GEOLOGY LECTURE NOTES

Geologic Time (Ch 9)

 

 

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 (absolute) 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. 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:

      Assigning an age of formation to a rock layer (dating)

      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

 

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

 

 

B. The Fundamentals of Radiometric Dating (cont)

 

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.

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. 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. Most of the divisions in the geologic timescale

represent significant events in the Earth's history,

mainly biological ones, like mass extinctions.

 

VIII. Geologic Time Vocabulary - Chapter 8