Transcript Chapter 3

William E. Ferguson
Geologic Time
A major difference between
geologists and most other
scientists is their attitude about
time.
A "long" time may not be
important unless it is > 1 million
years.
Amount of Time
Required for
Some Geologic
Processes and
Events
Some geologic
processes can be
documented
using historical
records
(brown area is new
land from 1887-1988)
Uniformitarianism
The present is the key to the past.
— James Hutton
Natural laws do not change—
however, rates and intensity of
processes may.
Two ways to date
geologic events
1 RELATIVE DATING (relative
position of fossils, structure)
2 ABSOLUTE DATING (isotopic,
tree rings, varves, etc.)
RELATIVE GEOLOGIC TIME
Steno Laws (1669) developed to
arrange rock units in time-order
• Principle of Superposition
• Principle of Original Horizontality
• Law of Cross -Cutting Relationships
• Law of Inclusions
Laws apply to both sedimentary and
volcanic rocks.
Principle of Superposition
In a sequence of undisturbed
layered rocks, the oldest rocks
are on the bottom.
Principle of Superposition
Youngest rocks
Oldest rocks
Jim Steinberg/Photo Researchers
Principle of Original Horizontality
Layered strata are
deposited horizontal or
nearly horizontal or nearly
parallel to the Earth’s
surface.
Principles of original
horizontality and superposition
Law of Cross-Cutting
Relationships
• A rock or feature is younger
than any rock or feature it
cuts across.
Law of
Cross-cutting Relationships
Fig. 9.9
LAW OF INCLUSIONS
• Included rocks are older than
surrounding rocks.
PRINCIPLE OF FAUNAL
SUCCESSION
Principle of Faunal Succession - groups of fossil
plants & animals have followed one another in
a definite & discernable order so certain fossil
assemblages characterize a specific time.
INDEX FOSSILS - fossils used to correlate a
specific time period
Based on distinct preservable parts, lived a
short time , in a specific environment with
wide distribution - MICROFOSSILS
Ammonite Fossils
Chip Clark
Petrified Wood
Tom Bean
Using Fossils to Correlate Rocks
Correlating beds using
index fossils
Unconformity
A buried surface of erosion
Separates much older, eroded
strata from younger ones
Hiatus - the time gap or the time
lost in the record
Unconformitites - 3 kinds
• Disconformity - undeformed beds
• Nonconformity - sedimentary
over igneous or metamorphic rx.
• Angular Unconformity - overlying
tilted beds
Formation of a Disconformity
South rim of the Grand Canyon
250 million years old
Paleozoic Strata
550 million years old
1.7 billion years old
Precambrian
South rim of the Grand Canyon
250 million years old
550 million years old
Nonconformity
1.7 billion years old
Nonconformity in the Grand Canyon
Nonconformity in the Grand Canyon
Tapeats Sandstone
(~550 million years old)
Vishnu Schist
(~1700 million years old)
Angular unconformity, Grand Canyon
The Great Unconformity of the Grand Canyon
Geoscience Features Picture Libraryc
Formation of an
Angular Unconformity
Reconstructing
Relative
Sequence
of
Events
CORRELATION
• Process used to tie separated strata
together
• Based on matching physical features
such as
– Physical continuity - trace of rock unit
– Similar rock types - marker beds, coal
seams, rare minerals, odd color
South rim of the Grand Canyon
Generalized
Stratigraphic
Section of Rocks
Exposed in the
Grand Canyon
after: Beus & Moral (1990)
Some of the Geologic Units
Exposed in the Grand Canyon
Michael Collier
The Geologic Time Scale
• Divisions in the worldwide
stratigraphic column based on
variations in preserved fossils
• Built using a combination of
stratigraphic relationships, crosscutting relationships, and absolute
(isotopic) ages
The
Geologic
Column
and
Time Scale
Absolute geochronology
• Adds numbers to the
stratigraphic column based on
fossils.
• Based on the regular
radioactive decay of some
chemical elements.
Isotopic dating
• Radioactive elements (parents) decay to
nonradioactive (stable) elements
(daughters).
• The rate at which this decay occurs is
constant and knowable.
• Therefore, if we know the rate of decay
and the amount present of parent and
daughter, we can calculate how long this
reaction has been proceeding.
Isotopes
Different forms of the same
element containing the same
number of protons, but varying
numbers of neutrons.
i.e.:
235U, 238U
87Sr, 86Sr
14C, 12C
Naturally Occurring
Isotopes of Carbon
Beta Decay
Electron Capture
Alpha Decay
Production and Decay
of Radiocarbon
Radioactive
Decay of
Rubidium to
Strontium
Half-life
The half-life of a radioactive
isotope is defined as the time
required for half of it to decay.
Proportion of
Parent Atoms
Remaining as a
Function of
Time
Geologically Useful Decay Schemes
Parent
235U
Daughter
207Pb
Half-life (years)
4.5 x 109
238U
206Pb
0.71 x 109
40K
40Ar
1.25 x 109
87Rb
87Sr
47 x 109
14C
14N
5730
PROBLEMS
• NEED A CLOSED SYSTEM!!!
