(This article originally appeared in the first quarter 1997 STAR Newsletter and was written by Eric Douglass – Ian)
Geologic Processes on the Moon
by Eric J. Douglass
I – INTRODUCTION
This article discusses the processes that form the features we see on the moon. Its sister paper, ‘The Geologic History of the Moon’, deals with how each of these processes fit into the moon’s
The primary geologic processes that shaped the moon are the formation of craters, volcanic activity, and tectonic activity. Each of these will be dealt with in their respective sections below.
II – CRATERING ON THE MOON
Introduction
Craters cover the surface of the moon. They are the result of high velocity impacts on the surface by meteorites. The velocity of meteorites upon lunar impact varies, but is generally between 10 and 40 km/sec. The variations result from differences in the native velocity of the object prior to reaching the moon and the direction with which it approaches the moon. For example, it if is catching up with the receding moon, then one must subtract the velocity of the moon from the impactor (the moon’s average velocity through space is 30 km/sec). On the other hand, if it is coming at the moon head on, then the two velocities are added together. The velocity is further increased by the gravitational pull of the moon (escape velocity = 2.4 km/sec).
The velocity of a bolide (the technical name for a body that strikes any planetary surface) is important for it is the major determinant of the amount of energy released upon impact. Bolides possess ‘kinetic energy’, and the value of this is proportional to the mass of the bolide multiplied by the square of the velocity. Thus, if there are two meteorites of the same mass striking the lunar surface, but one has twice the velocity of the other, then the faster one possessed four times (not two times) the kinetic energy of the slower one. Because of the high velocities, the value of its kinetic energy tends to be very high.
When a meteorite strikes the moon, all of this kinetic energy must ‘go’ somewhere (conservation of mass and energy). Upon impact it is transferred to a massive shock wave which goes both down into the moon’s surface and rearward into the bolide itself. The shock wave that goes rearward into the bolide is so high that it exceeds the strength of the rock–the bolide itself vaporizes. The shock wave that goes forward into the moon vaporizes part of the surface of the moon (several times the mass of the bolide), melts some of the surface deeper down (up to 100 times the mass of the bolide), and shocks (fractures) the surface deeper yet. On the surface, part of this shock escapes around the edges forming a route of escape for some of the vaporized/melted/shocked rock. This escape of material creates the crater itself, and the material that escapes will form the ejecta which goes outward onto the moon surface. Finally, part of the shock wave from the impact travels further throughout the bedrock of the moon, acting now as a seismic wave, creating effects further away (such as activating older faults, creating landslides, etc.). Note that part of the energy is also transformed into waste heat.
From this brief description on the kinetics of crater formation, we will now look at the types of craters and the unique morphology of each. While craters can be divided into a variety of different classes based on their size and morphology, I am going to use a fairly simple one. I will divide craters into simple craters, complex craters, and basins.
Simple Craters
Simple craters are bowl like depressions in the lunar surface. They occur from submillimeter size to approximately 15 km in diameter (15-20 km is the transition zone between simple and complex craters).
Simple craters form when smaller meteorites strike the moon at high velocities. The bolide is vaporized along with the surface struck (the target). This vaporized rock will go two directions: out along the sides, creating a part of the ejecta blanket, and inward, injecting the crater itself with bits of vaporized rock which will later cool. The next layer created in the crater is melted rock. This rock again will go in both directions. Below this is the fractured rock, of which some is again pushed in both directions. The crater itself is formed by decompression along the sides of the crater, allowing vaporized, melted, and shocked fragments to escape. This material will lay itself down as the ejecta blanket, which has four distinct parts. Just outside of the crater rim is the zone of continuous ejecta, which is formed from the last material ejected from the impact. The next layer out is the discontinuous ejecta, which inter-fingers with the surrounding lunar surface. Further out yet is the bright ray system, which is formed from the first material of the ejecta. The fourth part of the ejecta is found in the area of the discontinuous ejecta and just further beyond it–this is the area of ‘secondary cratering’, which results from ‘chunks’ of material which are thrown out from the crater. This typically forms a ‘herringbone’ pattern on the lunar surface, with multiple craters in a line which is tangential (not radial) to the crater itself. The crater rim is composed of wall material which is pushed up from the impact shock wave. The crater formed once the ejecta has exited is called the temporary crater, for other things will modify its final form. For simple craters, this comes down to impact melt, which was pushed against the side wall, sliding back down into the bottom of the crater, along with any other unstable material on the crater’s sides or rim. This will be the craters final form.
