1. The History of Earth Relative Dating Absolute Dating
Relative dating does NOT look for exact dates but instead looks for a sequence of events (order) – what came first…
Absolute Dating
Absolute dating determines and age or age range, using chemical and physical properties of an object
Carbon based objects can be dated by using carbon based radioactive dating

2. Relative Dating : Which Came First?
Earth Science Chapter 8.1

3. Objectives: Explain how relative dating is used in geology.
Explain the principle of superposition.
Describe how the geologic column is used in relative dating.
Identify two events and two features that disrupt rock layers.
Explain how physical features are used to determine relative ages.

4. Uniformitarianism
Coined by 18th century Scottish physician and farmer James Hutton
Made lots of observations on his farm and in his native land
SO what is UNIFORMITARIANISM?!?!?!
Big word that simply means – The geological events that occur today (volanoe eruptions, erosion etc.) were also happening in the past.
Although the events are the same, the rate of occurrence is different

5. Game of Clues
Geologists try to determine the order in which events have happened during Earth’s history.
They rely on rocks and fossils to help them in their investigation.
Relative dating
The process of determining whether an event or object is older or younger than other events or objects.

6. The Law of Superposition
Layers of sedimentary rock, such as the ones shown below, are stacked like pancakes.
As you move from the top to the bottom in layers of sedimentary rock, the lower layers are older.
Superposition
Law that states that younger rocks lie above older rocks, if the layers have not been disturbed.
Theoretically – the fossil findings that are buried on the lower layers was on earth first

7. The Geologic Column Geologic Geologists use the geologic column to:
an ideal sequence of rock layers that contains all the known fossils and rock formations on Earth, arranged from oldest to youngest.
Geologists use the geologic column to:
Interpret rock sequences
Identify the layers in puzzling rock sequences.
Principle of Original Horizontality: Sedimentary rock left undisturbed will remain in horizontal layers – if its not that way, something disrupted/disturbed the land - now geologist must look for clues…..

8. Disturbing Forces – messes with Superpositioning
Not all rock sequences are arranged with the oldest layers on the bottom and the youngest layers on top.
Some rock sequences have been disturbed by forces within the Earth.
These forces can:
Push other rocks into a sequence
Tilt or fold rock layers
Break sequences into moveable parts.

9. Some of the Clues… Cross Bedding Graded Bedding Ripple Marks
Size of the particles
Bigger pieces are on the bottom
If they are on the top, then the land was disturbed
Shape of the Bedding Planes
Sand deposits at angles & usually erode before new layer form
Scientists look for the shape of the cross bed to figure out the original position
Surface Waves
Caused by wind and/or water
If crests are not upwards, then there has been a disturbance

10. Disturbed Rock Layers Law of Crosscutting Relationships
The fault or intrusion rock is younger than the rock above and below because it cut into the middle of something already there.
The features must be younger than the rock layers because the rock layers had to be present before the features could cut across them.
DON’T MIX Up – Principle of Inclusion: a rock body that contains inclusion of preexisting rocks is younger than the rocks from which the inclusion came from…

11. Events That Disturb Rock Layers
Four ways that rock layers may become disturbed.
A fault
break in the Earth’s crust along which blocks of the crust slide relative to one another.
An intrusion
molten rock from the Earth’s interior that squeezes into existing rock and cools.
fault
intrusion

12. Gaps in the Record -- Unconformities
Missing Evidence
Sometimes, layers of rock are missing, creating a gap in the geologic record. This is called...
Unconformity
break in the geologic record created when rock layers are eroded or when sediment is not deposited for a long period of time.

13. Types of Unconformities
Most unconformities form by both erosion and non- deposition, but other factors may be involved.
Geologists place them into three major categories:
Disconformities
Nonconformities
Angular unconformities
Disconformities exist where part of a sequence of parallel rock layers is missing. (layers are parallel)

14. Nonconformities Angular Unconformities
exist where sedimentary rock layers lie on top of an eroded surface of non-layered igneous or metamorphic rock.
Angular Unconformities
exist between horizontal rock layers and rock layers that are tilted or folded.

15. Well that was confusing…

16. Rock-Layer Puzzles
Rock-layer sequences often have been affected by more than one geological event or feature.
For example, intrusions may squeeze into rock layers that contain an unconformity
Determining the order events
is like solving a jigsaw puzzle.
piece together the history of the Earth.
Key terms 8.3: Principle of faunal succession: groups of fossil plants and animals occur in the geologic record in a definite & determinable order. A period of geologic time can be recognized by its respective fossils

17. What do you know?
Why is it important or scientist to be able to understand the relative age of rocks?
What is the difference between unconformity and non conformity?
In which type of unconformity does the rock layers above and below the unconformity run parallel
So they can understand the age and history of Earth
Unconformity is a break in the rock record, non conformity is a type of unconformity where metamorphic and igneous rock are overlain by sedimentary rocks
Disconformity

18. Which of the following statements best describes the relationship between the law of superposition and the principle of original horizontality?
A. Both describe the deposition of sediments in horizontal layers
B. Both conclude that Earth is more than 100,000 years old
C. Both indicate the absolute ages of layers of rock
D. Both recognize that the geologic processes in the past are the same as those at work now.
A. Both describe the deposition of sediments in horizontal layers