August 3, 2011
Exotic volcanoes on the Moon
Top: Map of thorium concentrations near Compton crater on the lunar far side. Bottom: LRO view of the felsic highland volcano. After Jolliff et al. (2011), Nature Geoscience 4, 566.
The flood of new data from the Moon continues to enlighten and puzzle lunar scientists. Members of the Lunar Reconnaissance Orbiter Camera team have noticed an unusual landform on the far side of the Moon that was as unexpected as it might be significant.
We’ve known for many years that early in its history, the Moon was volcanically active. The dark, smooth maria of the Moon is made up of lava flows, individually erupted over at least a billion year time span and possibly for much longer. In total volume, the mare lavas make up only a percent or so of the crust, so the Moon is not the volcanic cauldron that Io, the large moon of Jupiter, appears to be. But these lavas indicate that the early Moon was hot and that melted material spilled onto its surface in the past.
The volcanic rocks of the Moon are what geologists call mafic, meaning that they are enriched in iron and magnesium. Mafic lavas (basalt) are commonly found as plains and low relief shield volcanoes, such as the Hawaiian islands. On Earth, volcanic rocks can be mafic (and in fact, are the most abundant rocks on Earth, comprising the ocean floor bedrock) or they can be felsic, meaning enriched in silica (SiO2) and depleted in iron. On Earth, felsic rocks occur in mid-continental volcanoes and the stratovolcanoes that are found along the margins of the giant tectonic plates that make up Earth’s outer, rigid layer (lithosphere). Felsic lavas are often associated with explosive, violent eruptions, such as the Mt. St. Helens eruption of 1980.
All of the volcanic rocks returned from the Moon by the Apollo astronauts are mafic. Most of them are basalts – lava erupted as quiet, fissure-fed sheets. A few of the samples are tiny glass beads of mafic composition, erupted when low viscosity (runny) fluid lava squirted into space as a spray of lava called a fire-fountain. Sprayed droplets of lava cool in ballistic flight and land on the Moon as a uniform deposit of tiny (40 micron diameter) glass beads, forming a lunar ash bed. Although we did find tiny fragments of felsic material in some of the complex breccias from the highlands of the Moon, no felsic lavas or ash were collected on Apollo.
In April 1972, the Apollo 16 mission was sent to the Descartes highlands on the near side of the Moon. Pre-mission mapping and studies indicated to the geology team that the plains and mountains of Descartes are felsic volcanoes, having the morphology of lava domes and ash flows on Earth. The Apollo 16 crew was given intensive instruction in the recognition and mapping of volcanic units on the Earth, so they would recognize the abundant felsic volcanics thought to make up the Descartes highlands.
John Young and Charlie Duke put their geological training to good use when they landed on the Moon. Not only were the rocks at Descartes not silica-rich volcanics, they weren’t even volcanic! The crew immediately recognized that all the rocks they found were breccias – aggregates of many rocks assembled by impact. As Command Module Pilot Ken Mattingly wryly noted, “Well, it’s back to the drawing boards – or wherever geologists go!” (They usually go for a beer.)
After that sobering experience, lunar geologists were hesitant to map felsic volcanoes on the Moon again. In fact, the pendulum swung away from such a process ever having occurred on the Moon at all. Nevertheless, we continued to note small geological anomalies around the Moon, hills and domes that are difficult to explain as impact features. Additionally, some of these small landforms apparently have unusual composition as they have unique spectral properties, being anomalously “redder” (i.e., higher reflectance at longer visible wavelengths) than surrounding terrain. These features were imaginatively named “red spots.” Although we could determine that lunar red spots were compositionally distinct, we did not know exactly what those compositions were. Now, with new data from the orbiting lunar missions, the mystery of the red spots is finally solved.
The red spots are small volcanoes made up of felsic rocks. We know from data returned by the DIVINER thermal imaging spectrometer on Lunar Reconnaissance Orbiter that these landforms are rich in silica. From the Lunar Prospector gamma-ray data, we have determined that they are also enriched in the element thorium, a key indicator of chemically evolved rock types. Finally, data from both Earth-based telescopes and from the Moon Mineralogy Mapper on the Chandrayaan-1 mission, show that some of the red spots are made of almost pure glass. On Earth, silica-rich volcanic glass forms a deposit called obsidian; its crystallized form is rhyolite. New, remotely sensed compositional data show that the lunar red spots are felsic domes of obsidian and rhyolite. Red spots occur mostly on the western near side of the Moon, the area in and around Oceanus Procellarum. The new finding of an isolated felsic volcano on the far side of the Moon indicates that such eruptions were a global phenomenon.
