When was absolute dating first used

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.

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.

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.

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.

Many substances, however, both biological and chemical, conform to a different mechanism: In a given time period, half of the substance will disappear in a fixed time no matter how much is present to start with.

which decay into lead-206 and lead-207 respectively.

If you want to know how old someone or something is, you can generally rely on some combination of simply asking questions or Googling to arrive at an accurate answer.

This applies to everything from the age of a classmate to the number of years the United States has existed as a sovereign nation (243 and counting as of 2019).

The half-life of uranium-238 is 4.47 billion years, while that of uranium-235 is 704 million years.

Because these differ by a factor of almost seven (recall that a billion is 1,000 times a million), it 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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