I did a post last year that took a look at volcanoes from the perspective of a heat engine, something that transferred warmer material to its colder surroundings. As such, they are not an earth-only phenomenon. This includes the terrestrial planets, Pluto and some of the larger moons. When the vents are large enough, we call those things volcanoes. When they are smaller, we call them vents, geysers, plumes, etc. https://volcanohotspot.wordpress.com/2016/06/27/volcanoes-as-heat-engines/
We have such a body around 384,000 km above our heads called the moon. It is tidally locked, so we see one face (the near side). And that face is pock-marked with basalt filled impact basins. If there was that much basalt billions of years ago, where did it come from? Better yet, what other volcanic structures are there up there? Finally, is there any current volcanic activity on the moon?
One of the things we have found with the last half century of space exploration, is the closer we look at bodies that are not the earth, the more incorrect our initial assumptions about those bodies becomes.
For example, little Mercury hosts a variety of unexpected ancient volcanic activity. The closest planet to the sun also appears to have water ice at the permanently shadowed poles. Venus is not a jungle planet. Rather it is very close to the classic description of Hell, with a thick, heavy CO2 layer that acts like a liquid ocean at its surface, and volcanic activity that resurfaces the entire planet from time to time. Mars is not a dry, barren planet at all, as the closer we look at it, the more water ice we find. Even the near earth asteroids participate, with fully half of them thought to be inactive comets.
The point I am trying to make however poorly, is that as we look at things with new eyes, new knowledge, more times than not our initial assumptions turn out to be wrong.
With that in mind, let’s take a look at volcanic activity on the moon.
The Earth – Moon system has been called a double planet due to the significant size of the moon compared to that of earth. The only other planet even close (and yes, I assume a dwarf planet is still a type of planet) is Pluto – Charon. The difference is that the center of rotation of the Earth – Moon system is under the surface of the Earth, while that of Pluto – Charon is in space between the two bodies.
So how do you form such a system?
For this, we have to go back to the formation of the solar system itself some 4.6 billion years ago.
It began with a large, cold cloud of dust and gas. Something pushed the cloud, likely the shock wave from one to several nearby supernova, and it started to collapse and rotate. Very shortly, the cloud arranged itself into what is called an accretion disk with what was to become the sun in the center. The collapse was quick and violent, generating heat and triggering fusion in the very center of the proto-sun.
As you traveled out in the disk away from the sun, temperatures dropped quickly. There is a zone where rocky elements predominate, a zone where water and other gasses predominate, and a zone where ices predominate. The bits of dust and gas started to gather together and build solid objects. Initially, the clumping was electrostatic in nature (we’ve seen this sort of clumping in ISS experiments). As the grains got larger, they started creating their own gravity, accelerating the growth. Once the planetesimals got large enough, they started attracting gas in the disk and clearing lanes. We’ve seen this around other newly formed stars.
The first two planetesimals were also the largest – Jupiter and Saturn. They formed as quickly as 10 million years. And their gravity influenced the growth and early history of the rest of the solar system.
Toward the end of the growth process, the solar system was an incredibly violent place. The growing planetesimals collided with one another. The larger and faster the participants, the more violent the collision. We can see results of the violence everywhere we look. The Pluto – Charon system appears to be the remains of a collision, as its axis of rotation is rotated over 100 degrees from perpendicular to the plane of the ecliptic (plane of the accretion disk). Something hit Uranus hard enough to turn it on its side too. Something hit Mars on its north pole that was large enough to turn most of the northern hemisphere into a basin. Several somethings hit Vesta and removed a significant portion of its southern hemisphere. All of this took place over the first 100 Ma of the solar system’s formation.
And then we get to the Earth – Moon system. Best description of its formation is the impact of a large body, something the size of Mars, in a glancing blow, destroying itself, and sending sufficient material into orbit of the newly formed system to form the moon.
The nature of that impact is still under intense discussion, with the older slow, glancing blow impact theory being replaced in recent years with a higher velocity, more direct impact. For purposes of this discussion, the details of the impact don’t much matter, as we know some portion of whatever was removed from both bodies ended up in orbit around the newly formed, larger body, and some percentage of it ended up forming the moon.
