Scientists use potassium-argon method to determine the age of a rock sample. Three separate tests showed the rock to be 2.5 billion years old. However, scientists know the rock must be less than 1.25 billion years old based on the rocks around it. How is this possible?

Question

The rock is sedimentary, but is included in an igneous rock formation.

The radiometric dating was off due to loss of daughter isotopes.

The rock is igneous, but has been weathered, eroded, and incorporated into younger sedimentary rock layers

The radiometric dating was done incorrectly.

Answer 1

The most plausible explanation for the discrepancy between the potassium-argon dating results and the known age constraints based on surrounding rocks is:

The rock is igneous, but has been weathered, eroded, and incorporated into younger sedimentary rock layers.

In this scenario, the igneous rock may have originally formed 2.5 billion years ago, but subsequent geological processes could have eroded it and incorporated it into younger sedimentary layers. This would allow for the rock sample to be dated with potassium-argon methods, giving an older age, while the context in which it is found (the younger sedimentary layers) indicates that it must be younger. The potassium-argon method measures the decay of potassium-40 to argon-40, and if the rock has been exposed to weathering processes, it could have lost some of the argon gas, leading to inaccurately older dates.