Cones and Domes
While there are no large volcanoes on the Moon, a few smaller volcano-like features have been recognized. These features are mostly fairly small. Few are more than a few thousand feet (few hundred meters) high, or more than 6 to 10 miles (10 to 15 km) across. They are also somewhat irregular in outline, and most are not very striking in appearance. Few show any large central pit or vent structures, but many do have very small central pits or craters.
These lunar constructs resemble small cinder cones and volcanic domes on the Earth. However, such cones and domes may form differently on the Earth and Moon. On the Earth, cinder cones form when small explosive eruptions pile up pieces of lava around a central vent. On the Moon, however, such eruptions will throw things much further, leaving little to pile up near the vent. Instead of a volcanic cone, such lunar eruptions should form a broad, thin layer around the central vent (a dark mantling deposit). Similarly, lava domes on Earth form from very thick, pasty lavas. Basaltic lavas are more liquid, and tend to form broad, flat lava flows. On the Moon, most of the domes and cones appear to be made of basalts. Thus, they can not have formed like Earth domes from thick, non-basaltic lavas. Instead, the lunar domes/cones may mark places where the erupted basalts were just barely molten.
Shown here are the Marius Hills in Oceanus Procellarum. These hills are the largest group of volcanic cones and domes on the Moon. Note the fairly uniform size and appearance of these features. Also note the sinuous rilles which have formed within the reg ion. (Lunar Orbiter image IV-157-H2, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)
This image shows the Rumker Hills in northern Oceanus Procellarum (arrows). These hills are low, flat mounds which formed on a small plateau. Their age is poorly known. Note the vary small central pits in the hills near the image center. (Lunar Orbiter image IV-170-H2, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)
This image shows two mare domes in northwestern Mare Imbrium. They are located on the rim of an old, mare-filled impact crater. Note -- These domes differ in color both from mare basalts and from typical highland rocks (see mare types). They may mark a rare instance of non-basaltic lunar volcanism. (Lunar Orbiter Image V-182-M, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)