The submarine volcanic emplacement of the Ontong Java Plateau (OJP) is the suggested cause of Ocean Anoxic Event 1a (OAE 1a). However, no precise timing and duration exists for the formation of OJP, and its connection to OAE1a relies mainly on proxies in the sedimentary record. We provide high-precision 40Ar/39Ar data from OJP drill and dredge sites that considerably improve OJP's eruptive history. The ages determined from this work are as much as 10 million years younger than previous dates and indicate a protracted formation over at least 6 million years. OJP now appears too young to have caused OAE1a, but we suggest that it may have had a role in the later OAE1b. The protracted eruptive sequence has implications for the emplacement dynamics of OJP and other large igneous provinces.