JOURNAL #25Read pages 191- 192 and answer the following questions:

What is the difference between relative and absolute age?

What are varves? How do they form?

REVIEW

ABSOLUTE DATING METHODS

TODAY’S LEARNING GOAL Today, we will describe the process of

radioactive decay and how it can be used to determine the absolute age of rocks

ABSOLUTE DATING METHODSTo learn about Earth’s history,

scientists often need to determine the age of a rock formation.

This is known as absolute age (numeric age)

Scientists use a variety of methods to measure age.Rates of ErosionRates of DepositionVarve Count

RATES OF EROSIONIf scientists measure the rate at

which a stream erodes its bed, they can estimate the age of the stream.

Example: Niagara Falls, Grand Canyon.

RATES OF DEPOSITIONAnother way to estimate absolute age

is to calculate the rate of sediment deposition.

By using data collected over a long period of time, geologists can estimate the average rates of deposition for common sedimentary rocks.

Not always accurate because a flood (for example) can deposit meters of rock in one day.

VARVE COUNTA tree’s age can be estimated by

counting the growth rings in its trunk.

Similarly, scientists look at layers of sediment deposits called varves.

They consist of light-colored band of coarse particles and a dark band of fine particles.

RADIOMETRIC DATINGRocks generally contain small

amounts of radioactive material that can act like natural clocks.

Atoms of the same element that have different numbers of neutrons called isotopes.

Radioactive isotopes have nuclei that emit particles and energy at a constant rate regardless of surrounding conditions.

RADIOMETRIC DATINGDuring the emission of the particles,

large amounts of energy are released. The method of using radioactive

decay to measure absolute age is called radiometric dating.

Scientists measure the concentrations of the original radioactive isotope and the newly formed isotope.

Comparing these proportions gives the absolute age of the rock.

CARBON DATINGYounger rock layers may be dated

indirectly by dating organic material found within the rock.Bones, shells, wood

Cabron-14 or radiocarbon dating can determine the age of these organic remains

CARBON DATINGCarbon-14 combines with oxygen to

form CO2.Remember the carbon cycle?

The find the age of the organic material, scientists determine the ratio of C-14 to C12 in the sample

While organisms are alive, the ratio remains constant. The half life of C-14 is 5730 years.

Since the organism is dead, it no longer absorbs carbon.

REVIEW:What is absolute age?

The known numeric age of a substance

REVIEW:What is radiometric dating?Method of using radioactive decay to measure absolute age

REVIEW:What are varves?Layers of sediment deposits that consist of light-colored band of coarse particles and dark band of fine particles. They help scientists estimate age.

RADIOACTIVE DECAY ACTIVITYUse a stopwatch to record time.Wait 20 seconds and then use

scissors to carefully cut a sheet of paper in half. Select one piece and set the other piece aside.

Repeat step 2 until nine 20 second intervals have passed (9 times)

Answer the questions on the activity sheet.