Research reveals that during the latter part of Holocene – a term used to describe a period that began about 11,700 years ago – the volcanic systems on the Reykjanes peninsula have erupted every 900 to 1100 years. Less is known about the first part of Holocene.
Each eruption period appears to have lasted about 500 years, and during that time most of the volcanic systems appear to have been active, albeit generally not simultaneously.
Ah. Most of them APPEAR to have been active. It doesn't get more conclusive than that. Thanks, science.