Volcanism on the Moon
Section M.4.2.
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Lunar Volcanism

Volcanic rocks are produced by the melting of other rocks. On Earth, this can be performed in a number of ways including the complicated method of subduction. On the moon, it appears that volcanic rocks were produced in only one way: that of partial mantle melting. Recycling of lunar crust did not occur or it was very rare. The melts produced on the moon are basalts.

Basalts are common products of mantle partial melting on the terrestrial planets. This is mainly due to broad similarity of their mantle compositions. For partial melting to occur on the moon, temperatures greater than 1100oC at depths of about 200km are required. The bulk eruption styles appear to be in the form of lava flows. There is also widespread evidence of fire-fountaining forming pyroclastic deposits (typically glass beads).

Volcanic Landforms

Mare lava flows

The mare plains were formed by low viscosity basalt lavas being erupted in large volumes. Current usage describes the lavas as maria and the basins as mare.

The best descriptions of flows come from the Apollo J missions. In Mare Imbrium, lavas were erupted from the southwest edge, in three successive flows. The flows extend for 1200km, 600km, and 400km, and at a slope of about 1:1000. Flow scarps bounding these flows vary from 10m to 63m in height (Gifford and El Baz, 1978). Combined, these flows cover about 2x105 km2 with a volume of about 4x104 km3. The concept of very long flow distances over flat terrain is supported by leveed flow channels and localized lava ponds (dammed by wrinkle ridges).

Such thin flows would be expected to cool quickly. The apparently contradictory long flow distances was explained by the unusually low basalt viscosities (Weill et al, 1971).

Supporting evidence for lava flows about 20m thick came from petrological analysis of samples from Apollos 11, 12, and 15 (Brett 1975a). Also, three distinct lavas flows were observed in the wall of Hadley Rille. These three flows were within 60m of the mare surface. Crater densities vary between different flows, indicating substantial periods of time between individual eruption cycles. Crater-density ages (Boyce, 1976) indicate that lava production in the main mare basins lasted about 500Ma. Infra-red reflectance also indicates a large variety of compositions (Pieters, 1978). Apollo has only sampled some of these varying compositions. Also, Apollo has "missed" the oldest and the newest lavas.

Radiometric dates show that the prominent basalt eruptions over the entire moon lasted from about 3.9Ga to 3.1Ga (Head, 1976a). Crater densities suggest that minor eruptions may have continued to 2Ga (Schultz & Spudis, 1983). The start of maria eruption is unknown, but the oldest basalt sample is dated at 4.2Ga (Taylor et al, 1983). This was extracted from a highland breccia. No corresponding outcrop has been found.

Sinuous Rilles

These are meandering channels which commonly begin at craters. They end by fading into the mare surface, or into chains of elongated pits. Sizes range from a few tens of meters to 3km in width. Lengths range up to 300km in length. Channels are U-shaped or V-shaped, but fallen debris (from the walls, or crater ejecta) have generally modified their cross-sections. Most sinuous rilles are near the mare basin edges, although they are found in most mare deposits. Apollo 15 confirmed the theory that sinuous rilles were analogous to lava channels and collapsed lava tubes.

Lunar rilles are much larger than their terrestrial equivalents. This is thought to be due to a combination of reduced gravity, high melt temperature, low viscosity, and high extrusion rates.

Mare Domes

Domes are defined as broad, shallow landforms. These are convex, and circular to oval in shape, and occur on the mare basins. Eighty low domes (2-3 degree slopes) have been mapped (Guest & Murray, 1976). Diameters range from 2.5 to 24km, and heights vary from 100 to 250m. Most of these domes are in the Marius Hills area where they accompany other interesting geological features (possible collapsed lava tubes, etc). Some domes have summit craters or fissures.

Observation and mapping of lava domes still continues. Due to their low profile, Orbiter and Clementine images do not tend to show them very well, due to the typically high solar angle on such images (they have short shadows). Hence observation is better from Earth, where the solar angle can be deliberately chosen. A number of amateur groups, such as the British Astronomical Association, run active mapping and observation programs. Note: These should not be confused with terrestrial "volcanic domes". On Earth, the term "volcanic dome" implies a silicic, highly viscous lava (cf. Mt. St. Helens). No such lavas have been found on the moon. Formation of lunar domes is still an unknown area. They might be due to the eruption of more viscous basalt lavas; by intrusion to form shallow laccoliths; or by mantling of large blocks of older rocks.

Lava Terraces

Small lava terraces have been observed within some craters and along mare-highland boundaries. These have been interpreted as "shorelines" left as lava has withdrawn (e.g., back into a vent or lower basin).

Cinder Cones

Terrestrial cinder cones are formed from lava bombs, "cinders", etc., erupted explosively from a central vent. The volume of each cinder cone is much smaller than the total basalt erupted from the cone.

On the moon, cone-like landforms have been observed in association with rilles. Most are less than 100m high with diameters of 2-3km, and have a low albedo. Some have summit craters (with diameters of less than 1km). Lines of cones are thought to be fissure vents.

