Geology Reference
In-Depth Information
CHAPTER 3
Planetary morphologic processes
geomorphology of the surface re
ects events that hap-
pened in the past but are no longer
3.1 Introduction
taking place.
Learning to recognize the landform
left by
tectonism, gradation, volcanism, and impacts is one of the
main goals of planetary geomorphology.
The relative importance of the surface-modifying pro-
cesses among the planets is a function of many factors,
including the history of the object and the local environ-
ment. For example, gradation on Earth is dominated by
water, but in the current cold, dry martian environment,
wind dominates. Thus, we must understand how gravity,
surface temperature, the presence or absence of an atmo-
sphere, and other variables in
uence the manner in which
the processes operate and lead to speciclandforms.
In the following sections, tectonism, volcanism, grada-
tion, and impact processes are described and illustrated,
using examples taken primarily from Earth to serve as a
basis for planetary comparisions.
“
signatures
”
Earth is a dynamic planet. That simple statement can be
supported by our own direct observations. Earthquakes,
river banks collapsing during
flooding, erupting volca-
noes
-
all are experienced or documented on the news
every year and show that our planet is everchanging.
These examples represent three of the four fundamental
processes that shape Earth
'
s surface: tectonism, grada-
tion, and volcanism.
The fourth fundamental process, impact, which is gen-
erally less often observed, is also documented, sometimes in
quite newsworthy events as when a meteoroid plunges
through the roof of a house. As the geologic record shows,
the history of Earth can be profoundly altered by impacts,
such as the well-known Chicxulub structure in the Yucatan
peninsula of Mexico. This structure, now buried beneath a
kilometer of sediments, has been mapped by geophysical
methods and drill-holes to be more than 80 km in diameter
and is estimated to have formed from an impact that released
the energy equivalent of some 10 billion tons of TNT. The
resulting
fireball ignited world-wide
res, generated enor-
mous amounts of CO
2
from the vaporization of limestone
present at the impact site, and triggered tsunamis throughout
the Gulf of Mexico and adjacent waters. As is now widely
accepted, these catastrophic events led to mass extinctions,
including that of the dinosaurs, and marked the boundary
between the Cretaceous Period and the Tertiary Period 65
million years ago. It was not so much the direct impact that
led to extinctions, but the effects on the surface environ-
ment, including
restorms, enhanced greenhouse processes,
and disruption of the food chain.
Geologic exploration of the Solar System shows that
the surfaces of the terrestrial planets, satellites, and small
bodies, such as asteroids, have been subjected to one or
more of the four fundamental processes. In some cases,
the processes are currently active; in other cases, the
3.2 Tectonism
Road-cuts along many highways show ample evidence for
rock deformation, or tectonic processes. As can be seen in
Figs. 3.1
and
3.2
, Earth
s crust can be broken along faults
or bent into distinctive folds. These and other features can
be related, in part, to the style of tectonic deformation of
the crust, such as tension or compression. However,
knowledge of global-scale crustal deformation on Earth
was not gained until the unifying concept of global plate
tectonics was formulated in the 1960s. This insight was
important for understanding the evolution of Earth
'
s crust
and is critical in the interpretation of other planets.
Determining the styles of tectonic deformation on the
planets provides clues to their general evolution and the
con
guration of their interiors.
Seismic and other geophysical data show that the inte-
rior of Earth consists of distinctive zones. The outer zone,
'
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