194 Encyclopedia of the Solar System
(e.g., erosion and weathering) have tended to rearrange,
bury, or destroy pre-existing continental landscapes at all
spatial scales. Thus, while often retaining the imprint of
pre-existing forms, subaerial landscapes on the Earth are
constantly being reinvented.
Because the Earth’s crust is so dynamic, one must realize
from the planetary perspective that any geomorphic survey
of the Earth’s surface may be representative only of the
current continental plate arrangement, and currently asso-
ciated climatic and atmospheric circulation regimes. Plate
tectonics is a powerful force in setting scenarios for conti-
nental geomorphology. For instance, during early Cenozoic
times the global continental geography was characterized
by the warm circumglobal Tethys Sea and higher sea lev-
els than now (possibly linked to higher rates of midoceanic
spreading), which strongly biased the overall terrestrial cli-
mate toward the tropical range (Fig. 5).
The rearrangement of continental landmasses in the
later Cenozoic closed the Tethys Sea, produced a circum-
Antarctic ocean, and set up predominantly north–south cir-
culation regimes within the Atlantic and Pacific Oceans.
FIGURE 5 Continental geography through time. Modern plate
tectonic theory is consistent provides the scientific framework for
observations of continental drift. Geologic evidence records the
breakup of the supercontinent Pangaea about 225–200 million
years ago, eventually fragmenting over time to create our familiar
continental geography. The Tethys Sea referred to in the text is
labeled. (Courtesy of the United States Geological Survey).
This global plate geography, combined with greater ocean
basin volume (linked to lower ridge spreading rates) and the
onset of continental glaciation, lowered sea levels, expos-
ing large marine continental self-environments to subaerial
erosion. Our current global surface environment reflects a
kind of “oceanic recovery” after the last Ice Age, with some-
what higher sea levels. Thus, our current perception of the
Earth’s subaerial geomorphic landform inventory is strongly
biased by our temporal observational niche in its environ-
mental history. Hypothetical interstellar visitors who arrived
here 50 Myr ago or may arrive 50 Myr in the future would
likely have a much different perception because of this dis-
tinctive dynamic character.
Terrestrial subaerial landform suites are the classic
landscapes studied in geomorphology. These are listed in
Table 1. Currently, on the Earth, globally dominant sub-
aerial geomorphic regimes are related to the surface trans-
port of liquid water and sediment due to the action of
rainfall. Thus drainage basins dominate terrestrial land-
scapes at nearly all scales, from the continental scale to
sub-100-m scales. These include currently active drainage
basins in humid and semiarid climatic zones, to only occa-
sionally active or relict drainages in arid zones. Drainage
basin topographies and network topologies, however, are
strongly influenced by the interplay of the orogenic aspects
of plate tectonics (i.e., mountain building) and prevailing
climatic regimes, including the biogenic aspects of climate
(e.g., vegetative ground cover). Clearly, areas of rapid uplift
(e.g., San Gabriel Mountains, California), have character-
istically steep bedrock drainages, where gravitational en-
ergies are high enough to scour stream valleys, generally
have parallel or digitate (hand-like) drainage patterns, have
high local flood potentials, and respond strongly to local
weather (e.g., spatial scales 10–100 km in characteristic di-
mension). At the other spatial extreme, major continental
drainages (e.g., Amazon River, Mississippi River, Ob River
in Siberia—Table 1), with highly dendritic (tree-like) over-
all pattern organization, are low average gradient systems
that integrate the effects of a variety of climatic regimes at
different spatial scales and tend to respond to mesoscale
and larger climatic and weather events (e.g., 100-to 1000-
km scale).
Subaerial volcanic processes produce characteristic
landforms in all terrestrial climate zones (see Fig. 2b). They
tend to occur in belts, mainly at plate boundaries, with a
few notable oceanic (e.g., Hawaiian Islands) and continen-
tal (e.g., the San Francisco volcanic field in Northern Ari-
zona; the Columbia and Snake River volcanic plains in the
U.S. Pacific Northwest; the Deccan Traps in India), excep-
tions that occur within plate interiors. Although not as mas-
sive or as topographically high as their planetary counter-
parts (e.g., Martian volcanoes such as Olympus Mons), they
provide some of the most spectacular and graceful land-
forms on the Earth’s surface (e.g., Mount Fujiyama, Japan;
Mt. Kilimanjaro, Kenya). Our planet’s central vent volcanic
landforms range from the majestic strato-cone volcanic