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So, we start out with two isotopes of uranium that are unstable and radioactive. They release radiation until they eventually become stable isotopes of lead. These two uranium isotopes decay at different rates. In other words, they have different half-lives. The half-life of the uranium to lead is 4. The uranium to lead decay series is marked by a half-life of million years.
These differing rates of decay help make uranium-lead dating one of the most reliable methods of radiometric dating because they provide two different decay clocks.
This provides a built-in cross-check to more accurately determine the age of the sample. Uranium is not the only isotope that can be used to date rocks; we do see additional methods of radiometric dating based on the decay of different isotopes. For example, with potassium-argon datingwe can tell the age of materials that contain potassium because we know that potassium decays into argon with a half-life of 1.
With rubidium-strontium datingwe see that rubidium decays into strontium with a half-life of 50 billion years. By anyone's standards, 50 billion years is a long time.
In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon. So, we see there are a number of different methods for dating rocks and other non-living things, but what if our sample is organic in nature?
For example, how do we know that the Iceman, whose frozen body was chipped out of glacial ice inis 5, years old? Well, we know this because samples of his bones and hair and even his grass boots and leather belongings were subjected to radiocarbon dating. Radiocarbon datingalso known as carbon dating or simply carbon dating, is a method used to determine the age of organic material by measuring the radioactivity of its carbon content.
So, radiocarbon dating can be used to find the age of things that were once alive, like the Iceman. And this would also include things like trees and plants, which give us paper and cloth. So, radiocarbon dating is also useful for determining the age of relics, such the Dead Sea Scrolls and the Shroud of Turin.
With radiocarbon dating, the amount of the radioactive isotope carbon is measured. Compared to some of the other radioactive isotopes we have discussed, carbon's half-life of 5, years is considerably shorter, as it decays into nitrogen Carbon is continually being created in the atmosphere due to the action of cosmic rays on nitrogen in the air.
Carbon combines with oxygen to create carbon dioxide. Because plants use carbon dioxide for photosynthesis, this isotope ends up inside the plant, and because animals eat plants, they get some as well.
When a plant or an animal dies, it stops taking in carbon The existing carbon within the organism starts to decay back into nitrogen, and this starts our clock for radiocarbon dating. A scientist can take a sample of an organic material when it is discovered and evaluate the proportion of carbon left in the relic to determine its age.
Let's review. Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. The decay rate is referring to radioactive decaywhich is the process by which an unstable atomic nucleus loses energy by releasing radiation. Each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life or, in other words, the time required for a quantity to fall to half of its starting value.
There are different methods of radiometric dating. Uranium-lead dating can be used to find the age of a uranium-containing mineral. Uranium decays to lead, and uranium decays to lead The two uranium isotopes decay at different rates, and this helps make uranium-lead dating one of the most reliable methods because it provides a built-in cross-check.
Additional methods of radiometric dating, such as potassium-argon dating and rubidium-strontium datingexist based on the decay of those isotopes. Radiocarbon dating is a method used to determine the age of organic material by measuring the radioactivity of its carbon content.
With radiocarbon dating, we see that carbon decays to nitrogen and has a half-life of 5, years. To unlock this lesson you must be a Study. Create your account. Already a member? Log In.
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Radiometric dating. The time that it takes for half of a sample to what is known as the half life of the isotope. Some isotopes have half lives longer than the present age of the universe, but they are still subject radiometric the same are of quantum what and will eventually decay, even if doing so at a time when all remaining atoms in dating universe are separated by astronomical distances. Examples of Radiometric Dating. Uranium-lead (U-Pb) dating: Radioactive uranium comes in two forms, uranium and uranium The number refers to the number of protons plus neutrons. Uranium's atomic number is 92, corresponding to its number of protons. which decay into lead and lead respectively. The earth is billions of years old. The most useful methods for measuring the ages of geologic materials are the radiometric methods-the ones that make use of radioactive parent isotopes and their stable daughter products, as preserved in rocks, minerals, or other geologic materials. The main condition for the method is that the production rate of isotopes stays the same through ages, i.e.