These features are not volumetrically major and occur as small geological oddities set within the predominantly mafic, basaltic volcanic terrain of the lunar surface. Their presence was not predicted by the prevailing model of lunar volcanism. After the fiasco of the mistaken Apollo 16 prediction, geologists were hesitant to pronounce any dome on the Moon to be a felsic volcano. Suitably chastened, they re-interpreted those dome-like landforms of the highlands to be by-products of basin-forming impacts. We see by the existence of these features that in some cases, the volcanic interpretation is viable. This information adds to our understanding of the incredibly rich and complex geological story of the Moon.
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A lunar base could allow us to have dozens of Earth controlled rovers continuously exploring practically all regions of the lunar surface. Such remote controlled vehicles could also return their rock and soil samples back to the Moon base for study and eventual export back to Earth.
I’m always particularly interested in any concentrated thorium deposits on the lunar surface that could potentially be used to power lunar nuclear reactors.
Thanks for the very interesting article Dr. Spudis!
I also hope that you’ll have some comments about the new hypothesis in Nature about the possibility that the Earth once had two moons!
Comment by Marcel F. Williams — August 3, 2011 @ 9:07 pm
It seems recent volcanic activity could have value in respect to lava tubes- older volcanic area are more likely to have them collapsed. In terms the possibility of forming lava tubes, is there differences between the mafic formation and felsic?
Generally does felsic volcanoes indicate relative shallow areas of volcanic magma whereas mafic generally originates from deeper levels.
Oh, also I was wondering if the regolith gardening involves mixing of regolith at some rate, and latest volcanic activity on the Moon is around billion year ago. How did Apollo astronauts find those “tiny glass beads”?
Comment by gbaikie — August 4, 2011 @ 12:05 am
I also hope that you’ll have some comments about the new hypothesis in Nature about the possibility that the Earth once had two moons!
Yes, that topic will be be the subject of a future post. That idea, by the way, is not new — I remember the idea of two sub-moons coalescing to form the Moon was proposed by John Wood at a conference back in 1982. Seems like our ideas get continually recycled. That’s why being a lunar scientist is great — if your idea falls out of favor, just wait — it will appear again!
Comment by Paul D. Spudis — August 4, 2011 @ 3:44 am
In terms the possibility of forming lava tubes, is there differences between the mafic formation and felsic?
Lava tubes are found almost exclusively in mafic (basaltic) lava flows. The sinuous rilles seen in the basaltic maria of the Moon are former lava channels and/or tubes.
Generally does felsic volcanoes indicate relative shallow areas of volcanic magma whereas mafic generally originates from deeper levels.
Not necessarily. Felsic magmas are evolved chemically and are the end products of long episodes of mafic igneous activity (this is an oversimplification, but it applies to felsic magmas on the Moon). If you start crystallizing a mafic magma, the remaining liquid becomes more felsic with time. The rhyolite domes I am describing here must be the end products of a long crystallization sequence. That’s one of the reasons we are interested in studying them.
Oh, also I was wondering if the regolith gardening involves mixing of regolith at some rate, and latest volcanic activity on the Moon is around billion year ago. How did Apollo astronauts find those “tiny glass beads”?
We found the volcanic glass both as a deposit on the Moon and as a component in the regolith by studying the soil after the missions (the Apollo 17 organge soil was recognized while the crew was on the Moon). Both the emerald green glass of Apollo 15 and the famous “orange soil” of Apollo 17 are glass beads made in a lunar fire fountain eruption. They have a mean grain size of about 40 microns, so they are very tiny.
Comment by Paul D. Spudis — August 4, 2011 @ 3:52 am
1. Is there any sign that these volcanoes might have been active in the recent past?
2. Any chance that they might erupt in the near future?
3. Might such volcanoes be a source of volatiles that could collect in the polar cold traps?
Comment by Warren Platts — August 4, 2011 @ 3:27 pm
Is there any sign that these volcanoes might have been active in the recent past?
No. They all date from the epoch of mare volcanism, mostly prior to 3 billion years ago.
Any chance that they might erupt in the near future?
No.
Might such volcanoes be a source of volatiles that could collect in the polar cold traps?
