Earth's Moon


Volcanism on the Moon

By Robert Wickman


Overview:

The Earth's Moon has no large volcanoes like Hawaii or Mount St. Helens. However, vast plains of basaltic lavas cover much of the lunar surface. The earliest astronomers thought, wrongly, that these plains were seas of lunar water. Thus, they were called " mare " (pronounced "mahr-ay"). Mare means "sea" in Latin. In addition, other volcanic features also occur within the lunar mare. The most important are sinuous rilles , dark mantling deposits,and small volcanic domes and cones . Most of these features are fairly small, however. They form only a tiny fraction of the lunar volcanic record.

 


1. Oceanus Procellarum 2. Mare Imbrium 3. Mare Cognitum 4. Mare Humorum
5. Mare Nubium 6. Mare Frigoris 7. Mare Serenitatis 8. Mare Vaporum
9. Mare Tranquillitatis 10. Mare Nectaris 11. Mare Humboldtianum 12. Mare Crisium
13. Mare Fecunditatis 14. Mare Marginis 15. Mare Smythii 16. Mare Australe
17. Mare Moscoviense 18. Mare Ingenii 19. Mare Orientale

 

Differences from Earth:

Volcanism on the Moon differs in several ways from volcanism on the Earth. First, there is the matter of age. Volcanism on the Earth is an ongoing process. Many of Earth's volcanoes are quite young in geologic terms, often less than a few 100,000 years old. In contrast, most volcanism on the Moon appears to have occurred between 3 and 4 billion years ago. Typical mare samples are ~3,500,000,000 years old. Even the youngest mare flows have estimated ages of nearly 1 billion years. These "young" rocks have not been sampled or directly dated, however, so this age is very poorly known. For comparison, the oldest dated rock on the Earth is ~3.9 billion years old. The oldest sea floor basalts on Earth are only about 200 million (0.2 billion) years old. Because the Moon does not show any evidence for recent volcanic or geologic activity, it is sometimes called a "dead" planet.

 

The settings of mare volcanism reveal another major difference from volcanism on the Earth. Specifically, Earth's volcanoes mostly occur within long linear mountain chains. Mountain chains like the Andes mark the edge of a lithospheric plate. Mountain chains like the Hawaiian Islands mark past plate movements over a mantle hotspot. In contrast, the mare typically occur in the bottoms of very large, very old impact craters. Thus, most of the mare are nearly circular in shape. Further, lunar mountain chains form the edges of these impact basins and tend to surround the lunar mare. There is no evidence that any system of plate tectonics ever developed on the Moon. Finally, the lunar mare are primarily found on one side of the Moon. They cover nearly one third of the lunar nearside (see figure), but less than 2% of the lunar farside. The surface is much higher on the farside, however, and the crust is typically much thicker there as well. Thus, the primary factors controlling volcanism on the Moon appear to be surface elevation and crustal thickness.

 

Finally, there are some major physical differences between volcanism on the Earth and on the Moon. First, lunar gravity is only one sixth that of the Earth's. This means that the forces driving lava flow are weaker on the Moon. Thus, the very flat and smooth mare surfaces imply that mare lavas were very fluid. They could both flow very easily and spread out over large areas. Also, the low gravity means that explosive eruptions can throw debris further on the Moon than on the Earth. Indeed, such eruptions on the Moon should spread lavas out into a broad flat layer and not into the cone-shaped features seen on the Earth. This gives one reason for why large volcanoes are not seen on the Moon. Second, the Moon has essentially no dissolved water. The lunar mare are all bone dry. In contrast, water is one of the most common gases in Earth lavas. Water also plays a major role in driving violent eruptions on the Earth. Thus, the lack of lunar water should strongly affect lunar volcanism. In particular, without water, violent explosive eruptions are much less likely on the Moon. Instead, lavas should just flow smoothly and quietly out onto the surface.

Mare

Due to their size, the lunar maria are the most obvious volcanic features on the Moon. These vast basalt plains cover over 15% of the lunar surface, mostly on the Moon's nearside. They are typically circular in outline because they tend to fill the bottoms of very large, very old impact basins. Smaller mare patches also occur in the floors of some impact craters. They also are very old, and have been battered by the impacts of many small meteorites for over 3 billion years.

