The Moon 241
the basins form the maria, as in Mare Imbrium (see Figs. 1
and 14) or Mare Ingenii (see Fig. 16). The basins, as in
the Imbrium Basin, were formed much earlier by impact
and have nothing to do with the generation of the mare
basalts. Thus, the mare basalt, which fills many basins, is
unrelated to the formation of the basins, a common miscon-
ception; instead, it is derived from the deep lunar interior
and merely floods into the low-lying depressions much later.
Some impact melt, distinct in composition from the lavas,
may be formed at the time of the impact, but it should not
be confused with the basaltic mare lavas, which differ both
in composition and age. Oceanus Procellarum (see Fig. 1)
is the type example of an irregular mare, where the lavas
have flooded widely over the highland crust. However, this
mare may be filling parts of an old, large, and very degraded
Procellarum Basin (3200 km in diameter), although the ex-
istence of this basin is questionable.
The mare lavas reach the surface because of the den-
sity difference between the melt and that of the overlying
column of rock. The scarcity of maria on the farside of the
Moon (see Fig. 3) is due to the greater crustal thickness.
An exception is part of the area of the deep depression of
the South Pole–Aitken multiring basin (2500 km in diam-
eter), on which is superimposed the Ingenii impact basin
(650 km in diameter), now occupied in part by the lavas
of Mare Ingenii (see Fig. 16). However, most of the South
Pole–Aitken Basin, which is deeper than the nearside maria,
is not flooded with lava. This argues for mantle heterogene-
ity and localized sources for the mare basalts, rather than
some moonwide melting of the interior, with consequent
flooding of lava to a uniform level.
Dark mantle deposits, which represent pyroclastic de-
posits formed probably by “fire fountaining” during lunar
eruptions, occur, for example, around the southern bor-
ders of Mare Serenitatis. These pyroclastic deposits are
composed mainly of glass droplets and fragments and can
be distinguished from the ubiquitous glasses of impact
origin by their uniformity, homogeneous composition, and
absence of meteoritic contamination. Over 25 distinct com-
positions have been recognized. They commonly have a su-
perficial coating of volatile elements such as Pb, Zn, Cl,
and F, derived from volcanic vapors during the eruption.
The dominant gas, however, was probably CO. The source
of the volatile elements is uncertain. They are rare in the
lunar samples, and the Moon is generally thought to be
strongly depleted in them. Possibly they come from local
cumulate sources, and so do not imply an enrichment of
the deep lunar interior in volatile elements. However, they
may have originated at a greater depth than the crystalline
mare basalts.
Some areas of mare basalts (so-called cryptomaria)
are covered by ejecta blankets of highland material
from multiring basins; their presence is revealed by the
haloes of dark basalt ejected from impact craters that
have punched through the light-colored highland plains
units of anorthositic composition into the underlying
basalts.
Although they are prominent visually on the Moon, the
maria typically form a thin veneer, mostly less than 1–2 km
thick, except in the centers of the circular maria where they
may reach maximum of 5 km as in the middle of Mare Im-
brium. The basalt thickness in Orientale is estimated to be
only 0.6 km. The total volume of mare basalt is usually esti-
mated at between 6 and 7× 106 km^3 or about 0.1% of lunar
volume. Cooling rates for mare basalts range from 0.1◦Cto
30 ◦C per hour, indicative of fast cooling in thin lava flows.
Sinuous rilles occur widely near the edges of the maria
and are either lava channels or collapsed lava tubes. They
have eroded into the surrounding lavas by a combination
of thermal and mechanical erosion. The classic example is
Hadley Rille (Fig. 18), visited by theApollo 15mission.
The rille is 135 km long and averages 1.2 km in width and
370 m in depth. Massive lava bedrock is exposed in the
rille wall at theApollo 15site. The sinuous rilles should not
be confused with the straight or arcuate rilles, which are
grabens of tectonic origin.7.1 Mare Basalt Ages
The oldest ages for returned lunar mare basalts are from
Apollo 14breccias; aluminous low-Ti basaltic clasts in theseFIGURE 18 Hadley Rille, a typically sinuous rille, about 1 km
wide, at theApollo 15landing site, close to the base of the
Apennine Mountains. (Courtesy of NASA, Lunar Orbiter
IV-102H3.)