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).
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.
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.
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.
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.
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).
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).
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.
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