Rock DatingRock Dating

How old is Earth?

What are some tools or methods that scientists could use to figure out the age of Earth?

Think About It . . .Think About It . . .

There are two types of time There are two types of time dating dating Geologists use Geologists use relative datingrelative dating to figure out if a to figure out if a

rock is older or younger than another rock.rock is older or younger than another rock.

• In rock that is not disturbed (moved around by tectonic forces)

•The older rocks are on bottom.• The younger rocks are on top. (law of superposition)

•Just like laundry in a laundry basket

Index fossil can also be used to show the approximate ages of rock. Fossils in different rock

layers can be compared to determine one rock layers age in relation to another’s

Problems…Problems… Rock layers are not undisturbed. Rock layers are not undisturbed.

Earthquakes, faults, flooding, erosion, Earthquakes, faults, flooding, erosion, volcanic activity all change the landscape volcanic activity all change the landscape (some quickly and some slowly)(some quickly and some slowly)

UnconformityUnconformity: a gap in the rock : a gap in the rock sequence that happens because sequence that happens because agents of erosion (water, wind, glaciers) move agents of erosion (water, wind, glaciers) move

layers or parts of layers away layers or parts of layers away or or

because no deposition occurs in that area.because no deposition occurs in that area. Deposition does not evenly distribute sediments Deposition does not evenly distribute sediments

(higher areas may not get as much as lower etc..)(higher areas may not get as much as lower etc..)

oror Earthquakes/plate movement has caused Earthquakes/plate movement has caused

ground to be upliftedground to be uplifted

But What If You Want to Know But What If You Want to Know the Exact Age? the Exact Age?

Geologists often also need to know the Geologists often also need to know the exact exact ageage of a rock or fossil. of a rock or fossil.

Finding the exact age of an object is called Finding the exact age of an object is called absolute datingabsolute dating..

Remember “absolute” means Remember “absolute” means exactexact or or definitedefinite..O

AtomsAtoms Everything is made of Everything is made of atomsatoms.. The defining characteristic of The defining characteristic of

an atom is the number of an atom is the number of protons. protons.

The number of neutrons can The number of neutrons can vary from one carbon atom to vary from one carbon atom to another or from one another or from one hydrogen atom to another. hydrogen atom to another. These are called isotopes.These are called isotopes.

Over time, some of these Over time, some of these atoms may atoms may changechange into a into a differentdifferent type of atom. type of atom.

The original atom is called The original atom is called the “the “parentparent.”.”

The new form of the atom is The new form of the atom is called the “called the “daughterdaughter.”.”

HalfHalf of the of the parentparent atoms atoms turnturn into into daughterdaughter atoms once the object has atoms once the object has reached a certain reached a certain ageage..

Like how Like how every year every year on your b-day on your b-day you you turnturn a a different different ageage..

Parents and DaughtersParents and Daughters

ParentsParents and and DaughtersDaughters Scientists can learn the Scientists can learn the

age of a rock by age of a rock by countingcounting the the numbernumber of of parentparent and and daughterdaughter atoms.atoms.

Half-LifeHalf-Life A A half-lifehalf-life is the time needed for one-half is the time needed for one-half

of the of the parentparent atoms to turn into atoms to turn into daughterdaughter atoms. atoms.

Different atoms have Different atoms have differentdifferent half-lives. half-lives. Some have half-lives are more than 4 billion Some have half-lives are more than 4 billion

years; others have half-lives of only 6,000 years; others have half-lives of only 6,000 years.years.

Scientists figure out which atom would be Scientists figure out which atom would be best to use to determine the exact age of a best to use to determine the exact age of a rock.rock.

Half-Lives of Two AtomsHalf-Lives of Two Atoms Potassium-40 has a half-life of 1.3 billion

years. It can be used to date rocks older than 100,000 years.

Uranium-238 has a half life of 4.5 billion years. It can be used to date rocks older than 10 million years.

•The half-life of carbon-14 is known to be 5720 years. Why do you think it is a good element to use to date fossils and some rock?