Dark-haloed craters & Pyroclastic Deposits

Extrapolation from terrestrial cinder cones suggests that lunar deposits should broad, pancake-shaped, and low. This is mainly due to the low gravity. Although no such structures have been identified for certain, this might be an explanation for the numerous dark-haloed craters. Again, these are usually along the margins of mare basins, or along rilles and other lineaments. Most are shallow with dark rim deposits 2-10km in diameter.

Apollo 17 managed to sample the dark-haloed crater "Shorty". This turned out to be non-volcanic, but it penetrated a loose layer of glassy and partly crystalline droplets. These were orange and black in color. They proved to be volcanic in origin (Delano, 1986) and to predate "Shorty". It is unknown whether other dark-haloed craters might be volcanic. Unambiguous dark-haloed impact craters can be found in highland areas adjacent to the mare. It has been suggested that these areas are due to highland-derived debris (i.e., regolith) overlying very early mare basalts at the rims.

Whether these black deposits are from the central crater, or exhumed by an impact, they are still of interest due to their relation to explosive volcanic activity. Such active eruptions imply a high volatile content in the melt. The material sampled from "Shorty" proved to be low-viscosity, but rich in iron and titanium. These droplets tend to be mantled in condensed volatiles such as zinc and sulfur. Compositions of the droplets themselves match those predicted by partial melting of the mantle. It is still possible that the melt was contaminated by assimilation of wall rocks (which would probably be basalts).

Filling of the Mare Basins

The Mare Basins were formed by large impacts. These impacts would have formed large cavities which collapsed almost immediately. These collapses led to large-scale fracturing in the crust. These fractures are thought to be the conduits along which the basaltic magmas ascended to the surface. Such conduits appear to occur just within the basin edges -- i.e., where large slump zones would be expected.

What caused the melting? This might be due to the thinner overlying rock (i.e., basin-filling ejecta) having a lower thermal bulk conductivity. Hence, the mantle isotherms would move upwards (cf. a house with a thinner roof would have a warmer roof than a house with a thick roof; hence the loft would be even warmer!). This warming in the upper levels of the mantle might be sufficient for partial melting to occur (Arkani-Hamed et al 1973). Another alternative is that the sudden removal of the overburden led to an isostatic uplift (cf. a terrestrial mountain range lifts up as the top is eroded). This reduction in pressure could lead to the formation of partial melts (Brett 1976). On Earth, ocean ridges form melt by releasing the confining pressure on the upwelling mantle underneath.

Infilling may have occurred for quite a while after the initial basin excavation. The Imbrium Basin is currently dated at 3.9Ga, but the Mare Imbrium lavas are dated at 3.3Ga (Apollo 15).

Most of the mare basalts are confined to basins, but occasionally they overflow into adjacent terrains. Also, many of the basins are not-infilled -- especially on the far side. This is thought to be due to the moon's thicker crust on the far side (gravity data: Bills & Ferrari, 1976; Solomon et al, 1982). There is also a correlation with age. The youngest basins tend not so show any volcanic activity, whilst older basins are infilled. There are exceptions to both of these trends.

Mare Orientale, the youngest large lunar basin, is only partly filled with basaltic lavas. Its interior is dominated a flat layer of impact melts -- possibly non-basaltic in composition. Later basaltic melts have overspilled onto these impact melts. This is thought to represent the early stages of basin infilling. Eruptions begin with infilling of the innermost ring, and then along the ring's concentric fault systems. Vents and domes are also well-preserved along the next ring (the Rook Mountains). I.e., eruption vents are along ring faults. (Lunar Sourcebook, Fig 4.28 for detailed pictures).

Mare Imbrium is older. Its ejecta deposits cover a large part of the near side. No impact melt is visible. Lava flows cover most of the area between the inner ring, and the outer (topographic) ring (the Appennines). The Serenitatis Basin is older still. Its interior rings are completely flooded. Solomon & Head (1980) have modelled the evolution of this basin with three stages. They suggest that the Stage I lavas erupted in the southern basin rim. Some spilled over into Mare Tranquillitatis (sampled by Apollo 11: 3.65-3.85 Ga). During this time, the center of the basin sank under the load of the Stage I lavas. Stage II lavas erupted into this newly made depression. These are approximately concentric to the Stage I lavas, and clearly show fewer craters (hence are younger). Further subsidence occurred due to the extra loading. Stage III lavas occupy the very center of the basin. Some Stage III lavas managed to spill into the Imbrium Basin, illustrating the very low gradients present.

This model suggests that central mare thicknesses may exceed a few kilometers. It also accounts for the compressional ("wrinkle") ridges (Muehlberge, 1974).

