Earth's Moon
Earth's 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.
 
 
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
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
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