Geology Reference
In-Depth Information
Figure 2.9. A view of Rima Ariadaeus, taken by the Apollo 10
astronauts. This linear rille, formed by faulting, is about 2 km wide
and cuts across older terrain (NASA AS10
Figure 2.8. View of the Taurus
-
Littrow region of the Moon, showing
flooding by dark mare lavas into the more heavily cratered highlands
on the right. The arrow points to a lava-
-
4646).
crater. The paucity
of craters in themare (left side) indicates the relative youth of the lava in
comparison with the highlands. The star indicates the location of the
Apollo 17 landing site. The darker part of the mare surface marks the
presence of dark mantle deposits of volcanic origin (NASA AS17
“
embayed
”
rocks decay or convert to more stable isotopes at a known
rate. If we know this rate and can measure the amounts of
unstable and stable isotopes in a sample, it is possible to
determine the age of the rock. Of course, radioactive iso-
topes of the right type must be available for measurement in
the rock sample and, unfortunately, not all samples contain
these isotopes. Consequently, only some rocks can be
dated. Typically, radiogenic dating provides the age of a
rock in years from its formation.
How is geologic time assessed on other planets and satel-
lites? Practical limitations require that rock samples be ana-
lyzed in laboratories on Earth to determine radiometric ages
because automated dating systems for robotic spacecraft
have not yet been developed. Consequently, planetary abso-
lute dates have been obtained only for rock samples from the
Moon and for meteorites, some of which are from Mars.
Because no direct samples are available from other
planets, only relative ages can be assigned with confi-
-
dence. The principles of superposition, embayment, and
cross-cutting relations are routinely applied to planet and
satellite surfaces for this purpose.
An additional method for establishing the relative ages
of planetary surfaces is based on the size
-
frequency dis-
tribution of impact craters. Old surfaces have been
exposed to the impact environment for longer than have
younger surfaces and statistically should contain more
impact craters (
Fig. 2.8
). By counting the number of
craters superposed on planetary surfaces, their age relative
-
0939).
valid (at least until new data are available), but the inter-
pretation part could change or be different, depending on
the analyst.
The first planetary geologic maps by Hackman and
Shoemaker set the stage for a series of mapping programs
that extended to Mars, Mercury, Venus, and outer planet
satellites and that continues today through the USGS. In
these programs, each planet is divided into map quadran-
gles of systematic scales, which serve as cartographic
bases for geologic mapping.
2.4 Geologic time
Geologic time can be considered from two perspectives,
absolute time and relative time. In absolute time, rocks
and geologic events, such as faulting, are determined as
being of a speci
c age expressed in years. In relative geo-
logic time, rocks and events are simply stated as being older
or younger than other rocks or events, without expressing
their age differences in years. Determining the absolute
ages of rocks is accomplished primarily by using radiogenic
“
clocks
”
that are based on the principle that certain unstable
radioactive elements (i.e.,
isotopes) contained in some