Observation of such a crater will reveal a bowl shaped depression with a sharp rim, some rim deposits (blocks of material thrown out at the end of excavation), a discrete ejecta blanket grading from continuous to discontinuous (with secondary craters which will be so small to see from earth based telescopes) and a bright ray system. Across time, parts of this crater will degrade due to the erosive rain of micrometeorite impacts. The first to go will be the ray system, followed by the discontinuous ejecta and the sharp rim. This will continue until only a bowl shaped depression with a gentle slope remains.
Complex Craters
Complex craters begin at 20 km. They are characterized by the morphology of a bowl like depression with a central uplift on one or more massifs (small, mountain like structures).
The origin of complex craters is by medium sized meteorite impact. The impact occurs as discussed in the simple crater above, though the energies involved are much greater. The differences begin after the formation of the temporary crater. At this point the rim is much greater (heavier) than in a simple crater. Because the subsurface rock is extensively fractured, this rim material cannot be supported. It slides down these fractures (called ‘slumping’) creating a series of ‘terraces’ on the crater’s inner walls. In addition, there is a central peak or peaks formed during this time. While the mechanism for central peak formation is not well understood, it appears that the impact compresses the underlying rock, and this rock rebounds after the shock energy is dissipated. Its size is also modified by slumping of the rim material, which pushes more rock down and into the bedrock beneath. At the same time these formations are occurring, the impact melt on the sides of the crater is also sliding down along with other unstable side/rim material. This again covers the bottom of the temporary crater as well as pooling in some of the terraces.
The parts of the complex craters now are the central uplift which can be one or several peaks which may attain heights of over a 1000 meters. This is followed outward by the flattened floor of impact melt which grades into the terraced sides. The rim occurs at the top of the crater and grades out into the continuous ejecta, the discontinuous ejecta, the larger secondary craters (which now can be seen by earth based telescopes; e.g.: Copernicus), and the ray system.
Degradation occurs in complex craters as in simple craters. First the ray system goes, followed by discontinuous ejecta and the sharp rim. The continuous ejecta erodes later along with the terracing and central peak. Across time, the crater will become a simple bowl like depression.
Basins
Basins begin at 150 km in diameter. They are characterized by one or more inner rings from the main outer ring. So, instead of forming a central peak/peaks, they form a ring or a series of rings. Multi-ring basins are the largest cratering events on the moon, and can span up to 2500 km in size.
Basins originate from large meteorite impacts. They impart so much kinetic energy to the surface of the moon, that the shock wave not only excavates a crater, but also causes the inner surfaces to act more like a fluid (material with low inherent strength). Because of this, when the central rebound occurs, it spreads out from the center in a ring until its energy is dissipated by the rock it is passing through (friction). At this point the ring ‘freezes’ in place. In the truly massive impacts, the center can rebound multiple times, with each rebound forming another ring.
Note that the formation of multi-ring basins is poorly understood, and competing theories do exist. The problems with the models are that the amount of kinetic energy released is so much greater than any event known on earth (atomic bombs only release a fraction of this amount of energy), and that it is difficult to predict exactly how solid surfaces behave under its influence. The above model assumes that the energy is sufficient to make the solid surface act like a fluid surface (one with low inherent tensile strength), and so the rings form like a stone dropped into still water. The basin with the greatest amount of rings in our solar system is on the moon Callisto, where around 25 rings are found.
