The oldest volcanic rocks in Haruj appear to be not older than Pliocene, although the presence of buried Miocene age flows in the northern sector of the field has been suggested. The oldest eruptions have been dated to be either 6.4 million years old or of Late Pliocene age and activity was originally thought to have continued to the Late Pleistocene; Wau an Namus may be 200,000 years old. Most of the field is younger than 2.2 million years ago and output appears to have decreased over time. Some eruptions may have been large enough to impact the regional environment.