Earth as a Planet: Surface and Interior 193
FIGURE 4 (a) Global altitude diagrams. At left are
histograms of land altitudes and seafloor depth as a
percentage of the Earth’s surface area (50-m intervals),
illustrating the classic continent–sea floor dichotomy. The
interface between the two, subject to tidal and climatic
fluctuation stress, is thought to have provided, in part, stimuli
for biological evolutionary adaptations. At right is the global
hypsometric curve, showing cumulative frequency of global
topographic heights. (b) Ocean basin schematic Principal
features of the ocean floor shown in schematic form—height
is greatly exaggerated. (c) Topography of the submarine
Monterey Canyon, California, USA. The continental shelf
offshore of Monterey California showing the Monterey and
other canyons. Such canyons are common on shelves on both
Atlantic and Pacific margins, often cutting through the shelf
and down the continental slope to deep water. Figures used
with permission of the Monterey Bay Aquarium Research
Institute (MBARI).they are probably less well-explored than the well-imaged
surfaces of Mars, Venus, and the satellites of the Outer Plan-
ets. Dominant features of oceanic basins are the oceanic
ridge and rise systems, which have a total length of about
60,000 km (∼1.5 times the equatorial circumference of the
Earth), rise to 1–3 km above the average depth of the ocean,
and can be locally rugged. In the Atlantic Ocean, oceanic
rises exhibit a central rift valley that is at the center of the
rise, whereas in the Pacific Ocean this is not always present
(see Fig. 2a).
Older crust within oceanic basins can have gently rolling
abyssal hills, which are generally smoother than the ridge
and rise systems. These may have been much more rugged
originally, but are now buried beneath accumulated sedi-
ment cover. Perhaps the most areally dominant feature of
ocean basins (with the largest ones occurring in the At-
lantic Ocean) is the predominantly flat abyssal plains that
stretch for thousands of kilometers, usually also covered
with accumulated marine sediments. Generally character-
ized by little topographic relief, in places they are punctu-
ated by seamounts (Fig. 4b), which are conical topographic
rises sometimes topped by coral lagoons, or which some-
times do not reach the oceans’ surface. These features are
subsea volcanoes associated with island arcs or with mid-
plate hot spots, such as the famous Emperor Seamount
chain, the southeastern end of which terminates in the
Hawaiian Islands. Such large hotspots are probably the re-
sult of persistent hot upwelling plumes from the upper man-
tle. Smaller “petite spot” subsea volcanoes may form above
flexure cracks in oceanic plates.
Oceanic margins represent another important, although
more areally restricted, submarine landform province
(Figs. 4b and c). “Atlantic style” continental margins tend
to exhibit substantial ancient sediment accumulations and a
shelf-slope-rise overall morphology, which probably repre-
sents submerged subaerial landscapes remnant from the last
Ice Age, when the sea level was lower (about 135 m below
current sea level, worldwide). Continental shelves are usu-
ally less than about 100 km in width and have very shallow
(∼0.1◦) topographic slopes. They typically end in a slope
break that merges into the continental slope (∼ 4 ◦slope,
about 50 km wide), which in turn merges into a gentle
continental rise (∼0.2◦slope, about 50 km wide), which
then typically transitions into an abyssal plain. Submarine
canyons (also probably remnant from the last Ice Age, e.g.,
Hudson Canyon off the coast of New York) can deeply cut
the continental shelf and slope and terminate in broad sub-
marine sediment fan deposits at the seaward canyon out-
let. “Pacific style” oceanic margins can be even more nar-
row. Along the margins of continents of the Pacific Rim,
a short shelf and slope can terminate into deep subma-
rine trenches, manifested by subduction zones (e.g., South
America, Kamchatka), up to 10 km deep. Similar fore-arc
submarine morphology is observed along the margins of
Pacific island arcs (e.g., Aleutians and Kurile Islands). Much
shallower “back-arc” basins occur behind the arcs, on the
over-riding plate (e.g., Sea of Okhotsk).2.2.2 SUBAERIAL LANDFORMS
The subject of classic geomorphological investigations, and
historically far more well studied because they are where
people on Earth live, are the “subaerial” landscapes—the
quarter of the Earth’s surface that is not submerged. These
terranes exist almost exclusively on continents; however,
some important subaerial landscapes (particularly volcanic
ones, e.g., Hawaii, Galapagos Islands) exist on oceanic
islands. Most continental landscapes are predominately
Cenozoic to late Cenozoic in age, because over that time
scale (65 Myr or so), the combined action of plate tectonics,
constructive landscape processes (e.g., volcanism and sed-
imentary deposition), and destructive landscape processes