Astronomers believe that the deposit of lava in the giant mare basins did not happen in one flow but in many different eruptions spanning some time. indeed, in any one mare, we find a variety of rock ages, typically spanning about 100 million years. the individual lava flows as seen in hadley rille by the apollo 15 astronauts were about 4 m thick. estimate the average time interval between the beginnings of successive lava flows if the total depth of the lava in the mare is 2 km.