Age Dating Rocks

Two Types of Dating Relative Dating Absolute Dating

Relative Dating

Relative and Absolute Dates Relative age: states whether one rock formation is older or younger than another formation. Absolute age: specific number of years. (example: like 150 million years ago.)

How old is the Earth??? People used to think that the earth was only a few thousand years old! Now we believe that the earth is billions of years old! video From: Greatest Discoveries with Bill Nye: Earth Science. Discovery Channel School (2005). Retrieved October 24, 2006, from unitedstreaming: http://www.unitedstreaming.com/

Which layer of rock is oldest? B C! C

Answers for Relative Dating Diagram (Oldest to Youngest)Deposition of A, B, C, D, E, F, G,Tilting of these strata; H (a fault which cuts across the strata); I (an unconformity produced by erosion); Deposition of J, K, and L; M (an igneous intrusion which cross cuts all older features); N (an unconformity produced by erosion); Deposition of P and Q;Both R (an igneous intrusion) and S (a lava flow) must come after Q but it is impossible to say whether R came before or after S since they do not cross cut; Deposition of T

Radioactive Decay

Radioactive decay is one of Earth’s sources for heat. Remember… Accretion Differentiation Radioactive decay Sun’s radiation

Before we get to crazy…

U 92 238 Uranium # of Protons Atomic Symbol Name of Element # of Protons + Neutons

Radioactive decay is the spontaneous change of the nucleus of an unstable isotope to a stable one. Isotope: an atom with the same number of protons, but a different number of neutrons. As protons and neutrons leave atoms, energy is produced.

When protons are lost during radioactive decay, the atom becomes a different element. Example: When Uranium-238 decays, it loses 32 particles (10 protons and 22 neutrons). What element is it now?

What element is it now? Lead (Pb)! What is its new atomic weight? (238-32) 206! So, the new product is Lead-206! Lead-206 is stable, so it won’t decay any further.

Parents and Daughters The original element is called the parent isotope, and the product of the decay is called the daughter isotope.

The moment an igneous rock crystallizes, radioactive atoms start to decay to a stable product.

Half life: The amount of time it takes for half of the parent to decay to the daughter. After one Half-life, half of the atoms of a radioactive element will have decayed to a stable product, and half will remain unchanged.

Half Life

A rock that has a lot of parent, but very little daughter is very young! A rock that has a lot of daughter, but very little parent is very old!

As the # of parent goes down, the # of daughter goes up!

Radioactive Dating

How we can get the age… Many minerals contain radioactive isotopes. 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.

Parent Daughter Half-Life 14C 14N 5730 years 3H 3He 12.8 years 40K 40Ar 1.25 Billion Years 87Rb 87Sr 48.8 Billion Years 238U 206Pb 4.5 Billion Years

100 g of 14C @ time=0   @ 1 half life, 50 g of 14C, 50 g of 14N So: 5730 years have passed @ 2 half lives, 25 g of 14C, 75 g of 14N So: 5730 (x2) or 11460 years have passed @ 3 half lives, 12.5 g of 14C, 87.5 g of 14N So: 5730 (x3) or 17190 years have passed Total Years Passed: 17190 years!

Lab: Radioactive Decay

Materials Jar 100 pennies

Procedure Obtain all the materials needed for the lab. The jar represents your rock. The pennies represent the minerals in the rock. Heads will represent parents, tails will represent daughters. Place all 100 pennies in the jar. Imagine that they are all heads-up (all parents). Dump the pennies onto the table and separate the parents from daughters. Count how many parents and how many daughters you have. Record the results on a data table. Place all the parents back in the jar, shake them, and dump them out again. Repeat procedures 3-5 until all the parents have changed to daughters, or until you have only one parent left. If you end up with only one parent left, put it back into the jar, shake it, and no matter what, imagine it changes to a daughter.

# of Half-lives (one half-life is 1000 years) Data # of Half-lives (one half-life is 1000 years) # of Parents # of Daughters 1 2 3 4 5 6 7 8 9 10 11 12…

Conclusion How many half-lives did it take for all your parents to change to daughters? How old is your rock when all the parents have changed to daughters? What did you learn from this lab?

Known Half-lives