Geologic Time Lesson

Unit 1: Earth and Space Science
Lesson 5: Geologic Time Scale

Multiple Choice

Someone took the last cookie in the cookie jar last night. The last person to leave the scene is the culprit. Who was it?

Clues:

Person A walks to work.

Person B rides a bike.

Person C rides a motorcycle.

Person D drives a car.

Person E travels with a guide dog.

• Person A

• Person B

• Person C

• Person D

• Person E

Let's talk about it!

• Who is the culprit?

• What evidence did you see that led you to your answer?

Answer

• Person E is the culprit.

• We know that Person E and the dog left most recently because their footprints were on top of all the others.

That brings us to... The Law of Superposition!

​Uniformitarianism

​Geological processes that occur today are similar to those that have occurred in the past.

​

​The same processes that we see today have been occurring in Earth's past.

Multiple Select

Which geological processes occurring today also occurred in Earth's past?

• Weathering

• Erosion

• Volcanic Eruptions

• Extinction of trilobites

Fossils

• A fossil is the preserved remains or impressions of an organism such as a plant, animal, or insect.

• Almost all fossils are preserved in sedimentary rock.

• Studying fossils helps scientists to learn about the past history of life on Earth.

​Index Fossil

​Species that existed on Earth for a short length of time, were abundant, and inhabited many locations.

Multiple Choice

An index fossil needs to...

• ...exist in only one spot on the planet.

• ...be difficult to find.

• ...have lived for a short period of time.

Multiple Choice

The brachiopod would make a good index fossil.

• True

• False

Multiple Choice

The horseshoe crab would make a good index fossil.

• True

• False

​Principles of Relative Age Dating

• ​Superposition

• ​Original Horizontality

• ​Lateral Continuity

• Erosional Unconformities

• ​Inclusions

• ​Cross-cutting Relationships

Law of Superposition

• Recall that fossils are present in layers of sedimentary rock.

• The Law of Superposition tells us that in a series of undisturbed rock layers, the oldest layer is at the bottom, and the youngest layer is at the top.

Multiple Choice

Which layer is oldest?

• A

• B

• C

• D

• E

Multiple Select

Which layer is the youngest?

• A

• B

• C

• D

• E

Open Ended

According to the Law of Superposition, layer 1 was the first layer deposited, so it is the oldest layer. The last layer deposited was layer 12, so it is the youngest layer.

What can this tell us about the age of the fossils that we find in these layers?

Answer

• The Law of Superposition gives us information about the relative age of different fossils.

• Relative age does not tell the exact age of something, but it tells us which object is oldest and which is youngest. 

Multiple Choice

Which of the following is an example of relative age?

• You are older than your sibling

• You are 14 and your sibling is 12

​Original Horizontality

​Most rock-forming materials are deposited in horizontal layers.

​Lateral Continuity

​Sediments are deposited in large, continuous sheets in all lateral directions.

​Erosional Unconformities

• ​Breaks or gaps in the rock record.

• Angular unconformity can also be called a "folded rock strata"
  

Multiple Choice

Which of the following is not an unconformity?

• Disconformity

• Angular unconformity

• Inconformity

• Nonconformity

Multiple Choice

What type of unconformity is shown in the picture?

• Disconformity

• Angular Unconformity

• Nonconformity

​Inclusions

​If one rock contains pieces of another rock, the rock containing the pieces is younger than the pieces.

Multiple Choice

Which picture shows an inclusion?

​Cross-cutting Relationships

​If one geologic feature cuts across another feature, the feature that cuts across is older.

Faults and Intrusions

• Sometimes, rock layers are disturbed by natural events, creating faults or intrusions.

  • In these cases, determining relative age can be more difficult.

• The Law of Crosscutting Relationships tells us that a fault or intrusion is always younger than all the rocks it cuts through.

Faults

• Faults occur when rocks break due to the forces acting on them

• There are 3 basic types of faults

  • Normal Fault

    • Found at divergent plate boundaries

    • Caused from extension stress; aka "thinning"

  • Thrust Fault

    • Found at convergent plat boundaries

    • Caused from compression stress; aka "folding"

  • Slip Fault

    • Found at transform plate boundaries

    • Caused from shear stress; aka "

Multiple Choice

Which letter represents the fault?

• A

• B

• C

• D

• E

Intrusions

• Intrusions: when magma (lava) penetrates existing rock, crystallizes, and solidifies, forming igneous rock.

• Also known as "volcanic dike"

Multiple Choice

Which letter represents the intrusion?

• A

• B

• C

• D

• E

Multiple Choice

Fault lines are younger than the rock they cut through.

• True

• False

Open Ended

Let's pull it all together! Based on what we just discussed, put the layers in the correct order, starting with the oldest layer.

Let's talk about it!

• What order did you put the layers in?

• What evidence did you use to put the layers in this order?

Answer

• K  youngest

  J

  I

  H

  P

  L

  M

  N

  O

  Q  oldest