When such a massive impact occurs on the moon, the kinetic energy transfer creates a massive shock wave which causes vaporization of the bolide and the surface of the moon. As in the simple crater, this material is both injected into the next layer down, and allowed to escape out the sides. The next layer is the melted rock, with its material also going in both directions. The final layer is of the shocked and fractured bedrock, of which some again goes in both directions. The temporary crater which forms is bowl like in form. Then a central uplift occurs in the center from rebound of the underlying rock. This rebound cannot come into equilibrium, and so is collapsed back down where this excess of rock forms a wave which propagates out across the inside of the temporary crater. As this propagation is occurring, other rebounds may occur, depending on the mass of the impactor (from one to three interior rings). The wave(s) finally freezes in place as its kinetic energy is dissipated by friction. At this time there is other modification of the basin, which involves both slumping of the rims of the rings and impact melt sliding down the sides and pooling either in the terraces or between the rings. The secondary craters that form can be up to 20 km in diameter.
The morphology of a multiring basin is best illustrated by the Orientale basin, which while being one of the most recent and so the least degraded, is also on the limb of the moon where only a fraction of it can be seen. However, it is well photographed by spacecraft. The center of the basin is flat, and probably covered with impact melt (it has since been modified by volcanism). Further out, at a spacing of the square root of two (no one knows why, but the rings are spaced at the square root of two times the inner ring radius) one will come to each successive ring. The rings each have terraced sides and pools of impact melt. Beyond the outer rim, there is the usual ejecta blanket, with continuous/discontinuous/secondary impacts/ray system. However, here it is much more massive (the secondary craters can be 10-20 km across, and the continuous ejecta can be hundreds of meters thick). Also, note that the ejecta can now form whole ‘mountains’ on the moon, called ‘hummocky’ terrain (and is easily seen in the Janssen formation, which is part of the Nectaris Basin ejecta sheet). Part of the ejecta blanket of the Imbrium basin can also be seen, and here it is called the Fra Mauro formation. Finally, a lot of damage is done when the ejecta blankets strike other formations already existent on the moon. Examples are clearly seen around the Nectarian, Imbrium, and Crisium basins (if you look in the correct areas).
Across time, the parts of the basins all became degraded from continued micrometeorite erosion. Indeed, as the basins are all very old, none of them have a ray system or discontinuous ejecta. Secondaries can still be seen.
III – OTHER EFFECTS OF CRATERING ON THE MOON
The process of cratering has several effects on the moon besides the creating of the crater and its ejecta. These are examined in this section.
First, the cratering event creates a shock wave which continues to travel across the moon. If this wave contains sufficient energy, it will cause faulting in the bedrock (the Straight Wall is an example of this). It can also activate faults that already exist. Finally, it can loosen semi-stable materials sufficiently to allow movement under the influence of gravity. An example of this is the landslide in Copernicus which was caused (it is thought) by the shock wave from the impact of Tyco.
Upon impact, basins spread a thick ejecta blanket over a huge section of the moon. The accumulation of these formed a layer, which is thought to be several kilometers thick, called the megaregolith. On top of it is yet another layer composed of fine, dusty material called the regolith. It was created by the continual rain of small meteorites/micrometeorites which slowly, over billions of years, eroded the top layers of the surface rocks. This layer can be over 15 meters thick on the lunar highlands, and up to 8 meters thick on the mare. Thus, while the appearance of many features is ‘sharp’ in an earth based telescope, it is actually quite rounded in the Apollo photographs taken from ground level.
The regolith, as a discrete layer, actually acts as a protective shield to the underlying structures (megaregolith, lava). Micrometeorites and small meteorites are not able to pierce this thick layer in making their craters. This protects the underlying structures from further degradation. However, meteors around 2 meters in diameter would be able to pierce the 10 meter highland regolith (depending on its velocity).
In an above section, we examined the erosive effects of micrometeorite impacts on craters/basins, and noted that this occurs in an orderly fashion. By knowing the level of degradation, one can predict the general age of the crater. A few other notes on this process need to be stated.
