Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 39

zone is initially beneath the continent, as the oceanic plate moves toward the continent,
it can carry an older inactive or mature island arc on it to eventually collide with the
continental margin. Alternatively, where the subduction zone dips away from the contin-
ent and beneath the island arc, the closing of the small oceanic basin between contin-
ent and island arc will eventually suture the island arc to the continental edge. In both
cases, because of its buoyancy, the island arc will not subduct beneath the continental
plate but will instead plow into the continent, deforming both blocks. When no further
convergence is possible, a new trench and subduction zone can develop on the seaward
side of the accreted island arc (Dewey and Bird 1970; Strahler 1998).
In other cases of accretion of continental fragments, both small and large, their low
density and buoyancy prevent their descent into the subduction zone. The ensuing colli-
sion of buoyant crustal fragments with the continent forces the stacking of crustal slabs
into thrust slices embedded in the Andean-type margin. Small microcontinents can be
diminutive islands or as large as Madagascar; there can even be submerged continent-
al fragments known as oceanic plateaus. There may also be submerged volcanic plat-
eaus created by massive outpourings of lava from hot-spot activity, or extinct volcanic
islands such as the Hawai'ian Island-Emperor Seamount chain. Some smaller crustal
fragments can ultimately become termnes accreted to the main continental mass; these
commonly have a geologic history quite distinct from that of the adjoining regions.
Alternatively, large continental masses, such as the well-known example of the Indi-
an plate—which broke from the Gondwanaland supercontinent in Mesozoic time and
moved north through the Indian Ocean to dock with Asia in Cenozoic time—represent
massive continent-to-continent collisions (Fig. 2.13). The majestic Himalaya and the
massive Tibetan Plateau resulted from this continent-to-continent collisional conver-
gence. In collisional convergence, the remains of ancient spreading centers, sea-floor
basalts, and overlying deep-sea sediments can be caught up in the collision and sutured
into the collisional mountain range. These ophiolite suites are a clue to the presence
of the former ocean bottom and help to decipher the sequence of collisional events, al-
though whether most ophiolites are typical sea floor, or due only to backarc spreading,
is much debated.
Erosional Mountains
Mountain building as a result of plate-tectonic divergent and convergent stress is
the traditional understanding of orogenesis. Nevertheless, at crustal and lithospheric
scales, gravitationally induced stress is as important as tectonic stress, with the result
that syntectonic deformation can be modified significantly by erosion (Beaumont et al.
1992). Paradoxically, the shaping of mountains can depend as much on the destructive
forces of erosion as on the constructive power of tectonics (Pinter and Brandon 1997).
In fact, after more than a century of viewing erosion as the weaker brother of tec-
tonics, many scientists now elevate erosion to the head of the family. As Hoffman and
Grotzinger (1993) noted, “savor the irony should those orogens most alluring to hard-
rock geologists owe their metamorphic muscles to the drumbeat of tiny raindrops.”
Erosion actually builds mountains in a variety of ways. Isostasy represents a kind of
crustal buoyancy in which the crust of the Earth floats on top of the denser, deform-able
rocks of the mantle. Mountain belts stand high above the surrounding lowlands because