Major Lunar Maria


1. Oceanus Procellarum 2. Mare Imbrium 3. Mare Cognitum 4. Mare Humorum
5. Mare Nubium 6. Mare Frigoris 7. Mare Serenitatis 8. Mare Vaporum
9. Mare Tranquillitatis 10. Mare Nectaris 11. Mare Humboldtianum 12. Mare Crisium
13. Mare Fecunditatis 14. Mare Marginis 15. Mare Smythii 16. Mare Australe
17. Mare Moscoviense 18. Mare Ingenii 19. Mare Orientale

Shown here is a map of the major lunar maria. These maria range from over 200 km to about 1200 km in size. They are typically about 500 m to 1500 m thick. However, each mare appears to contain many thinner basalt flows. Typical flow thicknesses appear to be 10-20 m. Thus, each mare records hundreds of overlapping eruption events. The map also shows a clear lack of major maria on the lunar farside. This probably reflects two changes in the lunar crust. First, the lunar surface is higher on farside than on the nearside. Second, the crust seems to be thicker on the lunar farside than on the nearside. These differences should make it harder for mare magmas to reach the surface on the lunar farside. They also explain why small mare patches are grouped together on the farside. The mare patches represent lava-filled craters. Most such craters lie in the bottoms of much larger and much older basins. On the nearside, such basins contain circular mare. On the farside, such basin filling volcanism is rare. Still, these basins contain both the lowest surfaces and the thinnest crust. Thus, mare volcanism is most likely inside these basins, especially where younger craters have dug into the basin floor. (Map prepared by G.W. Colton; published in NASA SP-362 (1978) and NASA SP-469 (1984).)

Mare Surface

 



This is an Apollo photo of the surface in southern Mare Imbrium. It shows some young, fairly pristine mare lava flows. These lavas are probably 1 to 2 billion years old. Still, individual flow lobes can be clearly seen at the top of the image. Similarly, the gully-like features in the lower left do not mark any kind of erosion. Rather, they mark shallow lava channels (sinuous rilles) which formed at the lava flow surfaces. The image also shows many small circular impact craters. While meteorite impacts on the Earth and Moon are rare, such craters are quite common within the lunar mare. The mare are so old that a large number of meteorite impacts have occurred. Indeed, the number of impact craters within a mare provides a method for guessing its age. Because older surfaces are more likely to have been hit by meteorites, older mare should contain both more craters and larger craters than younger mare. Note -- Younger mare lavas can bury craters formed on older lavas. This image shows one such example near the crater in the bottom center. The rough ejecta unit surrounding this crater is cut and partially buried by younger lava flows. (Mosaic of Apollo photographs A17 M-2295 and A15 M-1701)






Mare Humorum

 




This image shows Mare Humorum and the western edge of Mare Nubium. Mare Humorum is a small circular mare on the lunar nearside. It is about 275 miles (~440 km) across. The mountains surrounding Mare Humorum mark the edge of an old impact basin. This basin has been flooded and filled by mare lavas. These lavas also extend past the basin rim in several places. In the upper right are several such flows which extend northwest into southern Oceanus Procellarum. Note the large fractures arcing around Mare Humorum on the right. These fractures are believed to mark a bending of the lunar surface due to the weight of Mare Humorum. Such a sinking of the mare may also explain the two large, partly flooded craters that seem to slope into Mare Humorum. (Earth-based telescopic photo from the Consolidated Lunar Atlas)







Mare Moscoviense

 





This is an oblique image of the lunar farside. It shows the impact basin that holds Mare Moscoviense. Like Mare Marginis, this mare appears to be fairly thin. However, it is clearly centered within a large impact basin. It is also much lower than either the outer basin floor or the farside highlands. The great depth of this mare beneath the nearby highlands probably explains why mare units are so rare on the lunar farside. Very few basins on the farside were deep enough to allow mare volcanism. Such a contrast in mare and highland elevations also exists on the nearside. Still, it is much smaller than that found on the farside. This may be because the Moon's crust is much thinner on the nearside. Thus, while large impact basins are found on both the nearside and farside, large maria are mostly found on the nearside. Mare lavas apparently could reach the surface more often and more easily there. (Lunar Orbiter image IV-103-M)












