Page 128 - Fundamentals of Geomorphology

LARGE-SCALE TECTONIC AND STRUCTURAL LANDFORMS 111

The basic landforms connected with convergent
margins are island arcs and orogens. Their specific
form depends upon (1) what it is that is doing the
converging – two continents, a continent and an island
arc, or two island arcs; and (2) whether subduction of
oceanic crust occurs or a collision occurs. Subduction is
deemed to create steady-state margins in the sense that
oceanic crust is subducted indefinitely while a continent
or island arc resists subduction. Collisions are deemed
to occur when the continents or island arcs crash into
one other but tend to resist subduction.

Steady-state margins
Steady-state margins produce two major landforms –
intra-oceanic island arcs and continental-margin orogens
(Figure 4.10).
Intra-oceanic island arcs result from oceanic litho-
sphere being subducted beneath another oceanic plate.
The heating of the plate that is subducted produces vol-
canoes and other thermal effects that build the island
arc. Currently, about twenty intra-oceanic island arcs sit
at subduction zones. Most of these lie in the western
Pacific Ocean and include the Aleutian Arc, the Marianas
Arc, the Celebes Arc, the Solomon Arc, and the Tonga
Arc. The arcs build relief through the large-scale intru- Figure 4.10 The two kinds of steady-state margins.
sion of igneous rocks and volcanic activity. A deep trench (a) Intra-ocean island arc formed where an oceanic plate
often forms ahead of the arc at the point where the is subducted beneath another oceanic plate. These are
oceanic lithosphere starts plunging into the mantle. The common in the western Pacific Ocean. (b) A continental
MarianasTrench, at −11,033 m the deepest known place margin orogen formed where an oceanic plate is subducted
on the Earth’s surface, is an example. beneath a continental plate. An example is the Andes.
Source: Adapted from Summerfield (1991, 58)
Continental-margin orogens form when oceanic
lithosphereissubductedbeneathcontinentallithosphere.
The Andes of South America are probably the finest
example of this type of orogen. Indeed, the orogen is collision are possible: a continent colliding with another
sometimes called an Andean-type orogen, as well as continent; an island arc colliding with a continent;
a Cordilleran-type orogen. Continental-margin island a continent colliding with an island arc; and an island
arcs form if the continental crust is below sea level. arc colliding with an island arc (Figure 4.11):
An example is the Sumatra–Java section of the Sunda
Arc in the East Indies. 1 Continent–continent collisions create interconti-
nental collision orogens. A splendid example is the
Collision margins Himalaya. The collision of India with Asia produced
an orogen running over 2,500 km.
Landforms of collision margins vary according to the 2 Island arc–continent collisions occur where an island
properties of the colliding plate boundaries. Four types of arc moves towards a subduction zone adjacent to