Geology Reference
In-Depth Information
of undisturbed rocks those on the bottom must be the
oldest because they had to be present before the subse-
quent rocks could be put on them.
Geologic mapping on Earth began in the eighteenth
century. In the ensuing years, stratigraphic columns
re
ecting local sequences of rocks have been de
ned for
most regions of our planet and have been correlated
(connected) into regional and global associations. From
this synthesis, a generalized geologic time scale that
divides the history of the Earth into formal eras, periods,
and epochs was de
ned. Early versions of the geologic
time scale indicated only the relative ages of rocks, and it
was not until dating methods based primarily on radio-
active decay were developed that absolute ages could be
assigned, as will be discussed below.
As for the Earth, planetary geologic maps are critical for
understanding surface histories and for providing a frame-
work for other observations. This understanding was rec-
ognized very early in Solar System exploration, and the
first planetary geologic maps were compiled by the US
Geological Survey (USGS) for the Moon by Robert
Hackman and Gene Shoemaker, from telescopic observa-
tions (
Fig. 2.7
). Their techniques were later codi
ed and
standardized for planetary mapping by Don Wilhelms and
Ken Tanaka, also of the USGS.
Geologic maps of Earth are commonly assembled from a
combination of remote sensing data and
field work, all
combined on standard maps, which usually include topog-
raphy. The identi
cation of formations and other rock units
on planets is based primarily on remote sensing data using
photogeological techniques that are commonly employed
for Earth. Images enable obvious features, such as lava
flows, to be identi
ed, while the general appearance of
terrains is used to infer different rock units. Compositional
mapping on the basis of infrared and other data is also used
to distinguish units, when such data are available. Further
insight is provided by quantitative studies of albedo
(a measure of the re
ectivity of the surface) and surface
textures at the sub-meter scale derived from radar signa-
tures. Unfortunately, a full suite of remote sensing data is
seldom available for planetary surfaces.
After the rock units and structures have been identi
ed
and their distributions mapped, the next step is to place
them in a chronological sequence. In addition to super-
position, embayment and cross-cutting relations are
used to determine the relative ages among units. For
example, embayment refers to the
“
“flooding”
”
aspect of
some units, as seen on the Moon (
Fig. 2.8
), in which the
“
“flooding”
”
unit is the younger. For the application of
Figure 2.7. Part of the geologic map of the Kepler region on the
Moon, published in Geotimes in 1962. This was one of the early
prototype maps to show that geologic maps could be produced for
planetary surfaces (reprinted with permission from Geotimes
magazine, a publication of the American Geological Institute).
cross-cutting relations, a rock unit, fault, or other structure
that cuts across another must be the younger, as shown in
Fig. 2.9
.
Planetary geologic mapping is hampered by a lack of
field observations except for those at the local Apollo sites
and a handful of
sites gained from robotic
landers. Consequently, planetary geologic maps are for-
matted a little differently in comparison with those for
Earth. On planetary maps the rock unit descriptions are
divided into two parts, observations and interpretations.
The observation part describes the characteristics of the
unit in objective terms on the basis of the available data,
while the interpretation part explains the possible origin
and evolution of the unit according to the opinion of the
author. In principle, the observation part should remain
“
ground-truth
”
