Age of Fossils Essential Standard 8.E.2.1 Unit A, 2.2

Warm-up How do ice cores tell scientists about Earth’s changing atmospheric makeup? How do fossils form? Take out your homework, pg. 51

Vocabulary Relative age Law of Superposition Index fossil Absolute age

Rocks provide Earth’s Timeline

I. Relative Age The age of an event or object relative to other events or objects. It does NOT tell actual age.

How old is Agnes? Mary was born first, then Agnes, then John.

II. Law of Superposition the older beds (layers of rock) are on the bottom and the newer are on the top.

Which layer is older the shale or sandstone?

Fault- Break in the layers of rock (rocks must be older than the fault).

Folded or Overturned Rock- Rock layers that were disturbed by plate movements, can reverse order of age.

III. Igneous Intrusions When magma intrudes other rocks, it must be younger than the rock affected.

IV. Unconformity When a layer of rock is missing from a sequence. Usually due to erosion. Can represent thousands to millions of years of missing time.

G, D, F, E, Fold, Erosion, C, H, B, A, Erosion, Fault

D, C, I, H, Fold, Erosion, B, A, F, Erosion, G, E, Erosion

Place events in order… Q, O, N, M,L, P, Tilt, Erosion, H, I, J, Erosion, K

V. Index Fossils Fossil associated with a particular span of geologic time Would not extend far vertically in layers. Are widespread (found across the entire globe)

VI. Absolute Age Tells the actual age of an event or object. Uses radioactive dating. Elements that form rock decay and become a different element, this is measured in half lives.

I. Half Life time required for half of the atoms in a sample of radioactive element to decay.

How many years will it take for two half-lives to pass? What % of C14 will remain at this point?

Answer If you can see in a rock sample there is one quarter C 14 and three quarters N 14, the sample must have gone through two half-lives. 5,700 x 2 = 11,400 The sample is 11,400 yrs old.

EXAMPLE You Try… If you can see in a rock sample there is 1/16 th of C 14 remaining, the sample must have gone through ________ half-lives and is ___________ yrs old.

Half-LifeFraction of C 14 Remaining Number of years /25,700 21/411,400 31/817,100 41/1622,800

Radioactive Element Half-lifeDecay Product Rubidium billion yearsStrontium-87 Potassium billion yearsArgon-40 Uranium billion yearsLead-206 Uranium billion yearsLead-207 Carbon yearsNitrogen 14 1.Which of the following elements has the longest half? Which has the shortest? 2.Which of the elements turns into lead-206? 3.If a sample had half of its original uranium-235 left, about how old would the sample be? 4.Why do you think scientists might use so many different kinds of radioactive minerals to date rocks?

Assignment/Homework Complete the worksheet on half lives. Read 53-59, questions 1-6. Quiz next class TEST ON 19 TH & 20 TH Bring Skittles to next class for a half life activity, will need 100 Skittles.