This is the type of growth experienced by a population that increases at a rate proportional to its size. the larger the population gets, the faster it grows.

The “population growth rate” is the rate at which the number of individuals in a population increases in a given time period as a fraction of the initial population. Specifically, population growth rate refers to the change in population over a time period, often expressed as a percentage of the number of individuals in the population at the beginning of that period.

Changes in population growth are part of exponential growth. Exponential growth is exhibited when the rate of change (the change per instant or unit of time) of the value of a mathematical function of time is proportional to the function's current value, resulting in its value.  In this way, each increase in the exponent by a full interval can be s  n to increase the previous total.

i'd say that the moon rock is about 2.2 billion years old.