So we have a gravitationally captured ring or a synestia of solid, liquid, and gaseous rock and metal around the center of the collision. Some unknown percentage of that material falls back onto the surface. The rest of it quickly collects into one or more proto-moons, which in turn combine into the body we see today.
Initially, much of both bodies was molten. The moon forms some 20 – 30,000 km from the earth. Tidal forces quickly lock its rotation rate to one hemisphere facing the earth continuously. Tidal forces also start marching it outward. Being that close, the moon raised significant tides on the surface. The small distance between the two molten bodies has been suggested by some as one of several reasons the crust on the moon is thinner on the near side and thicker on the far side.
Over time, both bodies cool, creating solid surfaces, an atmosphere and oceans on the Earth.
While all this is taking place, interaction with the accretion disk has started moving Jupiter inward. It got within 1.5 Astronomical units (AU), around the current orbit of Mars, before gravitational interaction with Saturn started moving both planets outward ending up at their current locations. Needless to say, this movement wreaked havoc with the new solar system, scattering planets, asteroids, comets, and planetesimals all over the place. This period, some 3.8 – 4 billion years ago is called the Late Heavy Bombardment, with comets and asteroids ejected from their original orbits. A lot of them were ejected from the solar system. A lot of them entered the inner system, colliding with the newly formed planets. It is this bombardment that created the basalt filled impact basins on the near side of the moon. It is this bombardment thought to supply much of the waters of the oceans on earth.
Smaller bodies like the moon cool quicker, and do not have the heat output over geologic time or oceans necessary to set up active plate tectonic systems. As such, outside a few compressional grabens, it has not had significant tectonic activity within the last 3 billion years. The moon is thought to have shrunk some 180 m as it cooled, driving early tectonic activity. We see the effects of this shrinkage with linear scarps, fault lines, and grabens.
Magma sources for the flood basalts surfacing the mare were initially thought to be a combination of deep melting due to the impact events and a release of residual magma from the interior to the surface. Like most initial guesses, this has been updated with analysis of actual samples telling us that the basalt magma came from the interior of the moon. Typical mare samples date around 3.5 billion years. The youngest mare flow is thought to date around 1 billion years.
The flood basalts cover nearly a third of the near side and less than 2% of the far side, leading to the conclusion that the primary factors controlling volcanic activity on the moon are surface elevation (lower on the near side) and crust thickness (thicker on the far side).
A combination of low gravity and very low water content on the moon tend to drive eruptive activity to be more effusive than we see on the earth. The lack of an atmosphere leads to frothing of basalt magmas at the tip of rising dikes. The deeper the magma source, the more water, up to 150 ppm on average, is mixed with the magmas. This caused significant explosive pyroclastic eruptions toward the later stages of flood basalt activity.
Volcanic activity listed next is roughly in order of when the activity was most prevalent. The vast majority of lunar volcanic activity took place between 3 – 4 billion years ago, with the commonly accepted end of lunar volcanism being 1 – 1.5 billion years ago. Some of the most recent cone, dome, IMP, and TLP activity may date as recently as a few tens of millions of years.
There are 23 Maria on the moon, the vast majority of them on the near side. Diameters range from 132 – 2,568 km. There are another 20 smaller basaltic plains called Lacus ranging in diameter from 50 – 396 km. All are thought to be formed by massive impact events.
One of the things that happen when a crater of 20 km in diameter is formed, is that the crater rim slumps as surrounding rock is not strong enough to support a raised rim. Generally the larger the crater, the more pronounced the slumping. As you get over 100 km in diameter, magma formation, lava flows, and possible density current involvement due to frothing of the magma becomes more likely. The crater is then floored by some combination of impact-created breccias, fallback breccias, post impact basalts and ignimbrites.
The magmas filling the maria are primarily interior magmas. The impact events tend to create deep fissures into the cooling crust and upper mantle allowing deeper magmas to reach the interior bowl of the crater. Typical flow thicknesses appears to be 10 – 20 m. The majority of the flood basalt activity went on for nearly a billion years, mostly stopping some 3 billion years ago. Smoothness of the maria demonstrates the fluidity of the lavas. Elevation differences less than 150 m over a distance of 500 km are common.
