Geo101 Lab Lecture Notes -
Geologic Dating
I. Lecture Content
The Concept of Time
Means of Framing Time
Geologic Time - Deep Time
The History of Historical Geology
Fundamentals of Relative Age Dating
Relative Age Dating Applications
Correlating Rock Columns
Fundamentals of Absolute Age Dating
Absolute Age Dating Applications
The Development of the Geologic Time Scale
II. The Concept of Time -
A. Can Time be Defined?
1) Ever-Continuous Linking of a Sequence of Events
2) The
"thing" that connects past moments with Now; the
"thing" that causes new moments to occur.
3) Arrow or Circle?
4) Is there a start or finish to time?
5) One thing is certain: Time does not stand still.
B. How is Time Measured - How to
"Frame" Time.
1) Measure by Cyclic events:
o
Biorhythms - heartbeats; generations
o
Earth cycles - 1 day = 1 rotation; 24 hours
o
Lunar cycles - 28 days = 1 orbit; 4 phases
o
Solar cycles - 365 days = 1 orbit; 4 seasons
o
Celestial cycles - comets; meteor showers
2) Measure by periodic
Chronographic means:
Pendulum; wound
spring; vibrating crystal;
nuclear decay
C.
The Difference between Relative and Absolute Time
1) Relative
time only depends upon "sequence of events" -
· Oldest versus youngest
2) Absolute
time depends upon finite chronographic
increments of time, e.g. years, minutes, seconds, while
using a set frame of reference, e.g. today, or A.D. versus
B.C.
D. The Abstract of Geologic Time
1) Grasping the
"depth" of geologic time is an extremely
difficult thing to actually do.
2) Demonstrate with a paper
tissue roll.
3) Bridging the deep abyss between perspectives of
Earth
Time versus Human Time.
4) Difficulty over human historical times of
grasping
or
defining Earth Time.
E. The History of Establishing our Modern
Concept of
Geologic
Time and the Age of the Earth.
1)
Early humans perspective.
2)
Classics time perspective.
3)
Early Christian perspective.
4) 1700
science perspective.
5)
1800's science perspective.
6)
Modern-day science perspective.
III.
Establishment of Geological Principles
A. James Hutton and the Principle of
Uniformatarianism
1) Hutton's Legacy and
Contributions to Geology
o Father of Modern Geology
o Pioneered principle of
uniformatarianism
o Modern concepts of plutonism
o Modern concepts of mountain
building
o Modern concept of deep
geologic time
o Believed the Earth to be
100's millions old.
2)
Hutton's field experiences in Scotland.
·
Siccar Point, Scotland
3) Charles Lyell was big
supporter of Hutton's Ideas.
·
Promoted the principle of uniformatarianism as the
most fundamental geological principle.
B. The Principle of Uniformatarianism
1) Defined: the view that all geological
processes that are
occurring today (the rock cycle) were operating in
the
past, and produced similar results.
2) "The" guiding
principle in geology
3) Important implications for historical
geology
C. Nicholas Steno and the Principles of
Stratigraphy
1)
Steno's Legacy and Contributions to Geology
·
Father of Modern Stratigraphy
·
Pioneered Principles of Stratigraphy
·
Published "Prodromus" - the nature and explanation of
"solids entrapped within other solids".
·
Later became a priest; totally blew-off geology
2) Steno's field experiences in
Italy and abroad.
·
Fossilized shark's teeth started his interest
·
He later focused on the nature of rock "strata"
D. Relative Age and the Principles of
Stratigraphy
·
Stratigraphy defined: the
study of stratified rocks; the
science
of describing, ordering, and placing of layered
rock (strata) within the
context of Earth's history.
·
Stratigraphy is a two-step process:
ü
Assigning an age of formation to a rock layer (dating)
ü
Determining which other layers exposed in other places are
equivalent in age (correlation)
·
The ultimate goal of stratigraphy is the development of a global
geological timescale with the relative age and physical
relations
between all known strata of the world.
1) The Principle of Superposition -
2) The Principle of Original Horizontality -
3) The Principle of Cross-cutting Relationships -
4) The Principle of Exotic Inclusions -
5) The Principle of Fossil (floral and faunal) Succession
E. The Central Role Fossils Play in
Stratigraphy and Rock Dating
1) There is nothing temporally
unique about the lithology of
a rock.
2) The fossil record reflects a
unique sequence of the
evolution
and extinction of ancient species through time.
3) Crucial in working out the succession and age of
strata.
4) Fossils provide the only
reliable means for assigning
strata their proper position in geological
time.
F. The Significance of Unconformities
Unconformity
defined: Surfaces of discontinuity in
the
rock deposition sequence
which encompass significant
periods of time.
