Dating Question For Members

The sand would date to the time of formation of the igneous parent rock.

If there were multiple parent rocks with different dates of formation contributing to the beach, the different grains would have different dates.

However, I'm not sure that it's actually possible to date most grains of ordinary sand, which is mostly composed of quartz. In order for radiometric dating to work, you'd need long-lived radioactive isotopes to be including in the mineral at the time of its formation, and I don't think that happens with quartz. You'd want to find a grain of garnet or something like that.

Thanks for the replies, folks. What led up to my question was Moose's statement that fossils were dated from the time of deposit of the organism. From searching, I learned that most fossils consisted of sedimentary rock and were void of original organic matter. This raised the question in my mind as to why the aggregate age of the rock or sand grains in the sediment would not be calculated as the age of the fossil rather than the time the organism was deposited.

Fossils aren't dated by looking at the sedimentary rock in which they're buried, though of course such a date would provide a maximum possible age for the fossil.

Instead, you look at igneous rocks underlying or overlying the fossil. If you find, for example, that whenever you have a lava flow or a layer of volcanic ash or whatever under a particular species, it is ≥ 100my old, and when you find such rocks over the species, it is ≤ 95 my old, and whenever you find such rocks within strata containing the fossil, they are between 95 and 100my old, then that allows you to put a date on the species of 100-95mya.

Having put a date on that species by this method you can then use its presence to date sedimentary rocks in the absence of igneous clues and so to put dates on the other fossils that they contain (the method of "index fossils").

You should look at a geology textbook sometime, they contain lots of interesting information about geology.

But the sedimentary rock consist of aged grains of sediment much older than the organism which formed the sediment into a fossil. It's all particles of rock compacted into sediment instead of loose, for example, in an ant hill or a beach. I would assume that since the organic organism is no longer there that what is being calibrated would be the old particles of the sedimentary rock.

And you assume wrong, as you would know if you had read the message to which you're replying. What is being measured is the ages of igneous rocks.

In short, if dating methodology would date the ant hill by the rock in it ...

It wouldn't.

Dr Adequate, I thought most fossils are formed in sedimentary rock.

They are. But they're dated with reference to igneous rock, as I explained in my second post on this thread.

OK, Dr Adequate, I've been thinking this through and doing some reading. So it appears that as per my OP question, what is being dated in order to determine the time the organism was deposited is the intrusive igneous rock which is above the fossil. This seems to imply that the igneous rock likely originated from the earth's mantel and rose by plate tectonics so as to be above the fossil. So it appears that what would ultimately determine the age of the fossil is whether the plate tectonic activity was relatively sudden via catastrophe or over the millions of years, inch by inch. What think you?

Well my first thought is to wonder what you can have been reading to end up with your brain that full of nonsense.

How the heck did plate tectonics get involved in this?

This is incorrect, they are being dated by extrusive igneous rock. Intrusive rocks would be useless for this purpose.

Intrusive rocks aren't necessarily completely useless. If you have an intrusive rock sticking up through a sedimentary layer then it is:

* Younger than the sedimentary rock if it intruded into it, and
* Older than the sedimentary rock if it is a buried outcrop.

Now it is possible to tell which it is: the presence or absence of contact metamorphism would be one clue; its shape and the way it fits into the sediment would be another --- a sill, for example, would be a sign of intrusion.

Of course, this has nothing to do with Buz's blather.

Edited by Dr Adequate, : No reason given.

From what I read, the intrusive igneous originated from the hot mantel of the planet's core. I would assume that plate tectonics would be one cause of the rise of the intrusive igneous from the mantel to above the fossil. How else does it end up above the fossil sediment being studied?

Plate tectonics doesn't move it, and when it rises it's magma, not rock.

Which raises more questions. What forces cause the lava to rise to the extent that it raises the plates?

It doesn't. Plates are moved sideways by convection of the mantel. They buckle upwards where they collide.

That's the short version, anyway.

What causes magma to rise, at volcanos for example, is heat and pressure.

How far above the planet's mantel does the magma rise before it becomes cooled enough to harden into intrusive igneous rock?

Well, if you think about it, if you have a bit of magma embedded anywhere in the crust, it's surrounded by rock which is below the temperature and pressure needed for it to be ductile; and eventually the magma will cool to that temperature.

(This is a very slow process, because of the low thermal conductivity of rock, and so incidentally constitutes another proof that the Earth is not young; it would take millions of years for a really big intrusion to cool down to the temperature of the surrounding crust.)

If it is the cooled magma rock that is tested by the radiometric dating, does it date differently than the rising magma and if so, why?

You can't date magma.

To see why, consider as an example uranium-lead dating. This works by looking at how much uranium has decayed into lead, as hopefully you know. Now when rock is in its liquid form (magma) there's uranium and lead mixed all through it. But when it cools into rock it crystallizes, and for chemical reasons some of the crystals formed can accept uranium into their crystal structure but not lead. This starts the clock --- we know that when those crystals were formed, they contained no lead. Obviously this doesn't apply to molten rock.

Yes, I suppose there must be a point at which it has a sort of slushy texture. Is there a word for that?