- The period of geologic time between 4.6 and 3.8 billion years ago, when the solar system was forming and the Earth was solidifying. No rocks are known from this time, as they were presumably eroded or drawn deep into the Earth and melted through the processes of plate tectonics. See Chart at geologic time.
The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.