While the most vigorous portions of the basalt flows into the maria were initially described as forming lava lakes, because they inundated neighboring craters, at this point, it appears the maria were filled by massive individual flows layered one on another. Later lava flows are easier to identify as individual lava flows. The smallest mapped basalt flow into Mare Imbrium is nearly the same size as the Columbia River Basalts.
Mare basalts may be more extensive than currently thought, extending under some of the highlands mountains which may be debris from basin formation. This is a possible explanation for dark halo craters in the neighboring highlands.
The mare basalts may end up being quite thin, especially around the edges of the filled basins. Depths toward the centers are not so readily determined. Average depths of the flood basalts in the maria is in the neighborhood of 2 km.
The oldest mare basalt dated from an Apollo sample is 3.85 billion years for an Imbrium basalt, which is a bit older than the generally accepted date for the impact event. More than 20 distinct basaltic types have been identified from Apollo samples. They show extreme variation in titanium content. Depths for the magma source is in the vicinity of 400 km. Note that the basin-filling eruptions started some 600 Ma after the formation of the moon itself, so there is a high likelihood that a significant portion of the interior of the moon was still molten.
The chemical composition and temperature of lunar basalts defines their viscosity. The basalts are low in aluminum and alkali elements and high in iron. This coupled with relatively high temperatures give them a very low viscosity similar to motor oil at room temperature, about ten times more fluid than terrestrial lavas.
Sinuous Rilles and Lava Tubes
Rilles are linear features on the lunar surface. There are generally two types ; straight and sinuous. Straight rilles are generally thought to be grabens, fault troughs, and are not generally connected to volcanic activity.
Sinuous rilles and lava tubes are connected to volcanic activity. They can be quite large. Most have a small pit at one end. Rilles and lava tubes are thought to be two flavors of what we see here on earth with highly effusive basalt eruptions. Rilles are simply lava tubes without roofs. In most instances, it appears the lava flow has cut a channel into older lava flows underneath it.
The majority of rilles are at the edges of the lunar maria or their boundaries with other maria. There are numerous rilles in the floors of lunar craters. Rilles and lava tubes do not appear to be features formed early in the moon’s eruptive history.
The length of lunar rilles is astounding. The longest is some 566 km. In contrast, the longest lava tube on earth at Barker’s Cave in Australia, is 35 km long. There are more than 200 sinuous rilles known. They vary in width from 160 m to 4.3 km. Oceanus Procellarum contains 48% of the rilles, with most of those associated with known volcanic centers within Procellarum KREEP Terrain, the Aristarchus Plateau, and the Marius Hills.
Most of these formed between 3.0 – 3.8 billion years ago. Some associated with the Aristarchus Plateau may be as recent as 1.0 – 1.5 billion years ago.
Lava flows here in Earth are generally additive, as they add new layers on top of older, cooled flows. The larger lunar rilles carried very hot lavas, which were able to cut channels into older, cold lava flows.
Apollo 15 landed on the edge of Hadley Rille. Due to the loose debris and the steep slopes, they did not sample the bedrock cut by flowing lava into the rille.
The final type of flowing lava construct is the lava tube. If the roof is intact, they are very difficult to observe from orbit. Fortunately, we have been able to start identifying them when portions of the roofs have collapsed. In some cases, this gives us a twisting line of small craters. In others, it is a single round hole (black pit) in the lunar regolith, called a window or a skylight. One has been identified in Marius Hills. Gravity sensing spacecraft like Grail helped identify at least 10 candidates for subsurface lava tubes.
The discovery of lunar lava tubes triggered a speculative cottage industry suggesting their use as habitats for future lunar visitors.
Pyroclastics and Dark Mantling Deposits
There are nearly 100 known explosive or pyroclastic eruption deposits. They are spread widely and intermixed with products of more effusive eruptions which have covered older deposits. They are generally very dark and smooth. While pyroclastics are widely dispersed, the dark deposits are thought to be volcanic vents.
The Apollo 17 Taurus – Littrow landing site is one of the most famous pyroclastic sites. It was chosen as a landing site because it was thought to be a young volcanic deposit. Actual dating of the samples showed it dated at 3.5 billion years.
