Domes occur singly or in clusters. The best known dome cluster is the sprawling blister-like Rumker plateau in the Oceanus Procellarum, around 70 km across. The Copernicus region is an excellent location for domes. Here, many domes in clusters can be seen, notably north of Hortensius and south of Tobias Mayer. Famous single domes include the dome Pi near the crater Milichius, 200 km due west of Copernicus, which has a small summit craterlet discernable with a 150 mm reflector at high power. A 10,000 sq km field of dome-like features, elongated ridges and low rounded hills lies to the west of Marius in Oceanus Procellarum. Even a 60 mm refractor is capable of resolving any of the above domes when the illumination conditions are suitable.
Early lunar observers concentrated on the more obvious lunar features in their efforts to produce basic maps of the Moon. They really weren't too concerned about charting finer detail like domes or rilles which can only be discerned at low angles of illumination. The 18th/19th century German lunar observers Schroter and Gruithuisen both recorded features at the locations of known domes, yet they attributed them to lunar clouds because they vanished as the sun climbed higher over the Moon's landscape. Ironically, both lunar features named after these two astronomers - Schroter's Valley and Gruithuisen crater - have rather large domes nearby! Mons Gruithuisen Gamma on the northwest border of Mare Imbrium is the loftiest single dome on the Moon, and like the dome near Schroter's Valley it boasts its own summit craterlet.There are a number of ways lunar domes could have been formed. Localised pressure of magma under the Moon's crust may have caused the parts of the surface to arch upwards. Alternatively - and most likely in the case of the majority of domes - they may be examples of true lunar volcanoes, with their summit craterlets representing the remnants of volcanic vents. It also appears that many of the more irregular domes or low hills are actually ancient hills or mountains that have been overlain with more recent lava flows. As the seas contracted and slumped, the previous topography became evident.
Perhaps early next century an astronaut will bravely abseil down into one of these summit craterlets and report on the interior of a real lunar volcano - they have long been extinct, so thankfully there is no chance of the visitor being incinerated in any of these features.
You can observe for yourself these fascinating remnants of ancient lunar activity. The Hortensius and Milichius area is best observed near the terminator on the evening of 7 March or morning of 23 March. The Marius region can be seen in all its splendour on the eveining of 9 March or morning of 25 March, and the Rumker plateau is on display during the evenings of 10/11 March.
