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- Potassium-Argon Dating
So it won't necessarily seep out. But the argon will seep out. So it kind of resets it. The volcanic event resets the amount of argon So right when the event happened, you shouldn't have any argon right when that lava actually becomes solid. And so if you fast forward to some future date, and if you look at the sample-- let me copy and paste it. So if you fast forward to some future date, and you see that there is some argon there, in that sample, you know this is a volcanic rock.
You know that it was due to some previous volcanic event.
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You know that this argon is from the decayed potassium And you know that it has decayed since that volcanic event, because if it was there before it would have seeped out. So the only way that this would have been able to get trapped is, while it was liquid it would seep out, but once it's solid it can get trapped inside the rock.
And so you know the only way this argon can exist there is by decay from that potassium So you can look at the ratio. And so for every one of these argon's you know that there must have been 10 original potassium's. And so what you can do is you can look at the ratio of the number of potassium's there are today to the number that there must have been, based on this evidence right over here, to actually date it.
And in the next video I'll actually go through the mathematical calculation to show you that you can actually date it. And the reason this is really useful is, you can look at those ratios. And volcanic eruptions aren't happening every day, but if you start looking over millions and millions of years, on that time scale, they're actually happening reasonably frequent.
And so let's dig in the ground. So let's say this is the ground right over here. And you dig enough and you see a volcanic eruption, you see some volcanic rock right over there, and then you dig even more. There's another layer of volcanic rock right over there. So this is another layer of volcanic rock. So they're all going to have a certain amount of potassium in it. This is going to have some amount of potassium in it. And then let's say this one over here has more argon This one has a little bit less.
And using the math that we're going to do in the next video, let's say you're able to say that this is, using the half-life, and using the ratio of argon that's left, or using the ratio of the potassium left to what you know was there before, you say that this must have solidified million years ago, million years before the present. And you know that this layer right over here solidified. Let's say, you know it solidified about million years before the present.
And let's say you feel pretty good that this soil hasn't been dug up and mixed or anything like that. It looks like it's been pretty untouched when you look at these soil samples right over here. And let's say you see some fossils in here. Then, even though carbon dating is kind of useless, really, when you get beyond 50, years, you see these fossils in between these two periods.
It's a pretty good indicator, if you can assume that this soil hasn't been dug around and mixed, that this fossil is between million and million years old.grupoavigase.com/includes/288/2381-instituto-sorolla.php
Then you have these fossils got deposited. These animals died, or they lived and they died.
And then you had this other volcanic event. So it allows you, even though you're only directly dating the volcanic rock, it allows you, when you look at the layers, to relatively date things in between those layer. Herein lies the true advantage of the radiocarbon method. Potassium-Argon K-Ar dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks.
The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. It is based on the occurrence of a small fixed amount of the radioisotope 40 K in natural potassium that decays to the stable Argon isotope 40 Ar with a half-life of about 1, million years. In contrast to a method such as Radiocarbon dating, which measures the disappearance of a substance, K-Ar dating measures the accumulation of Argon in a substance from the decomposition of potassium. Argon, being an inert gas, usually does not leech out of a mineral and is easy to measure in small samples.
This method dates the formation or time of crystallisation of the mineral that is being dated; it does not tell when the elements themselves were formed. It is best used with rocks that contain minerals that crystallised over a very short period, possibly at the same time the rock was formed. This method should also be applied only to minerals that remained in a closed system with no loss or gain of the parent or daughter isotope.
Uranium-Lead U-Pb dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils particulary outside the range of radiocarbon. Quaternary geology provides a record of climate change and geologically recent changes in environment. U-Pb geochronology of zircon , baddelyite , and monazite is used for determining the age of emplacement of igneous rocks of all compositions, ranging in age from Tertiary to Early Archean. U-Pb ages of metamorphic minerals, such as zircon or monazite are used to date thermal events, including terrestrial meteoritic impacts.
U-Pb ages of zircon in sediments are used to determine the provenance of the sediments. The Fission track analysis is based on radiation damage tracks due to the spontaneous fission of U.
Potassium-argon (K-Ar) dating
Fission-tracks are preserved in minerals that contain small amounts of uranium, such as apatite and zircon. Fission-track analysis is useful in determining the thermal history of a sample or region.
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By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved. As long as the mineral has remained cool, near the earth surface, the tracks will accumulate.
If the rock containing these minerals is heated, the tracks will begin to disappear. How is the Atomic Clock Set? When rocks are heated to the melting point, any Ar contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K in the rock decays into Ar, the gas is trapped in the rock. The Decay Profile In this simulation, a unit of molten rock cools and crystallizes.
The ratio of K to Ar is plotted.
Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph. Click on the "Show Movie" button below to view this animation. How are Samples Processed?