Richard J. Lloyd Nov. 18, 2008 Edinboro University of Pennsylvania The Modern Age of the Earth.

Richard J. LloydNov. 18, 2008

Edinboro University of Pennsylvania

The Modern Age of the Earth

ObjectivesHistory of dating methods, pre-radioactive era

The era of radioactive discovery

Dating methods and their applicability

How to date a rockThe Age of the Earth

History of Dating Methods (Western)

Biblical Chronologiesa) Chronologie de

l’historie sainte (1738) collected over 200 computations with creation dates ranging from 3483 B.C. to 6984 B.C.

b) Johannes Kepler combined biblical and astronomical arguments. Result: 3993 B.C.

Mostly based on determining the time elapsed between known historical events—i.e. the Flood, Abraham, etc. by summing lists of generations and the reigns of various rulers. Assumed roughly 3 generations per century and about 25 years/ruler typically

History (cont.)Kepler’s Chronology

Based on the belief that Earth was created at the summer solstice when the solar apogee was at the head of the constellation Aries. Using the known rate at which the solar apogee moved, a date could be calculated.

Ussher’s Chronology

Most famous for his prediction of creation in October 22, 4004 B.C. at midnight using astronomical cycles, historical accounts and biblical chronology.

Progress of the Sun

The Modern, Pre-Radioactive Era

Credibility of biblical chronologies eroded in 17th and 18th centuries when observations of nature were becoming more precise in concert with the theories that explained natural phenomenon.

Methods based on scientific explanation became popular, variously depending on observations of:

• Declining sea levels• Cooling of Earth and

Sun• Scenarios involving

the Moon’s origin• Salt clocks and

sedimentation

Declining Sea levelsDe Maillet (1720’s)

• Based on assuming Earth was once entirely covered with water

This was NOT stupid!Marine strata bearing sea

shells were found in inland mountains

• Tried to measure present rate of sea level decline from historical records and modern data. Result: 2.4 Gyr—half the modern result!

Why It Was WrongWe know (now!) that most

sea level declines locally are due to land UPLIFT. There are other places where the land level is falling. These effects are due to geological forces not understood in the day of De Maillet. The modern theories of geology were over a century away, and almost 250 years before plate tectonics.

Cooling of Earth and SunComte de Buffon

(1749) Why It Was Wrong

Measured the rate of cooling of iron spheres of various diameters—found a nearly linear relationship between diameter and cooling time.

Extrapolated this to a molten Earth of known diameter—96,700yrs.

Buffon himself didn’t trust his results and noted that the thickness of sedimentary rocks would affect cooling rate. In other calculations, he estimated ages up to 3 Gyr.

Cooling, cont.William Thomson, Lord Kelvin (1860) Why It Was Wrong

Postulated Sun was receiving energy from infall of meteoric material.

Observations showed too little material to maintain the Sun’s temperature—Sun must be cooling.

Variously estimated between 10-500 million years for the age of the Sun

• There was no way to measure the Sun’s temp. at the time (Kelvin put it at 2.3 times the actual value)

• He assumed the specific heat of the Sun was the same as water

• He disregarded any possible INTERNAL heat sources.

Cooling, cont.

Kelvin, round 2 Wrong, again!

Applied similar reasoning to the Earth’s cooling.

Used data collected from mines, showing increasing temp. with depth.

Using various guesses and supplementary evidence, the result was about 98 Myr.

Kelvin used inadequate theory of heat conduction

Data available was sparse, unreliable.

Dismissed INTERNAL sources of heat, specifically lunar tidal friction, chemical action, etc.

Radioactivity was still unknown.

Other MethodsMoon’s Origin

(1890) Why Wrong

Hypothesis was that Moon was formed by rapid rotation of Earth in Early history—mat’l flew off

Tidal friction slowed Earth’s rotation at rate which could be measured—resulted in Earth less than 1 Gyr.

