Encyclopedia of the Solar System 2nd ed

Earth as a Planet: Surface and Interior 195

TABLE 1 Classification of Terrestrial Geomorphological Features by Scale

Approximate

Approximate Time Scale of

Order Spatial Scale (km^2 ) Characteristic Units (with examples) Persistence (years)

1107 Continents, ocean basins 108 –10^9

2105 –10^6 Physiographic provinces, shields, depositional plains, continental-scale

river drainage basins (e.g., Amazon, Mississippi Rivers, Danube, Rio

Grande)

108

3104 Medium-scale tectonic units (sedimentary basins, mountain massifs,

domal uplifts)

107 –10^8

4102 Smaller tectonic units (fault blocks, volcanoes, troughs, sedimentary

sub-basins, individual mountain zones)

107

5 10–10^2 Large-scale erosional/depositional units (deltas, major valleys, piedmonts) 106

610 −^1 –10 Medium-scale erosional/depositional units or landforms (floodplains,

alluvial fans, moraines, smaller valleys and canyons)

105 –10^6

710 −^2 Small-scale erosional/depositional units or landforms (ridges, terraces,

and dunes)

104 –10^5

810 −^4 Larger geomorphic process units (hillslopes, sections of stream channels) 103

910 −^6 Medium-scale geomorphic process units (pools and riffles, river bars,

solution pits)

102

10 10 −^8 Microscale geomorphic process units (fluvial and eolian ripples, glacial

striations)

10 −^1 –10^4

Modified from Baker, 1986, and Bloom, 1998.

structures just mentioned to large collapse and resurgent
caldrons or caldera features (e.g., Valles Caldera, New
Mexico; Yellowstone Caldera, Wyoming; Campi Flegrei,
Italy; Krakatau, Indonesia). More areally extensive and
lower subaerial shield volcanoes, formed by more fluid lavas
(and thus with topographic slopes generally less than 5◦)
exist in the Hawaiian Islands, at Piton de la Fournaise (Re-
union Island), in Sicily at Mount Etna (compound shield
with somewhat higher average slopes, up to∼ 20 ◦), and
the Galapagos Islands (Ecuador), for example. Often their
areal extent corresponds strongly to the rate of their effu-
sion. Subaerial and submarine volcanoes occur on the Earth
at nearly all latitudes. Indeed some of the world’s most ac-
tive volcanoes occur along the Kurile-Kamchatka-Aleutian
arc, in subarctic to arctic environments, often with signif-
icant volcano–ice interaction. High-altitude volcanoes that
occur at more humid, lower latitudes (e.g., Andean vol-
canoes like Nevado del Ruiz in Columbia) can also have
significant magma or lava–ice interactions. Volcanoes also
occur in Antarctica, Mt. Erebus being the most active, with
a perennial lava pond). [SeePlanetaryVolcanism.]

2.3 Summary: Terrestrial vs. Planetary Landscapes

Overall, the Earth’s geomorphic or physiographic provi-
nces, as compared to those of the other planets in our
solar system, are distinguished by their variety, their rel-
ative youth, and their extreme dynamism. Many of the
other terrestrial-style bodies, such as the Moon, Mars, and

Mercury, are relatively static, with landscapes more or less

unchanging for billions of years. Although this may not have

been the case early in their histories, as far as we can tell

from spacecraft exploration, this is the case now. Other land-

scapes, such as those on Venus and Europa and a few of

the other outer planets’ satellites, appear younger and ap-

pear to be the result of very dynamic planet-wide processes,

and possibly for Venus, a planet-wide volcanic “event.” Cur-

rently most of these bodies appear relatively static, although

this point may be credibly debated. For instance, the Jovian

satellite Io has vigorous ongoing volcanic activity as was first

discovered in Voyager spacecraft imaging, and the Satur-

nian satellite Enceladus appears to be erupting water from

relatively warm spots in its southern hemisphere, as seen in

recent Cassini spacecraft data. Nevertheless, it seems that

the crusts of all of these bodies are currently somewhat less

variegated than that of the Earth. Be aware, however, that

this last statement may turn out to be just another example

of “Earth chauvinism,” and will be proved wrong once we

eventually know the lithologies and detailed environmental

histories of these bodies as well as we know the Earth’s. [See

Venus:Surface andInterior.]

3. Earth Surface Processes

The expenditure of energy in the landscape is what sculpts

a planetary surface. Such energy is either “interior” (en-

dogenic) or “exterior” (exogenic) in origin. The combined