Learning Objective: I will be able to explain how we use fossils to date rocks.

Success Criteria: I can explain how fossils help us to date rocks. I can explain how index fossils differ from fossils. I can identify fossils on the ESRT and use them to date rocks.

Correlation Matching similar rock layers in different locations to see if they formed at the same time

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1. Which layers are the same? 2. Which layer is older E or F? 3. What is correct sequence of rock layers from oldest to youngest? 4. An unconformity is represented by the interface between which 2 layers?

February 19 Fossils Naturally occurring preserved remains or impressions of living things Generally only hard parts get preserved Bones, teeth, shells Hard parts are replaced by naturally occurring minerals Allen

Eurypterus NY State Fossil Silurian index fossil

Which fossil might be found in Devonian rock layers?

Fossils Why don’t fossils exist in most igneous or metamorphic rock? February 19 Fossils Generally found in sedimentary rock layers Why don’t fossils exist in most igneous or metamorphic rock? Allen

February 19 Fossils The living organism that made the fossil lived during the time the rock layer was forming i.e. when the sediment was being deposited Allen

Fossils Fossils can provide information about ancient environments Marine fossils=marine environment, wooly mammoths= cold environment, etc…

Fossils A study of the fossil record shows: February 19 Fossils A study of the fossil record shows: A great variety of plants, animals and simpler life forms have lived on Earth in the past Life forms have evolved through time Most life forms have become extinct Allen

Index Fossils Index Fossils--used to date rock layers (strata) Fossils from creatures that existed for a geologically short period of time Ex. less than 2.0 x 107 Fossils from creatures that had a wide geographic distribution

Which letter would make a good index fossil? Location A Location B Location C Rock layer 1 W W Z Rock layer 2 W Z Y Z Rock layer 3 W X X X Z Which letter would make a good index fossil?

Volcanic Ash Falls Can also be used to correlate rock layers over a large area Ash is a good indicator because: The ash from one explosion has distinct characteristics ash can be deposited around the globe The event occurs at one, geologically brief, time

Summary: Success Criteria Recap I can explain how fossils help us to date rocks. I can explain how index fossils differ from fossils. I can identify fossils on the ESRT and use them to date rocks.

Absolute Dating

When an unstable radioactive element changes into a stable element Radioactive Decay When an unstable radioactive element changes into a stable element

Example Unstable Radioactive Isotope Stable Decay Product Carbon 14 (C14) Nitrogen 14 (N14) Uranium 238 (U238) Lead 206 (Pb206) Parent Daughter

Radiometric Dating Half life: The time required for 1/2 of a parent material to break down to daughter material

Another look

Radiometric Dating The age of objects can be determined by measuring the relative amounts of radioisotope and decay products

Radiometric Dating The half lives of radioisotopes vary depending upon the isotope Examples: C14 = 5,770 years U238 = 4,500,000,000 years Rb87 = 47,000,000,000 years

Carbon 14 is used to date biological remains Carbon is incorporated into the cells of living organisms and begins to decay when the organism dies Uranium 238 is used to date rocks Larger half life

Half Life Practice Problems

If half the K -40 of a rock has decayed into Ar-40, how old is the rock? How many years would it take for 75 % of a sample of K-40 to disintegrate?

A sample that has ½ c-14 and ½ n-14? How old are: A sample that has ½ c-14 and ½ n-14? A sample that has ¼ K-40 and ¾ Ar-40? A sample that has ½ U-238 ½ Pb-206?

How old are: A sample that has 1/8 Rb-87 and 7/8 Sr-87? A sample that has ¼ C-14 and ¾ N-14? A sample that has ¾ C-14 and ¼ N-14?

What fraction remains of: February 19 A 50 billion year old sample has ____ Rb-87 and ____ Sr-87. A 2.6 billion year old sample has____ K-40 and ____Ar-40. Allen

Stranz should take a break ;) This is so boring Stranz should take a break ;)