Geology Reference
In-Depth Information
such as sulfur and chlorine, which are indicative of vol-
canic origins (
Fig. 4.20(c)
). Thus, dark mantle deposits
were probably produced from pyroclastic eruptions sim-
ilar to the
fire-fountain eruptions seen in Hawaii. In the
low-gravity, airless environment of the Moon, such erup-
tions would have spread the pyroclastics over wide areas.
In addition to basaltic maria, anorthositic highlands,
and pyroclastic deposits, other surface materials include
breccias and impact melts. Glassy beads in lunar soils and
glasses incorporated in other samples re
ect melted rocks
generated during impacts, some of which
onto
the surrounding terrain (
Fig. 4.22
). Unlike the orange
pyroclastic glass beads, impact melt materials lack the
coatings of volatile elements.
Breccias are composed of angular rock fragments,
which, on the Moon, result primarily from impact pro-
cesses that both break up and consolidate rocks
(
Fig. 4.20(d)
). Many of the lunar samples show breccias
within breccias, indicative of repeated breakup and con-
solidation by multiple impact events.
“
splashed
”
90
1001−49000 (v large) (20)
401−1000 (large) (15)
201−400 (medium) (9)
101−200 (small) (17)
1−100 (v small) (14)
(75)
−90
−180
0
180
Figure 4.21. Distribution of lunar pyroclastic deposits analyzed by Lisa Gaddis et al.(
2003
), classi
ed by areal extent on the basis of Clementine
UV
-
VIS data (reprinted from Icarus, 161, Gaddis, L. R. et al., Compositional analyses of lunar pyroclastic deposits, 262
-
280,
2003
, with
permission from Elsevier).
(a)
Figure 4.22. (a) A view of the central peak of
Tycho from the Kaguya orbiter; Tycho is a very
young, bright-rayed crater in the near-side
southern hemisphere and has a diameter of
85 km; as seen in this image, the crater
oor
surrounding the central peak is hummocky
and fractured, considered to be fall-back melt
deposits from the impact. (b) A high-resolution
image from the Lunar Reconnaissance Orbiter
camera showing melt deposits on Tycho
oor
and the direction of motion as the melt slid
into place (NASA/GSFC/Arizona State
University).
'
s
