Law of Superposition Age of Rock

Law of superposition

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

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!

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.

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

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.

Faults

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

Intrusions

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

Faults and Intrusions

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

Multiple Choice

Which letter represents the fault?

• A

• B

• C

• D

• E

Multiple Choice

Which letter represents the intrusion?

• A

• B

• C

• D

• E

Multiple Choice

Which layer is oldest?

• A

• B

• C

• D

• E

Multiple Select

Which layer is the youngest?

• A

• B

• C

• D

• E

Answers

• "E" represents the fault

• "D" represents the intrusion

• "C" is oldest because it is the lowest layer

• "E" is youngest because it cuts through all of the layers.

Review

• 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.