Geologic Time The Geologic Time Scale Geologists have divided the history of the Earth in time units based on fossil evidence. The time units are part

Geologic TimeGeologic Time

The Geologic Time ScaleThe Geologic Time Scale

• Geologists have divided the history of the Earth in time units based on fossil evidence.

• The time units are part of the Geologic Time Scale.

• The scale starts from Earth’s origin, 4.6 billion years ago, to present day.

The GeolThe Geologic Timogic Time Scalee Scale

Eons Eras Periods

Earth’s History Compared to a Football Field

Pages 8 and 9 of your Earth Science Reference Tables!!

FossilsFossils

• A fossil is any evidence of earlier life preserved in rock.

• Fossils are ONLY found in sedimentary rocks!

• Why?• Igneous rocks are cooled magma/lava and

metamorphic rocks have undergone too much heat and pressure to preserve a fossil.

FossilsFossils• Types of Fossils:• 1. Original Remains – unchanged remains

of a plant or animal.• 2. Amber – animal or plant caught in sticky

amber and amber hardens and preserves it – think Jurassic Park!

• 3. Replaced Remains – soft parts of animal have disappeared and the hard parts are replaced by minerals. Ex. – petrified wood

FossilsFossils

• 4. Molds and Casts – a mold is a hollow depression left in the rock from a shell or bone. A cast is when the mold fills in with material and makes a cast of the mold.

• 5. Trace Fossils – evidence of life other than the actual remains of the plant or animal. Ex. – trails, footprints, burrows, etc.

Mold and Cast

Burrows

Footprints

Petrified Wood

Carbonized Remains

Index FossilsIndex Fossils

• Index fossils are very useful for dating rocks.

• These fossils are used to correlate (match-up) rock layers from one geographic area to another.

• This is very useful to geologists because they can then determine when a specific rock layer formed.

Fossil CorrelationFossil Correlation

time

grandpa

father

son

Time when all three generations coexist

1900 2000

Fossil CorrelationFossil Correlation

Different species overlap in time

Time (millions of years)

species A

species B

species C

Age of rock containing these 3 fossils

5 4 3

Fossil Correlation

Index FossilsIndex Fossils

• To be considered a GOOD index fossil, it must have these three things:

• 1. Be easily recognized

• 2. Be abundant (many of them found)

• 3. Be spread-out over a large geographic area.

• Someday, we (humans) will make great index fossils!!

Relative Dating of RocksRelative Dating of Rocks

• James Hutton proposed the principle of uniformitarianism.

• This states that the processes occurring on Earth today, have been occurring since Earth first formed.

• In other words, “the present is the key to the past”.

Geologic PrinciplesGeologic Principles

• Relative dating of rocks is getting an approximate age of the rock formation, without being able to get an EXACT age.

• In this picture of the Grand Canyon, each horizontal layer of rocks represents a different chunk of time.


• The first geologic principle is the Principle of original horizontality. It states that sedimentary rocks are always deposited in horizontal layers, unless something has happened to fold or fault them.

• The second principle goes along with the first. Because sedimentary rocks are deposited horizontally, the one on the bottom must be the oldest and each layer on top of that gets younger, with the youngest on the top.

• This is called the Principle of Superposition.

Geologic Principles

Superpositionoldest layers are on the bottom

Original horizontalityrocks form horizontal layers

youngest

oldest


• The third principle is the Principle of Cross-cutting Relationships.

• This states that an igneous rock intrusion or a fault must be younger than the rock it cuts across.

• In other words, the pizza or cake has to be baked before you can cut it!!

Igneous IntrusionsIgneous Intrusions• There are 2 types of

igneous intrusions.• 1. Dyke – molten

rock that cuts vertically into rock layers.

• 2. Sill – A horizontal sheet of molten rock that gets squeezed between 2 rock layers.

Geologic Principles

Cross - cutting features are younger

canyons intrusions faults

UnconformitiesUnconformities

• Earth’s surfaces are constantly being eroded.

