The buildup of soil on bare rock by the decomposition of early colonizers is characteristic of succession.

primary succession

Explanation:

In ecology, succession is a phenomenon used to describe the process by which the structure of an ecosystem changes from time to time. New features, species and components are being introduced into an ecosystem.

There are 2 major types of succession: primary and secondary. While secondary succession involves a series of changes that occur in a biological community that is not new or that has been once colonized by several features and forms of life, primary succession involves the changes that occur from a bare land that has no life form.  

Primary succession occurs on barren lands and bare rocks. The buildup of soil on bare rock by the decomposition of early colonizers is characteristic of primary succession .