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(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 history.
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.
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