First, small craters degrade more quickly than larger ones. Second, ray systems degrade faster on mare surfaces. However, given these two problems, we can still tell much about the age of intermediate sized craters given the amount of erosion each one exhibits.
Medium sized craters which are only rounded bowls with no rim crest are Pre-Nectarian in age. Ones with a rounded, dull rim crest but little else are Nectarian. Ones with a sharp rim, a central uplift, terracing, and a smooth continuous ejecta are Imbrium. Ones with a sharp rim, rim deposits, terracing, a central peak, continuous and discontinuous ejecta are Eratosthenian. Ones with sharp rim, rim deposits, terracing, a central peak, continuous and discontinuous ejecta and a bright ray system are Copernican. The dates for these periods, given our application of radio dating are: Copernican–present to 1 billion years of age; Eratosthenian–1 billion years to 3.2 billion years of age; Imbrium–3.2 billion years to 3.85 billion years of age; Nectarian–3.85 billion years to 3.92 billion years of age; Pre-Nectarian–3.92 billion years of age to moon’s formation.
Basins, on the other hand, are all much older. The youngest basin on the near side is Imbrium, which is 3.85 billion years of age.
There are no other significant erosive forces on the moon. The atmosphere in nearly non-existent resulting in no aeolian (wind) erosion, there is no hydrologic cycle, and tectonic forces are minimal in the present period (these will be discussed below).
IV – VOLCANISM
Next to cratering effects, the effect of volcanism is the next major geologic force on the moon. Radioactive elements (such as uranium, potassium, and thorium) reheated sections inside the mantle of the moon creating a series of partial melts. These melts were less dense than the surrounding mantle rock, and so they began rising toward the surface under the effects of pressure. Their surface eruptions preferentially occurred in basins, because these massive impacts sent cracks deep into the lunar crust (tens of kilometers down) which acted as conduits for the lava. Further, these partial melts were closer to the surface in basins because of mantle rebound. So, lava preferentially filled the basins because the mantle was closer to the surface and the surface there was deeply faulted.
As lava erupted into the basins, it sometimes flowed long distances before it finally ‘emplaced’. It could do this because lava on the moon has a low viscosity (it is very thin and runny). Indeed, when lava material was melted on earth, it was shown to have the consistency of motor oil. This is because lunar lava is low in silicates (‘mafic’), whereas earth lava is higher. So, when lunar lava erupted on an inclined surface, it would flow downhill, eventually creating a river-like channel. This formation is called a sinuous rille. These channels run up to several hundred kilometers before finally spilling their lava on a flatter surface (obviously, if lava erupted on a flat surface, these sinuous rills would not occur).
This process of mare flooding from fissure eruptions resulted in large flat lava sheets which covered the basin bottoms. Because the basins were concave in shape, lava was thicker in the center of the basin and thinner towards the edges. Now lava is denser (heavier) than the surrounding crustal rock, so it begins to ‘compress’ the bedrock underneath it (a process called ‘subsidence’). The thicker areas would do this more than the thinner areas, creating a gentle bowl shaped depression in the basin. This process allowed for three unique formations to occur.
First, across time other lava flows would occur onto the same surface. When the next one did occur, it would follow the inclination of the depression and fill the center of the basin. This flow would now also be thicker in the center, and thinner out toward the edges, and the process of subsidence would again occur. This kind of flowing into the more central areas and then subsiding made the basin lava look like the rings in a target. The outer rings would be the older ones and the inner rings would represent the youngest flows (Note that each lava period produced lava with slightly different compositions, and this made each flow appear different to earth based telescopes).
Second, this subsidence created stresses within the lava flow itself. So as the lava in the center sank in, it would cause a compression force at the juncture areas when outer lava didn’t sink in as much. These forces would cause the lava to ‘buckle’, and we see these as ‘mare ridges’. While there are several types of mare ridges (discussed below), these ones occur around the edges of lava filled basins, and consist of a wide, gentle sloping arch with a thinner, sharply twisting spine on top.
