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Absolute Dating: Grab a book and read pages 656-658 and answer the prelab questions
Radiometric dating
Radiometric dating of rocks
• Marie Curie 1900’s• Discovered all Rocks have
Radioactive isotopes:unstable atoms
that decay, release energy
and change into different element over a specific period of time
Half-life – the time for half of the radioactive nuclei to decay
Rubidium-87 47 billion yearsPotassium-40 1.3 billion years
Radiometric dating can be done, but only reliably on IGNEOUS ROCKS and volcanic ash
5. Carbon-14 dating• All living things contain
carbon• Carbon-14 is the radioactive
form (“evil twin”) of carbon-12
• It’s half-life decay rate is 5730 years.
• Organic materials less than 70,000 yrs old can be dated this way.
The radioactive decay curve
First half life
2nd half life
“Parent” is the radioactive isotope that decays
“Daughter” is new atoms that have formed from the decay of the parent
Half-life time is constant
Practice
A radioactive isotope has a half-life of 10,000years.
The percent of parent atoms left is 25%
How OLD is the rock containing the radioactive material?
1 half-life = 50% remains, age of rock is 10,000 years old2 half-life = 25% remains rock is ANOTHER 10,000 yrs. oldSo……. 20,000 years old!
Radioactive Half-life (Candy) Lab
Parentisotope
TotalNumber decayed
Start #of
atoms 48 0
1st half life
2nd half life
3rd half life
PercentParent remaining
Age of rock
100%________
24 24 50%
12 36 25%
6 42 12.5%
If the half life of this radioactive isotope was 3000 years, how old is this rock?
3000 yrs old
6000 yrs. Old
9000 yrs. old
Directions: determine the AGE of your
rock using radioactive atoms in the rock
1. Remove any “mutant” atoms that do not have a letter on them.
2. Put radioactive “parent” atom’ in the box facing UP.
3. Count the number of atoms to start. Record on data table
4. Put lid on and gently toss to decay the atoms.
5. Remove the “daughter” atoms that have ‘flipped over’ (decayed). Count and record the number of remaining atoms in the box .
6. Continue until all the atoms have ‘decayed’
7. Determine the age of your rock using the half-life time.
Make your prediction
Half-lives that have occurred
Predict The # of “parent”
(radioactive) atoms that will remain after the next half-life (shake)
Actual # of “parent” (radioactive) atoms that REMAINING in the box
after the half-life (shaking)
Percent % of “parent” atoms remaining
Age of your rock
0 50 50 100% 0
1
2
3
4
5
6
Half-life of the rock is _____200_____________ years
Dating sedimentary strata using radiometric dating
Figure 11.21
Which type of rock has been dated:Igneous, sedimentary of metamorphic?
How old is the Dakota Sandstone?
How old is the Wasatch Formation?
1) How does Sedimentary Rock form?
2) What are the 3 types of Sedimentary rock?
3) How can you distinguish between the 3 types of sedimentary rock?
Clastic rocks NON-ClasticChemical Organic rock
Hint: They can all form in water!
Because sediment is sorted by size in waterParticle Size determines Environment that
the rocks would be found
Boulders, cobbles, gravel
moved by swiftly flowing water, rockslides or glaciers
River beds,
Sand
moved by rivers, streams, windblown,
Edges of lakes, oceans
Beaches dunes,
Silt / clay
found in quiet waters, lake BOTTOMS, lagoons, swamps, ponds
2. Chemical Sedimentary Rocks(pg. 129)
Step 1. Begins with…
Step 2. Then the water…
Examples:
-
-
-
Minerals (chemicals) that are dissolved
in sea, water, lakes,
underground Evaporates and the minerals precipitate out
Rock salt
Gypsum
limestone
Organic Sedimentary rock
Step 1. Begins in swamps with….
Remains of living things: plants and animals, shells
Organic material
that contain carbon.Step 2.
In swamps, they material doesn’t decompose
Compressed (without oxygen)
Bituminous coal
A
B
C
D
QUIZ: For each Clastic Rock. Name it, Identify where it is made, and what sediments it is made from.
Describe the Features of Sedimentary rocks
Stratification
Ripple marks, mud cracks
Fossils
Geodes
Sedimentary
rocks
Pg.130
Bellringer: Match the following:
a. Leaf print in shale
b. Mammoth tusk
c. Rabbit burrow
d. carbon print of fish
e. Petrified wood
1. Original remains
2. Replaced remains
3. Molds / casts
4. Trace fossils
5. Carbonaceous films
4. Write the letters that the rock formed in order.
Quiz1. What is an Index Fossil? Why are they important?
2. What are the three major Eras of Geological Time?
3. Tell the story of how the Sedimentary Rock formed below using terms like deposition, strata, folding, erosion, and unconformity.
F
ED
C
A
B
G
First…..
A
B
C
Layers A,B,C, were laid down in succession.
Layer A was deposited first, then B, then C.
Geologic time scale Subdivisions of Eons
• Era• Eras of the Phanerozoic eon
• Cenozoic (“recent life”) • Mesozoic (“middle life”)• Paleozoic (“ancient life”)
• Eras are subdivided into periods• Periods are subdivided into epochs
http://www.youtube.com/watch?v=SY3MZ_wNFW8Overview song
Figure 11.19
The Geologic Time Scale
http://www.youtube.com/watch?v=j2r55qCPbDo