Origins of Mountains - Mountain Geography: Physical and Human Dimensions - page 34

The essential feature in mountain building through plate tectonics is the plate boundary,
of which there are three main types, each having different characteristics. Divergent
rifting occurs where spreading centers develop beneath a lithospheric plate, such as
the rift valleys of East Africa (Wright et al. 2006). As magma rises beneath a contin-
ent, the overlying crust is weakened and domes upward above the hot, rising plumes.
Such uplift stretches and thins the crust, which ruptures along linear rift valleys, allow-
ing formation of multiple normal fault scarps, down-dropped grabens, upraised horst
mountains, and extrusive volcanoes from escaping magmas. Where such orogenic pro-
cesses continue for a long time, the rift valley will eventually open to an invasion by the
sea. Further progressive rifting and slow movement of the continental plate down from
upraised spreading centers will cause subsidence of the fault mountains and volcanoes
beneath the sea, where they become part of a new passive-margin accumulation wedge
of thick, shallow-water sandstones, limestones, and shales, similar to the Atlantic Ocean
and its coasts. Over time, some passive margins can become unstable and begin to sub-
side into a newly developed subduction zone, where the oceanic part of the plate now
begins to dive beneath the continental portion.
In some cases, however, a number of passive margins seem to have maintained their
fault or warped mountain relief for a long time, perhaps as a result of enigmatic swell-
ing forces (isostasy, thermal expansion, intrusion, renewed uplift) on passive continental
margins. The Western Ghats of southern India, the Great Escarpment of eastern Aus-
tralia, and the Drakensberg of South Africa constitute erosional mountains of passive-
margin plateaus (Oilier and Pain 2000).
In a transform plate boundary, plates slide past each other; here, small volcanoes
can occur and other small mountains are produced by buckling, especially where faults
change direction. Convergent plate margins, where plates come together and subduc-
tion takes place, are the most important for mountain building. Subduction convergence
of oceanic lithosphere produces two basic different kinds of orogenic belts, depending
upon the nature of the overriding plates: the Aleutian-type island arc or the Andean-
type mountain belt (Kearey and Vine 1996).
Aleutian-Type Island Arc
Island arc chains of volcanic islands curve in a convex arc toward the open ocean,
with a deep-sea trench on the descending plate. In this ocean-ocean convergence, sub-
duction beneath an oceanic plate generates a magmatic arc of volcanoes and associ-
ated features on the opposing plate. Related processes include partial melting of the
mantle wedge located above the subducting plate; sporadic volcanic activity; emplace-
ment of magmas as batholiths at depth; high-temperature metamorphism of the deeper
surrounding rock; and the accumulation of an accretionary wedge of sediment scraped
of the subducting plate into the chaos of folded and faulted marine sediment, along
with pieces of oceanic crust that are subjected to low-temperature, high-pressure meta-
morphism.
Regional extension or spreading can also occur behind or within island arcs. Such
backarc spreading can tear an arc in two, move an arc away from a continent, or split
of a small part of a continent, which seems to be how Japan formed. This spreading
also forms oceanic crust similar to that at the crest of mid-ocean spreading ridges,