That’s an interesting possibility. However, we think (but are not sure) that the current spin axis orientation of the Moon is only about 2 billion years old. This means that the polar dark areas (cold traps) did not exist prior to that date. Unless there was some late stage volcanism that we haven’t recognized yet, it is unlikely that volcanic volatiles would be preserved in the cold traps.
Comment by Paul D. Spudis — August 4, 2011 @ 4:11 pm
Great read Dr. Spudis. I was wondering about differences between the mafic and felsic volcanoes/fields as far as does one type or the other throw out gemstones or PMG more often? I looked online and couldn’t see which type of lava tube is mined more often for gemstones.
I still believe lunar gemstones is one of the few things light enough with enough value to ship home. The rarity of lunar gemstones will automatically give them a higher value than terrestrial gemstones. I can’t wait to see how DeBeers markets them.
“But honey, Bill bought Mrs. Gates a lunar diamond, when do I get one”
lol
Comment by Vladislaw — August 5, 2011 @ 4:27 pm
Most lunar minerals are fairly common rock-forming minerals and few are often used as gemstones. However, peridot is the gem form of olivine and olivine is abundant in the lunar crust. We might find gem quality olivines there; they would likely be part of deeply derived, igneous layered intrusions, which are often exposed in the central peaks of lunar craters.
Volcanic terrains do not offer much for gem prospects, but again, olivine is the most likely.
Diamond (carbon), opals (cryptocrystalline quartz), and amber (fossilized tree sap) are highly unlikely to occur on the Moon. Emeralds are a variety of beryl, and beryllium is a rare element on the Moon. However, rubies and sapphires (forms of corundum, an aluminum oxide) might occur. These gems are often associated with hydrothermal deposits, which of course, do not occur on the Moon (as far as we know.)
Comment by Paul D. Spudis — August 5, 2011 @ 5:32 pm
Thank you, Dr. Spudis, for the informative comments on the progress of lunar geology. Even an economist, such as I, could understand it.
Comment by Arthur C. Barker — August 7, 2011 @ 1:34 am
We found the volcanic glass both as a deposit on the Moon and as a component in the regolith by studying the soil after the missions (the Apollo 17 organge soil was recognized while the crew was on the Moon). Both the emerald green glass of Apollo 15 and the famous “orange soil” of Apollo 17 are glass beads made in a lunar fire fountain eruption. They have a mean grain size of about 40 microns, so they are very tiny.
Interesting. On topic of gardening, is impactors the only mechanism involved with this process.
Only perhaps I should say impacts and lunar quakes. And other than impactors, it seem the long duration cooling of the Moon seems to me the main mechanism involved lunar quakes. And it seems likely the rate of lunar cooling over million to billions of years isn’t constant, and that rate magnitude of lunar quakes of today is not the same as 1 billion years ago.
I was reading somewhere that there is more volcanic activity on farside of the Moon as compared to near side, and some supposed this have to do with uneven density of the Moon [it's more more massive on the side that faces Earth]. It seems to me that on the side with higher density should more volcanic activity, rather than less- unless the lunar core is migrating away from Earth.
Comment by gbaikie — August 7, 2011 @ 6:39 pm
there is more volcanic activity on farside of the Moon as compared to near side
No, it’s the other way around. Most of the volcanic maria is on the near side of the Moon. It is thought that’s because the crust on the far side is thicker and it is more difficult for magma to reach the surface on the far side. Of course, that begs the question of why the crust is thicker on the far side. A recent paper suggests that a giant asteroid/small sub-moon was plastered onto the far side, thickening the crust. I am skeptical of such an interpretation and this topic will be discussed in a future post here.
Comment by Paul D. Spudis — August 8, 2011 @ 3:44 am
“No, it’s the other way around. Most of the volcanic maria is on the near side of the Moon. It is thought that’s because the crust on the far side is thicker and it is more difficult for magma to reach the surface on the far side. Of course, that begs the question of why the crust is thicker on the far side. A recent paper suggests that a giant asteroid/small sub-moon was plastered onto the far side, thickening the crust. I am skeptical of such an interpretation and this topic will be discussed in a future post here.”
Could it be that large asteroids headed directly towards the near side of the Moon are or were more frequently intercepted or deflected by its larger co-planet, the Earth, with the Earth receiving the impact damage that the Moon should have received?
Comment by Marcel F. Williams — August 9, 2011 @ 2:25 pm
Could it be that large asteroids headed directly towards the near side of the Moon are or were more frequently intercepted or deflected by its larger co-planet, the Earth, with the Earth receiving the impact damage that the Moon should have received?
