Geology Reference
In-Depth Information
Figure 4.26. A geologic map of the Orientale basin, showing the main ejecta deposits and mare units. The units of the Orientale Group
are (a) Maunder Formation, (b) Montes Rook Formation, knobby facies, (c) Montes Rook Formation, massif facies, (d) Hevelius
Formation, inner facies, (e) Hevelius Formation, outer facies, (f) Hevelius Formation, transverse facies, and (g) Hevelius Formation,
secondary crater facies (reprinted from Phys. Earth Planet. Inter., 15, McCauley, J. F., Orientale and Caloris,
1977
,220
-
250, with
permission from Elsevier).
impact craters by mare lavas, the thicknesses of the lava
flows are estimated to be generally less than a few
kilometers. Data from recent missions to the Moon
demonstrate a complex stratigraphic history of volcan-
ism, as reviewed by Hiesinger et al.(
2011
), including
estimates of dates based on sophisticated crater-counts.
Results show that volcanism apparently ceased at about
1.2 Ga.
Lunar lavas are very similar to terrestrial basalts but
have slightly higher abundances of iron, magnesium, and
titanium in some areas. Analysis of lunar samples sug-
gests that the basalts were derived from the mantle at
depths of 150
-
450 km below the surface.
Even before the return of samples, photogeologic evi-
dence suggested that the lunar lavas were very fluid at the
time of their eruption, as exempli
ed by the
flows in Mare
Imbrium (
Fig. 4.30
). These
ows extend more than
1,200 km but are less than 10
-
65m thick. When the lava
compositions were determined from the Apollo samples,
synthetic batches of lunar lavas were found to be
extremely
fluid (about 10 poise), equivalent to motor oil
at room temperature. This is substantially more
fluid than
typical basalt flows on Earth and can account for the great
lengths of the
ows on the Moon.
For the most part, the source vents for the lunar lava
flows are not known, but are mostly inferred to be associ-
ated with impact-basin fractures. Similar to
flood eruptions
on Earth, most of the mare lava flows probably buried any
vestiges of their vent structures. There are, however, some
well-known volcanic vents, such as those associated with
lunar sinous rilles.
4.5.4 Sinuous rilles
Sinuous rilles are channel-like features found on some
mare surfaces and (rarely) in highland terrains. The
