“How do you know how old it is?”

Dating in Geology

• 1) Relative dating– a) Superposition– b) Correlation

• 2) Numerical (“absolute”) dating

– radiometrics

Miocene-Pliocenescallops

PaleoceneCalcareousNannoplankton

Permo-TriassicConodonts

Cretaceous Bivalves

Neogene Calcareous Nannoplankton

http://www.priweb.org/ed/ICTHOL/timescale.htm

Stratigraphic Concepts and Terminology

Lithostratigraphic Group Formation Member Bed

Biostratigraphic Biozones

Chronostratigraphic Rocks TimeErathem Era System Period Series Epoch Stage Age

Mesozoic Cretaceous Cenomanian

Paleozoic Devonian Givetian

Cenozoic Paleocene Thanetian

Using Index Fossils

Dating in Geology

• 1) Relative dating– a) Superposition– b) Correlation

• 2) Numerical (“absolute”) dating

– radiometrics

Radiometric Dating: the basic idea

• A product is detectable

• The rate of production of that product is known

• Product amount is 0 (or known) at age 0 of sample

• Product / Rate = Time– example: 10g / 0.5g/yr = 20 years

Isotopes: elements with the same atomic number but

different atomic mass (number of neutrons).

Some isotopes unstable (40K, 14C; radioisotopes);

others are stable (39K, 41K, 12C, 13C)

Unstable isotopes (parent) decay to another stable

or unstable isotope (daughter)

Rate expressed as half-life -- time in which ½ of

radioisotopes decay.

Half Lives for Radioactive Elements

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

Carbon 14 Nitrogen 14 5730 years

Measuring decay in half-lives

Measuring decay in half-lives

Parent Daughter Half Life Dating Range

_________________________________________________________________

Carbon-14 Nitrogen-14 5730±30 0100-70,000

________________________________________________________________________

Potassium-40 Argon-40

Calcium-40 1.3 billion 50,000-4.6 billion________________________________________________________________________

Uranium-238 Lead-206 4.5 billion 10 million-4.6 billion

Uranium-235 Lead-207 710 million Uranium-232 Lead-208 14 billion

________________________________________________________________________

Rubidium-87 Strontium-87 47 billion 10 million-4.6 billion

Minerals you can datePotassium 40 is found in:

potassium feldspar (orthoclase)

muscovite

amphibole

glauconite

amphibole

orthoclase

muscovite

glauconite

Minerals you can date

Uranium may be found in:

zircon

zircon

Combining Radiometric and Paleontological Dating

Dating in Geology

• 1) Relative dating– a) Superposition– b) Correlation

• 2) Numerical (“absolute”) dating

– radiometrics

http://www.priweb.org/ed/ICTHOL/timescale.htm

Dating in Geology

• 1) Relative dating– a) Superposition– b) Correlation

• 2) Numerical (“absolute”) dating

– radiometrics