Absolute and Relative
 A numerical (or "absolute") age
is a specific number of years, like
150 million years ago.
 A relative age simply states
whether one rock formation is
older or younger than another
formation.
The Geologic Time
Scale was
originally laid out
using relative
dating principles.
Absolute numerical dating takes
advantage of the "clocks in
rocks" - radioactive isotopes
("parents") that spontaneously
decay to form new isotopes
("daughters") while releasing
energy.
An unstable atom that
will give off energy
and decay into
another type of atom.
Decay of the parent isotope Rb-87 (Rubidium)
produces a stable daughter isotope, Sr-87
(Strontium), while releasing a beta particle
(an electron from the nucleus).
("87" is the atomic mass number = protons +
neutrons.
 Radioactive isotopes decay at constant rates
 The rates are different for each type of
isotope
 A graph of isotope decay over time is called a
decay curve
Many minerals contain radioactive isotopes.
In theory, the age of any of these minerals
can be determined by:
 1) counting the number of daughter
isotopes in the mineral, and
 2) using the known decay rate to calculate
the length of time required to produce that
number of daughters.
The amount of time it takes for
50% of the parent radioactive
isotope to decay to its stable
daughter isotope
 The radiocarbon dating method
has evolved into the most
powerful method of dating
fossils, artifacts and geologic
events up to about 50,000 years
in age.
 N-14 in the
atmosphere is
converted to C-14 by
cosmic radiation
 C-14 enters food
chain through
photosynthesis
 C-14 decays back to
N-14
 In order to date older fossils, scientists must use
other radioactive isotopes.
 The element potassium 40 (K40) is found in most
rock-forming minerals
 half-life = 1.25 billion years
 allowing measurable quantities of Argon 40 (Ar40)
to accumulate in potassium-bearing minerals of
almost all ages.
 Absolute Dating can only be performed in igneous
rocks, not sedimentary rocks
 Fossils older than 50,000 years cannot be dated,
their age must be estimated using the surrounding
rocks
 Most fossils are found in sedimentary rock layers
 Scientist then age the igneous rock layers above
and below the fossil to determine an age range
 The rate of decay (half-life), and the branching ratio,
of K-40 have not changed.
 The material in question lost all its argon at an
identifiable time, the reset time.
 No argon has been lost since the time the rock was
reset, or set to zero.
 No argon except atmospheric argon, with today's Ar-40
/ Ar-36 ratio, has been gained since the reset time of
the rock.
 No potassium has been gained or lost since the reset
time, except by decay.
 The ratio of K-40 to total K is constant.
 The total K, Ar-40, and Ar-36 in the material in
question can all be measured accurately.