190 Encyclopedia of the Solar System
FIGURE 1 Blue Marble view of the Earth from
Apollo 17. Earth as seen from the outbound Apollo
17, showing Mediterranean Sea to the north and
Antarctica to the south. The Arabian Peninsula and
the northeastern edge of Africa can also be seen.
Asia is on the northeast (upper right) horizon. Most
striking is the prevalence of liquid water (thus
evidence of an average surface temperature
> 273 ◦K), not now present in the arid landscapes of
the other solid bodies within our solar
system.(Courtesy of NASA)is dominated by its oceans of liquid water: approximately
75% of the Earth’s surface is covered by liquid or solid
water. The remaining 25% of nonmarine subaerial land,
the subject of nearly all historical geological and geomor-
phological study, lies mainly in its Northern Hemisphere,
where most of the world’s population lives. The South-
ern Hemisphere is dominated by oceans, some subaerial
continental and archipelago land masses (mainly parts of
Africa, South America, southeast Asia, and Australia), and
the large, mainly subglacial, island continent of Antarctica
(Fig. 2a).
Remarkably, despite the fact that geological and geo-
graphical sciences have been practiced on the Earth for
about 200 years, it has only been during the last 40 or so
that scientists have begun detailed mapping and geophysical
explorations of the submarine land surface. Subsea remote-
sensing technology has provided one of the most profound
discoveries in the history of geological science: the paradigm
of “plate tectonics.” The extent, morphology, and dynam-
ics of the Earth’s massive tectonic plates were only realized
after careful topographic and geomagnetic mapping of the
intensely volcanic midoceanic ridges and their associated
parallel-paired geomagnetic domains.
Similar topographic mapping of the corresponding sub-
marine trenches along continental or island-arc margins was
equally revealing. The midoceanic ridges were found to be
sites of accretion of new volcanically generated plate mate-
rial, and the trenches the sites of deep subduction, where
oceanic crust is consumed beneath other over-riding crustal
plates. Tectonic plates represent the most fundamental and
largest geomorphic provinces on Earth.
The Earth’s crustal plates come in two varieties: oceanic
and continental (Fig. 3).
Oceanic plates comprise nearly all of the Earth’s ocean
floors, and thus most of the Earth’s crustal area. They are
composed almost exclusively of iron and magnesium-rich
rocks derived from volcanic processes (called “basalts”).
Oceanic plates are created by volcanic eruptions along the
apices of the Earth’s midoceanic ridges: 1000-km-long sinu-
ous ridges that rise from the flat ocean floor (called “abyssal
plains”) in the middle of oceans. Oceanic plates are typically
less than 10 km thick. Here, nearly continuous volcanic ac-
tivity from countless submarine volcanic centers (far more
than the 1000 or so active subaerial volcanoes) provides a
steady supply of new basalt, which is accreted and incorpo-
rated into the interior part of the plate.
At plate edges, roughly the reverse occurs, where the
outer, oldest plate margins are forced below over-riding ad-
jacent plate edges. Usually, when two oceanic plates collide,
the resulting subduction zone forms an island arc along the
trace of the collision. The islands, in this case, are the result
of the eruption of lighter, more silica-rich magmas gener-
ated as part of the subduction process. The subducted plate
margin is consumed along the axis of the resulting trench.