Although the mare formed from large effusive lava flows, there is some evidence for explosive volcanism on the Moon. In places, the lunar surface is covered by dark layers of material. The largest of these areas are near the edges of the lunar mare. They cover many thousands of square kilometers. They also include a range of knobs and other highland features. Thus, because lavas only flow downhill, these units can not be lava flows. Instead, they seem to mark areas where a thin layer has been draped over an older surface. Apollo 17 brought back samples from one such unit. They contain many small spheres of orange and black glass. These spheres probably formed from small drops of lava that cooled very quickly. Such droplets are thrown out of an eruption when bubbles of gas burst near the surface. Due to the size of the dark mantle deposits, however, some of these spheres may have been thrown hundreds of kilometers. Thus, despite the low gravity and lack of air on the Moon, some of lunar eruptions must have been quite violent. They may have resembled Hawaiian fire fountains, but on a much larger scale. There are also many smaller dark mantling units on the Moon. Most of these features are only a few kilometers in diameter. They are almost always located near the mare or in large crater floors. Many also lie along clear fault lines. Since most have a small central pit or crater, they are likely sites for small volcanic explosions. Some of these small eruptions may have released gases from shallow lunar intrusions.
Map of Large Dark Mantling Deposits
This is a map of the central nearside. It shows the sizes and locations of the largest known dark mantling deposits. Note that most of these units are near the edges of major maria. Some may in fact be partly buried by younger mare lavas. For reference, Apollo 17 landed in the Taurus Littrow unit on the southeastern edge of Mare Serenitatis. (This map is based on the work of Head, 1974 and Gaddis et al, 1985, as presented in Hawke et al, 1990.)
This image shows one example of a large dark mantling deposit. It is located in Sinus Aestum on the central nearside, and is just east of the crater Copernicus. (Part of Copernicus lies on the leftmost edge of the photo.) Note how the dark areas in this photo (arrows) resemble the brighter highlands nearby. This suggests that the dark unit is relatively thin, and that it was dropped uniformly across an older, non-volcanic surface. (Earth-based telescopic photo from the Consolidated Lunar Atlas).
This oblique photo shows the crater Alphonsus on the eastern edge of Mare Nubium. Note the five dark patches along the crater floor edge. Each of these patches has a central pit. Three are located along clear fracture trends. Also, most of these pits are also elongated or irregular in shape. Thus, they are unlikely to be small impact craters. Instead, they probably mark the sites of small vulcanian eruptions. Note: The long corkscrew-like pole pointing into the image from the left is part of the Apollo Orbiter. (Apollo 16 image M-2478, looking southwards.)
This figure shows two styles of volcanism that might leave dark mantling units. (1) Strombolian eruptions feature bubbly, frothy magmas, and spatter large volumes of magma over large distances. Thus, such eruptions may have formed the largest dark mantle deposits on the Moon. (2) Vulcanian eruptions tend to be smaller than strombolian eruptions. They feature short explosions of gas and rocks. Because gases need to build up near the vent, they do not involve large volumes of magma. They also are somewhat episodic. Thus, vulcanian eruptions are more likely to form the smaller patches of dark materials on the Moon. They also are more likely to open up a recognizable central pit or vent structure. In the figure, red depicts new lavas in an eruption, blue represents old, colder lavas, and purple denotes mixtures of old and new lava. (figure modified from Wilson and Head (1981) J. Geophys. Res, vol. 86)