NASA Earth Observatory provides a satellite view as well and gives a few more details regarding the very noticeable dark area:
Es Safa contains numerous vents that have been active during the Holocene Epoch (beginning approximately 12,000 years ago). The most recent recorded activity was a boiling lava lake observed in the area around 1850. The dark lava flow field (center) likely represents the latest activity of the volcanic field, and is emplaced over older, lighter colored flows.