Chapter 6 BASALTIC VOLCANISM Basaltic volcanic activity is very common in the solid planets in the solar system, appearing at the planetary surface as a consequence of partial melting deep within the silicate mantles. It is a manifestation of the internal heat budget of a planet, and may continue for billions of years, contrasting sharply with the briefer episodes of planetary evolution triggered by accretionary melting. The evidence for extraterrestrial volcanism was first detected on the Moon by observers with vision acute enough to identify the dark patches on the lunar surface as lava flows [I], despite many contrary suggestions includ- ing asphalt, dust and sedimentary rocks [2]. In this chapter, most emphasis is placed on the well-studied lunar lavas. Like the highland crustal materials described in Chapter 5, these samples have provided enough scientific debate to engender caution about the age and composition of apparent volcanic landforms on unvisited or little explored planets. For this reason, most of the discussion in this chapter is concerned with the lunar mare basalts. These are well characterized by petrographical and geochemical studies. In contrast, the evidence for Martian and possible Mercurian lavas is based mainly on photo- geological interpretation, so that the description of these inferred basalts is addressed more properly in Chapter 2. 6.1 Floods of Basaltic Lava The vast plains, which cover 17%of the surface of the Moon (Fig. 6. l), contrast with many terrestrial volcanic landforms, a difference which delayed 264 Planetary Science 6.1 Far-side lunar maria and highlands. The large circular crater, filled with mare basalt, is Thomson (112 km diameter) in the northeast sector of Mare Ingenii (370 km diameter, 34'5, 164"E). The large crater in the right foreground is Zelinskiy (54 km diameter). The stratigraphic sequence, from oldest to youngest, is (1) formation of highland crust, (2) excavation of Ingenii basin, (3) formation of Thomson crater, (4) formation of Zelinskiy, (5) flooding of lngenii basin and Thomson crater with mare basalt, (6) production of small craters on mare surface including a probable chain of secondary craters (NASA AS15-87-11724). their acceptance as lava flows. The relief on the mare surfaces is nowhere great, and they are, in fact, exceedingly smooth. Slopes of 1:500 to 1:2000, and differences in elevation of less than 150 m over distances of 500 km are common. This smoothness of the maria is due to the low viscosity of the mare lavas. The mare basins are not all at the same level. The surfaces of Mare Crisium and Mare Smythii are about 4 km below the surrounding highlands, but Oceanus Procellarum averages about 1 km below the highlands. The distribution of mare basalts on the lunar surface is shown in Fig. 6.2. An outstanding feature of the maria [3] is their tendency toward circular form caused by the flooding of the large ringed basins (Fig. 6.1). A clear distinction exists between the craters or multi-ring basins excavated by mete- Basalric Volcanism 265 orite impacts and the maria formed by lava flooding. The Imbrium basin was formed by impact; Mare Imbrium is the "sea" of lava which filled it later. Mare basalts are much more common on the earth-facing side of the Moon [4] (Fig. 6.2). This dichotomy has been remarked upon frequently. It is probably due to the presence of a thinner (60-70-km-thick) highland crust on the near side compared with possible 80-90 km thicknesses on the far side. The paucity of far-side lavas is thus explained by their failure to reach the surface, except in deep basins, the height to which lavas can rise being governed by the density difference between the melt and that of the overlying column of rock. In the southern hemisphere of the Moon, the far side contains more mare basalt than the near side, which is the reverse of the normal situation. This concentration of mare basalt is due to flooding of the deep depressions in the Australe and South Pole-Aitken basins. Mare basalts may be more extensive than their surface exposure indicates and extend under some of the highland light plains units, covered by debris from basin-forming collisions. The principal evidence for this comes from the presence of dark halo craters (Section 3.1 1). A summary of the areal distribu- tion of mare basalts has been given by Whitford-Stark [5]. 