Given that the Earth’s gravitational capture cross-section is bigger than the Moon’s, that is certainly possible. However, there is little evidence that any one part of the Moon has received significantly more impacts than any other. The impact flux seems to be pretty isotropic.
Comment by Paul D. Spudis — August 9, 2011 @ 2:32 pm
“Could it be that large asteroids headed directly towards the near side of the Moon are or were more frequently intercepted or deflected by its larger co-planet, the Earth, with the Earth receiving the impact damage that the Moon should have received?”
Everything in the inner planetary systems is strongly dominated by the sun’s gravity, and this domination lessens when you get about 1 million miles from earth.
Obviously, before any object can hit the Earth or Moon it first needs to get within 1 million miles. And the earth’s gravity will affect it’s trajectory, but there no preference of this gravitational effect which should cause it to hit the Moon- it could cause it to hit as much as it could cause to miss. Asteroids hit the Moon and Earth mainly due to it’s size as target rather than either of their gravitational influences.
Because Earth is much larger than the Moon, the Earth will hit more often than the Moon from rocks orbiting the Sun, and because the earth is hit more often this could increase amount rocks hitting the Moon [in terms of secondary impact- the relatively large impactor that killed the dinosaur may have caused secondary impactors which got as far as the Moon's orbit [most would have re-entered Earth]. Related to secondary impacts, rocks could have had near misses with Earth atmosphere, the tidal affect and/or hitting Earth’s atmosphere, could made a rock pile type asteroid into into cloud-like object and slightly increasing chance of it hitting the Moon.
In terms of secondary impacts, it seems the Moon would have higher percentage of these leaving the Moon orbit- increasing the amount objects hitting Earth. But none of these secondary impacts should be statistically significant in terms of hitting the near side of the Moon. If there is anything statistically significant in regard to secondaries it’s probably related to rocks hitting the Moon or Earth at a low angle.
Comment by gbaikie — August 9, 2011 @ 6:31 pm
The Moon is an extremely fascinating & intriguing world unto itself, geologically. Selenology would make heavy leaps and bounds if U.S. astronauts would be allowed to go back, after the lamentable fourty year absence. Groups like the Planetary Society really sicken me by their condemnation of renewed manned Lunar exploration. They act as if all this time, for the last four decades that we’ve been sending men there to Luna, and that it has been “completely” explored as well as we know the Isle of Wight, England. So they make all these dumb manifestos for “roadmaps to go beyond the Moon”; forgeting clearly, that NO ONE, nobody has been there since December of 1972! This Anti-Moon attitude has done much to disinform & mislead the public at large. The common man on the street is deluded into thinking that Obama’s sinking of the Lunar-going ship was justified, when in high fact IT WASN’T. Compare the 135 flights that the Space Shuttle made to Low Earth Orbit, and contrast that to the mere 9 manned flights made to the Moon and its vicinity, in the whole history of the world. Obama declares further manned journeys to the Moon worthless, and then he sets NASA on the course of bigger & expanded operations in LEO. An LEO on steroids program that gets us to nowhere, but 200 miles up, in endless circles. In the grand Apollo days, LEO was just the beginning of the journey, not the end of it!
Comment by Chris Castro — August 10, 2011 @ 12:32 am
@ Chris Castro
Ironically, I believe that a Moon base is actually the fastest way to take the next step towards manned interplanetary missions. And any delay in building of a lunar base and the exploitation of polar water resources on the Moon will actually delay the establishment of similar bases on the surface of Mars.
Comment by Marcel F. Williams — August 10, 2011 @ 10:55 pm
It’s good to see more attention being given to volcanic processes on the Moon. I have long believed that volcanism has played a much bigger role in the moon’s history, apart from the obviously volcanic maria, than was generally believed. In previous posts I called attention to an apparently unnamed crater chain extending to the NW of Tyco, a large, curving ridge between Copernicus and Tobias Mayer with a well developed crater chain on it’s summit, and a large mountain near Godin with a mare patch on it’s summit. Indeed, summit craters and crater chains are ubiquitous on radial ridges east and south of Mare Imbrium, as well as in the fields of isolated peaks and domes south and west of Copernicus, and in Mare Imbrium itself, to name just a few examples.
Comment by Dick Morris — August 11, 2011 @ 5:49 pm