Dating Question For Members

Yeah, this will probably confuse Buz, but intrusive layers are used for dating where appropriate: The black basalt intrusion in the middle obviously is younger than the rock through which it cut, and can easily be dated. Oh, typically much slower below the surface. Big plutons can take millions of years to cool to near the temperature of the surrounding rock. There are lots of interesting thing to be said about that, but Buz is confused enough already.

At least that photo makes pretty clear the difference between an intrusion and superimposed layers.

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If you have nothing to say, you could have done so much more concisely. -- Dr Adequate

The same forces that cause hot water to move from the bottom of a pot to the top. It is convection currents. Ever been to the Hawaiian Islands? For K/Ar dating, most assuredly. Liquid magma does not hold on to argon gas. It escapes into the atmosphere. Magma can only hold onto argon produced by the decay of potassium-40 after the magma has solidified.

Yes. I thank you and so many who've been good spirited and informative in this thread. It's all giving me a lot to mull over and try to make sense of.

Just thought I would show off a little of geology from my neck of the woods. Here is a picture of Lizard Butte near Marsing, ID along the Snake River:Lizard Butte, Idaho | Snake River Oregon Trail Significance. | FlickrWhat you are seeing is a flood basalt that solidified on top of a lot of sediment. It is called Lizard Butte because . . . well . . . it looks like a lizard resting on a hill. Anyway, the flood basalt on top of the sediments can be dated, and that gives a minimum age for the sediments below. If you were to dig below the sediments and find another basalt or igneous rock layer this would allow you to construct an age range for the sediments between the igneous layers.Edited by Taq, : No reason given.

One cause. Not the only cause. Certainly, plate boundaries offer more openings for magma to migrate to the surface and we tend to see a lot of volcanic activity around those boundaries; eg, the Pacific "Ring of Fire" -- on Google Earth, check out the Aleutian Islands some time. However, magma may also migrate in the middle of a plate, two famous examples being the mid-Pacific "hot spot" (currently just east of Hawaii) that has created a long chain of islands and seamounts as the Pacific plate moved over it, and the Yellowstone cauldera (http://en.wikipedia.org/wiki/Yellowstone_Caldera#Volcanism):

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Yellowstone, like Hawaii, is believed to lie on top of an area called a hotspot where light, hot, molten mantle rock rises towards the surface. While the Yellowstone hotspot is now under the Yellowstone Plateau, it previously helped create the eastern Snake River Plain (to the west of Yellowstone) through a series of huge volcanic eruptions. Although the hotspot's apparent motion is to the east-northeast, the North American Plate is really moving west-southwest over the stationary hotspot deep underneath.

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At any rate, the speed of plate tectonic activity or even the very presence of activity has absolutely no bearing on the age of any igneous formations. Rather, it is when those formations had solidified that determines that. Here is an excellent article on isochron dating, http://www.talkorigins.org/faqs/isochron-dating.html, which explains that.Also, here is a link to a textbook, An Introduction to Physical Science at http://books.google.com/books?id=1LvMLoaN0HQC&pg=PA710&lp.... That page gives an example of interpreting two igneous intrusions -- it's in Google books, so I'm unable to copy-and-paste. The graphic it refers to is on the next page, so simply scroll down a little bit.

That was a trick question, one that nobody else caught, which surprises me. The age of a rock is how long ago it had formed. In the case of igneous rock, which we are talking about here, it formed when it solidified from a molten state. Solidified magma, AKA "igneous rock", has started to age and its age is tested by radiometric dating. Magma has not yet solidified and hence is still molten doesn't need to be tested. We already know its age: zero.Now, igneous rock that is many millions of years old could be re-melted and mixed thoroughly, in which case its clock has been reset, its age is back to zero and when it solidifies then its clock will start running again.Read that article on isochron dating at http://www.talkorigins.org/faqs/isochron-dating.html . It explains all that. That and Dr. Wiens' ASA article, "Christian Perspective on Radiometric Dating" (title cited from memory) should be required reading.

Thanks, Taq and Dwise. More food for thought.

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. 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.)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.

Magma can indeed contain crystals since not all crystals form at the same temperature. That being said recovering and actually dating those crystals is beyond current technology. But in generally it is not impossible just difficult.

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

I have on idea if it has a separate name since most magma would have it. That being said diffusion would be a huge problem so you could only date young magma or see which crystals are least affect. But hey its a moot point unless you can go sample a piece of magma.

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I have on idea if it has a separate name since most magma would have it. That being said diffusion would be a huge problem so you could only date young magma or see which crystals are least affect. But hey its a moot point unless you can go sample a piece of magma.

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To be a bit more precise here. Radiometric ages give us cooling ages, and not solidification ages. A given mineral has a 'trapping' temperature above which daughter elements are mobile, even in the solid state. Most of those temperatures would be below the temperature that the rock is solid (called the 'solidus'). Hence each mineral would give a different age based on its trapping temperature and its cooling history.This, of course, would confuse YECs...There are ways around this mineral issue based on isochron dating and whole rock methods, but it has been very long since I studied them and I'm too lazy to look up the information. I think that RAZD has it handy.

And, of course, once the mineral is well below its closure temperature diffusion is not a problem.Closure temperature varies by mineral and atomic species. I.e. the temperature at which U is immobilized will be different from the temperature at which Sr is immobilized. This fact is used to track the cooling history of rocks which cooled over times greater than the uncertainty in radiometric ages, by dating different minerals from the rock by different methods. E.g. Tectonic geomorphology.

Actually if you look at Zircons with uranium lead the lead would move a few nanometers in millions of years at 900C or so. In that case you do get a solidification age. Diffusion can be a problem but not in all cases.