240 Encyclopedia of the Solar System
FIGURE 16 The distinction
between the maria and highlands is
clearly shown in this view of the
lunar farside. The large circular
crater, filled with dark mare basalt,
is Thomson (112 km in diameter),
within the partly visible Mare
Ingenii (370 km in diameter, 34◦S,
164 ◦E). The large crater in the
highland terrain in the right
foreground is Zelinskiy (54 km in
diameter). The stratigraphic
sequence, from oldest to youngest,
is (a) formation of the white
highland crust, (b) excavation of the
Ingenii Basin, (c) excavation of the
Thomson Crater within the Ingenii
Basin, (d) excavation of Zelinskiy
Crater in the highland crust, (e)
flooding of Ingenii Basin and
Thomson Crater with mare basalt,
and (f) excavation of small craters,
including a probable chain of
secondary craters, on the mare
basalt surface. (Courtesy of NASA,
AS15-87-11724.)After centuries of speculation during which the maria were
thought to be composed of sediments, dried lake beds, as-
phalt, or other unlikely materials, they were conclusively
identified following theApollo 11sample return in 1969 as
being formed of basaltic lavas. This conclusion had already
been reached by earlier workers such as R. B. Baldwin and
G. Kuiper and was strongly suggested by the data from the
Surveyorlanders. These vast plains cover 17% (6. 4 × 106
km^2 ) of the surface of the Moon, and they are exceedingly
smooth, with slopes of 1:500 to 1:200 and elevation differ-
ences of only 150 m over distances of 500 km. This smooth-
ness and the lack of volcanic constructional forms, which
litter many terrestrial volcanic fields, remind one of plateau
basalts on Earth and are probably due to several factors.
These include a combination of high eruption rates and the
low viscosity of the lunar lavas, which is about an order of
magnitude lower than that of their terrestrial counterparts
and is close to that of engine oil at room temperature. The
lava flows (Fig. 17) are thin (10–40 m) and up to 1200 km
long, a consequence of the low viscosity and probable long
duration of the eruption. Flow fronts are generally less than
about 15 m in height. Occasional small volcanic domes and
cones occur on the mare surface. The classic example is the
region of the Marius Hills.
The maria are not all at the same level, and this is in-
dicative of independent eruptions from diverse sources at
differing depths in the interior. They are mostly subcircular
in form owing to their filling of the multiring basins, orig-
inally excavated by impact. The dark basaltic lavas that fill
FIGURE 17 Mare basalt flows in southwestern Mare Imbrium.
Flow thicknesses are in the range 10–30 m. The source of the
flow is about 200 km southwest of crater La Hire (5 km in
diameter), seen at right center. This crater is superimposed on
Mount La Hire (about 30 m long at its base), a highland remnant
that is partially submerged by lavas. Note the prominent
concentric wrinkle ridges on Mare Imbrium. (Courtesy of
NASA, AS15-1556.)