How does knowing the half life of an isotope help a geologist establish the age of a rock or fossil?

Answer 1

In general, the shorter the half-life of a radioisotope, the less far back in time it will be useful in dating things from the past. Radioisotopes with short half-lives decay away more quickly, and if "too much" time has passed, there isn't a sufficient amount of the radioisotope left to "count" it with sufficient accuracy to date something. Radiocarbon dating is a fairly well known method of dating living thing back a few tens of thousands of years. It looks at ("counts") the carbon-14 decay in a sample and determines how long ago the living material died. When we are alive, our bodies are built of materials that include carbon. And carbon as it occurs has a bit of the C-14 isotope in it. That means there will be a give "concentration" of C-14 in all our tissues. When we die, C-14 intake stops, and the C-14 in our tissues decays without being replaced. We know how long it takes for C-14 to decay (we know its half-life), and we can measure what is left and make a fairly accurate estimate of how long the material has been dead. After a while, there just isn't enough C-14 left to make for accurate dating. Anyone who says living material has been C-14 dated to 100,000 years is pulling your leg. If we were dating rocks, we could use uranium-lead or lead-lead dating. They take as way back in time. But these methods have limits on their applications, just like carbon dating. There are always limits on what we can do as regards radiometric dating, and one of them depends on which radioisotope we choose to illuminate the past.

Answer 2

In general, the shorter the half-life of a radioisotope, the less far back in time it will be useful in dating things from the past. Radioisotopes with short half-lives decay away more quickly, and if "too much" time has passed, there isn't a sufficient amount of the radioisotope left to "count" it with sufficient accuracy to date something.

Radiocarbon dating is a fairly well known method of dating living thing back a few tens of thousands of years. It looks at ("counts") the carbon-14 decay in a sample and determines how long ago the living material died. When we are alive, our bodies are built of materials that include carbon. And carbon as it occurs has a bit of the C-14 isotope in it. That means there will be a give "concentration" of C-14 in all our tissues. When we die, C-14 intake stops, and the C-14 in our tissues decays without being replaced. We know how long it takes for C-14 to decay (we know its half-life), and we can measure what is left and make a fairly accurate estimate of how long the material has been dead.

After a while, there just isn't enough C-14 left to make for accurate dating. Anyone who says living material has been C-14 dated to 100,000 years is pulling your leg. If we were dating rocks, we could use uranium-lead or lead-lead dating. They take as way back in time. But these methods have limits on their applications, just like carbon dating. There are always limits on what we can do as regards radiometric dating, and one of them depends on which radioisotope we choose to illuminate the past.

Answer 3

The half-life of an isotope is only useful in dating if the half-life is a reasonable fraction of the actual age. For example, a half-life in the range of days or even a few years is meaningless if the sample is several thousand years old. On the other hand, a half-life in the hundreds of thousands of years is also not useful if the sample is, say, 15,000 years old. All of this has to do with statistical validity of the measurement of the ratio remaining.

That brings me to the second factor. Half-life is only meaningful if you have something to compare it to. You need to be able to correlate the measurement of the sample against some reference point. Since quantity of the sample is a function of both half-life intervals (age) and original quantity, attempting to measure the age without the original quantity will fail.

The most commonly used method, carbon-14 dating, solves this problem by comparing the quantity of carbon-14 against carbon-12 and carbon-13, both of which are stable. However, this only works because we, as scientists, have accepted that there is a known ratio of carbon-14 to carbon-12/carbon-13, said ratio starting the clock "ticking" at the point of age=zero.

This works for carbon-14 dating because the ratio is relatively constant when the biochemical composition of the sample represents an "alive" sample. Carbon-12 dating depends on plant material that is associated or attached to the sample. We call this carbonaceous material. When the plant material "dies", the constant refreshing of the ratio stops. This allows us to date a sample within a short region of time that is not an excessively long number of carbon-14 half-lives. (5730 years, give or take)

Complicating this is the fact that the ratio varies over time, due to variations in the solar wind activity. Scientists calibrate the ratio using independent samples of materials that are dated by other means.

Answer 4

The half life is a time standard.

Answer 5

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