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Section 8.2

Determining Absolute Age

ObjectivesObjectives

SummarizeSummarize the limitations of using the rates the limitations of using the rates of erosion and deposition to determine the of erosion and deposition to determine the absolute age of rock formations.absolute age of rock formations.

DescribeDescribe the formation of varves. the formation of varves.

ExplainExplain how the process of radioactive decay how the process of radioactive decay can be used to determine the absolute age of can be used to determine the absolute age of rocks.rocks.

Absolute AgeAbsolute Age

The numeric age The numeric age of an object or of an object or event, often event, often stated in years stated in years before the before the present.present.

Absolute Dating MethodsAbsolute Dating Methods

A variety are used, A variety are used, either by either by observing geologic observing geologic processes or by processes or by measuring the measuring the chemical chemical composition of a composition of a rock.rock.

Rates of ErosionRates of Erosion

By measuring the By measuring the rate at which a rate at which a stream erodes its stream erodes its bed, we can bed, we can estimate the age of estimate the age of the stream. the stream. Practical only for Practical only for features that features that formed within the formed within the past 10-20,000 past 10-20,000 years.years.

ExampleExample

Niagara Falls has been eroding at an average rate Niagara Falls has been eroding at an average rate of 1.3 m per year for the past 9,900 years.of 1.3 m per year for the past 9,900 years.

For Older Surface FeaturesFor Older Surface Features

The method is less The method is less accurate because accurate because rates of erosion rates of erosion may vary greatly may vary greatly over millions of over millions of years.years.

Rates of DepositionRates of Deposition

Geologists can Geologists can estimate the average estimate the average rates of deposition for rates of deposition for common sedimentary common sedimentary rocks (about 30 rocks (about 30 cm/1000 years).cm/1000 years).

It Provides Only An Estimate It Provides Only An Estimate BecauseBecause

Any given Any given sedimentary layer sedimentary layer may not have been may not have been deposited at the deposited at the average rate.average rate.

The rate of The rate of deposition may deposition may change over time.change over time.

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Varve Count Sediments can show

definite annual layers that consist of a light-colored band of coarse particles and a dark band of fine particles. We can count these like tree rings.

Generally Form In Glacial LakesGenerally Form In Glacial Lakes

Large light-colored Large light-colored sediments deposited sediments deposited quickly in the summer. quickly in the summer. Small particles and Small particles and organic matter settle out organic matter settle out in the winter, forming a in the winter, forming a dark layer. One light dark layer. One light and dark layer thus and dark layer thus equals one year.equals one year.

Radiometric Dating

A method of determining the absolute age of an object by using radioactive decay.

Isotopes

Atoms of the same element with different numbers of neutrons.

Radioactive Isotopes

Atoms that break down and emit particles and energy. Since the decay rate is known we can use radioactive materials as natural clocks by comparing the ratios of two isotopes.

Parent Isotope

The original, radioactive isotope.

Daughter Isotope

The result of radioactive decay; the newly formed isotope.

Using The Known Decay Rate

Scientists compare the proportions of the parent and daughter isotopes to determine the absolute age of the rock.

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An Example Think of an hourglass

filled with sand. The sand on top is the parent material and the sand falling to the bottom is the daughter product.

As the parent material decreases the daughter product increases in proportionate amounts.

Half-Life The time it takes half the mass of a given amount of a parent isotope to decay into its daughter isotopes.

The rate is assumed to be constant for each substance and is not effected by changes in temperature, pressure, or other environmental factors.

This Only Works If

The sample has not gained or lost either parent or daughter isotopes through leaking or contamination.

Radioactive Isotopes (Table 1, p. 195)

The amount of time that has passed since a rock has formed determines which radioactive element is used to date a rock.

Time Is The Key

Too little: Not enough daughter isotope to measure.

Too much: Not enough parent isotope to measure.

So the estimated age of the rock must be correlated to the dating method used.

Carbon Dating

Younger rocks may be dated by the remains of organic material found within the rock. Also known as radiocarbon dating, it is used for material less than 70,000 years old.

14C Is Created In the Atmosphere

It then becomes part of carbon dioxide, which all organisms use as a carbon source. The ratio of 14C to normal 12C is a known constant, and is the same in all organisms.

When The Organism DiesWhen The Organism Dies

The ratio between the two isotopes begins to change because 14C is radioactive, with a half-life of 5,730 years. Thus the amount of 14C loss tells us how long ago the organism was alive.

Assignment

Due EOP Thursday:

Directed Reading

8.2 Key Terms Due EOP Friday:

Licorice Lab Due BOP Monday:

“Shaky” Chapter 20