In Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, La-Be, La-Ce and K-Ca dating, we check that the points we plot on the isochron diagram lie on a straight line.
We began our discussion of absolute dating by saying that sedimentation rates could not be relied on for absolute dating.
There is no particular reason to suspect that this will turn out to be the case when it comes to the laws underlying absolute dating; nonetheless, an argument from principle alone can never be entirely convincing. You will recall from our discussion of sea floor spreading that the sea floor spreads out from mid-ocean rifts, and so ought to be younger nearer the rifts and progressively older further away from them.
What is more, we can measure the rate of spreading directly by GPS, SLR, and VLBI.
Consider the following analogy: a clockmaker sells us an electric clock, a pendulum clock, and a spring-driven clock, and guarantees that they are shockproof.
Skeptical of the clockmaker's claim, we subject the clocks to shock: we shake them, drop them, hit them with hammers and shoot them out of a cannon.
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We also see close agreement between dendrochronology and uncalibrated radiocarbon dates.