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John Grotzinger, Thomas H. Jordan Frank Press and Raymond Siever
Understanding Earth
Fifth Edition
Chapter 8:
The Rock Record and the
Geologic Time Scale
Copyright © 2007 by W. H. Freeman & Company
Concepts you will need to know for the exams
CHAPTER 8
Geologic Time Scale
Relative age
Absolute age
Principle of original horizontality
Principle of superposition
Principle of cross-cutting relationships
Unconformity
Angular unconformity
Disconformity
Half-life
Geologic Time Scale
Geologists differ from other professional
scientists in their view of time.
Geologists deal with periods of time that go as
far back as the origin of the universe.
Before we had the ability to read the “atomic
clocks” of nature, geologists used the concept of
relative ages.
We can see changes in the landscape over time.
We can see one type of organism replace
another type of organism through the fossilized
remains of these changes over time.
With the advent of atomic/radioactive decay
techniques we began to obtain absolute dates.
Absolute dates or ages have specific numbers
Relative dating has been done in the past
through the study of layers of sedimentary rock
(aka “the field of stratigraphy”)
Principle of Original Horizontality
sediments deposited innearly flat layers. Crossbedded layers are contained within overall flat
beds.
If layers are faulted (broken) or folded it means
that they were deformed AFTER sediments
were deposited.
Principle of Superposition
Lowest layers are older
If this is not so, then the layers have been
overturned by tectonic activity:
There are limitations for using stratigraphy
to keep time because,
(1) sedimentation rates are variable
e.g. Mississippi can deposit 1m of sediment in
1000 y
The deep ocean may deposit 1mm / 1000 years
We need to estimate time in another way and be
able to recognize when the representation of
time via sediments is incomplete.
Paleontology is the study of ancient life from
fossilized remains.
William Smith in the 19th century was an
engineer who noted that similar rocks in
different areas had similar fossils and that
different rock types had different fossils.
He made a composite (ideal, pieced-together)
stratigraphic succession as in the following
figure
Most of geologic time goes unrecorded
Sometimes NO SEDIMENT is DEPOSITED over
an interval of time (hiatus)
Other times sediment is eroded away.
This is happening here: Mississippi Valley is
filling whereas the higher ground is being eroded
away.
The previous geological
boundary is called …
1. paraconformity
2. unconformity
3. Cross-bedding
An unconformity in which the layers above an
erosional surface are parallel to the layers below
is known as a DISCONFORMITY
One way of producing a disconformity
is to change global sea-level by
glaciation and glacial melting.
Cross-cutting relationships in
sedimentary rocks allow is to date the
relative timing of events such as
faulting and igneous intrusion
http://www.nvcc.edu/home/cbentley/geoblog/2009_08_01_ar
chive.html
The Geologic Time Scale
…..is still under cosntruction
Life began only 2500 Ma
Radioactive Dating Methods
Mining
Uranium Oxide
U3O8
U-238 99.3%, U-235 0.7%
Milling
crushed
soaked in sulfuric acid
Yellow Cake U3O8
CONVERSION:
yellowcake is heated to about
147 F, converted into uranium
hexafluoride gas (UF6).
Nuclear Reactors: enriched 3-5% U-235
Nuclear Weapons: enriched 90% U-235
ENRICHMENT:
The UF6 gas centrifuged
heavier U-238 moves outside
U-235 concentrates inward
Convert to UO2 powder
Rb-37
Sr-38
All lead, except 204Pb (1.4%),
is the end product of a
complex radioactive decay.
The most common isotope of
Lead is: 208Pb 52.4% (stable)
The END ….. For now