Most petrological models suggest that the basalts formed by partial melting at depths of 200-400km. The melts are of lower density than the surrounding rocks -- hence they tended to rise to the surface. In the brittle upper crust, faults and fractures would have formed the conduits. Fissue vents only have to be 10m wide (Head & Wilson, 1979) to produce the observed high eruption rates. Vertical speed of the rising melts had to be greater than 0.5-1m/s in order to maintain lava fountains at the fissures. Vesicles (bubbles) are observed with lavas and pyroclasts. Unlike on Earth, water could not have been a significant gas component due to its scarcity. Sato (1978) proposes CO as the main gas phase which drove the volcanic activity. Head & Wilson (1979) found that only 250-750 ppm CO was required to disrupt magmas at 15-40m depths. I.e., bubbles grow so big that they burst and spatter the melt (fountaining).

Mare Volume Estimates

The volume of erupted melt is dependent on the degree of partial melting, and how much of the mantle experienced partial melting. Mare basalts cover about 6.4x106 km2 of the moon's surface. There are no direct measurements of the mare thicknesses, but radar sounding (Peeples et al, 1978) found discontinuities at 1.6-2km (Mare Serenitatis); 1.4km (center of Mare Serenitatis); 0.8-1.0km (peripheral shelf of Mare Serenitatis). Isopach modelling of (ideal) crater shapes (DeHon & Waskom 1976; DeHon, 1979) indicate thicknesses of 0.5-1km in shelf areas, increasing to a few kilometers in the basin centers. This method has deficiencies, though. This estimates the total volume of erupted basalts to be ~6.5x106 km3 (i.e., less than 1% of the lunar volume).

Another method is to assume that basins have the geometry seen in Mare Orientale. This gives a figure of about 6x107-7x107 km3. By assuming all such basins are filled to the brim (clearly not the case), a maximum volume of 3x107-4x107 km3 is obtained.

Solomon & Head (1980) have combined most geophysical, photogeological, and petrographic data with rheological and thermal modelling of the lunar crust. Their latest models allow about 6x106 km3 basalt to be produced by the moon.


The Lunar Sourcebook

Arkani-Hamed et al (1973) "On the thermal history of the Moon" Proc. Lunar Sci. Conf. 4th, pp2673-2684
Bills & Ferrari (1976) "Lunar crustal thickness" Proc. Lunar Sci. Conf. 7th, frontispiece
Boyce (1976) "Ages of flow units in the lunar nearside maria based on Lunar Orbiter IV photographs." Proc. Lunar Sci. Conf. 7th, pp 2717-2728
Brett (1975a) "Thicknesses of some lunar mare basalt flows and ejecta blankets based on chemical kinetic data." Geochim. Cosmochim. Acta, v39, pp 1135-1143
Brett (1976) "Reduction of mare basalts by sulfur loss." Geochim. Cosmochim. Acta, v40, pp 997-1004
DeHon & Waskom (1976) "Geologic structure of the eastern mare basins." Proc. Lunar Sci. Conf. 7th, pp 2729-2746
DeHon (1979) "Thickness of the western mare basalts" Proc. Lunar Planet Sci. Conf. 10th, pp 2935-2955
Delano (1986) "Pristine lunar glasses: Criteria, data, and implications" Proc. Lunar Planet. Sci. Conf. 16th, in J. Geophys. Res., v91, D201-D213
Guest & Murray (1976) "Volcanic features of the nearside equatorial lunar maria." J. Geol. Soc. Lond., v132, pp 251-258
Head (1976a) "Lunar volcanism in space and time" Rev. Geophys. Space Phys., v14, pp 265-300
Head & Wilson (1979) "Alphonsus type dark-halo craters: Morphology, morphometry and eruption conditions." Proc. Lunar Planet. Sci. Conf. 10th, pp 2861-2897
Muehlberge (1974) "Structural history of southeastern Mare Serenitatis and adjacent highlands." Proc. Lunar Sci. Conf. 5th, pp 101-110.
Peeples et al (1978) "Orbital radar evidence for lunar subsurface layering in Maria Serenitatis and Crisium." J. Geophys. Res., v83, pp 3459-3468
Pieters (1978) "Mare basalt types on the front side of the Moon." Proc. Lunar Planet. Sci. Conf. 9th, pp 2825-2849
Sato (1978) "Oxygen fugacity of basaltic magmas and the role of gas forming elements" Geophys. Res. Lett., v5, pp 447-449.
Schultz & Spudis (1983) "Beginning and end of lunar mare volcanism." Nature, v302, pp 233-236.
Solomon & Head (1980) "Lunar mascon basins: Lava flooding, tectonics, and evolution of the lithosphere." Rev. Geophys. Space Phys. v20, pp 411-456
Solomon et al (1982) "The evolution of impact basins: Viscous relaxation of topographic relief." J. Geophys. Res., v87 pp 3975-3992
Taylor et al (1983) "Pre- 4.2Ma mare basalt volcanism in the lunar highlands." Earth Planet. Sci. Lett., v66, pp 33-47
Weill et al (1971) "Mineralogy-petrology of lunar samples: Microprobe studies of samples 12021 and 12022; Viscosity of melts of selected lunar compositions." Proc. Lunar Sci. Conf. 2nd, pp 413-430

Volcanism on the Moon

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