Section 1 Determining Relative Age Chapter 8 Objectives State the principle of uniformitarianism. Explain how the law of superposition can be used to.

Section 1 Determining Relative AgeChapter 8Objectives

• State the principle of uniformitarianism.

• Explain how the law of superposition can be used to determine the relative age of rocks.

• Compare three types of unconformities.

• Apply the law of crosscutting relationships to determine the relative age of rocks.

Section 1 Determining Relative AgeChapter 8Uniformitarianism

uniformitarianism a principle that geologic processes that occurred in the past can be explained by current geologic processes

• Geologists estimate that Earth is about 4.6 billion years old, an idea that was first proposed by James Hutton in the 18th century.

• Hutton theorized that the same forces that change Earth’s surface now, such as volcanism and erosion, are the same forces that were at work in the past.

Section 1 Determining Relative AgeChapter 8Relative Age, continued

relative age the age of an object in relation to the ages of other objects

• One way to learn about Earth’s past is to determine the order in which rock layers and other rock structures formed.

• Layers of rock, called strata, show the sequence of events that took place in the past.

Section 1 Determining Relative AgeChapter 8Relative Age, continued

• Scientists can determine the order in which rock layers formed by using a few basic principles.

• Once they know the order, a relative age can be determined for each layer.

• Relative age indicated that one layer is older or younger than another layer but does not indicate the rock’s age in years.

Section 1 Determining Relative AgeChapter 8Law of Superposition, continued

law of superposition the law that a sedimentary rock layer is older than the layers above it and younger than the layers below it if the layers are not disturbed

• Scientists commonly study the layers in sedimentary rocks to determine the relative age of rocks.

• Sedimentary rocks form when new sediments are deposited on top of old layers of sediment.

• As the sediments accumulate, they harden into layers called beds. The boundary between two beds is called a bedding plane.

Section 1 Determining Relative AgeChapter 8Law of Superposition, continued

• The law of superposition states that an undeformed rock layer is older than the layers above it and younger than the layers below it.

Chapter 8Law of Superposition, continued

Section 1 Determining Relative Age

Section 1 Determining Relative AgeChapter 8

Principle of Original Horizontality• Scientist know that sedimentary rock generally forms in horizontal layers.

• The principle of original horizontality states that sedimentary rocks left undisturbed will remain in horizontal layers.

• So, scientists can assume that sedimentary rock layers that are not horizontal have been tilted or deformed by crustal movements that happened after the layers formed.


Principle of Original Horizontality, continuedGraded Bedding

• In some cases, tectonic forces push older layers on top of younger ones or overturn a group of rock layers. So, scientists must look for clues to the original position of the layers.

• One clue is the size of particles in the layers. In some environments, the largest particles of sediment are deposited in the bottom layers.


Principle of Original HorizontalityGraded Bedding

• The arrangement of layers in which coarse and heavy particles are in the bottom layers is called graded bedding.

• If larger particles are located in the top layers, the layers may have been overturned by tectonic forces.


Principle of Original Horizontality, continuedCross-Beds

• Another clue is the shape of the bedding planes.

• When sand is deposited, sandy sediment forms curved beds at an angle to the bedding plane.

• These beds are called cross-beds. Scientists can study the shape of the cross-beds to determine the original position of the layers.


Principle of Original Horizontality, continuedRipple Marks

• Ripple marks are small waves that form on the surface of sand because of the action of water or wind. When the sand becomes sandstone, the ripple marks may be preserved.

• In undisturbed sedimentary rock layers, the crests of the ripple marks point upward. Scientists study the orientation of the ripple marks to determine the original position of the layers.


Unconformities, continuedunconformity a break in the geologic record

created when rock layers are eroded or when sediment is not deposited for a long period of time

• Movements of Earth’s crust can lift up rock layers that were buried and expose them to erosion.

• Then, if sediments are deposited, new rock layers form in place of the eroded layers.

• The missing layers create a break in the geologic record, called an unconformity.

• There are three types of unconformities.

• A nonconformity is an unconformity in which stratified rock rests upon unstratified rock.


