Geo101 Lab Lecture Notes - Geologic Dating 
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