Four radioisotopes commonly used radiometric dating
To date a radioactive rock, geologists first measure the “sand grains” in the top glass bowl (the parent radioisotope, such as uranium-238 or potassium-40).
Most people think that radioactive dating has proven the earth is billions of years old.These basalts yield ages of up to 1 million years based on the amounts of potassium and argon isotopes in the rocks.But when we date the rocks using the rubidium and strontium isotopes, we get an age of 1.143 billion years.This is the same age that we get for the basalt layers deep below the walls of the eastern Grand Canyon.4 How could both lavas—one at the top and one at the bottom of the Canyon—be the same age based on these parent and daughter isotopes?One solution is that both the recent and early lava flows inherited the same rubidium-strontium chemistry—not age—from the same source, deep in the earth’s upper mantle.Based on these observations and the known rate of radioactive decay, they estimate the time it has taken for the daughter isotope to accumulate in the rock.
However, unlike the hourglass whose accuracy can be tested by turning it upside down and comparing it to trustworthy clocks, the reliability of the radioactive “clock” is subject to three unprovable assumptions.
No geologist was present when the rocks were formed to see their contents, and no geologist was present to measure how fast the radioactive “clock” has been running through the millions of years that supposedly passed after the rock was formed.
No geologists were present when most rocks formed, so they cannot test whether the original rocks already contained daughter isotopes alongside their parent radioisotopes.
Yet this view is based on a misunderstanding of how radiometric dating works.
Part 1 (in the previous issue) explained how scientists observe unstable atoms changing into stable atoms in the present.
When we look at sand in an hourglass, we can estimate how much time has passed based on the amount of sand that has fallen to the bottom.