How did Victorians estimate the ages of fossils?

[sophiecentaur]

They didn't have Carbon 14 and I think fossil tree rings (dendochronology?) is a modern method so where did they get their millions of year from?

 

[billiards]

They didn't know absolute ages. They only knew what was older than what, based on their knowledge of geology.

 

[sophiecentaur]

Ah. That makes sense now. Jeez, they must have had some surprises when they found out the real time intervals involved!

 

[dorlomin]

Ah. That makes sense now. Jeez, they must have had some surprises when they found out the real time intervals involved!

By the 1790s James Hutton had introduced the idea that things like mountain errosion and other geological features would take millions of years. By the time Victoria sat on the throne the scales involved were being contemplated if not widely accepted.

 

[billiards]

In 1862 Lord Kelvin famously calculated the age of the Earth was between 20 million and 400 million years old based on how long it would take a molten ball the size of the Earth to cool to its present temperature. He was wrong, of course, because his assumptions were too simplistic, in fact we know now the Earth is much older; but it shows the most advanced thinking in Victorian times (1837-1901), and that they were just beginning to get a grapple hold on questions of absolute age.

 

[sophiecentaur]

There was an earlier estimate of the age of the Solar System, based on the assumption that the Sun was burning. The bestfuel they could think of was good quality coal. That yielded a figure of a few thousand years - confirming what they had read in the Bible!

 

[Gokul43201]

By the mid to late 1800's there were several estimates of the age of the Earth (Lord Kelvin, Helmholtz, other physicists and astronomers) that were in the tens of millions of years. Geologists considered these numbers too small, and proposed longer times, at least in the hundreds of millions of years. The discrepancy between these estimates and Biblical time was one of the big factors that influenced Darwin's skepticism of the theological account of life.

As for the "how", I'm not familiar with the methods used by geologists (stratification rates, etc.), but physicists did some simplistic calculations such as the one by Lord Kelvin, described by billiards (where the error was that convection was not properly modeled into the calculation), or the gravitational condensation of the Sun (the problem with that calculation was that nuclear fission was unknown until several decades after Helmholtz came up with his number, but it still gave a number in the millions of years).

 

[Studiot]

I thought the question was about Victorian estimates of the age of fossils, not the age of the earth?

I am sure palaeontologists will come up with better references, but Professor Benton of Bristol University discusses this question in chapter 4 of his delightfully readable book
When Life Nearly Died.

His approach is refreshingly open minded.

 

[ideasrule]

On the topic of the age of the Earth, Darwin was one of the scientists arguing that Earth must be substantially older than 20 million years because otherwise, life wouldn't have time to evolve.

 

[Subductionzon]

A further note on Lord Kelvin. He calculated the age of the Earth based on assuming that it was a molten ball of magma originally and cooled until the present. His age satisfied neither geologists, who using Uniformitarianism calculated an age of at least several hundred million years, and Christians, who had an upper limit of 10,000 years for the Earth at that time. The discover of radioactivity eventually changed all of that.

 

[Jon Richfield]

I have an idea that the first reasonably quantitative estimates of ages were based on counting sedimentary rock strata, on the assumption of one layer per year. Combine those with fossils peculiar to particular strata, and it permitted cross-estimation. I probably am omitting about dozens of other types of information that they used with varying degrees of validity and success, but that is the main one I remember.
People often overlook the stunning ingenuity of some of the workers of those days, not to mention the sheer sodden hard work it took to produce results we take for granted today.

Note in particular that the work of the likes of Kelvin are not for you and me to sneer at; its shortcomings mainly reflected fields of knowledge unknown at the time. His real simplistic error was not to overlook radioactivity, but to fail to appreciate the fact that his estimate left some very troublesome evidence unexplained. Not too long after us, armchair critics may be wondering why we were so similarly bog-stupid about dark matter. Quite unlike *their* deep and natural insights of course.

If there is no other reason to study the history of science, that would be enough.

