Earth as a Planet: Surface and Interior 191
FIGURE 2 (a) Physiographic map of the Earth.
This image was generated from digital data bases of
land and sea-floor elevations on a 2-minute
latitude/longitude grid. Assumed illumination is
from the west, and the projection is Mercator.
Spatial resolution of the gridded data varies from
true 2-minute for the Atlantic, Pacific, and Indian
Ocean floors and all land masses to 5 minutes for
the Arctic Ocean floor. (Courtesy of NOAA). (b)
Volcanoes and the Crustal Plates. Global map of
the major tectonic plate boundaries and locations of
the world’s volcanoes (Courtesy of the U.S.
Geological Survey).Because the more silicic island arcs tend to be less dense
and thus more resistant to subduction, they can be accreted
onto plate margins and can thus increase the areal extent
at the edges of oceanic plates or can enlarge the margins of
existing continental plates.
Continental plates tend to consist of much more silicic
material, and are thus lighter, as compared with oceanic
plates. Because of their lower density and the fact that
they are isostatically compensated, they are much thicker
than oceanic plates (30–40 km thick) and tend to “float”
over the denser, more mafic (ferromagnesian—of the met-
als iron and magnesium) subjacent material in the Earth’s
upper mantle. When continental plates collide with oceanic
plates, deep subduction trenches, such as the Peru–Chile
trench along the west coast of South America form, as the
oceanic plate is forced under the much thicker and less
dense continental plate. Usually, the landward side of the
affair is marked by so-called Cordilleran belts of mountains,
including andesitic-type volcanoes, which parallel the coast-
line. The Andes Mountains are an example of this type of
tectonic arrangement.
When continental plates collide, a very different tectonic
and geomorphic regime ensues. Here, equally buoyant and
thick continental plates crush against each other, resulting in
the formation of massive fold belts and towering mountains,
as long as the tectonic zone is active (e.g., the Himalayan
Range in Asia). When aggregate stresses are tensional rather
than compressive, extensional mountain ranges can form,
as tectonic blocks founder and rotate. The western U.S.
Basin and Range Province is a good example of that type of
mountain terrane. Another large subaerial extensional tec-
tonic landform is the axial rift valley and associated inward-
facing fault scarps, which form when aggregate tensional
stresses tend to pull a continental plate apart (e.g., the East
African Rift Valley).
The geomorphic provinces just discussed generally tend
to be very dynamic, with lifetimes that are intrinsically short
(100–200 Myr) relative to the age of the Earth (4.56 Byr).