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DO NOW: Discuss with your table: Where in this picture would you expect to find the OLDEST rocks? Where would you expect to find the NEWEST rocks? What kinds of events might change that?. Chapter 3 The Rock and Fossil Record Section 1: Earth’s Story and Those Who Listened.

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DO NOW: Discuss with your table: Where in this picture would you expect to find the OLDEST rocks? Where would you expect to find the NEWEST rocks? What kinds of events might change that?

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Chapter 3

The Rock and Fossil Record

Section 1: Earth’s Story and Those Who Listened

the rock and fossil record
The Rock and Fossil Record
  • Geology- Study of Earth’s history
paleontology
Paleontology
  • The science of studying fossils to learn about past life on Earth.
how does earth change
How does Earth Change?

Most changes on the Earth are slow – erosion, deposition, pressure, melting.

Some changes are quick – volcanos, earthquakes, asteroid impacts,

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Chapter 3

The Rock and Fossil Record

Section 2: Relative Dating: Which Came First?

relative dating
Relative Dating
  • Finding an estimated age of objects on Earth by comparing it with rocks and fossils.
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The Law of Superposition

  • Superposition - Fossils/Rocks closer to Earth’s surface will be younger than Fossils/Rocks found closer to Earth’s center.
disturbing forces
Disturbing Forces
  • The law of superposition does not always hold true!
  • Sometimes there are disturbing forces that cause rocks to tilt, fold, or get flipped upside down.
  • This can be due to:
    • Earthquakes
    • Magma intrusions
    • Folding or tilting of rock layers
law of crosscutting relationships
Law of Crosscutting Relationships

Any rock layer that cuts across any other rock layer is youngerthan the one it cuts across.

What can cut across a rock layer?

Intrusions– magma can seep in between rocks and then harden to intrusive igneous rock

Faults– cracks in rocks along which the rocks move

F, A, and C are older than B

E is older than B

disturbed rock layers
Disturbed Rock Layers

1.) Fault- A break in Earth’s crust, that force the crusts to slide opposite of each other.

2.) Intrusion- Molten rock pushes up between existing rock layers.

disturbed rock layers1
Disturbed Rock Layers

3.) Folding- When Earth’s layers bend and buckle from internal forces such as tectonic plates

4.) Tilting- Internal forces slant rock layers instead of folding them.

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All of these disturbances are younger than the rock layers they affect!

The rock layers had to have been there already for the change to take place.

Disturbed Rock Layers

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6

What is the story here?

the geologic column
The Geologic Column
  • A collection of undisturbed fossil/rock layers from all over the world with oldest rocks at the bottom.
  • An “IDEAL” picture of what rocks would be present if there had been no disturbing forces.
  • Geologists use it to compare to other rock sequences and find out what’s missing.
index fossils

Index Fossils

Index fossils are fossils of organisms that lived for a short period of time all over the world.

If you find an index fossil in a piece of rock, you know how old that rock is.

common index fossils
Common Index Fossils

Trilobites are fossils that are found in the MAUV LIMESTONE layer of the Grand Canyon. Trilobites are known to have only existed on Earth 543 to 505 million years ago, so the Mauv Limestone must be 543-505 million years old!

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This layer of Bryce Canyon is called the Winsor Layer. It contains fossils of cephalopods which were only on Earth from 199 to 145 million years ago. So, the Winsor Layer is 199-145 million years old.

missing pieces of the record
Missing Pieces of the Record
  • Missing rock layers create gaps in rock layer sequences called unconformities.
  • Unconformity - a break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.
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3 Types of Unconformities:
  • Disconformity
  • Nonconformity
  • Angular unconformity
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1.) Disconformity- Sequence of parallel rock is missing! It is hard to see but very common.

3 Types of Unconformities

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2.) Nonconformity - Sedimentary rock layers lie on top of an eroded surface of a non-layered igneous or metamorphic rock.

Layers are on top of non-layered rock

3 Types of Unconformities

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3.) Angular Unconformity- exists between horizontal rock layers and eroded tilted or folded rock layers.

The tilted or folded layers were eroded before horizontal layers formed above them.

3 Types of Unconformities

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5

What is the story here?

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Chapter 3

The Rock and Fossil Record

Section 3: Absolute Dating: A Measure of Time

absolute d ating
Absolute Dating
  • Absolute Dating-A very accurate way of dating and measuring the age of rocks and fossils.
    • Geologists do this by using Isotopes and Radioactive Decay
isotopes
Isotopes
  • Isotopes are unstable forms of elements.
  • They change, at a predictable pace, into stable forms of the elements.
  • Some isotopes change quickly and some change slowly.
  • When the isotope changes, it’s called radioactive decay.
radioactive decay
Radioactive Decay
  • An unstable atom turns into a stable atom.
  • Unstable = parent
  • Stable = daughter
radioactive decay1
Radioactive Decay
  • Because radioactive decay occurs at a steady pace, scientists can use the relative amounts of stable daughter and unstable parent atoms present in an object to determine the object’s age.
radiometric dating
Radiometric Dating
  • Using radioactive decay to determine how old a rock is
  • Scientists determine a ratio of the unstable isotope is present compared to how much of the stable isotope is present.
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In other words….
    • An element changes forms over time helping scientist accurately date things
half life

Half Life

The amount of time it takes for one half of the parent isotope to turn into daughter isotope

Newly formed rock = 100% parent

After 1 half life = 50% parent

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EXAMPLE

The element we measured has a half life of 10,000 years.