6.1.1 Thickness of Mare Fill A basic question is how thick are the basaltic lavas. In many areas, of which Oceanus Procellarum is a prime example, the flows are clearly rather thin, embaying the rugged highland topography. Around the edges of the circular maria, flooding of craters provides a measure of depth of mare fill. Early estimates of great thicknesses of mare basalts have not been substan- tiated and they form a comparatively thin veneer on the surface of the highlands. Thickness of mare basalts in the irregular maria (e.g., over much of Oceanus Procellarum) are probably only a few hundred meters [6-91. These values can be estimated from the partially buried "ghost" craters on the mare surface. The depth of mare basalt fill in the centers of large basins cannot be so readily identified, although an estimate of their thickness is significant for the mascon problem (Section 7.4.2). Although the mare basalt fill in the circular basins is a component in providing the positive gravity anomaly, mantle uplift beneath the basin appears to be the major factor. It should be noted that the crater Kepler did not penetrate through the lava fill in Mare Imbrium [lo]. In Mare Crisium, no basin peak ring remnants are observed and hence the mare basalt fill must be 2-4 km thick [ll]. The central peak, if present, is also covered [12], although the probable basin diameter is so large that a central peak is unlikely (Sections 3.5,3.6). Six large craters ranging in diameter from 6.2 to 22.5 km, with depths from 1.3 to 1.8 km, occur within the basalt fill of Mare Crisium. The two large craters, Peirce 266 Planetary Science ndarum (19 km diameter, 1.49 km deep) and Picard (22.5 km diameter, 1.9 km deep) appear to bottom in highland material, and hence give a maximum thickness (near the edge of Mare Crisium) of 1.5-2.0 km. Minimum thicknesses of 1.6 km for Mare Serenitatis and 1.4 km for Crisium are indicated from the Lunar Sounder experiment [l3]. A recent estimate for the thickness of the basaltic fill in Mare Imbrium is 1.5-2.5 km [14]. No indications of buried central peaks appear on the surfaces of the lava plains of the large circular maria, but probably no central peaks were formed, from analogy with Mare Orientale. 6.1.2 Age of the Oldest Mare Surface The oldest established crystallization age for a mare basalt is for sample 10003 which has an 40~r-39~rage of 3.85 IIC 0.03 aeons. This age overlaps ages of some of the highland samples and is older than the generally accepted date of 3.82 aeons for the Imbrium event. The rock is an Apollo 11 low-K mare Basaltic Volcanism 267 FAR SIDE 6.2 Distribution of mare deposits on the Moon. [M = Mare,0 = Oceanus,S = Si- nus, P = Palus, L = Lacus. From Basaltic Volcanism (1981)Fig. 5.4.1.1 basalt, but its relationship to the geology of Mare Tranquillitatis is unknown. The samples from the high-K Apollo I1 suite have well established dates of 3.55 aeons, so that the surface of the mare may be of that age [15]. The basalts at the Apollo 17 site were carefully sampled and yield ages from 3.69 to 3.79 aeons (with one at 3.84 f 0.02), slightly but significantly younger than the Imbrium basin ages. This age provides, in turn, an absolute younger limit for the time of formation, both of the multi-ringed basins and of most of the highland craters. Baldwin [16] records only Hausen, Tsiolkovsky, Schrodinger, Antoniadi, Compton and Humboldt as younger than Mare Orientale, which in turn, is younger than Imbrium. Six large craters occur between the formation of Imbrium and Orientale (Section 3.17). High-Ti basalts dated from the upper surfaces of the maria are accordingly close in age to the final great cratering events. 268 Planetary Science 6.3 Mare basalt flow fronts illuminated under varying low sun angles in southwest- ern Mare Imbrium. Source of the flows is about 200 km southwest of La Hire crater (5 km diameter) seen right center in d. Flow thicknesses are in the range 10-30 m (Apollo pan photos 0269-0272). Basaltic Volcanism 269 6.1.3 The Lunar Lava Flows Individusl lava flows were noted by early observers [l], and some were observed in the Orbiter photographs [17], but it was not until the low sun angle photographs were taken on the Apollo 15 and 17 missions that the extent and nature of the flows were fully appreciated (Figs. 6.3 and 6.4). There were channels with levee banks along the centers of some flows larger than in terrestrial examples and typically 20-50 m high. Three stages of very late flows ca;x be distinguished at Mare Imbrium. Figure 6.5 shows their distribu- tion. They are of Eratosthenian age [18], and based on crater counting, dates as late as 2.5 aeons have been suggested.