Absolute Dating: Grab a book and read pages 656-658 and answer the prelab questions

Radiometric dating

How do they measure the ACTUAL age of rock?

Count Tree rings

Dates trees back to 8000 years

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

Each radioactive element has it’s own fixed half-life. Each decays at a predictable, regular RATE

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?

FBKNAJDMHCLGE

425 my

300 my

How old are layers D and J?

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.

Then they were uplifted and folded

Then glaciers moved through eroded the top

Then new layers of sediment were deposited on top at different times

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

www.quia.com/cm/555161.html

Quia Matching Columns ActivityOn Geological Time