Transcript GLG101online_03A_RelativeDating_MCC_Leighty

Relative Geologic Dating
GLG 101 - Physical Geology
Bob Leighty
These notes and web links are your primary “lecture” content in this class.
Additionally, various articles are assigned each week to supplement this “lecture”
information. I believe you’ll have enough information to reference without having
to purchase a costly textbook.
These lecture notes are very similar to the ones I use in my traditional classes.
You’ll find they are loaded with imagery and streamlined text that highlight the
most essential terms and concepts. The notes provide a framework for learning
and, by themselves, are not meant to be a comprehensive source of information.
To take advantage of the global knowledge base known as the Internet, I have
included numerous hyperlinks to external web sites (like the Wikipedia, USGS,
NASA, etc.). Follow the links and scan them for relevant info. The information
from linked web sites is meant to supplement and reinforce the lecture notes –
you won’t be responsible for knowing everything contained in them.
As a distance learning student, you need to explore and understand the content
more independently than in a traditional class. As always, I will help guide you
through this learning adventure. Remember, email Dr. Bob if you have any
questions about today’s lecture ([email protected]).
Leave no questions behind!
Explore and have fun!
Relative
Geologic Dating
Telling Time, Geologically
 Earth’s history is recorded in its rocks
 Uniformitarianism: “the present is the key to the past”
> Common, everyday processes operate over
immense periods of time
> Known in other disciplines as “actualism”
Charles Lyell (1940’s)
Sand dunes of today
Sand dunes of the past
Relative
Geologic Dating
Telling Time, Geologically
 To decipher Earth’s history we use:
> Relative dating: Common sense observations that put rocks &
events in a proper order (e.g., older vs. younger)
> Absolute dating: Lab analysis yields a specific age for rocks
(e.g., giga-annum [Ga], mega-annum [Ma], kilo-annum [Ka])
younger
older
View west from the Hermit trail of Eremita Mesa,
Grand Canyon, AZ
Relative
Geologic Dating
Relative Dating Principles
Original Horizontality
 Many rock layers are originally deposited FLAT (water-laid
sedimentary rocks, volcanic flows & airfall tuff)
An active river flood plain
A river flood plain of the past (the 230-Ma Moenkopi Formation at
the Vermillion Cliffs, AZ).
Relative
Geologic Dating
Relative Dating Principles
Original Horizontality
 So, if layers are TILTED, then something happened since their
deposition
?
?
Relative
Geologic Dating
Relative Dating Principles
Superposition
 In an upright sequence of sedimentary or volcanic rocks,
the layers get YOUNGER from bottom to top
younger
older
Relative
Geologic Dating
Relative Dating Principles
Cross-cutting Relations
 Faults & intrusions are YOUNGER than the layers they cross
(or “bake”)
Faulted volcanic tuff at Resting Springs Pass
near Death Valley, CA
A small dike cuts through conglomerate on
Highway 93 south of the Hoover Dam
Relative
Geologic Dating
Relative Dating Principles
Inclusions
 A younger rock (host rock) may contain pieces of older rocks
(inclusions)
Clasts in a conglomerate are
older than the conglomerate
Inclusions in an intrusion are
older than the intrusion
Relative
Geologic Dating
Relative Dating Principles
 What is the correct relative order of features A, B, and C?
B
B
(youngest)
C
A
(oldest)
C
A
A boulder in the Agua Fria River north of Black Canyon City, AZ
Relative
Geologic Dating
Unconformities
 An unconformity is a contact that represents a significant time
difference between adjacent rock units
younger
older
Relative
Geologic Dating
Unconformities
Angular Unconformity
 Contact separating younger & older beds that are at an angle to
each other
 Older beds typically dip more steeply
 Relatively easy to see
Relative
Geologic Dating
Unconformities
Angular Unconformity
Making an angular unconformity
Deposition
Folding/Uplift
Erosion
Subsidence/more erosion
Relative
Geologic Dating
Unconformities
Nonconformity
 Where sedimentary rocks overlie older plutonic igneous or
metamorphic rocks
sedimentary rx
metamorphic rx
Relative
Geologic Dating
Unconformities
Nonconformity
4) deposition
3) tilting & uplift &
erosion
2) intrusion
1) deposition
Relative
Geologic Dating
Unconformities
Disconformity
 A contact between parallel younger & older sedimentary layers
 Relatively hard to see; need to look for fossils & inclusions
sedimentary rx
sedimentary rx
Relative
Geologic Dating
Relative Dating Puzzle
A B C E D
(oldest)
F G H
I J
K
(youngest)
Relative
Geologic Dating
WWW Links in this Lecture
> Charles Lyell - http://en.wikipedia.org/wiki/Charles_Lyell
> Relative Dating- http://en.wikipedia.org/wiki/Relative_dating
> Absolute Dating - http://en.wikipedia.org/wiki/Absolute_dating
> Original Horizontality - http://en.wikipedia.org/wiki/Principle_of_original_horizontality
> Superposition - http://en.wikipedia.org/wiki/Law_of_superposition
> Unconformity - http://en.wikipedia.org/wiki/Unconformity