Unconformities, continued• An angular unconformity is the boundary

between a set of tilted layers and a set of horizontal layers.

• A disconformity is the boundary between horizontal layers of old rock and younger, overlying layers that are deposited on an eroded surface.

Chapter 8

Types of Unconformities, continuedThe diagram below illustrates the three types of unconformities.



Unconformities, continuedCrosscutting Relationshipslaw of crosscutting relationships the principle that a fault or body of

rock is younger than any other body of rock that it cuts through.

• A fault is a break or crack in Earth’s crust along which rocks shift their position. An intrusion is a mass of igneous rock that forms when magma is injected into rock.

• In these cases, scientists use the law of crosscutting relationships, or the fact that a fault or an intrusion is always younger than the layers it cuts through, to determine the order of the layers.

Chapter 8

Crosscutting Relationships, continuedThe law of crosscutting relationships can be used to determine the relative ages

of rock layers .


Objectives

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

Describe the formation of varves.

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

Drill: What is the purpose of Relative Dating?

Section 2 Determining Absolute AgeChapter 8

Absolute Dating Methods

absolute age: the numeric age of an object or event (how long ago it occurred)

This often is stated in years before the present (YBP)


Rates of Erosion

• One way scientists use to estimate absolute age is to study rates of erosion.

• Studying the rates of erosion is practical only for geologic features that formed within the past 10,000 to 20,000 years.

• For older surface features, the method is less dependable because rates of erosion can vary over millions of years.


Absolute Dating Methods, continuedRates of Deposition

• Scientists can also estimate absolute age by calculating 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.

• This method is not always accurate because not all sediment is deposited at an average; therefore it provides only an estimate of absolute age.


Absolute Dating Methods, continuedVarve Count

varve a banded layer of sand and silt that is deposited annually in a lake, especially near ice sheets or glaciers, and that can be used to determine absolute age.

• Some sedimentary deposits show definite annual layers, called varves.

• The varves can be counted much like tree rings to determine the age of the sedimentary deposit.


Radiometric Datingradiometric dating a method of determining the absolutes age of an object by comparing the relative percentages of a radioactive (parent) isotope and a stable (daughter) isotope.

• Rocks generally contain small amounts of radioactive material that can act as natural clocks.

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

• Radioactive isotopes can be used to determine age.

Radiometric Dating, continuedHalf-Life

half-life the time required for half of a sample of a radioactive isotope to break down by radioactive decay to form a daughter isotope.

• Scientists have determined that the time required for half of any amount of a particular radioactive isotope to decay is always the same and can be determined for any isotope.


Radiometric Dating, continuedRadioactive Isotopes • Uranium-238, or 238U, is an isotope of uranium

that has an extremely long half-life, and is most useful for dating geologic samples that are more than 10 million years old.

• Potassium-40, or 40K, has a half-life of 1.25 billion years, and is used to date rock that are between 50,000 and 4.6 billion years old.


Radiometric Dating, continued• Scientists use the natural breakdown of isotopes to accurately measure the absolute age of rock, which is called radiometric dating.

1000g

1 2 3 4 5 6 7 8 9 10

Radioactivematerial

remaining

0

20

40

60

80

100

Radioactive materialremaining


Radiometric Dating, continued • By comparing the amounts of

parent and daughter isotopes in a rock sample, scientists can determine the age of the sample.

• The greater the percentage of daughter isotopes present in the sample, the older the rock is.


Reading check

How does the half-life of an isotope affect the accuracy of the radiometric dating method?

Chapter 8

Reading check, continuedAn isotope that has an extremely long half-life will not show significant or measurable changes in a young rock. In a very old rock, an isotope that has a short half-life may have decayed to the point at which too little of the isotope is left to give an accurate age measurement. So, the estimated age of the rock must be correlated to the dating method used.

Section 2 Determining Absolute Age


Radiometric Dating, continuedCarbon Dating • Younger rock layers may be dated indirectly

by dating organic material found within the rock.

• Organic remains, such as wood, bones, and shells that are less than 70,000 years old can be determined by using a method known as carbon-14 dating, or radiocarbon dating.


