Geology Reference
In-Depth Information
(a)
Figure 4.42. (a) Floor-fractured craters bordering western Oceanus
Procellarum; largest crater is ~75 km across (NASA LO IV H-189).
(b) Diagrams showing the possible formation and evolution of
oor-
fractured craters through the intrusion of magma and uplift of the
crater
floor zone (with permission from Springer Science+Business
Media: The Moon, Floor-fractured lunar craters, 15,
1976
, 241
Figure 4.41. The Alpine Valley is a structural graben 150 km long by
10 km wide radial to the Imbrium basin, seen toward the top of
the image. The
floor of the graben has been
flooded with mare
deposits emplaced by a rille in the center of the valley (NASA LO IV
M-102).
-
273,
Schultz, P. H., Fig. 10).
Mare ridges constitute some of the most common
tectonic features on the Moon. Also called
“
wrinkle
ridges
”
(an apt name given their appearance;
Fig. 4.43
),
these features extend tens of kilometers in length across
mare surfaces. Mare ridges typically consist of a broad,
gentle arch surmounted by a steeper-sided, narrow crenu-
lated ridge crest. While most of the ridges are found on
mare surfaces, many extend into highland terrains
(
Fig. 4.44
). As shown in
Fig. 4.45
, most mare ridges
fractured the basalts well after the emplacement of the
lavas (at least after a crust of suf
cient thickness had
formed to support the preservation of the crater shown in
the
figure). However, in some places, mare ridges also
exhibit
flow lobes suggesting volcanic extrusions; thus,
some ridges probably formed on mare surfaces before
complete solidi
cation of the lavas. The consensus is
that most mare ridges are predominantly structural fea-
tures that re
ect deformation of basaltic rocks.
4.5.7 Gradational features
In the absence of wind and
flowing water, gradation on
the Moon occurs in the form of
“
space weathering
”
(see
Section 3.5
) and downslope mass wasting of debris under
the in
uence of gravity. Mass wasting of several forms is
seen in most areas of the Moon. These include landslides
(
Fig. 4.44
) and individual rocks that have rolled down
slopes, leaving tracks (
Fig. 4.46
).
The physical breakup and fusing of surface materials
occurs by impacts at all scales. The formation of soil agglu-
tinates is particularly important. This material involves gas-
ses formed by micrometeoroid bombardment, gasses
implanted by the solar wind, and subsequent fusing of soil
grains. With time, the amount of agglutinates in the regolith
increases, leading to soil maturation. Understanding the
effects of soil maturity is critical in the interpretation of
