ABSOLUTE AGE
RIVER EROSION RATES
SEDIMENT DEPOSITION
VARVE COUNTS
RADIOACTIVE DECAY
Absolute vs. Relative Age
•Relative dating tells us which rocks
are older by comparing position…
•…it does not tell us age in actual
years.
•Absolute dating assigns an age in
years; it gives rocks and fossils
“birthdays!”
Erosion Rates of Rivers
•Niagara Falls
erodes 1.3 meters
per year.
•…age is 9,900
years!
•Only works with
“younger” features.
Deposition of Sediment
•Rivers deposit
sediment into
oceans or lakes.
•Rates of
deposition can be
inconsistent over
time and thus
inaccurate.
Varve Count
• Sediments are
deposited yearly in
glacial lakes.
• Spring melt brings
coarse (large) sediment
first which settles on
bottom; fine (small)
material lands on top.
• One year = one layer of
big & small sediment.
Varve Count
…one band of dark & light = one year
Radioactive Decay
Radioactive Decay
•Radioactive elements act as
geologic clocks.
•Radioactive decay: the loss of
particles and energy from unstable
(radioactive) elements.
•Radioactive elements (such as
uranium) decay at their own
unique, constant rate to form new
elements…
Radioactive Decay
•Once that rate is known, scientists
compare the amount of original
radioactive material (“parent”) in a
rock sample to the amount of newly
created element present (“daughter”)…
•…and the age of the sample can be
determined by plotting amount versus
time on a logarithmic graph!
Half-Life
•Definition: the time it takes for
half of a radioactive sample to
decay to a different stable
element.
•Example: it takes 10 g of U-238
4.5 b.y. to “become” 5 g of Pb206 and 5 g of U-238.
Half-Life
Radioactive Parent Stable Daughter
Half life
Potassium-40
Argon-40
1.25 billion yrs
Rubidium-87
Strontium-87
48.8 billion yrs
Thorium-232
Lead-208
14 billion years
Uranium-235
Lead-207
704 million years
Uranium-238
Lead-206
4.47 billion years
Good to Know…
•The number that appears with the
element name (U-238) is the atomic
mass number of the element…the total
of protons & neutrons found in the
nucleus.
•During decay, an element that loses
neutrons becomes an ISOTOPE of that
element (like U-236).
•When that element loses protons, it
becomes a different element!
Good to Know…
•Radioactive elements become
concentrated in minerals found in igneous
rocks (granite, gabbro) which make up
continental crust.
•So…radiometric dating is not useful for
determining the ages of sedimentary rock
(you would simply be finding the “birthday”
of the igneous rock that got “changed” into
the sedimentary rock…)
Carbon Dating
Problem…
•Uranium, Potassium, & Rubidium
work great with very old (more
than 10 million year-old) ROCKS…
•…but what about younger rocks
and objects not containing
radioactive elements (like onceliving things)?
Carbon Dating
•Carbon dating is used for younger
samples that contain carbon such
as organic material and some
sedimentary rocks.
•The half-life of C-14 is 5,730 years.
•Works for wood, bone, shells, early
humans up to 50,000 years old.