James Hutton (1626–1797)
The Age of the Earth
Alan R. Rogers
February 16, 2016
I
Rocks of Scotland documented erosion, deposition, and
volcanism—processes still at work.
I
Perhaps the landforms and strata were not produced by
catastrophes, but by the processes still at work:
uniformitarianism.
I
Required lots of time: “no vestige of a beginning,—no
prospect of an end.”
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Charles Lyell (1797–1875)
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Charles Darwin (1809–1882)
I
1830: published Principles of Geology
I
Uniformitarianism applied to life rather than geology.
I
Supported uniformitarianism; convinced scientific community.
I
Also required an old earth.
I
Estimated the age of the earth at several hundred million
years.
Lyell and Darwin motivated several 19th century scholars to
estimate the age of the earth.
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Measuring geological time
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Estimates based on salinity
1. Measure rate at which salt enters ocean.
I
Total thickness of sedimentary strata
2. Measure salt now in the ocean.
I
Salt in sea water
3. Divide #2 by #1 to get age of earth.
I
Radiometric dating
4. Result: 90 million years (John Joly 1899).
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Counting annual layers
In reality, salt is lost from ocean: plate tectonics take sediment
down into earth, where it is melted and comes back again as
igneous rock. Salt is recycled. Salinity doesn’t measure age of
earth. No one knew this in 1918.
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Estimates based on rate of sedimentation
I
Measure rate of sedimentation in modern basins.
I
Measure total thickness of sedimentary rock.
I
Divide #2 by #1 to get age of earth.
Estimates from thickness of sediment
Date
Author
1860
1869
1871
1878
1883
1889
1890
1892
1892
Phillips
Huxley
Haughton
Haughton
Winchell
Croll
de Lapparent
Wallace
Geikie
Thickness
(feet)
72,000
100,000
177,200
177,200
—
12,000
150,000
177,200
100,000
Rate
(y/ft)
1,332
1,000
8,616
?
—
6,000
600
158
730–6,800
(Holmes 1912)
Time
(106 years)
96
100
1,526
200
3
72
90
28
73–680
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Estimates from thickness of sediment (cont.)
Date
Author
1893
1893
1893
1893
1895
1897
1899
1900
1908
1909
McGee
Upham
Walcott
Reade
Sollas
Sederholm
Geikie
Sollas
Joly
Sollas
Thickness
(feet)
264,000
264,000
—
31,680
164,000
—
—
265,000
265,000
335,800
Rate
(y/ft)
6,000
316
—
3,000
100
—
—
100
300
100
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William Thomson (Lord Kelvin) (1824–1907)
(Holmes 1912)
Time
(106 years)
1,584
100
45–70
95
17
35–40
100
26.5
80
80
I
Measure rate at which temperature increases with depth
below surface of earth.
I
Calculate rate of heat loss.
I
Calculate time to cool from molten to present temperature.
I
Answer: less than 100 million years.
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Loss of sun’s heat
I
Sun’s source of energy unknown in early 20th century.
I
Yet supply is surely not inexhaustible.
I
Persistent loss of energy must lower sun’s temperature.
I
Sun was hotter in past and will be cooler in the future.
I
Wild guess: planet has been habitable for 20–40 my.
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Discovery of radioactivity
1896 Henri Becquerel. Uranium exposes photographic
plates in total darkness. Coined term radioactivity.
1903 Pierre and Marie Curie. Radium always warmer than
surroundings.
1906 R.J. Strutt: Radioactivity in earth’s crust easily
accounts for Kelvin’s heat flow, which therefore tells
nothing about age of earth.
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Boltwood’s 1907 Radiometric Dates
Geologic period
Carboniferous
Devonian
Precarboniferous
Silurian or Ordovician
Precambrian
Sweden
Geological time scale
Lead/Uranium
0.041
0.045
0.050
0.053
United States
Celon
106 years
340
370
410
430
0.125
0.155
0.160
0.175
0.200
Period
Pleistocene
Pliocene
Miocene
Oligocene
Eocene
Carboniferous
Devonian
Ordovician
Algonkian
Archean
1,025
1,270
1,310
1,435
1,640
(Holmes 1912)
Time Scale in myr
Helium
Lead
1
—
2.5
—
6.3
—
8.4
—
30.8
—
146
340
145
370
209
430
—
1000–1200
710
1400–1600
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Early skepticism about radiometric dates
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Have rates of decay been constant?
“I venture to suggest—I do so with diffidence—that our
assumption of a constant rate of change for the parent
substances—uranium and thorium—is without any strong basis.
. . . The rate of change 150 million years ago may have been many
times what it is now.”
“If the recorded depth of sediments have taken 1400 million years
to collect, the average rate has been no more than one foot in
4000 years! This seems incredible: and if we double the depth of
maximum sedimentation it still remains incredible.”
(John Joly 1909?)
(John Joly 1909?)
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Have we underestimated radioactive decay?
Sedimentary record is incomplete
Sediment
accumulates below,
erodes above,
“baselevel,”
Suppose the earth is 6000 y old rather than 4.55 billion.
1. Radioactive decay underestimated by
2. Produces ∼106 × more heat,
(J Barrell 1917)
∼106 ×.
3. Thermal gradient ∼1000◦ C/km instead of ∼25◦ C/km.
which oscillates
with sea level.
4. Granite molten below 1 km.
5. No coal or petroleum.
6. Massive volcanism.
Each fall in sea level
erases sediment.
7. Pompeii destroyed by Vesuvius 22 hours ago.
(Thanks to Prof. David Chapman)
Only a fraction
remains.
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Earth & meteorite lead isochron: 4.55 × 109 y
(Patterson (1956)
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