Final answer:
The lunar maria are dark, basaltic plains on the Moon formed by ancient volcanic eruptions, filling in impact basins created by collisions with celestial objects. They provide insight into the Moon's geological history.
Step-by-step explanation:
The lunar maria are vast plains on the Moon formed from ancient volcanic eruptions. These dark basaltic regions were created after large objects collided with the Moon, creating impact basins that were eventually filled with lava. Over billions of years, lunar volcanism ceased, but the basaltic plains remained as key features on the Moon's surface.
Studies have found that the basalt on the Moon is similar to that beneath Earth's oceanic crust or the lavas from terrestrial volcanoes. Mare Orientale is notably the youngest of these impact basins. The maria's smooth surface, as compared to the heavily cratered highlands, implies that the lava flows covered and obliterated earlier craters, which has given scientists important clues regarding the Moon's geological history.