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.

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Obviously, these eruptions took place very recently, after the Canyon’s layers were deposited ().

These basalts yield ages of up to 1 million years based on the amounts of potassium and argon isotopes in the rocks.

Find additional lessons, activities, videos, and articles that focus on relative and absolute dating.

PART 1: Back to Basics PART 2: Problems with the Assumptions PART 3: Making Sense of the Patterns This three-part series will help you properly understand radiometric dating, the assumptions that lead to inaccurate dates, and the clues about what really happened in the past.

By dating these surrounding layers, they can figure out the youngest and oldest that the fossil might be; this is known as "bracketing" the age of the sedimentary layer in which the fossils occur.

Teach your students about absolute dating: Determining age of rocks and fossils, a classroom activity for grades 9-12.There are similar problems with the other radioactive “clocks.” For example, consider the dating of Grand Canyon’s basalts (rocks formed by lava cooling at the earth’s surface).We find places on the North Rim where volcanoes erupted after the Canyon was formed, sending lavas cascading over the walls and down into the Canyon.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?Radioactive rocks offer a similar “clock.” Radioactive atoms, such as uranium (the parent isotopes), decay into stable atoms, such as lead (the daughter isotopes), at a measurable rate.