Third, the subsidence of lava with refilling in the center and further subsidence put stresses on the bedrock underneath the lava. This rock was already deeply fractured from the basin impacts themselves, and this stress downward and inward caused some of these faults to activate. They opened up creating a series of ‘grabens’ (a type of faulting where extensional forces open up two parallel faults with the area between them falling in), which are called arcurate rills. These, also, are only found around the edges of lava filled basins (the best examples are those around the Mare Humorum), though they may extend from these onto the surrounding highlands.
Now we have the usual scheme for lava filling of the basins, with their sinuous rills, arcurate rills, and mare ridges. Next we need to examine a few other unusual features associated with mare volcanism.
The first of these is the lunar volcano. On earth, with our higher silicate lava, we have tall volcanoes–sometimes miles high–with steep inclines. Since lava on the moon is thin and runny, it doesn’t pile up much. So lunar volcanoes are generally low in relief (under 300 meters in height) with gentle sloping sides. Most are 5-20 kilometers across. They are most similar to earth ‘shield’ volcanoes, which form from more mafic (lower in silicates and hotter) lava. It is of note that some lunar volcanic structures do have steeper slopes (some of those in the Marius Hills region), though the causes of this are not well understood.
If one looks around the edges of some of the basins, one will see some isolated dark patches. These are called dark mantling areas. They were formed by the process of ‘fire fountaining’. When lava rises from the mantel, the tremendous pressures it has been under are suddenly released. This permits gasses that had been trapped in the lava to emerge and escape (called degassing). These gasses can act as propellants, shooting the lava high above the lunar surface (the same process occurs on earth). On the moon, the propellant gas is thought to be carbon monoxide. Above the surface the lava breaks apart into beads and cools. The Apollo missions did return some of these glassy volcanic beads (called orange glass). Some excellent examples of dark mantling are see around Mare Serenitatis..
Next, there are a few places where a series of ‘endogenous’ craters line up either in a line or along a rille (a good example is Hyginus Rille). These are interpreted as being volcanic in origin, but it isn’t clear whether they are eruptive fissure vents or collapse features which may not have actually extruded lava. Another possibility includes eruptive degassing near the surface. Generally, those with rims are interpreted as fissure vents, and those without as collapse features. But only a return to the moon with further geologic work will resolve their origin.
Finally, there are ‘dark halo’ craters on the moon (such as in Alphonsus). These features are composed of a small crater with a surrounding apron of dark material. There are several possible origins for these, but all involve volcanic materials. Some are definitely due to an impact piercing a thin surface layer to exhume volcanic material buried underneath. These provide evidence for highland volcanism, where the regolith/megaregolith has formed a thin veneer which covers over an older lava flow. Others are of less certain origin, especially those associated with rilles. Some of these may represent dike penetration by an impact, or pyroclastic venting, or degassing features, or even spatter cones.
V – TECTONIC PROCESSES
Tectonic forces refer to forces which deform the lunar surface. These can be endogenous (such as thrust faults) or external (such as the creation of faults by impact events). Most of the present day tectonic activity on the moon is the direct result of impacts and volcanism.
Crater Induced Processes
Impacts create a shock wave which propagates throughout the surface of the moon. These waves, if of sufficient energy, can induce faulting in the subsurface bedrock, can reactivate faults located elsewhere on the moon, and/or can induce local changes in semi-stable materials (e.g.: landslides in crater walls).
Examples of faulting in the subsurface layers are seen around a variety of basins. The faulting can be radial (straight out from the basin’s center) or concentric (around the basin’s sides). Examples of concentric faulting include ‘arcurate rills’. These are places where concentric fractures were created by the basins and then covered by the basin’s ejecta. At a later time, lava poured onto the surface and reactivated these faults (see above in the ‘Volcanic’ section) creating the grabens we see today. Good examples of this are around Mare Humorum and Mare Serenitatis. Similarly, structures radial to Imbrium, like the Straight Wall, the Alpine Valley, and the Cauchy rilles may have been formed by the Imbrium shock wave. The Straight Wall was probably activated at a later time by lava flowing and subsiding in that area (the ‘down’ side of that fault faces the deeper lava section). Another example is seen in the ejecta associated with the outer ring of the Imbrium basin, where one can see down-warping areas in radial and concentric patterns.
