Label 1a lies on the rim of a large crater, and 1b lies on the rim of a smaller one. Which crater must have formed first?

Label 1a

Explanation:

The craters are usually referred to as those large depressions that are created due to the impact by meteorites, asteroids or other astronomical bodies. These are of variable size and shape, which are controlled by factors such as the mass of the impact body, velocity at which the impact body strikes.

According to the rule of thumb, the geologically older surfaces are exposed to various types of impacting bodies such the meteoroids, asteroids, and comets, for a longer duration of time in comparison to the surfaces that are younger in age.

The bigger craters are formed first, and the smaller craters are formed later. Thus, the age of the craters is directly proportional to the size of the craters.

Hence, the larger crater (label 1a) was formed first.