The two horizontal black stripes are places where data were lost during transmission to Earth.
Calderas are depressions caused by collapse during volcanic eruptions.
The lava flows are similar in texture to lava flows within the caldera at Hawaii's Kilauea volcano. This suggests that the floor of Chaac has been covered by a combination of lava flows and lava lakes. Galileo scientists believe that the greenish color on the caldera floor is a form of contaminated sulfur created when sulfur-rich material escaping from volcanic vents reacts chemically with warm lava flows. The high-resolution view shows numerous lava flows. The darkest flows are thought to be the most recent because they have not been covered by the sulfurous materials which coat most of Io's surface.