Transcript Geologic Time

Classroom presentations
to accompany
Understanding Earth, 3rd edition
prepared by
Peter Copeland and William Dupré
University of Houston
Chapter 9
The Rock Record and the Geologic
Timescale
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.
Two ways to date
geologic events
1) relative dating (fossils,
structure)
2) absolute dating (isotopic, tree
rings, etc.)
Amount of Time
Required for
Some Geologic
Processes and
Events
Fig. 9.1
Some geologic
processes can be
documented
using historical
records
(brown is new land
from 1887-1988)
Fig. 9.2
Ammonite Fossils
Chip Clark
Fig. 9.4
Petrified Wood
Tom Bean
Steno's Laws
Nicolaus Steno (1669)
• Principle of Superposition
• Principle of Original Horizontality
• Principle of Lateral Continuity
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
Fig. 9.3b
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
Fig. 9.3a
Principle of Lateral Continuity
Layered rocks are deposited in
continuous contact.
Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
Principle of Lateral Continuity
Map view
Using Fossils to Correlate Rocks
Fig. 9.5
Unconformity
A buried surface of erosion
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
Formation of a Disconformity
Fig. 9.6
The Great Unconformity of the Grand Canyon
Geoscience Features Picture Libraryc
Fig. 9.7
Angular unconformity, Grand Canyon
South rim of the Grand Canyon
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)
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
Formation of an
Angular Unconformity
Fig. 9.8
Cross-cutting Relationships
Fig. 9.9
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
Reconstructing
Relative
Sequence
of
Events
Fig. 9.12
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
Time Scale
Fig. 9.13
Absolute geochronology
• Add numbers to the
stratigraphic column based on
fossils.
• Based on the regular
radioactive decay of some
chemical elements.
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
Radioactive
Decay of
Rubidium to
Strontium
Fig. 9.14
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
Fig. 9.15
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.
Major Radioactive Elements Used
in Isotopic Dating
Table 9.1
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
Uniformitarianism
The present is the key to the past.
— James Hutton
Natural laws do not change—
however, rates and intensity of
processes may.
Calculating
Relative
Plate
Motion
Fig. 9.16
1871
Fig. 9.17
1968
Fig. 9.17
Fig. 9.18
Areas with Potentially Hazardous
Amounts of Radon
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