Geology 1001-section 4
Dr. Dutrow
 
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GEOLOGIC TIME (abbreviated versions of overheads)

Geochronology - study and measurement of  time as it relates to the history of the Earth.

Relative time scales - defined by sedimentary rock sequences and  their included fossils; arranged events in order of occurrence.

Absolute time scales - defined as the actual number of years = old.

 

Time scales of geological processes
 
Event Time Unit Time in yrs
Earthquake 1 sec - several  min 10-8-10-5
Floods days 10-2
Measurable  erosion 1-100s  years 100-101
Opening of the  Atlantic Ocean 1 km 25,000  years 104
Uplift of mtn  range to 3000m at a rate of 0.2 mm/year 15,000,000  years 107
Age of the  Earth 4,500,000,000  years 4.5*109
Problems:

(1) geologic processes are

(2) Parts of the geologic record may be

 


PRINCIPLES USED FOR RELATIVE  AGE

(0) Priciple of Uniformitariansim - The Present is the Key to the Past.
 

(1) Principle of  superposition - in a sequence of rocks that are not overturned the oldest rocks

 

(2) Principle of original = horizontality - sediments are deposited in
 

(3) Principle of original = continuity - a sedimentary layer forms (at the time of deposition) a :

(a) thinning

(b) changing to a bed of a different composition (e.g. sandstone to shale)

(c) hitting a barrier

(4) Principles of cross-cutting = relationships - if an igneous body crosscuts a rock layer it must be
 
 

(5) Principle of inclusions - a  rock containing fragments of another rock must be

 


KINDS OF UNCONFORMITIES - erosional markers
 

(1) Angular = unconformity - angular discontinuity between

 

(2) Disconformity - irregular surface of

 

(3) Nonconformity -  strata lie on

 


STRATIGRAPHIC = CLASSIFICATION

Strata reflect the physical and biological = character of a part of the Earth at some past time.

Units of stratigraphic classification:

  1. Rock-stratigraphic unit - distinctive from rock above and = below.
  2. Formation - = distinctive rock unit that can be mapped (e.g. Navaho sandstone)
  3. Time-stratigraphic unit - all rock deposited in a given time = and may include other rock types.
  4. System - primary = time-stratigraphic unit involving time interval necessary for deposition of a = time-stratigraphic unit.
  5. Geologic time - interval of time necessary for deposition of = time-stratigraphic units.
Period - time = during which a geologic system is deposited.

 

A given area will not be complete but is broken by unconformities. = Must piece these areas together by: (1) Physically match grain size, color, sedimentary structures, = etc.

(2) Use key marker = beds - thin distinctive units such as ash falls.

(3) Use index = fossils - common widespread fossils of restricted age.


ABSOLUTE GEOLOGIC TIME

Philosophically, there is a contrast between = eastern and western cultural views of time.

Eastern example

Hindus view time as cyclic with a continuing = process of birth, life, death and rebirth.

Hindu philosophers believed that one cycle of the universe = equaled 1 day in the life of Brahma, the creator god, or =BB 4.3 = billion years.

Hindu scriptures postulate the Earth was about half through a = cycle; or about 2 b.y. old.

 

Western approaches Judeo-Christian cultures generally conceptualize = time as linear i.e. beginning to end

 

Early Estimates of the age of the Earth Xenophanes of Colophon - (570-470 = BC) - Greek philosopher who realized antiquity of fossils and sedimentary rocks.

Herodotus (450 BC) - Greek historian concluded the = Nile Delta was built by countless flood deposits over 1000s of years.

St. Augustine - followed these ideas to explain = natural phenomena through deductive reasoning.

During the late Middle Ages there was a mix of theology with = scientific thought (very repressive) e.g. In mid 1600s Archbishop Ussher of = Ireland counted up the ages of the various patriarchs in the Bible and concluded the Earth = was created at 9 am, October 26, 4004 BC.

In the mid 18th century Comte de Buffon calculated = the age of the Earth as 75,000 years based on cooling of a metal ball from the molten = state.

In the 19th century Lord Kelvin used estimated = cooling rates of the Earth/sun to give 20-40 millions years.

Breakthrough: discovery of radioactivity by = Becquerel and Madame Curie in the 1890s.

