TOPIC 11 - Changing Views on the Age

of the Earth and Solar System

 

 

WAYS WE CAN THINK ABOUT TIME

1. Is time Constant? Changing? (Greeks vs Relativity)

2. Is time Infinite? Finite? (Greeks vs Big Bang)

3. Historical Development of Our Views of Time.

One Characteristic of our Model of the Universe is its Age.

Example (of #2, 3, 4): Age of the Earth? Solar System? Universe?

 

 

 

AGE OF THE EARTH

Ways that we can determine the age of rocks:

1. Relative Dating and Development of Geologic Time Scale

Stenoís stratigraphic principles for layered rocks

Principle of Superposition (oldest rock layer on bottom if undeformed).

Principle of Original Horizontality

Principle of Lateral Continuity

Change of Fossils Through Time

Crosscutting Relationships (unconformities, faults, intrusions)

 

2. Absolute Dating Techniques

a. Historical Records

b. Seasonal Patterns

Tree Rings

Growth Rings (shell fish, corals, fish scales)

Varve Sequences (layers formed cyclically by deposition)

Average Growth Rates (Lichens)

c. Rates of Deposition plus Thickness of Units

Red Clay 0.1 cm/yr

CaCO3 ooze 1-3 cm/yr

Coastal 5-400 cm/yr

Deltas 7 m/yr

d. Paleomagnetic record (pattern of reversals of magnetic field)

e. Radioactive Decay

Cosmogenic:

Parent Daughter Half Lives

14C 14N 5730 y

32Si 32S 280 y

36Cl 36A 308 ky

7Be ? 55 d

10Be 10B 1.5 m.y.

26Al ? 716 ky

Naturally occurring:

238U 206U 4.5 b.y.

235U 207Pb 0.7 b.y.

147Sm 143Nd 108 b.y.

87Rb 87Sr 49 b.y.

K/Ar series:

40K 40Ar 0.7 by

U-Th series

230Th 226Ra 75 ky

210Pb 210Po 22 y

231Pa 207Ac 32.5 ky

Anthropogenic (produced by bombs, nuclear reactors)

239Pu ? 24.4 ky

137Cs ? 30 y

134Cs 137Ba 2 y

 

CHANGING PERSPECTIVES OF THE EARTHíS AGE

Early views (Greeks to middle ages):

Catastrophism accepted up to around1850

Great Flood caused mountains, fossils, etc.

Volcanic eruptions, earthquakes due to angry gods

These beliefs nicely explained a young earth.

Around 1600 Shakespeare (As You Like It) referred to age of earth being about 6000 yrs.

In 1654 Anglican Archbishop Ussher of Ireland proclaimed that earth was created in 4004 B.C. on the 26th October at 9 a.m. (now 6002 yrs old)

 

Beginnings of a Changing View:

Around 1500 Leonardo da Vinci:

fossils = ancient life, in communities such as those seen today,

many rock layers not formed by single great flood.

Formed by seasonal flooding.

 

In 1669 Nicolas Steno

Fossils in rocks beneath buildings in ancient Rome, so must be older than city, or > 3000 yrs and not flood related.

Principle of Superposition (oldest rock layer on bottom if undeformed).

Principle of Original Horizontality

Principle of Lateral Continuity

 

1670 Robert Hooke: fossils remains of old living organisms, not all living at same time. Could use to date rocks.

 

1700ís Modern geologic mapping started (e.g., Robert Smith)

 

1787 A.G. Werner (German) Neptunian view

Rocks formed from receding ocean

Primary (Primitive), Transitional, Secondary, Tertiary

Assumed Age much greater than Bible

 

1785-1795 James Hutton (Scottish farmer) ìFounderî of modern geology

Cyclic view of earth evolution by slow processes

Dynamic view (internal heat engine) rather than static

Gradual (and thus slow) processes rather than catastrophic

Primative and Transitional rocks had complex history

Plutons, tilted layers, unconformities, mountains build over time.

 

1830-1870 Charles Lyell, Published book called Principles of Geology.

Uniformitarianism or steady-state evolution of earth.

 

Charles Darwin, born 1809, famous travels 1831-1835, Origin of Species 1859,

Evolution (takes a very long time)

Also saw evidence for slow geologic processes: earthquake volcanic eruptions

 

Modern Dating: the Discovery of Radioactivity

 

Henri Becquerel (1896) Discovered radioactivity

 

In 1907 E. Rutherford (British physicist) suggested to

 

B. Boltwood (American chemist) that could use radioactive decay to date rocks

 

Rock Age = amount of daughter/amount of parent X decay rat e

 

N o t e t h a t a g e = t i m e m i n e r a l s t r u c t u r e c l o s e d s y s t e m .

 

A s s u m p t i o n s :

k n o w d e c a y c o n s t a n t

s y s t e m ( m i n e r a l ) c l o s e d t o p a r e n t a n d d a u g h t e r

A c c u r a t e l y m e a s u r e p a r e n t a n d d a u g h t e r

 

B o l t w o o d d a t e d 1 0 r o c k s g o t a g e s f r o m 4 0 0 , 0 0 0 t o 2 . 2 b . y . ! !

1 0 t o 100 times older ages than any previous guess

 

1934 first isotopic time scale published (see handout)

 

Summary of Important Attempts at Dating the Earth

1654 Anglican Archbishop Ussher of Ireland ~6000 yrs

1760 Buffon and others (guessing) 75,000 yrs

1846 Lord Kelvin (using cooling of earth) 20-30 m.y. old

1890ís Joly, (accumulation of salt in sea) 100 m.y. old

Sedimentation rates & sediment

thicknesses since Cambrian 75 m.y. old

1907 Boltwood (First use of radioisotopes)

got rocks as old as 2.2 b.y.!!

1940 to Present: (Using Radioisotopes)

Age of Moon Rocks 3.5-4.2 b.y.

Meteors 4.5 b.y.

Primordial Isotopic Ratios of Earth 4.5-5 billion

Accepted Age of Earth ~4.55 + 0.7 billion Oldest Rock ~4 billion

Organic Life ~3 billion

Humans A few m.y.

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