Radiometric Dating, continuedCarbon Dating • All living organisms have both the carbon-12 and carbon-14 isotope.

• To find the age of a sample of organic material, scientists compare the ratio of 14C to 12C and then compare this with the ratio of 14C to 12 C known to exist in a living organism.

• Once a plant or animal dies, the ratio begins to change, and scientist can determine the age from the difference between the ratios of 14C to 12C in the dead organism.

Drill:Fill in the following circles with the appropriate

amounts of radioactive material

If the half life of this material is 5730 years; how old would the material be when only .78125g remain?

25g

3 Half Lives

Time 0

4 Half Lives5 Half Lives

1 Half Life2 Half lives



10 Half Lives

100 %50 %25 %

12.5 %6.25 %

3.125 %1.625 %.8125 %

.406125 %.2030625 %

.10152125 %

Calculate the age, amount of parent material remaining,

And the amount of daughter material remaining if:

The Half life of the parent isotope is 1000 yrs

The original sample contained 50 mg of parent material

And .81255% of the parent material remains

• Objective: SWBAT model the half-life of radioactive decay IOT determine how geologists can utilize this science to date rocks and geologic events.

• Drill: 1.40625mg` of a C-14 sample remain from a 45mg sample. If the half live of C-14 is 5730yrs how old is the sample?

• Objective: SWBAT model the half-life of radioactive decay IOT determine how geologists can utilize this science to date rocks and geologic events.

• Drill: A sample of a meteorite was found to contain 30mg of lead. The ratio of lead to the isotope uranium 238 was 7:1.

• 1) Identify the parent and daughter materials• 2) How much uranium is in the sample• 3) How old is the meteorite? (you will need to know

the half life of Uranium238)

• Safety concerns for the lab:– Its all fun and games till someone gets a wooden peg in

their eye.• Make sure the lid is on the container before shaking it!• It’s a simple lab: No need to shake the container in a ridiculous

manner. Make sure that the bottom of the contain is always pointed to the ground!

– If you spill pegs, Pick em up! You need to have 100 in your container to start each data set.

– Stay in your lab groups.• Be sure to push in your chairs when getting supplies or cleaning up

• This is going to be a multi-day lab, so do not rush. Work on getting good data and answering the questions to the best of your ability.

Objective: SWBAT…

- describe the geologic timeline of earth

- identify where humans fall within the geologic timeline

- explain how geologists use “index fossils to determine the relative age of rock layers

4.6 billion years ago….

Precambrian time the interval of time in the geologic time scale from Earth’s formation to the beginning of the Paleozoic era, from 4.6 billion to 542 million years ago.

• The time interval that began with the formation of Earth and ended about 542 million years ago is known as Precambrian time, which makes up 88% of Earth’s history.

• The Precambrian rock record is difficult to interpret (ITS OLD!!!), therefore we do not know much about what happened during that time.

• Most Precambrian rocks have been so severely deformed and altered by tectonic activity that the original order of rock layers is rarely identifiable.

Precambrian Life

• Fossils are rare in Precambrian rocks mostly because Precambrian life-forms lacked bones, or other hard parts that commonly form fossils.

• One of the few Precambrian fossils that have been discovered are stromatolites.

• The presence of stromatolite fossils in Precambrian rocks indicates that shallow seas covered much of Earth during that time.

Index fossils

• Index fossil a fossil that is used to establish the age of rock layers because it is distinct, abundant, and widespread and existed for only a short span of geologic time.

• Paleontologists can use index fossils to determine the relative ages of the rock layers in which the fossils are located.

• Practice. Turn to page 208, and complete the Activity on the Geologic Map of Ohio.

• Pg 232 complete the exercise on trace fossils.

• To be turned in at the end of class.

Section 1 Determining Relative Age Chapter 8 Objectives State the principle of uniformitarianism. Explain how the law of superposition can be used to.

Documents

relative age slide

relative age chapter

layers of rock

relative age of rocks

rocks age

older layers

sedimentary rock layers

horizontal layers