Radiometric dating sedimentary rock
This lower limit is at least concordant with the independently derived figure of 4.55 billion years for the Earth's actual age.
The most direct means for calculating the Earth's age is a Pb/Pb isochron age, derived from samples of the Earth and meteorites.
The oldest rocks which have been found so far (on the Earth) date to about 3.8 to 3.9 billion years ago (by several radiometric dating methods).
Some of these rocks are sedimentary, and include minerals which are themselves as old as 4.1 to 4.2 billion years.
This causes the data points to separate from each other.
The higher the uranium-to-lead ratio of a rock, the more the Pb-206/Pb-204 and Pb-207/Pb-204 values will change with time.
If the source of the solar system was also uniformly distributed with respect to uranium isotope ratios, then the data points will always fall on a single line.
And from the slope of the line we can compute the amount of time which has passed since the pool of matter became separated into individual objects.
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Changes in the environment cause changes in the rocks. Sometimes, the layers are twisted or tilted because of the movement of the Earth’s tectonic plates. Scientists study these layers to learn more about the Earth’s history.
For example, a volcanic eruption may create a layer made of hardened ash. Radiometric dating measures radioactive elements in the rocks.
This involves measurement of three isotopes of lead (Pb-206, Pb-207, and either Pb-208 or Pb-204).
A plot is constructed of Pb-206/Pb-204 versus Pb-207/Pb-204.