How would we use isotopic ages to infer the ages of the upper and lower unit in this diagram?
A)The lower unit would be younger than an age on the crosscutting dike.
B)The upper unit would be older than an age determined on pebbles of the dike in that unit.
C)The dike is younger than the lower unit but older than the upper unit.
D)An age on the dike would not help us because it is an igneous rock.

Question

How would we use isotopic ages to infer the ages of the upper and lower unit in this diagram? 
A)The lower unit would be younger than an age on the crosscutting dike. 
B)The upper unit would be older than an age determined on pebbles of the dike in that unit. 
C)The dike is younger than the lower unit but older than the upper unit. 
D)An age on the dike would not help us because it is an igneous rock.

Quizplus · Accepted Answer

Isotopic ages can be used to infer relative ages of rocks. A crosscutting dike is a younger feature, so we know the dike is younger than the lower unit. If we find an age on the dike, we know it must be younger than that age. If we find an age on the pebbles of the dike within the upper unit, we know that the upper unit must be older than that age. Therefore, the dike is younger than the lower unit but older than the upper unit.

How Would We Use Isotopic Ages to Infer the Ages