Moon Volcanic Features
Maria, domes, rilles, and a few craters display indisputable characteristics of volcanic origin. Maria are plains of dark-colored rock that cover approximately 40 percent of the Moon's visible hemisphere. The maria formed when molten rock erupted onto the surface and solidified between 3.16 billion and 3.96 billion years ago. This rock resembles terrestrial basalt , a volcanic rock type widely distributed on Earth, but the rock that formed the maria has a higher iron content and contains unusually large amounts of titanium. The largest of the maria is Oceanus Procellarum, an oval-shaped plain on the near side of the Moon 2,500 km by 1,500 km wide. Photographs of the side of the Moon not visible from Earth have revealed a startling fact: The far side generally lacks the maria that are so conspicuous a feature of the visible side. This probably reflects the fact that the Moon's crust is thicker on the far side than on the near side, and therefore the lavas that form the maria wer