_Physics/Gifs/Uranium_decay.gif
Radioactive isotopes
-found in the minerals that
crystallize when rocks form
-as time goes on amount of parent
material ___creases and amount of
decay products ___creases
- Decay rate of each isotope is known
amountof parentisotope
- Can use the ratio
amountof decay products
to determine age
This method of determining age is called
radioactive dating.
Half-Life
Half Life – The amount of time
required for half the radioactive
nuclei in a sample to decay.
Isotopes used for Finding Ages
-Carbon-14 only present in organic
(once living) material-can’t be used for
igneous rocks (not in lava)
-no contamination (chemical weathering)
Argon 40 is a gas-can escape.
-Isotopes with short half-lives good
for only “young” rocks
-Those with long half-lives good for
only “very ancient” rocks
Do “Review 12.4” on page 274 of
text.
For igneous rocks (eg. granite), and
metamorphic rocks, radioactive
dating shows the age of the rock
since the crystals actually formed.
(eg. cooling of magma)
Sedimentary Rocks are formed
from the weathered grains of much
older rocks. Radioactive dating gives
the age of the older rocks, not the
sedimentary ones!
Do Activity 12F on pages 275-277
of Textbook.