This rock is four half lives old because 15/16 of it has changed to daughter. The rock is 40,000 years old.

This rock is two half lives old because ¾ of it has changed to daughter. The rock is 20,000 years old.

This rock is three half lives old because 7/8 of it has changed to daughter. The rock is 30,000 years old.

This rock is one half life old because half of it has changed to daughter. The rock is 10,000 years old.

This rock is newly formed

example the half life of the element we measured is 8 years
ExampleThe half life of the element we measured is 8 years.
  • If ¼ of your sample is parent material then ______ is daughter material.
  • If ¼ of the your sample is parent material, how many half lives has it been through?

32 years

16 years

8 years

0 years

24 years

3/4

2

example the half life of the element we measured is 2000 years
ExampleThe half life of the element we measured is 2000 years.
  • If 1/16 of your sample is parent material then how many half lives has it been through? ___________
  • How old is it? ______________

4000 years

2000 years

8,000 years

0 years

6000 years

4

4 x 2000 = 8000 years

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Example

If the mineral you’re studying has a half life of 12,000 years, identify the fraction of parent and daughter isotopes and the ages of each of these rock samples.

examples of elements used in radiometric dating
Examples of Elements used in Radiometric Dating
  • Uranium-238 - decays to lead-206
    • 1/2 life is 4.5 billion years
  • Potassium 40- decays to Argon and Calcium
    • 1/2 life of 100,000 years
  • Carbon-14 -decays to carbon-12
    • 1/2 life of 5,000 years
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Chapter 3

The Rock and Fossil Record

Section 4: Looking At Fossils

fossils
Fossils
  • Any naturally preserved evidence of life.
  • Fossils can indicated changes in the environment and can give us a time frame for the life span of certain plants and animals
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Fossilization

Formation of fossils

1 mummification
1. Mummification
  • In dry areas organisms can die and be preserved because of low humidity and most bacteria can not survive in these places.
2 preservation in amber
2. Preservation in Amber
  • Amber is hardened tree sap
  • Amber traps insects and preserves them
3 tar seeps tar pits
3. Tar Seeps/Tar Pits
  • Thick petroleum oozes to Earth’s surface and traps animals
  • You can see the fossils of ice age animals from 10,000 to 40,000 years old, such as a saber tooth Tiger
4 freezing
4.) Freezing
  • Low temperatures protect and preserve organisms and keep bacteria out
5 petrification
5.) Petrification
  • Minerals replace an organism’s tissues.
  • Petrified wood is actually stone and fossilized.
6 imprints
6.) Imprints
  • Made in soft mud or clay and preserved in sedimentary rock.
7 casts
7.) Casts
  • Formed when sediments fill an imprint (mold) and then cement to form rocks with the reverse impression of the organism
8 coprolites
8.) Coprolites
  • Fossilized waste materials
9 gastroliths
9.) Gastroliths
  • Fossilized stones from inside an organism’s digestive system to help break food into smaller parts.
    • The stones become gastroliths when the organism is dead
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Chapter 3

The Rock and Fossil Record

Section 5 – Time Marches On

geologic time scale
Geologic Time Scale
  • Divides Earth’s 4.6 billion year history into time intervals (4 Eons)
divisions of time
Divisions of Time
  • Eons Eras  Periods Epochs

Biggest Smallest

what determines a new era
What Determines a New Era?
  • The geologic column is divided into EONS, ERAS, PERIODS, & EPOCHS based on major changes in:
    • Earth’s surface
    • Climate
    • Type of organisms
the phanerozoic eon is all of the time that earth has been here 4 6b years it s divided into

The Phanerozoic Eon is all of the time that Earth has been here – 4.6B years. It’s divided into:

Paleozoic Era – 251M-542M years ago

Mesozoic Era – 65M-251M years ago

Cenozoic Era – 65Myears ago to today

542MYA 251MYA 65MYA TODAY

Paleozoic Mesozoic Cenozoic

paleozoic era
Paleozoic Era
  • 542 to 251 million years ago
  • Begins - with dramatic increase in plant and animal species.
  • Ends - with landform called Pangaea and mass extinction of 90% marine species and 70% land species.
mesozoic era
Mesozoic Era
  • 251 to 65.5 million years ago
  • Known as “Age of Reptiles”
    • Ex. Dinosaurs
  • Pangaea breaks up
cenozoic era
Cenozoic Era
  • 65.5 million years ago to present
  • “Age of Mammals”
  • Continents move to present day positions.