– MINERAL MAY LEAK PARENT OR
DAUGHTER
– MINERAL MAY BE CONTAMINATED
WITH EITHER PARENT OR DAUGHTER
Another Clock
Paleomagnetism
• Earth’s magnetic field reverses every
half million years
• Reversals are recorded in rocks that are
forming at that time - seafloor
• Time scale calibrated by both relative &
absolute time methods
Earth’s
Magnetic
Field
Lavas
record
magnetic
reversals
Calculating
Relative
Plate
Motion
1871
1968
Paleontology
The study of life in the past based on
fossilized plants and animals.
Fossil: Evidence of past life
Fossils preserved in sedimentary rocks are
used to determine:
1) Relative age
2) Environment of deposition
Many methods have been used to
determine the age of the Earth
1) Bible: In 1664, Archbishop Usher of
Dublin used chronology of the Book of
Genesis to calculate that the world began
on Oct. 26, 4004 B.C.
2) Salt in the Ocean: (ca. 1899) Assuming
the oceans began as fresh water, the rate
at which rivers are transporting salts to the
oceans would lead to present salinity in
~100 m.y.
Many methods have been used to
determine the age of the Earth
3) Sediment Thickness: Assuming the rate of
deposition is the same today as in the past,
the thickest sedimentary sequences (e.g.,
Grand Canyon) would have been deposited in
~ 100 m.y.
4) Kelvin’s Calculation: (1870): Lord Kelvin
calculated that the present geothermal
gradient of ~30°C/km would result in an
initially molten earth cooled for 30 – 100 m.y.
Flawed assumptions
• Bible is not a science text or history book
• Salt is precipitated in sedimentary
formations
• Both erosion and non-deposition are
major parts of the sedimentary record
• Radioactivity provides another heat
source
The heat inside the Earth
The discovery of radioactivity at the turn of
the century by Bequerel, Curie, and
Rutherford not only provided the source of
the heat to override Kelvin’s calculations
but provided the basis for all later
quantitative estimates of the ages of
rocks.
Oldest rocks on Earth
Slave Province, Northern Canada
• Zircons in a metamorphosed granite dated
at 3.96 Ga by the U-Pb method
Yilgarn block, Western Australia
• Detrital zircons in a sandstone dated at 4.10
Ga by U-Pb method.
Several other regions dated at 3.8 Ga by
various methods including Minnesota,
Wyoming, Greenland, South Africa, and
Antarctica.
Age of the Earth
Although the oldest rocks found on Earth
are 3.96 Ga (or even 4.1), we believe that
the age of the Earth is approximately 4.6
Ga. All rocks of the age 4.6 to 4.0 Ga
have been destroyed (the rock cycle) or
are presently covered by younger rocks.
Age of the Earth
This is based on the age of rocks brought
back from the Moon (4.4 Ga), and
meteorites (4.6 Ga), that are thought to
be good representatives of the early solar
system as well as more complicated
geochemical modeling. This data
suggests that the present chemical
composition of the crust must have
evolved for more than 4.5 Ga.
Double it and add 1
number of
half-lives
0
1
2
3
4
5
number of
parents
number of
daughters
D/P
64
32
16
8
4
2
0
32
48
56
60
62
0
1
3
7
15
31
The geologic
timescale
and absolute ages
Isotopic dating of intebedded
volcanic rocks allows assignment
of an absolute age for fossil
transitions
The big assumption
The half-lives of radioactive
isotopes are the same as they
were billions of years ago.
Test of the assumption
Meteorites and Moon rocks (that are
thought to have had a very simple
history since they formed), have been
dated by up to 10 independent isotopic
systems all of which have given the
same answer. However, scientists
continue to critically evaluate this data.
Frequently used decay schemes
have half-lives which vary by
a factor of > 100
parent
235U
daughter
207Pb
half life (years)
4.5 x 109
238U
206Pb
0.71 x 109
40K
40Ar
1.25 x 109
87Rb
87Sr
47 x 109
147Sm
144Nd
106 x 109
What if the rates have varied?
What we think happened:
rate
of 
decay
 time
What if the rates have varied?
What we know didn’t happen:
rate
of 
decay
 time
Best initial D = 0
Two ways around this problem:
1) Choose minerals with no initial
daughter.
2) Use a method that tells you the
initial concentration of D and P.
Minerals with no initial daughter
• 40K decays to 40Ar (a gas)
• Zircon: ZrSiO4
ion
radius (Å)
4+
Zr
0.92
U4+
1.08
2+
Pb
1.37
Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
Sedimentation of Beds A-D
Beneath the Sea
Fig. 9.6
Uplift and Exposure of D to
Erosion
Fig. 9.6
Continued Erosion
Removes D and Exposes C
to Erosion
Fig. 9.6
Subsidence and
Sedimentation of E over C
Unconformity:
a buried surface of erosion
Fig. 9.6
Sedimentation of Beds A-D
Beneath the Sea
Fig. 9.8
Deformation and Erosion
During Mountain Building
Fig. 9.8
Erosional Surface Cuts
Across Deformed Rocks
Fig. 9.8
Subsidence and Subsequent
Deposition Buries Erosional Surface
Angular
Unconformity
Fig. 9.8
Schlumberger Executive Communications
Fig. 9.10
Sequence A forms during
lower sea level
Fig. 9.11a
Sequence B forms during
higher sea level
Fig. 9.11b
The Geologic
Time Scale
Fig. 9.13
Areas with Potentially Hazardous
Amounts of Radon