The extreme darkness ends up being due to black glass beads. There were also orange beads collected at Taurus – Littrow. The beads are thought to be a product of fire fountain eruptions lasting days or weeks. Quenched glass is rapidly cooled. Beads are more slowly cooled. Generally, the darker the beads, the higher the titanium content.
The source of the pyroclastic magmas is thought to be very deep, perhaps more than 400 km, as they represent ancient volcanic materials which have a higher volatile content than more recent magmas.
As we have not gone to the moon in nearly half a century, most of the recent analysis of these deposits have been done remotely. Analysis of the remote sensing is informed by that actual samples taken from Taurus – Littrow.
Pyroclastic deposits are divided into two types – regional (over 1,000 km2) and localized (less than 1,000 km2). The regional deposits are thought to have been emplaced by fire fountain or Strombolian style eruptions. The localized deposits are thought to be the product of intermittently explosive or Vulcanian style eruptions, with explosive decompression acting to remove a plug of lava in a conduit.
There are also a small number of pyroclastic deposits on the lunar far side. Most are found on the near side around the periphery of the maria.
Remote sensing from India’s Chandrayaan-1 orbiter suggest a widespread occurrence of water in pyroclastic materials from deep sources. Water up to 150 ppm in the large deposits with local values in the 300 – 400 ppm range is in line with estimates of bulk water abundance in lunar mantle. This means that the water-bearing volcanic glasses from the Apollo samples are not an anomaly, and pyroclastic eruptions may represent a significant pathway for transport of water to the surface.
Cones and Domes
While the moon does have the functional equivalent of cinder cones, domes, and shield volcanoes, the volcanic action behind their formation is different.
The majority of erupted material on the moon was placed on the surface as flood basalts and pyroclastic flows. As the temperature of the magmas decreased, we start seeing more recognizable volcanic structures. Cones, domes, and shield volcanoes were formed with cool magmas toward the end of active eruptible life for that particular volume of magma.
Cones and domes are fairly low and wide, seldom more than a few hundred meters high, and in the range of 10 – 20 km in diameter. Sometimes they have a small central crater on top. Sometimes they don’t. I haven’t found anything so far describing calderas. Many of the cones are horseshoe-shaped, perhaps created by breaching by lava flows like we see in cinder / scoria cones on the Earth. The gentle slopes of these volcanic structures make them difficult to observe during most of the lunar day. Best observation is early in the morning or late in the afternoon when the sun is low.
The Marius Hills in Oceanus Procellarum is the largest group of cones and domes on the moon.
Cones are slightly steeper, associated with pyroclastic deposits, and appear to be formed by magmas with higher volatile contents.
Most domes are found in clusters and those clusters in turn are found around the borders of the lunar maria.
A 2013 paper by Spudis et al. described large regional rises in the lunar maria measuring tens to hundreds of kilometers in diameter and up to several kilometers in height as shield volcanoes. Like the domes and cones, they are found at the edges of the maria. Volcanic centers include Mons Rumker, Marius Hills, the Aristarchus Plateau, Hortensius, and Cauchy.
Marius Hills has long been known as a volcanic center with over 300 cones, domes, numerous sinuous rilles, and collapse pits. It is the site of some of the youngest lunar lava flows. The entire complex is on an elliptical topographic rise 330 km across. The rise tops out 2.2 km above the surrounding mare plain. The structure has been dated 2.5 – 3.3 billion years for its main lavas, and activity as recent as 0.8 – 1.1 billion years. The complex is also on top of a pair of gravity anomalies. The abundant cones suggest an extended magma evolution and partial crystallization.
Rumker has up to 30 domes over 70 km in diameter. It is 1,000 – 1,200 above the surrounding mare surface. Rumker looks like a miniature version of Marius Hills, though does not have any rilles.
Cauchy is the largest of the newly detected shields with numerous cones, domes, and sinuous rilles. It occupies the eastern half of Mare Tranquillitatis, 560 km across and 1.8 km high.
The Aristarchus Plateau is different, occupying the uplift of a structural block caused by the formation of the Imbrium basin. It has been covered with the products of massive eruptions and is the source of a regional pyroclastic deposit covering 49,000 km2. The Plateau has been described as a proto-shield, formation halted partway. It measures 240 km in diameter up to 2 km high.
