Radioactive Half-life Absolute Dating Radioactive Half-life

Superposition tells us who is older but not the actual ages. To find the actual age of rocks we use 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

Absolute time A natural clock is necessary -- 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. -- Dendrochrolonology: using tree rings to date (only useful to about 10,000 years ago) -- Radiometric dating (very accurate) (nuclear clock: decay of radioactive isotopes) 4

Radioactive Decay Isotope = atoms that have gained or lost neutrons and whose nucleus has become unstable Decay = when the nucleus of an isotope releases energy and a particle (proton, neutron or both) and becomes a new element

Half-Life The amount of time it takes for one half of a radioactive substance to decay into a nonradioactive substance.

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

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. 8

After death the C-14 in an organisms body decays After death the C-14 in an organisms body decays. After one half-life (5730 years), the amount of atoms gets cut in half. 9

One half-life (5730 years later) only one half of the C-14 is left 10

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 12

Four half-lives (4 x 5730 = 22,920 years later) 1/16 of the C-14 is left 13

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 14

Elements Used for ½ Life dating Carbon-14 is in all living things but can only be used for things about 60,000 years old To age the Earth we use elements with longer half lives. 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

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 16

Dating Igneous Rock Some radioactive elements turn into a gas after they decay. Any gases will escape from lava. All remaining radioactive elements are trapped in crystals To find out how many half lives remain we just have to compare the amount of gas trapped to the amount of radioactive atoms left

Half-life Questions A rock has 100 radioactive atoms and 100 daughter atoms. How many half-lives have gone by? Answer = 1 A rock has 25 radioactive atoms and 175 daughter atoms. How many half-lives have gone by? Answer = 3 If the half-life of Carbon-14 is 5,700 years and three half-lives have gone by, how old is the rock? Answer = 17,100 years

1. An isotope has a half-life of 30 years. If a rock started with 1 1. An isotope has a half-life of 30 years. If a rock started with 1.0 grams and 90 years goes by, how many grams of the parent isotope will be left? 1.0g 0.5g0.3g0.15 2. A 2.5g sample of an isotope was formed by the explosion of an atomic bomb at Johnson Island in the Pacific ocean, in 1960. The half-life of this element is 28 years. How much of this sample will still be remaining in the year 2016? In the year 2072? 2.5g 1.3g.07g 0.07g0.04g0.02g