192 Encyclopedia of the Solar System
FIGURE 3 Tectonic plate interactions. Tectonic plate interactions and the three fundamental
kinds of plate boundaries. (Left) A convergent boundary caused by the subduction of oceanic
material as it is overridden by another oceanic plate. (Center left) A subplate hot spot capped by a
shield volcano (e.g., Hawaiian Islands). (Center right) A divergent plate boundary, in particular, a
midoceanic spreading ridge. (Right) Another kind of convergent plate boundary, where the
oceanic crust is being subducted by overriding continental crust, producing a chain of volcanic
mountains (e.g., Andes Mountains). (Far right) A continental rift zone, another kind of divergent
plate boundary (e.g., East African Rift). Finally, a transform plate boundary is shown at the upper
middle of the scene, where two plates are sliding past each other without subduction. The three
relationships are shown as block diagrams at the top of the figure. (Courtesy of the U.S.
Geological Survey.)
Some of the stable interior areas of continental plates, or
cratons, however, do possess landforms and associated litho-
logic regions with ages comparable within a factor of two
or three to the age of the Earth (2–3 Byr). The interior of
the Canadian Shield and the Australian Continent are two
such special areas. Despite having been scoured repeat-
edly by continental ice sheets, the granitic craton of the
Canadian Shield possesses a record of giant asteroidal and
cometary impacts that are about 2 Byr old. [SeePlanetary
Impacts]. These interior cratonic areas, in contrast to most
of the rest of the Earth, which is mobile and active, provide
a chance to view a part of the long sweep of the Earth’s sur-
face history. They are thus important, particularly in trying
to understand how the environmental history of the Earth
compares to that of the other terrestrial planets.
The distribution of the earth’s landscape altitudes, rel-
ative to the mean geoid, is bimodal—continental and sea
floor (Fig. 4a). Although limited in percentage of surface
area coverage, the interface between the two modes is a
relatively high-energy place called the littoral or tidal zone.
Ocean tides in this zone generate frequent (twice daily)
environmental stresses on its residents that profoundly en-
courage evolution and natural selection, and may have been
a key influence on the origin and early evolution of life here.
It is interesting that Mars is another planet with a global bi-
modal highland/lowland dichotomy and may have had early
oceans, although the absence of large lunar tides may be
significant in this context. [SeePlanets and theOrigin
ofLife.]2.2 Landform Types
2.2.1 SUBMARINE LANDFORMS
Geomorphically, submarine oceanic basins comprise the
areally dominant landform of the Earth, but, ironically,