Absolute Dating Absolute Dating

Radioactive Half-lifeRadioactive Half-life

Superposition tells us Superposition tells us who is older but not the who is older but not the actual ages. actual ages.

To find the actual age To find the actual age of rocks we use of rocks we use radioactive elements.radioactive elements.

Objectives

Identify methods used for absolute dating

Describe the process of radioactive decay

Calculate the age of an material using known ½ life of a radioactive isotope

A natural clock is necessary

-- Radiometric dating (very accurate)(nuclear clock: decay of radioactive isotopes)

-- Dendrochrolonology: using tree rings to date (only useful to about 10,000 years ago)

Absolute time

-- Rates of Erosion or Deposition Good for relatively small time frames

(100-10,000 years, but not reliable for large time frames)Example: Niagara Falls…. We can measure how fast it erodes--- Varves: Light/dark or course/fine layers of sediments that vary annually (each year) due to summer and winter. These mainly form in glacial lakes.

Radioactive DecayRadioactive Decay

Isotope = atoms that Isotope = atoms that have gained or lost have gained or lost neutrons and whose neutrons and whose nucleus has become nucleus has become unstable unstable

Decay = when the Decay = when the nucleus of an isotope nucleus of an isotope releases energy and a releases energy and a particle (proton, neutron particle (proton, neutron or both) and becomes a or both) and becomes a new elementnew element

Half-LifeHalf-Life The amount of time it takes for one half The amount of time it takes for one half

of a radioactive substance to decay into of a radioactive substance to decay into a nonradioactive substance.a nonradioactive substance.

Half-life GraphHalf-life Graph# half-lives x half-life = age of the # half-lives x half-life = age of the rockrock

Carbon 14 Dating

Constant generation of C-14 in the upper atmosphere by cosmic particle bombardment of N (nitrogen).

Nitrogen (N-15) emits a proton and becomes C-14. This is radioactive with a half-life of about 5,730 years.

Plants and animals ingest a constant amount of radioactive C-14. When they die, the ingestion stops, and the radioactive C-14 clock begins to count down.

After death the C-14 in an organisms body decays. After one half-life

(5730 years), the amount of atoms gets cut in half.

One half-life (5730 years later) only one half of the

C-14 is left

Two half-lives (2 x 5730 = 11460 years later) only ¼

of the C -14 is left

Three half-lives (3 x 5730 = 17,190 years later) how

much C-14 is left?

1/8 of the C-14 is left

Four half-lives (4 x 5730 = 22,920 years later)

1/16 of the C-14 is left

Carbon 14 dating is only useful to date objects 60,000 years or younger because too little is left to measure accurately past this

point

Elements Used for ½ Life Elements Used for ½ Life datingdating Carbon-14 is in all living things but can only Carbon-14 is in all living things but can only

be used for things about 60,000 years oldbe used for things about 60,000 years old To age the Earth we use elements with To age the Earth we use elements with

longer half lives. longer half lives.

Radioactive Radioactive ParentParent

Stable Stable DaughterDaughter

Half lifeHalf life

Potassium 40 Potassium 40 Argon 40 Argon 40 1.25 billion yrs1.25 billion yrs

Rubidium 87Rubidium 87 Strontium 87Strontium 87 48.8 billion yrs48.8 billion yrs

Thorium 232Thorium 232 Lead 208Lead 208 14 billion years14 billion years

Uranium 235Uranium 235 Lead 207Lead 207 704 million years704 million years

Uranium 238Uranium 238 Lead 206Lead 206 4.47 billion years4.47 billion years

Radioactive Half Life Lab: Read the information in the book about radioactive half-life

and finding a date by decay (Page 193 – 196, also see figure 4).

The beans in this lab represent radioactive atoms. Put 100 beans into a cup. Shake the cup then dump and spread the beans out. Any bean with a painted side facing upward has radioactively decayed. Remove these beans from the pile and put them back in their bag. Record the number of beans remaining in the table on your handout

Put the remaining beans in the cup and repeat like this until all beans have decayed or until you have done 12 tosses.

Complete the graph and questions on the handout

Dating Igneous RockDating Igneous Rocko Some radioactive elements turn into Some radioactive elements turn into

a gas after they decay.a gas after they decay.o Any gases will escape from lava.Any gases will escape from lava.o All remaining radioactive elements All remaining radioactive elements

are trapped in crystalsare trapped in crystalso To find out how many half lives To find out how many half lives

remain we just have to compare the remain we just have to compare the amount of gas trapped to the amount of gas trapped to the amount of radioactive atoms leftamount of radioactive atoms left

Half-life QuestionsHalf-life Questions

A rock has 100 radioactive atoms and 100 daughter A rock has 100 radioactive atoms and 100 daughter atoms. How many half-lives have gone by?atoms. How many half-lives have gone by?

Answer = 1Answer = 1 A rock has 25 radioactive atoms and 175 daughter A rock has 25 radioactive atoms and 175 daughter

atoms. How many half-lives have gone by?atoms. How many half-lives have gone by? Answer = 3Answer = 3

If the half-life of Carbon-14 is 5,700 years and three If the half-life of Carbon-14 is 5,700 years and three half-lives have gone by, how old is the rock?half-lives have gone by, how old is the rock?

Answer = 17,100 yearsAnswer = 17,100 years