You can't argue with rocks

[kaberett]

The age of the Earth and of the Sun and the Universe are central questions for mankind.

Until the 19th century the ages were unknown; however, at that time with the advent of modern time, a significant debate arose concerning the age of the Earth. Geologists with their prestigious leaders Lyell and Darwin defended an old age, estimated to be several tenths of billions of years, while physicists tended to defer to Lord Kelvin who claimed that the Earth was not older than a few tens of millions of years.

Whereas physicists have physical laws and calculations, geologists only have simple observations of geological phenomena: the erosion rates of mountains, sedimentation rates, and the amount of volcanic eruptions today compared to the total amount of volcanic rocks, and maybe just intuition, but geologists were right (Thomson (Lord Kelvin), 1899; Darwin, 1859; Lyell, 1930; Rutherford and Soddy, 1902). The Earth is not 20 to 100 million years old as Kelvin thought, but rather billions of years as Lyell (1930) claimed. the debate was resolved when the physicist Rutherford (1929) used the newly discovered radioactivity for measuring ages of rocks and soon obtained an age of billions of years using the U-He method.

In the middle of this century, Hubble's observations initiated a second debate. This time the target was the age of the Universe (Hubble, 1929). Astronomers and geologists fought about that, and again the geologists were right.

CJ Allègre, G Manhes, C Göpel. 1995. The age of the Earth, Geochim. Cosmochim. Acta 59:1445--1456

Comments

[kaberett]

GIC

[swaldman]

Am I right in thinking that Kelvin's estimate was based on "It should have cooled off by now", assuming a hot lump of rock without radioactivity?

(I have a vague memory of something along those lines, and your post brought it to mind)

[kaberett]

Yep - and I think he actually assumed conductive rather than convective cooling with no additional heat sources, so his estimate should've been even smaller... I might be misremembering, but if you ask it on es.se I promise to look up the answer properly for you ;)

[swaldman]

That makes sense - I'm guessing that at that point nobody realised the interior of the planet was fluid? Since you wouldn't get convection in solid rock... Although I'd have thought that assuming only conduction would give a longer time to cool? (in most areas convection speeds up heat transport, but I'm no geophysicist)

[kaberett]

Sorry, unclear -- his estimate should have been shorter based on no-heat-sources if he had included the effects of convection, which IIRC he didn't.

There's a glorious glorious anecdote about the Rutherford lecture at the Royal Society (taken from his letters):

I came into the room, which was half dark, and presently spotted Lord Kelvin in the audience and realized that I was in trouble at the last part of my speech dealing with the age of the earth, where my views conflicted with his. To my relief, Kelvin fell fast asleep, but as I came to the important point, I saw the old bird sit up, open an eye, and cock a baleful glance at me! Then a sudden inspiration came, and I said, 'Lord Kelvin had limited the age of the earth, provided no new source was discovered. That prophetic utterance refers to what we are now considering tonight, radium!' Behold! the old boy beamed upon me.

[swaldman]

Oh, that's wonderful!
And yes, thank you, what you said makes more sense now :-)

[kaberett]

Isn't it just lovely? :-)

HURRAH for managing to string words together in ways that communicate meaning, at least some of the time!

[cosmolinguist]

Aw! This makes me happy.

[kaberett]

I am charmed forever by it, for srs.

[birke]

Diplomacy at its finest!

[kaberett]

yesss

[sylvaine]

*giggles* Smart man!

[kaberett]

ayup :D

[silveradept]

The rocks do not, apparently, lie.

[kaberett]

and they are a good deal more stubborn than you :-)