Moon probably originated from impact of planetoid sized body

Hypothesis could not account for inclination of Moons orbit with Earth’s rotation

Moon would have been 3 times larger under this hypothesis than it really is.

Salt ClockVarious proponents

(1900) Why Wrong

Assume that the influx from rivers, precipitation of salts and other minerals into the oceans simply accumulates

Measures the present rate of influx and assume constant—results in dates of order of 100 Myr.

Ocean crustal material is recycled by the oceanic conveyor belt—unknown at time. The ocean “consumes” material influx

Rates of mineral influx are poorly known over large periods of time. Not constant at all.

SedimentationVarious (late 1800’s, early

1900’s) Why Wrong

Typically assumed constant average rates of erosion and deposition

Computed ages based on known thicknesses of, for example, Cambrian strata

Yielded ages on order of 100 Myr.

Rates of erosion highly variable

Thicknesses of Cambrian strata varied widely depending on location

SummaryMost estimates in the pre-radioactive

scientific era relied on theoretical models that were incomplete and had too little reliable measurements as inputs.

The consensus ages collected around predictions of a few hundred Myr—over 10 times too small!

What was needed was a “clock” that was unaffected by ordinary physical processes, including geological, solar, mechanical, etc. events that could affect chemical reactions, weather patterns, erosion and sedimentation that could change age estimates dramatically.

The Radioactive EraRadioactivity was discovered by Henri

Becquerel in 1896 in uranium salts.Radioactivity was thought to be like X-rays,

a form of electromagnetic radiation initially.Further experiments showed it was mostly

composed of charged particles—subatomic in nature

Subatomic processes are insulated from all ordinary physical processes, i.e. chemical interactions, including explosions!

Atomic structure

Basic picture Relative sizes

The nucleus is very smallIf the nucleus was

a pea at the 50 yd. line in a football field, the nearest electron would be in the endzone.

Chemical interactions involve only electrons, not the nucleus.

RadioactivityOver many years, the properties of

radioactive elements were catalogued and experiments were able to determine the statistical laws that governed their decay.

It became possible to know how long the sample had been present by looking at how much decay product had accumulated over time.

A CLOCK HAD BEEN FOUND!

Radioactivity Basics

Radioactive decays Change in Nucleus

Beta minus decayn0  →  p+  +  e−  + 

νeAlpha decay238U  →  234Th  +  αFissionVarious

(A, Z+1)

(A–4, Z–2)

A=Z+N, mass number

N=# neutronsZ=# protonsBAZ

Mathematics of Radioactive Decay

N=# of radioactive atoms left

/toeNN

presentoriginallyatomsofNo #lifetimeaveragemean )(

Half-livesAll radioactive elements have average

lifetimes, but often we speak of half-lives. A half-life is defined as the amount of time it takes for a radioactive material to decay to ½ of its original amount, i.e. when we have:

This implies that the half-life is:

02

1NN

2ln2

1

2/1

/2/1

t

e t

Analyzing the Rubidium-Strontium Clock

Rb-87 decays to Sr-87 in a half-life of 48.8 billion years.

A long-lived radioactive element is needed to date something that is very old.

Carbon dating is only good for relatively short time periods, about 70,000 yrs.

C-14 has a half-life of about 5700 yrs.

In about 5-10 half-lives, any radioactive element in terrestrial material falls below minimum detectable amounts (MDA)

How We Date Old StuffWhat we need to

know How We Know It

The amount of radioactive material originally present

Has the rock been disturbed in any way to add/subtract material since formation?

The decay series of the radioactive element

Sometimes unknown, but can be deduced assuming that its decay products were not originally present or it can be determined.

Re-melting can often be detected via crystallization.

Decay series is known from laboratory experiments.

References“The Age of the Earth,” G. Brent

Dalrymple, Stanford University Press; 1 edition (February 1, 1994)

“Introductory Nuclear Physics,” Kenneth S. Krane,Wiley; 1 edition (October 22, 1987)

“Finding Darwin’s God,” Kenneth R. Miller, Harper-Collins; (1999)