1) Unconformities may result
from nondeposition and/or
erosion.
2) Mechanisms, Processes, and
Events that create
Unconformities.
3) Several Types of
Unconformities
· Unconformity -
· Disconformity -
· Angular
Unconformity -
· Nonconformity -
4) Importance of unconformities
in the rock record.
IV.
Relative Age Dating Applications
A.
Use in correctly ordering a stratigraphic rock column
and interpreting the
successions of geologic events.
B. Examples of Column and Block Diagram
Problems
See Figures 8.11 and 8.12
V. Correlation of
Separately Located Stratigraphic Sections
A.
What is "Correlation"?
Defined: Demonstration of
time-equivalency of rock
units in different areas.
B. Geologic techniques used in stratigraphic
correlation
1) Cross-Matching several
different elements:
§
Key beds -- E.g. "ash beds"
§
Major Unconformities
§
Guide or Index Fossils
C. Fossils as a very powerful relative
age dating tool
1) A global-scale dating system
2) Index, or "guide" fossils are ancient species that lived
for a relatively short time span, but were very abundant
and
globally widespread in most marine environments.
3) Were originally useful only for relative
age-dating.
D. Techniques for correlating exposed
rock sections
with buried rock sections.
1) Use of borehole cores and well logs, and mine shaft
data.
VI. Absolute Age Dating Methods
A. History of Development of Absolute
Dating Methods
1)
The Curies - Discovery of Radioactivity (1903)
2) Research on radioactivity
leads to dating method
B. The Fundamentals of Radiometric Dating
1)
Stable versus Unstable (Radioactive) Isotopes
2) Radioactive
Decay and Half Lives
3) Using Radioactivity to date
rocks.
4) Radioactive element parent/daughter pairs in minerals.
P - D 1/2-life
U238 - Pb206 - 4510
Ma
U235 - Pb207 - 713
Ma
Th232 - Pb208 - 13900 Ma
Rb87 - Sr87 - 47000
Ma
K40 - Ar40 -
1300 Ma
5) Radioactive Carbon14 in
organically-derived materials
o
C14 - N14 - 5570 years
o
Tree rings and plant material
o
Carbon-rich sediments
o
Shell, Bone and clothe materials
C. Laboratory Analytic Techniques for Age Dating
Rocks
1) Crush, refine, extract
target minerals from rock.
2)
Dissolve the minerals in hydrofluoric acid bombs
3)"Wet"
chemistry to get total ppm for each of the
parent and daughter
elements.
4) Run the samples through a
Mass Spectrometer to
get the exact isotopic
ratios for both the parent
and daughter isotopes.
5) Use logarithmic decay
equations to calculate the
length of time it took to decay from parent to
daughter.
6) Assumptions and Uncertainties
in the calculations
· The initial amount of
daughter isotope.
· Was the rock/mineral a
closed system since
the time the rock formed? Or when it was
metamorphosed?
D. Usefulness of Absolute Age Dating in Stratigraphy
·
Provides the means of putting absolute age
brackets on most
of the world's stratigraphic sections.
·
Time-equivalent, absolute age-dated marker
surfaces
1) Volcanic layers (lava flow and ash beds) interbedded
within sedimentary rock sequence.
2) Cross-cutting plutonic intrusions (dikes and sills).
VII. Development of the Geologic Timescale
A. The Geological Timescale - Defined:
A hierarchical time-rock
scale in which the 4.6 billion-year
geological/biological history of the Earth is divided into time
units of varying duration and subdivisions, based primarily
on the changes of prehistoric life found in the rock record
B. Development of timescale was a long,
evolutionary
process made by a great number of
individuals
working on outcrops scattered
far and wide across
the entire globe.
C. First stages of development (1800's)
were done only
with the use of relative age
dating methods.
D. The use of "index" fossils
was, far and away, the prime
tool used in the development of the geologic timescale.
D. The modern geological timescale
developed in the 1900's
with the addition of absolute age
dating methods.
1) Interbedded lava flows and ash falls provided
excellent "marker" beds
2) The magnetic reversal
record another method
E. Illustration of the Geologic Timescale
· See Figure 8.1 in text
F. The Geologic Timescale is divided into
several
temporal hierarchies - from
longest to shortest:
1) Eons
-
2) Eras
-
3) Periods
-
4) Epochs
-
G. Each time hierarchy may be further
subdivided
H. Every time period has an absolute age
range.
I. Nearly all of the divisions in the
geologic timescale
represent significant biological
events in the Earth's
history, like mass
extinctions of ancient life forms,
and the appearance of new
life forms.
VIII.
Geologic Time Vocabulary -
Chapter 8