Go well,

Jon

 

[Jon Richfield]

Oh, and btw, C14 and dendrochronology are generally only for *very* recent work, tens of thousands of years mostly.

Jon

 

[sophiecentaur]

Is it too tenuous to use fossil tree rings further back than that? I guess you need to be able to span / fill the whole time interval with contiguous generations of trees in order to get back to a reliable time. AND, of course, that the divisions are very small; only a few years at a time for identifiable ring patterns and one tree life would only be a hundred years or so.

 

[Jon Richfield]

Hi sophiecentaur,
You have partly answered your own question of course. In theory we could use dendrochronology for at least a couple of hundreds of millions back (not say, 400MY; for dendrochronology you need trees, and they didn't exist then!) But there are problems. To work on a time period more than the lifetime of one tree, (usually about 100Y or less; not all trees are bristlecone pines!) you firstly need a number of overlapping trees. Secondly, if you want to know anything about say, climatic trends, you need several trees, because local conditions affect tree rings drastically and you need to apply corrections. Thirdly, not all tree rings are annual. In fact I think that in tropical regions only the minority are. Just hoiking a stump out of the ground gives you little guaranteed data, especially if it is a tree that went extinct fifty millions of years before the first palaeontologist was born, possibly twenty million years before the Himalayas began to rise, then it takes some doing to pronounce with any confidence what the significance of its rings may have been.
As I said, these things take LOTS of work! :-)
Seriously, they *swallow* the lives of research workers...
Anyway, so you now have your hundred million trees or so, all certified to produce annual rings etc and with plenty of overlap.
Would you care to budget for the labour of assessing and correlating the dates?
And now, someone finds a tree in a stratum. Can you see any difficulty about the conclusions to draw from its presence in that stratum?
Don't get me wrong; I take dating technology very, very seriously. It is work I would hate to do, and I would be very bad at it, but though it is work that can tell a great deal, it also is very challenging.

Cheers,

Jon

 

[sophiecentaur]

Yeah
Basically, I wouldn't start from there!
There must be a better way.

 

[mgb_phys]

Is it too tenuous to use fossil tree rings further back than that? I guess you need to be able to span / fill the whole time interval with contiguous generations of trees in order to get back to a reliable time.

Main problem is that you need preserved wood.
So in europe you are limited to roughly 1000years because any tree that isn't part of a building is going to have rotted away.
In places with very long lived trees you can go back a lot further, since individual trees can live for a few 1000years, but there isn't very much built from them so they aren't much use for dating.

As Jon said the ring pattern varies too much across the world/species to use a bristlecone ring pattern from mexico to date oak from a bog in europe.

Another technique useful for human ranges but beyond C14 is luminescence from electrons trapped in crystals in heated rock, that pushes back to about 100,000years.

The important and useful bit about tree rings/C14/luminescence is that it dates a particular event - te tree chopped down, the plant/animal dying in C14, the rock being heated in a fire for luminescence - otherwise simply knowing the age of a rock is pretty useless for dating. A caveman knocking flakes off a 100Million year old piece of flint doesn't tell you very much about when the caveman lived.

 

[DaveC426913]

http://en.wikipedia.org/wiki/The_Map_that_Changed_the_World" chronicles geologist William Smith's comprehensive map of the strata underlying Great Britain that was published in 1815. The book highlights the research and events that lead up to a profound reorientation in our understanding of the Earth's geology. It has a lot to do with cataloguing layer upon layer of subtlely different species of fossilized shells.

 

[hokie1]

If you read the book the Time Machine you'll see that the time traveler goes to a point some 30+ million years into the future to see the end of the world. The book was published the year before Becquerel discovered radioactivity. My guess is that Wells used the best estimate of the life time of the sun when he wrote the book.

 

[hokie1]

Wood found in bogs can be used to develop a ring database. At one point some place in Europe had the most complete ring database going back 10,000 years. I'm sure that has changed.