Relative Age
GEOLOGIC HISTORY
THE RULES OF THE GAME
© Copyright 2005 - 2006 M. J. Krech. All rights reserved.
Time
What is time?
Time is…
Either relative or absolute
Time is: a measured period during which an event, process, or condition exists or continues.
One Year =
the time it takes the
Earth to complete one orbit around the Sun.
Geologic Time
• Based on the same time units
• Generally much greater periods of time
• 100,000 years
• 1,000,000 years
Dating Techniques
One ways in which geologists look at time: Relative Age
Relative Age of a rock…
Example:
My niece,
Sara, is older than my other niece, Mariam.
…is its age compared to the ages of other rocks.
What happened first?
What happened next?
Absolute Age of a rock…
Example:
My niece, Sara, is 3 years old and my niece,
Mariam, is 3 months old.
…is the number of years since the rock formed.
Relative and Absolute Age
Law of Superposition
• The youngest rocks are on the top.
• The oldest rocks are on the bottom.
Q#1 Principle of Superposition
Younger Rocks
Older Rocks
• Which rocks are older?
• Which rocks are younger?
Principle of Cross-Cutting
Volcanic vents cutting across sedimentary rock layers -
Which is youngest?
• Fault is a break or cut in the rock.
• You can't cut something until that something exists, therefore the cut has to be younger.
• So faults are younger than the rock it cuts through.
Q#2 Principle of Cross-Cutting
• What is younger?
• The rocks or the fault line?
Intrusions
Principle of Inclusions
The inclusion is always younger than the rock layers around and beneath it.
Q#3 Principle of Inclusions
Which is older? The rock layer or the rocks included in the layer?
Principle of Unconformities
Unconformities are surfaces that represent gaps in the geologic record.
Example of Unconformity
• http://www.classzone.com/books/earth_scie nce/terc/content/visualizations/es2902/es29
02page01.cfm?chapter_no=visualization
Principle of Unconformities missing rock layer = missing time
Q#4
Where is the youngest rock?
How do you know?
Q#5
Where is the unconformity?
How do you know?
Q#6 What rock layer is the oldest?
How do you know?
Q#7 Which is younger?
The fault or the rock layers?
FAULT
How do you know?
Q#8
Which is younger?
The rock layer or the inclusion?
How do you know?
inclusion
Using Fossils to Date Rocks
• Give relative age to a layer of rock
• Match layers of rock at other locations
• Use index fossils to match rock layers
• Index fossils are distributed in different areas & only occur in one area
• Ex. pg. 117
