Some time ago I was watching a programme on geology which talked about the age of rocks. It seems that carbon dating enables you to determine the age of a rock formation.
If I understand what is going on with carbon dating, the story goes like this. There is a known ratio (or known history) of different isotopes of carbon present in the atmosphere. Some of these isotopes are radioactive and decay at known rates. If you come across the remains of an animal, the ratio of carbon 14 (or whatever it is) can tell you how long since it carked it, since from that date, no new carbon entered its system.
How would the deformation of rocks “reset” the date? Does a rock take in carbon from the atmosphere when molten? Or is the age of the rock determined by taking a sample from the middle of the rock and saying that this part of the rock has not been exposed to the atmosphere for x million years? Or do I misunderstand what is going on here completely?
I think you’re confusing two different concepts here.
Cabon dating only works on organic material, the remains of plants and animals, because living beings take in carbon through the normal metabolic cycle. Once they die, the amount of carbon14 in their remains decreases at a constant rate, allowing the dating.
Carbon dating doesn’t work on inorganic materials like rocks, since those don’t take in carbon. Geologists use other techniques to date layers of rocks.
but what do I know? I’m just a lawyer. Anyone more learned in the area care to comment?
Unfortunately, I’m not more learned in this field – but I have some vague ideas :
jti is right that carbon dating only works on organic material. Although I’m fairly sure it can be used on fossils, if we know enough about how fossils are formed to determine the total amount of carbon not replaced by minerals (or if the carbon itself remains intact while other structures disappear).
That’s just a guess, though I would say that I think the show might have been talking about is using knowledge of geology to back up (or dispute) the carbon dating method. Riverbeds, for example, are deposited in layers, oldest deepest, so that can provide clues. Geologists (or is that paleogeologists?) can figure in other factors, like erosion, seismic events, folding of the crust, etc. to help determine the age.
I guess we’re still hoping for the more learned to arrive.
pj
Come and tell me who and what you are.
Are you a politician asking what your country can do for you or a zealous one asking what you can do for your country?
-Kahlil Gibran
Most geologists never use Carbon-14 dating, mostly because its short half-life (5730 years) renders it only usefull for dating things less than ~57,000 years old (i.e., Holocene (Recent) things and barely reaching into the Pleistocene). That, plus (as has already been mentioned), it’s only valid for organic carbon. Charcoal from a forest mowed down by a pyroclastic flow, for instance, can reveal the Recent activity of a volcano. Otherwise, there aren’t many other practical applications for C-14 dating in Geology (give the Anthropology dept. a call).
Geology is more fond of K/Ar (or, really, Ar-40/Ar-39) dating for most rocks. That, and/or, Rb/Sr, U/Th, and a number of others which work well for very old rocks (since the isotopic systems involved have very long half-lives) and are just fine for inorganic things like feldspar.
As for “reseting” the isotopic age of a rock, metamorphic process (as opposed to simple deformation in which no metamorphism occurs) does do that. So, a date taken from a metamorphic rock reveals the age of metamorphism rather than the age of the protolith (although there are ways around this, too.)
Specifically regarding K/Ar or Ar/Ar dating, since argon is a gas, it can escape, particularly from micaceous minerals, when the rock is heated, thus throwing off the ratio of parent to daughter isotopes, resetting the “clock” either partially (giving a date later than original formation but earlier than metamorphism) or completely (giving the date of metamorphism).
Sure! Heck, I’ll give ya a freebie. Here’s a guaranteed conversation-stopper:
“Although the layer silicate mineral parageneses of low grade metapelites and their petrogenetic significance have been intensively studied for many years, comparable investigation of layer silicates in low grade metabasites has been a relatively recent development. This is surprising because these phyllosilicates, which include chlorite, smectite, and their interstratifications, are most abundant in sub-greenschist grade metabasites.”
Notthemama: by my count, the SDMB has three geologists in residence. We’re pretty well represented!
My favorite smectite? Montmorillonite. But whos isn’t? And let’s not forget everybody’s favorite consumer application for montmorillonite: Kitty Litter!
