Moon Volcanic Features

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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 were more easily erupted through the thinner crust. Rilles are of two basic types: sinuous and straight. Sinuous rilles are meandering channels that are probably lava drainage channels or collapsed lava tubes formed by large lava flows. Straight rilles are large shallow troughs caused by movement of the Moon’s crust; they may be up to a thousand kilometers long and several kilometers wide. Domes are small rounded features that range from 8 to 16 km (5 to 10 mi) in diameter and from 60 to 90 m (200 to 300 ft) in height. Domes, thought to be small inactive volcanoes, often contain a small rimless pit on their tops.

Magnetic and other measurements indicate a current temperature at the Moon’s core as high as 1600°C (2900°F), above the melting point of most lunar rocks. Evidence from seismic recordings suggests that some regions near the lunar center may be liquid. However, no evidence of recent volcanic activity has been observed.

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