Mare Marginis

 




Mare Marginis lies on the very edge of the lunar nearside. Thus, it lies halfway between the lunar nearside and farside. It also differs from most of the nearside maria. It has an irregular outline, and it appears to be fairly thin. Note the small circular and elongated features in the mare plains. These probably mark impact craters buried by less than 1000 - 1700 feet (300-500 meters) of lava. Further, Mare Marginis is not centered on any clear, large impact basin. Thus, Mare Marginis seems to mark a low-lying region of the highlands where mare lavas were just able to reach the surface. Several large mare-floored craters also occur nearby. In these craters, the crater floors lie below the surrounding highland surface. Thus, they mark sites around Mare Marginis where lavas were close to the lunar surface. (Lunar Orbiter image IV-165-H3)














 

Imbrium Flow Map

Lava flows within the lunar mare are quite large. Shown here is a map of 3 "young" lava flows in Mare Imbrium. These flows apparently record three separate eruptions within a period of ~500 million years over 2.5 billion years ago. The oldest group is the largest. Its furthest point lies about 750 miles (~1200 km) from the inferred vent in the lower left corner. The second group then buried parts of the first group. It extends for a distance of about 375 mile (~600 km). Finally, the youngest group is also the smallest. It is ONLY some 250 miles (400 km) in length. (NOTE: some areas contain a mixture of flows from the first and second flow groups and mapped here as "mixed.") No active Earth volcanoes have lava flows anywhere near this length. Still, a few older eruptions are of similar size. Due to their size, these features are called Flood Basalts. One example is the Columbia River Flood Basalts in the northwestern U.S. They extend from Idaho into the Pacific Ocean. Most of these flows formed about 16 million years ago, but some erupted as recently as 6 million years ago. The biggest flows are over 188 miles (300 km) long, and they collectively cover over 102,500 suqre miles (164,000 square kilometers). Thus, the Columbia River Basalts are nearly the same size as the youngest and smallest of the basalt flows in Mare Imbrium. (Map after figure 4.26 in the Lunar Sourcebook; based on Schaber, 1973)






Types of Mare Basalt

 

 

 

 

The lunar mare are very dark when seen with the naked eye. They are not all of the same color, however. Small differences are present in the amounts of ultraviolet, visible and infrared light reflected from the mare. Such color differences define 13 mare basalt types (shown here). These basalt types should mark changes in the minerals and chemistry of the mare basalts. However, the exact nature of over half of these mare units is poorly known. Most are located far from the Apollo landing sites. We have samples for only the 4 basalt types labeled Apollo 11, Apollo 12, Apollo 15, and Luna 20. Note -- The mare reflect only a small fraction (~7-10%) of visible light. Thus, most of the color differences in this map are invisible to the human eye. (Figure from Pieters (1978) Proceedings of 9th Lunar & Planetary Science Conf., vol. 3, p. 2826.)








Sinuous Rilles

Sinuous rilles are probably the most recognizable of small volcanic features on the Moon. Many partially resemble river valleys on the Earth. However, the lunar rilles usually flow away from small pit structures. Also, the lunar samples indicate that the Moon has always been bone dry. Thus, the sinuous rilles probably mark lava channels or collapsed lava tubes that formed during mare volcanism. Still, in some cases, the lunar flows may have melted their way down into older rocks, much like rivers cut into their flood plains on Earth. Similar lava channels and tubes are found in Hawaii, but these are all much, much smaller than those found on the Moon.