Irregular Mare Patches
Irregular mare patches (IMPs) appear to be the remains of small basaltic eruptions scattered across the volcanic plains. There are at least 70 of these features observed so far. IMPs are thought to be remnants of small basaltic eruptions. Like the cone, dome, and shield volcano activity, these tend to be located around the periphery of the lunar maria. IMPs are small, which is why they were discovered relatively late in lunar exploration. They average around 500 across.
The best known IMP, Ina or Ina-D was first spotted by Apollo 15. Originally thought to be the collapsed summit of a low shield volcano, the lack of craters suggest this is much younger than the surrounding mare basalt.
IMPs have smooth deposits which are sometimes connected to the surrounding mare basalt, and rough-looking deposits that end at steep edges of the smooth deposits. To some, it looks like the smooth materials are covering portions of the rough materials. Topographic measurements indicate that the smooth deposits average 8 m thick (2 – 20 m), or about the average thickness of lunar basalt flows.
Slopes at the edges of the smooth deposits are measured at 30 – 35 degrees, higher than the natural angle of repose. This is also used as evidence that these are relatively young features. This along with crater counts confirm that Ina may be as young as 33 Ma. Crater count modeling on other IMPs range between 18 – 58 Ma.
The study of IMPs suggest that young, small volume extrusions of mare basalt still happen. And if they do, this means that lunar volcanism didn’t stop quickly, but tailed off slowly over time. It also means that it may not be done yet. One anomaly from Apollo 15 and 17 heat flow measurements from buried thermometers in the regolith was that temperatures recorded were a bit higher than models suggested. This was written off as the result of local anomalies. Perhaps the measurements were accurate and the lunar interior is still hotter than expected.
Transient Lunar Phenomena (TLP)
This last category may include volcanic activity and / or outgassing of some sort. TLPs have been observed on the moon since 1540, perhaps as far back as 1178. They are a combination of quick flashes due to impacts on the lunar system, to changes in overall luminosity or colors of patches on the lunar surface. Descriptions use terms like mists, clouds and volcanoes for the events. Length of the events range from a few second flash to a few hours brightening.
The first reports were by five Canterbury monks on June 18, 1178 who reported a flaming torch springing from the crescent moon. This torch was tied in 1976 to an impact that created a 22 km diameter crater Giordano Bruno.
Over the years, numerous impact events have been observed by amateur astronomers, much like asteroid and comet impact transients on Jupiter.
The longer events are more difficult to explain and may involve some sort of gas release. A third of the 579 recorded events took place in the vicinity of Aristarchus crater on the near side. One was observed while Apollo 11 was orbiting the moon. They looked in that direction and reported the following:
“Hey, Houston, I’m looking north up toward Aristarchus now, and there’s an area that is considerably more illuminated than the surrounding area. It seems to have a slight amount of fluorescence.” NASA, Apollo 11 Transcripts
Apollo 15 reported a significant rise in alpha particles as it passed over the area. Alpha particles are caused by radioactive decay of Radon-222 which has a half-life just under 4 days. http://www.armaghplanet.com/blog/whatever-happened-to-transient-lunar-phenomena.html
Another possible description is electrostatically charged lunar dust, which is kicked high above the lunar surface at sunrise. https://science.nasa.gov/science-news/science-at-nasa/2005/30mar_moonfountains
At this point, the actual cause of the longer lived phenomena is unknown, though we have pretty good explanations for the short events (impact) and Apollo astronauts have observed the electrostatically levitating dust at sunrise. Nobody has tied outgassing or an eruption to a TLP yet.
For a seemingly dead body, the moon has quite the volcanic history. After its formation, the creation of the large impact basins of the maria triggered a billion years of flood basalt eruptions. As the volume of available magma tailed off, and that magma cooled, we started seeing more familiar volcanic activity and structures – pyroclastic flows, rilles, lava tubes, cones, domes, and shield volcanoes. We may or may not be past the end of volcanic activity on the moon, but the existence of IMPs and TLPs argue otherwise.
We simply don’t have enough information on the volcanic centers at Aristarchus, Marius Hills, Rumker, or Cauchy to know how recent the activity is. There are a lot of reasons for a return to the moon. Geologic study of volcanism is one of them.