• Because of this, often there is a gap in the rock record.

• These gaps are called unconformities.

• There are 3 types of unconformities.

1. Angular Unconformity1. Angular Unconformity

Rock layers above and below the un-conformity are at an angle to each other

horizontal layers

tilted layers

2. Disconformity2. Disconformity

Rock layers above and below the un-conformity are parallel to each other

horizontal layers

horizontal layers

3. Nonconformity3. Nonconformity

Rock layers above and below the un-conformity are different rock types

sedimentary

igneous or metamorphic

SedimentaryRocks

IgneousRocks

Nonconformity

Relative age problemsRelative age problems

OLD

metamorphicigneous

all other layers are sedimentary

W

L

F

C

A

P

XR

T

Z

1. C

2. R

3. T

4. Z

5. L

6. X

7. P

8. W

9. A

10. F

Oldest

Youngest

Absolute Dating of RocksAbsolute Dating of Rocks

• The main method used to get the exact age of a rock is called radioactive or radiometric dating.

• This method studies the decay rates of radioactive isotopes in order to determine the age of the rock.

Radioactive DatingRadioactive Dating

• During radioactive decay, the original material or “parent material” decays at a set rate into the new material or “daughter material”

• As the number of parent material decreases, the number of daughter material increases.

• The amount of time that it takes for exactly one-half of the parent to turn into daughter is known as a half-life.

• For example – the half-life of Carbon-14 is 5,700 years. Carbon-14 is the parent material and Nitrogen-14 is the daughter material it decays into.

• Therefore, this means that it will take 5,700 years for one-half of the Carbon-14 to turn into Nitrogen-14.


• THE GOOD NEWS!!

• Half-lives are completely and totally set in stone and DO NOT ever change!!

• The half-life of Cabon-14 is ALWAYS 5,700 years, no matter what!!

• ALSO – some common half-lives are given free of charge on page 1 of your ESRT!


• The smaller the half-life, the less useful the radioactive isotope is for dating really old stuff.

• For example, Carbon-14 half life is only 5,700 years. Therefore, it can only accurately date rocks no older than about 100,000 years old.

• Therefore, to date rocks that formed back at the beginning of the Earth, 4.6 billion years ago, we would use an isotope with a much larger half-life.

• Since Uranium-238 half-life is 4.5 billion years, anything containing it has undergone 1 half-life since the Earth first formed!

Example of Radiocarbon decay (decay of Carbon-14)

Analogy: “decay” of Bit-O-HoneyAnalogy: “decay” of Bit-O-Honey

“Original” Bit-O-Honey (Parent Material)

“Decayed” Bit-O-Honey (Daughter material)

100

50

25

13

1 2 3 4 5 6

Number of parent isotopes

Number of daughter isotopes

20 of original 100 left

100

50

25

13

1 2 3 4 5 6

Number of parent isotopes

Number of daughter isotopes

2.2

20

1

0.5

0.25

0.13

PARENT=1,000,000DAUGHTER=0

Start with a million parent atoms…

1

0.5

0.25

0.13

PARENT=500,000DAUGHTER=500,000

1

1

0.5

0.25

0.13


1 2

1

0.5

0.25

0.13


1 2 3

1

0.5

0.25

0.13


1 2 3 4

1

0.5

0.25

0.13


1 2 3 4 5

1

0.5

0.25

0.13


1 2 3 4 5 6

Let’s Try Some More…Let’s Try Some More…

Parent Daughter

1

1/2

1/4

1/8

1/16

0

1/2

3/4

7/8

15/16

1st half-life

2nd half-life

3rd half-life

4th half-life

Let’s Try Some More…Let’s Try Some More…

C – 14 N - 14

1

50%

25%

12.5%

6.25%

0

50%

75%

87.5%

93.75%

1st half-life

2nd half-life

3rd half-life

4th half-life

Documents

Geologic Time The Geologic Time Scale Geologists have divided the history of the Earth in time units based on fossil evidence. The time units are part