TIME IN GEOLOGY

I. Age of Solar System: about 4.5-4.6 billion years

a. Pb/Pb ratios

b. Meteorites contain excess Xe129

c. Xe129 is decay product of I129

(Isotopes=same atomic Co, different atomic mass due to difference in neutrons)

d. I129 forms only at nuclear temperatures

1. implies meteorite formed in orginal solar disc

2. cold planets formed from hot solar disc.

II. Relative Time--events occurring before or after each other
a. Superposition, cross cutting relationships, Law of inclusion

b. Dendrochronology: width and spacing of tree rings-depends on temperature, light and water

1. summer wood-thin cells with thick walls

2. spring-larger cell with thinner walls

(a) Sequoia Trees-3,000 yrs old

Bristle cone pines-4,800 yrs old

(b) Growth rings observed on clams (molusca), fish scales, corals, otoliths

3. Varves-seasonal changes
Winter-Spring-dark organiclly rich

Summer-Fall-light in color organically poor

 

 

Range 10-15,000 yrs

Ice Cores-Paleocllimatology

 
4. Runcorn-Coral as Paleontological Clock
a.Suggestion J.W. Wells (1963)
(1) growth rings on external surface (Epitheca) on Devonian Corals

(2) Devonian year 400 days, (385-410)

 

 

 

(3) Devonian Month 30.6 days 22 hr/day

(4) Cambrian 570 million years 428 days

Devonian 370 million years 400 days
 

 

 

*(5) Runcorn-recent Bahama corals have daily growth rings
b. Astronomical Observations Earth slowing down due to tidal friction
fa>fb-because closer to moon
 

 

 

Tides act as shallow water waves

As a result moon is moving away from earth- (G. Darwin the son) angular momentum transfered to moon-moving away 5 yds per century

 

ABSOLUTE TIME

I. defining a second- 9,192,631,770 cycles of vibration of a cesium atom

II. Radiometric dating (Isotope Geology)

Henry Becquerel 1896 discovered radioactivity
A. Assumptions
1. Parent Atom A by a random process becomes radioactive

2. Parent Atom decays to a (stable) daughter (Atom B)

3. Decay rate from A to B is constant and must be accuratel known

4. System remains (closed) with regard to the geologic event being studied

B. Radioactivity-Spontaneous Transmutation of one element into another by emission of particles from the atomic nucleus

C. Isotope-same Atomic number different atomic mass due to different number of neutrons

D. Half Life-time it takes for 1/2 of a radioactive material to decay to a (stable) daughter product

 

 

 

 

III. Stable Daughter Methods

1 Uranium-Lead T 1/2
U238-Pb206 4.51 X 109 yr

U235-Pb207 .73 X 109 yr

Th232-Pb208 13.9 X 109 yr

2 Rb87-Sr87 4.7 X 109 yr

3 K40-Ar40 1.3 X 109 yr

4 C14-N14 5,730 yrs

*Method is good from approximately 1/2 of a halflife to "10" Halflifes

 

Alpha Particle = 2 He4 (x)

Beta Paricles (e-)

Neutron--positron (+)

Electron (-)--Beta

Atomic No. increse by 1

Election capture-Nucleus capture an electon from innermost orbital (K capture)

E C reduces atomic number and mass remains constant

E C sometimes produces gamma radiation

Branching decay

 

 

 

Gamma ( ) rays originate from inside Nucleus

X-rays orginat from electron clouds

 

Decay Clock

 

 

 

 

 

Accumulation Clock

Dt = Po -PT

time zero

 

 

 

Uraninum--lead

U238--Pb206

U235--Pb207

 

 

If ages do not agree within 10% they are discordant and fall off of the concordia plot

Constant Diffusion

 

 

 

Weathering Model

 

 

 

Anything falling off of concordia plot is due to recent weathering

RUBIDIUM--STRONTIUM

Rb87---Sr87

In a rock Sr86 also is present

Isochron

 

 

 

Whole Rock

 

 

Standard pre 1890