"65 mya" etc? How do geologists get to date strata? What have they got to date the strata by?

Question

And don't say 'the fossils', because how can they measure their date if there's no organic material left?
No axe to grind. Just curious.

Answer 1

The dating is done via radiometric techniques, which provide an acurate estimate of the absolute age, in millions of years. The fossils only provide a relative order of the ages of strata they are found in (from oldest to youngest). It is entirely mistaken to believe the circular argument that the fossils provide the age of the strata, and the strata provides the ages of the fossils, and that the dates are somehow "made up", as alleged by the likes of Hans B.



You don't need "organic material" to do the dating. Actually, most fossils don't have any organic material left, having been recrystallized and altered, leaving behind imprints or traces as opposed to the original remains. Furthermore, the oft-quoted Carbon14 method only works over a period going back a hundred thousand years or so (due to it's relatively short half life) whereas geological processes are measured in millions to hundreds of million years. So we need isotopes with very long half lives; in particular K/Ar, U/Pb & Rb/Sr.



Now none of these isotopes are particularly associated with fossils, so how is it done??? The radioactive isotopes are measured, and ages calculated, on rock types (ideally unaltered igneous rocks such as lavas) that were formed at the same time as fossiliferous sedimentary strata in close proximity. This provides a time framework, and the ultimate ages.



The fossils, many of which are easily recognizable and show distinct changes in geological time due to evolution, can be used to provide the correct relative sequencing of geological stata in which they are found. This is a relative timescale, not an absolute one. The correct age sequence of geological strata, and the division into eras & periods & more (e.g. Jurassic, Cretaceous) was deduced by geologists well over 100 years ago. However it was only a relative sequence, and the geologists had no real clue as to the absolute ages (although some of the more far-sighted geologists knew that the timescale was vast...). It was only with the invention of the radiometric technique that the truly vast and remarkable timescales were established beyond doubt.



Of course, there are, and will always be, gaps in knowledge and extrapolations made. The geological record is actually incredibly complex, as hinted at by Jamie M with great foresight. There are usually big sections missing due to tectonic activity, uplift and erosion - what geologists call "unconformities" - and furthermore, many rock types are either not condusive to fossilization or didn't contain much in the way of living remains in the first place. And many fossils are no good for sequential dating anyway, particularly those that did not evolve rapidly, or were restricted to living in certain limited palaeo-environments. Big gaps in the geological record are (a) very common, and (b) entirely to be expected, so please do not believe the pseudo-creationist propaganda put out by the likes of Hans B (who seems to have not even a basic level of geological knowledge) that would have you believe otherwise.

Answer 2

U are very clever to notice that the absence of organic material limits the dating ability.



Scientist use "index fossils" found in rock to date that particular layer of rock strata.

The "geological column" is a list of index fossils that are listed from the simplest (oldest) to most complex (newest) fossils. Each of the fossils is given an age, so if it is found in a rock you know the rock is the same age as the fossil.



Interestingly, the ONLY place in the world you can find a "geological column" is in a geology textbook. In real life the fossils are never in the "correct" order.



Also, the way the fossils are dated is by what layer of rock they are found in.



Scientists use the fossils to date the strata and they use the strata to date the rocks.



So the answer to your question is, the dates are totally made up. but they usually pick dates that will support whatever theory they are trying to prove.

Answer 3

whereas the answers already given are perfectly correct that is not always the case as if there are unconformities then the fossils found may not correspond to the rock, some of the time it is just guess work however if the location of the area of the rock is known such as britain in the ordivician was near the tropics then you would expect to find rocks that had been deposited in that climate. geologists also use major events like the hercyninian orogeny to dat rocks and the surrounding area

Answer 4

There are a number of radioisotope decay measurements that geologists use to date strata and fossils, and there are also magnetic tests, and other isotope ratios that geologists use.



These include:



C14/C13/C12

K/Ar

U/Pb

Rb/Sr

Eu/Sm/Nd

Hf/La

Pb208/Pb207/Pb206

Cl36/Cl35



There's also a radioactive decay series for Si/Al, but the half life for radioactive aluminum is so long that it is used more by astronomers than by geologists



Geologists also study O16/018 ratios in living organisms, not because of radioactivity, but because many marine organisms make diurnal deposits of calcite or aragonite (both are CaCO3 minerals) in their shells. The Earths rotation rate is slowing down. Geologists have proved that the used to only a little 0ver 20 hours long by studying the oxygen isotope ratios in shells

Answer 5

The Fossils

Answer 6

a lot of times fossils give the period, and carbon dating , gas chromatography, and geological knowledge all factor into dating.





-hov

Answer 7

I believe they use fission track dating.