 


Three types of radioactive decay

(1) Alpha (a ) decay (   42He+2 nucleus)

corresponds to loss of 2 neutrons, 2 protons per each decay)

3D"alphadecay.gif

 

e.g. 238U decay

1st step: 23892U radioactive due to instability in the nucleus - i.e. will spontaneously  transform to a more stable isotope of the same  element or an isotope of a = different element.

Parent - atomic nucleus undergoing decay

Daughters - product  of the radioactive decay

Because the decay involves the nucleus and not the electrons  (define the nature of the chemical bonds), the rate of decay is

The rate of radioactive decay is measured by

 

 

The constant rate of decay of  different radioactive isotopes forms

 

 


MEASURING LONG HALF-LIVES OF ISOTOPES

Determining the value of a half-life of a given isotope is merely  a matter of counting the number of radioactive decay events per unit time for a given  amount of material. Below is a generalized procedure for doing this.

Count the radioactive disintegrations of individual atoms (e.g.  by counting the scintillations produced when alpha-particles from the decaying atoms  strike a ZnS screen).

Divide the number of disintegrations per second for a given  sample of radioactive element by the number of atoms in the sample (determined by  multiplying Avogadro's number (6.023*1023) and by the number of grams divided by  the atomic number of the element.

Obtain the probability that an individual atom will undergo a  radioactive disintegration in 1 second.

The half-life is then the time required for these probabilities  to build up to 50%.

 

RADIOMETRIC DATING

To be able to use radiometric dating, it is assumed that:

(1) There is a

 

(2) Once the mineral or rock crystallizes (or organism dies), 

 

  (a) Complication: In some situations the crystals can be

 

(3) It is possible to accurately measure proportions of

 

 

(4) Using isotopes with known half-lives (or decay constants) it  is possible to

 

 

 

Example: Assume there is a mineral that crystallizes today = (1998). The crystal traps 8 atoms of a radioactive parent. This parent decays to a = daughter atom at rate indicating a half life of 1000 years.
 
Date Half-lives Parent atoms Daughter atoms % parent remaining parent/ daughter ratio
1998 0
l l l l l l  ll 
  100 0
2998 1
l l l l 
 
50 1:1
3998 2
l l 
 
25 1:3
4998 3
l 
10 m.y - 4.5 b.y.
zircon
40K 40Ar 1.3  b.y. 50,000 y - 4.5  b.y. muscovite 

biotite 

hornblende 

volc. rocks

87Rb 87Sr 47 b.y.  10 m.y. - 4.5 m.y. muscovite 

biotite 

K-feldspar 

Ig./Met. rx

14C 14N 5730 y. 100 - 70,000 y. bone 

wood 

cloth 

glacier ice


Other geochronologic techniques

Magnetic Polarity Time Scale

Rock magnetism - magnetic minerals cool and have  their  

- particularly effective in lavas

- weaker magnetism in sedimentary rocks

 

Polarity-reversal time scale - some rocks contain a  record indicative of reversals in the Earth's magnetic field
record of reversal goes back

 

- must have additional information to calibrate reversals

 

Dendrochronology (tree ring record)      

 

Toilet Tissue Time Scale

ScotTissues contain 1000 sheets. If the age of the Earth is  4,600,000,000 years, then each sheet represents 4.6 million years (Ma) in time. In addition, 1  sheet 3D 4.6 inches or 1 inch represents 1 million years. Significant time lines in Earth history:
 
Age (Ma) Event Sheet #
4,200 Oldest material  dated - Australian zircons 87
3,996 Oldest dated rock  - NW Territories 131
2,500 Archean/Proterozoic boundary - appearance of multi-celled animals 456
570 Proterozoic/Phanerozoic - appearance of marine invertebrates 876
505 Appearance of  first fish 890
438 First land  plants 905
320 First  reptiles 930
245 Paleozoic/Mesozoic boundary - beginning of age of dinosaurs and conifers 947
66 Mesozoic/Tertiary  (K-T) boundary - disappearance of dinosaurs and rise of mammals and flowering  plants 985
23 Grasses become  abundant 995
2 First appearance  of humans last 2" of last sheet