SCIENCE NEWS

How to Determine Geologic Ages

Relative age dates

&

Numerical (absolute age) dates

Absolute age dating

• Enables scientist to determine the numerical age of rocks and other objects

• Measure the decay of radioactive isotopes

Radioactive Decay

Radioactivity: spontaneous changes (decay) in the structure

of atomic nuclei

Radioactive Decay

Radioactive decay: the emission of radioactive particles and the resulting change into other

isotopes over time

Review • Nucleus– Protons: positively charged particles with mass– Neutrons: neutral particles with mass– Electrons: negatively charged particles, orbit nucleus

• Atomic number– An element’s identifying number– Number of protons in the atom’s nucleus

• Mass number– Sum of protons & neutrons in atom’s nucleus

• Isotope– Variant of the same parent atom– Differs in the number of neutrons– Different mass number than the parent atom

Radioactive Decay

• Radioactive isotopes emit nuclear particles at a constant rate regardless of pressure, temp

or physical changes

• Change of protons with each emission the original isotope (parent) is gradually

converted into a different element called (daughter)

• See text p 601 figure 21.13

Radiometric Dating

Radiometric Dating: Method scientist use to date an object by radioactive isotopes

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• Types of radioactive decay•Alpha emission– Emission of 2 protons & 2 neutrons (alpha particle) – Mass number is reduced by 4 and the atomic number

is lowered by 2•Beta emission– Electron (beta particle) is ejected from the nucleus– Mass number remains unchanged and the atomic

number increases by 1•Electron capture– Electron is captured by the nucleus– Electron combines with a proton to form a neutron– Mass number remains unchanged and the atomic

number decreases by 1

Radiometric Dating

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Radiometric Decay

Electron Capture

AlphaEmission

BetaEmission

Half-Life

Half-life: measured length of time it takes for one half of the original isotope to decay.

After one half-life 50% or parent remains with 50% of daughter.

After two half-life one half of the remaining 50% of the parent decays result is 25% parent and 75%

daughter.Text p 602 figure 21.15

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Radioactive Decay Curve

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Radiometric Dating

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• Sources of error•To avoid potential problems, only fresh, unweathered rock samples should be used•Not useful for dating sedimentary rocks

Radiometric Dating

What do you think??Explain why radiometric dating is not

useful for sedimentary rocks?

Minerals in sedimentary rocks were formed in preexisting rocks. Their

dates provide the ages of the older rocks not the ages of the sedimentary

rocks

Radiocarbon dating

• Carbon-14 (radiocarbon) dating• Half-life of only 5730 years• Used to date very recent events• C14 is produced in the upper atmosphere• Used to determine the age of all living things

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Other Dating Methods

Shell GrowthRings

TreeRings

Rhythmic Layering in Glaciers

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Dendrochonology : Tree ring counting

Youngest Older

Oldest

Can you think of any other methods?

ANY QUESTIONS?