Imagine that you enjoy a certain kind of ice cream flavored with chocolate chips. You have a sneaky, but not especially clever, roommate who doesn't like the ice cream itself, but cannot resist picking out eating the chips - and in an effort to avoid detection, he replaces each one he consumes with a raisin. He is afraid to do this with all of the chocolate chips, so instead, each day, he swipes half of the number of remaining chocolate chips and puts raisins in their place, never quite completing his diabolical transformation of your dessert, but getting closer and closer.
Say a second friend who is aware of this arrangement visits and notices that your carton of ice cream contains 70 raisins and 10 chocolate chips. She declares, "I guess you went shopping about three days ago. Because your roommate eats half of the chips on any given day, and not a fixed number, the carton must have held 20 chips the day before, 40 the day before that, and 80 the day before that.
Calculations involving radioactive isotopes are more formal but follow the same basic principle: If you know the half-life of the radioactive element and can measure how much of each isotope is present, you can figure out the age of the fossil, rock or other entity it comes from.
Elements that have half-lives are said to obey a first-order decay process.
They have what is known as a rate constant, usually denoted by k. The relationship between the number of atoms present at the start N 0the number present at the time of measurement N the elapsed time t, and the rate constant k can be written in two mathematically equivalent ways:. In addition, you may wish to know the activity A of a sample, typically measured in disintegrations per second or dps.
This is expressed simply as:.
Types of radiometric dating methods
You don't need to know how these equations are derived, but you should be prepared to use them so solve problems involving radioactive isotopes. Scientists interested in figuring out the age of a fossil or rock analyze a sample to determine the ratio of a given radioactive element's daughter isotope or isotopes to its parent isotope in that sample.
With the element's decay rate, and hence its half-life, known in advance, calculating its age is straightforward.
The trick is knowing which of the various common radioactive isotopes to look for. This in turn depends in the approximate expected age of the object because radioactive elements decay at enormously different rates. Also, not all objects to be dated will have each of the elements commonly used; you can only date items with a given dating technique if they include the needed compound or compounds. Uranium-lead U-Pb dating: Radioactive uranium comes in two forms, uranium and uranium The number refers to the number of protons plus neutrons.
Uranium's atomic number is 92, corresponding to its number of protons. The half-life of uranium is 4. Because these differ by a factor of almost seven recall that a billion is 1, times a millionit proves a "check" to make sure you're calculating the age of the rock or fossil properly, making this among the most precise radiometric dating methods.
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The long half-lives make this dating technique suitable for especially old materials, from about 1 million to 4. U-Pb dating is complex because of the two isotopes in play, but this property is also what makes it so precise.
The method is also technically challenging because lead can "leak" out of many types of rocks, sometimes making the calculations difficult or impossible. U-Pb dating is often used to date igneous volcanic rocks, which can be hard to do because of the lack of fossils; metamorphic rocks; and very old rocks. All of these are hard to date with the other methods described here. Rubidium-strontium Rb-Sr dating: Radioactive rubidium decays into strontium with a half -life of Not surprisingly, Ru-Sr dating is used to date very old rocks as old as the Earth, in fact, since the Earth is "only" around 4.
Strontium exists in other stable i. But because rubidium is abundant in the Earth's crust, the concentration of strontium is much higher than that of the other isotopes of strontium. Scientists can then compare the ratio of the strontium to the total amount of stable strontium isotopes to calculate the level of decay that produces the detected concentration of strontium This technique is often used to date igneous rocks and very old rocks.
Potassium-argon K-Ar dating: The radioactive potassium isotope is K, which decays into both calcium Ca and argon Ar in a ratio of Argon is a noble gas, which means that it is nonreactive and would not be a part of the initial formation of any rocks or fossils.
Any argon found in a rocks or fossils therefore has to be the result of this kind of radioactive decay. The half-life of potassium is 1. Potassium is very abundant in the Earth, making it great for dating because it is found in some levels in most kinds of samples. It is good for dating igneous rocks volcanic rocks.