Figure 6-24 Mare Basalt This 1.53-kg (3.38-lb) specimen of mare basalt was brought back by Apollo 15 astronauts in 1971. Small holes that cover about a third of its surface suggest that gas was dissolved in the lava from which this rock solidified. When the lava reached the airless lunar surface, bubbles formed as the pressure dropped and the gas expanded. Some of the bubbles were frozen in place as the rock cooled.