Transcript cbs The-Fossil-Record-1415-23htdjf

Dating Fossils & Rocks
Fossils are the petrified remains of onceliving organisms. There are a few kinds:
 Mineralization: the bone or tissue is
replaced by minerals (there are several
types).
 Molds and Casts
 Trace fossils: tracks and burrows.

How Old?
There are two types of dating…relative
and absolute.
 Relative Dating: Sediment builds up over
time…if the layers are undisturbed, the
older stuff is on the bottom (this is called
Superposition).
 You can’t get an actual time in years this
way, but you can say fossil A is older or
younger than fossil B.

Absolute Dating: Radioactive isotopes
are used to figure out an actual age in
years.
 Isotopes are elements that are radioactive
and unstable. They decay at a
measurable rate. When this happens,
they breakdown into another element.
 By measuring the ratio of isotope to
breakdown product, we calculate the age
of the rock.
 The time it takes for half of an isotope
sample to break down is called…half-life.

Some Isotopes Used for Absolute
Dating
Parent-------Daughter
 Carbon-14>Nitrogen: half-life= 5,730 years
 Potassium-40>Argon-40: HL= 1.3 bill. Yrs.
 Uranium-235>Lead-207: HL= 700 mill. Yrs


That means for example that if you had 100 grams of U235, after 700 million years had passed you’d have 50
grams left (along with 50 grams of Lead-207)
That also means…you can’t use C-14 to calculate the
age of dinosaurs fossils. Why? Cuz…it breaks down
too quickly! You can use other isotopes though
As the amount of Parent
Decreases the amount of Daughter
Increases!
Try This…

1.
2.
Carbon-14 has a half-life of 5,730 years.
If you had a 10 gram sample of C-14,
how many years would take for there to
be 2.5 grams left?
If you had a 20 gram sample of C-14,
how much would remain after 22,920
years?