How did the craters on the moon form? John Lee 05.14.01 About 4 billion years ago, a series of major impacts occurred, forming huge craters on the moon. These craters are now the sites of basins call maria (e.g., Mare Imbrium, Mare Serenitatis). Between 4 and 2.5 billion years ago, volcanic activity filled these basins with dark-colored lavas, called basalts. CLICK HERE for more information! ©2001 NASAkids.com , Science@NASA  John Lee ©2001 The Aurora Collection, Inc. All Rights Reserved. Site Development by: Interactive Multimedia. Inc.