Examples are granites (formed by cooling under the ground) and basalts (formed by cooling of lava at the earth’s surface).
The next step is to measure the amount of the parent and daughter isotopes in a sample of the rock unit.
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These parent radioisotopes change into daughter lead-206, lead-207, argon-40, strontium-87, and neodymium-143 isotopes, respectively.
Thus geologists refer to uranium-lead (two versions), potassium-argon, rubidium-strontium, or samarium-neodymium dates for rocks.
Some isotopes are radioactive; that is, they are unstable because their nuclei are too large.
To achieve stability, the atom must make adjustments, particularly in its nucleus.
In some cases, the isotopes eject particles, primarily neutrons and protons.
(These are the moving particles measured by Geiger counters and the like.) The end result is a stable atom, but of a different chemical element (not carbon) because the atom now has a different number of protons and electrons.
It is the interpretation of these chemical analyses that raises potential problems.
To understand how geologists “read” the age of a rock from these chemical analyses, let’s use the analogy of an hourglass “clock” (Figure 2).
He walks into the room when half the sand is in the top bowl, and half the sand is in the bottom bowl.
Most people would assume that the “clock” started half an hour earlier.
These variations are called isotopes of that element.