Other already existent faults can be activated by the shock wave. These faults were caused by former massive impacts, and then covered by their ejecta.
Semistable material can be made unstable by a shock wave, creating a landslide in a crater. An example of this is the landslide in Copernicus which was though to be triggered by the impact Tyco.
Volcanism as a Tectonic Process
Other types of tectonic activity are found in association with volcanism. Lava, by coming from the mantle, is denser than the overlying crust (see discussion on the Magma Ocean Hypothesis in the associated paper ‘The Geologic History of the Moon’). When lava flows and emplaces on a basin, it is thicker (and thus ‘heavier’) in the center, and thinner on the rims. Because the lava is denser than the crustal rock, it compresses the rock beneath, and this occurs more where the lava is thicker–that is more toward the center of the basin and less on the periphery. Such a situation induces local stress fields within the subsurface rock, and this rock–if there is a fault in it–will activate and open up (crustal extension). If two parallel faults exist, then the opening will cause the center to slump and a graben will be formed. This is the cause of arcurate rills (as noted above).
Another formation caused by this subsidence of lava is the ‘mare ridge’. When the thicker, central lava subsides more than the thinner, peripheral lava, it creates a compressional stress in the lava bed above. At this place a mare ridge will form. These mare ridges have a wide, shallow ‘body’ with a thin, windy spine on top. They are generally concentric with the basin rims (for more information, see above under ‘Volcanic Processes’ above).
There are other causes for mare ridges, and we will examine these now. Mare ridges can also form over crater/basin rims. If a basin/crater rim is covered by lava, then the same situation exists as above–that is, we have a shallow shelf of lava over the rim and a much deeper shelf of lava over the area where the rim falls off. The lava will subside more over the deeper area and less over the shallow area, and this will induce local stress in the cooling lava above. At such a point a mare ridge will occur. Yet another cause for mare ridges is a volcanic intrusion just under a shelf of cooling lava or activation of a fault with slippage as the lava is cooling.
Tidal Interactions
Tidal forces refer to the stresses induced by gravity between bodies. For example, the Earth’s tides are caused by the tidal stress induced by the moon. As earth is larger, it induces proportionally larger stresses on the moon. In fact, the earth exerts sufficient force to distort the moon’s shape, so that it is not perfectly round. Before the moon was in a locked rotation with earth (the same side of the moon always faces the earth), this distortion changed the shape of the moon as it rotated, creating massive moonquakes and subsurface faulting. Unfortunately, this was long ago and the effects of this stage have been wiped away. This kind of distortion caused tidal slowing, meaning that the friction of these events slowed down the moon’s spin. Eventually, the moon locked into synchronous rotation, meaning that the same side of the moon faces earth. Interestingly, the moon is also causing tidal slowing of the earth, and our spin is ever so minutely slowing across time.
Now, if the moon were completely locked into rotation with earth, there would be no more seismic activity on the moon. However, the seismic monitors left by the Apollo missions revealed small moonquakes–Richter Scale 2-3. This is because the moon still has some wobble (librations), and this causes changing tidal stresses which produce the continuing moonquakes.
Endogenous Forces
The only endogenous tectonic forces are those induced by the moon’s continued cooling. With this cooling comes shrinkage. But the solid crust cannot shrink with it. Consequently, it faults where the stresses build up. In these, the crust on one side of the fault slides up diagonally on the other side, thus releasing the stress. This is called a thrust fault. Similar faults exist on Mercury where the shrinkage has been even higher. While these faults are small, there are many of them, and they are continuing to form.