 

 

Hadley Rille (from Orbit)

 

This photo shows the Hadley Rille on the southeast edge of Mare Imbrium. It is fairly well known because Apollo 15 landed there (see next image). The rille begins at the curved gash in the bottom left corner, and is clearest in the rectangular, mare-floored valley shown here. In the upper left, it gets much shallower and it slowly fades out of sight in Palus Putredinis. In all, the rille is over 75 miles (120 km) long. It is up to 5000 feet (1500 m) across and is over 950 feet (300 m) deep in places. It formed nearly 3.3 billion years ago . In contrast, lava channels on Hawaii are usually under 6 miles (10 km) long and are only 150 - 300 feet (50-100 m) wide. This contrast in channel size probably reflects (1) differences in the volume of erupted lava and (2) the difference in gravity. Note -- The bright bumps surrounding Hadley are peaks of the Montes Apenninus. These mountains mark the edge of the impact basin holding Mare Imbrium. They rise from 6000 to 15,000 feet (1800 - 4500 m) above the mare. (Apollo 15 image M-1135, arrow marks landing site of Apollo 15. Image taken from NASA SP-469, Geology of the Terrestrial Planets)

 

Hadley Rille (from Surface)

 

This is a photo of Hadley Rille from the lunar surface. Astronaut James Irwin is standing in the foreground by the lunar rover. At this point, the rille is nearly a mile wide (1.6 km). Note the many boulders on the Valley floor. These rocks have apparently rolled off of the Valley wall over time. Thus, any lava flows inside the rille are now buried by an unknown amount of rocks and soils. The rille appears to have cut down through a couple of older lava flows, however. These flows are probably the only actual bedrock seen by astronauts on the Moon. They form small cliff faces near the top of the rille wall in the distance. Due to the loose soil and steep slopes above these cliffs, however, no samples were taken of these units. (Apollo Photograph AS15-84-11450)

 

Posidonius Rille

 

This image shows a sinuous rille located in the crater Posidonius. The source of this rille is a shallow pit on the northern crater wall (arrow). The rille then meanders along the edge of the crater floor for over 60 miles (100 km). Finally, it reaches a hole in the crater rim and vanishes into Mare Serenitatis. Note that the rille is also only the last stage of mare volcanism within this crater. It lies on top of an older, smooth mare unit which buries part of the crater floor and much of the western crater wall. ( The black bar in the lower right is about 6 miles (~10 km) long.) (Lunar Orbiter image IV-86-H3)

 

Lava Tubes

 

This image shows a chain of small pits and ridge segments. It marks the likely site of a partially collapsed lava tube in western Mare Imbrium. Here, instead of a surface lava channel, lava flowed through a buried tunnel in the mare. After the eruption stopped, the tunnel then emptied. Where the roof of the tunnel has fallen in, we see pits. If the entire roof had fallen in, we might see a sinuous trough like other lunar rilles. Thus, it is believed that some lunar rilles mark collapsed lava tubes. These rilles are usually near other collapse pits, and some also merge with ridge-like features. Thus, uncollapsed lava tubes may still exist near some of these lunar sinuous rilles. (Lunar Orbiter image V-182-M, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)


Schroter's Valley 1

 

This photo shows Schroter's Valley (arrow) and the Aristarchus Plateau. It lies between Mare Imbrium and Oceanus Procellarum. This valley is the largest (widest) sinuous rille on the Moon. It also has a smaller rille inside it (see next photo). Both rilles come from the same vent, but they probably reflect two separate eruptions. Both rilles fade away into the plains of Oceanus Procellarum. (Apollo 15 image M-2611, from Wilhelms (1987) The Geologic History of the Moon, USGS Prof. Paper 1348.)

 

This is a detailed photo showing part of Schroter's Valley (see last image). It clearly shows a complex, highly contorted rille inside the Valley. Like many rivers on the Earth, this rille has many tight loops along its course. These loops are meanders, and they suggest a long-lived flow on a fairly flat surface. They also require active erosion of the valley floor. This channel may have partially melted its way into older lava flows. (Part of Apollo 15 image P-341.

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.


Marius Hills

 

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.)

Rumker Hills

 

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.)

Gruitheisen Domes

 

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.)


Dark Mantling Deposits

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.)

Sinus Aestum

 

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).

Crater Alphonsus

 

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.)

Eruption Styles

 

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)