Shorter half life means more accurate age

Radiometric Radiometric datingdating

Radiometric dating Radiometric dating uses the half-life of uses the half-life of atoms to figure out atoms to figure out the age of the rock the age of the rock layers the atoms are layers the atoms are in.in. Determines Absolute Determines Absolute

AgeAge

Age of EarthAge of Earth Scientists have used Scientists have used absolute datingabsolute dating to to

figure out the figure out the age of the earthage of the earth.. By counting the By counting the parentparent and and daughterdaughter

atoms in some very old rocks, scientists say atoms in some very old rocks, scientists say that the solar system (including Earth) is that the solar system (including Earth) is about about 4.6 billion years old4.6 billion years old!!

(4,600,000,000 years old!)(4,600,000,000 years old!) That’s a lot of birthdays!That’s a lot of birthdays! Before this method was Before this method was

invented, people believed invented, people believed Earth was only Earth was only 6,0006,000 yrs old. yrs old.

ExampleExample A scientist knows that A scientist knows that halfhalf of the of the parentparent

atoms will turn into atoms will turn into daughterdaughter atoms atoms every 10,000 years.every 10,000 years.

Originally, there is Originally, there is 16mg16mg of parent atoms of parent atoms 10,000 years go by . . . 10,000 years go by . . . HalfHalf of the of the 16mg16mg of parent atoms turn of parent atoms turn

into daughter atoms.into daughter atoms. 1616 ÷ 2 = ÷ 2 = 88 So there are So there are 8mg8mg of daughter atoms of daughter atoms nownow And there are And there are 8mg8mg of parent atoms of parent atoms leftleft.. Check your math: Check your math: 88 + + 88 = 16 mg = 16 mg

Then another 10,000 years go by . . . Then another 10,000 years go by . . . HalfHalf of the of the 8mg8mg of parent atoms turn into of parent atoms turn into

daughter atoms.daughter atoms. 88 ÷ 2 = ÷ 2 = 44 Now there are Now there are 4mg4mg moremore daughter daughter

atoms.atoms. Add this to the Add this to the 8mg8mg that has already that has already

formedformed 88 + + 44 = = 12 mg12 mg of daughter atoms of daughter atoms And there are only And there are only 4mg4mg of parent atoms of parent atoms

leftleft.. Check your math: Check your math: 1212 + + 44 = 16 mg = 16 mg

Parent: 16 Parent: 16 8 8 Daughter: 0 Daughter: 0 8 8

Then another 10,000 years go by . . . Then another 10,000 years go by . . . HalfHalf of the of the 4mg4mg of parent atoms turn into of parent atoms turn into

daughter atoms.daughter atoms. 44 ÷ 2 = ÷ 2 = 22 Now there are Now there are 2mg2mg moremore daughter atoms. daughter atoms. Add this to the Add this to the 12mg12mg that has already that has already

formedformed 1212 + + 22 = = 14 mg14 mg of daughter atoms of daughter atoms And there are only And there are only 2mg2mg of parent atoms of parent atoms leftleft.. Check your math: Check your math: 1414 + + 22 = 16 mg = 16 mg

Parent: 16 Parent: 16 8 8 4 4

Daughter: 0 Daughter: 0 8 8 12 12

Scientists compare the number of Scientists compare the number of parentparent atoms to the number of atoms to the number of daughterdaughter atoms. atoms.

The more The more daughterdaughter atoms there are, the atoms there are, the older the rock is.older the rock is.

This tells them how many times half the This tells them how many times half the parentparent atoms turned into atoms turned into daughterdaughter atoms. atoms.

In this example, each time this happened, In this example, each time this happened, 10,000 years10,000 years had gone by. had gone by.

So if it happened So if it happened 3 times3 times, then the object is:, then the object is:

3 x 10,000 = 30,000 yrs old3 x 10,000 = 30,000 yrs old

Parent: 16 Parent: 16 8 8 4 4 2 2

Daughter: 0 Daughter: 0 8 8 12 12 1414

Pop QuizPop Quiz1) What method is used to figure out if a rock is 1) What method is used to figure out if a rock is

older or youngerolder or younger than other rocks? than other rocks?

RELATIVE DATINGRELATIVE DATING2) What methods has helped scientists

determine the exact age of Earth?ABSOLUTE DATING

3) Fill in this 3) Fill in this tabletable::

5 15

Rock interpretationRock interpretation

Rock interpretationRock interpretation

Rock InterpretationRock Interpretation

Rock interpretationRock interpretation