Geology Reference
In-Depth Information
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-
sion of igneous rocks and volcanic activity. A deep trench
often forms ahead of the arc at the point where the
oceanic lithosphere starts plunging into the mantle. The
Marianas Trench, at
Figure 4.10
The two kinds of steady-state margins.
(a) Intra-ocean island arc formed where an oceanic plate
is subducted beneath another oceanic plate. These are
common in the western Pacific Ocean. (b) A continental
margin orogen formed where an oceanic plate is subducted
beneath a continental plate. An example is the Andes.
Source:
Adapted from Summerfield (1991, 58)
−
11,033 m the deepest known place
on the Earth's surface, is an example.
Continental-margin orogens
form when oceanic
lithosphere is subducted beneath continental lithosphere.
The Andes of South America are probably the finest
example of this type of orogen. Indeed, the orogen is
sometimes called an Andean-type orogen, as well as
a Cordilleran-type orogen. Continental-margin island
arcs form if the continental crust is below sea level.
An example is the Sumatra-Java section of the Sunda
Arc in the East Indies.
collision are possible: a continent colliding with another
continent; an island arc colliding with a continent;
a continent colliding with an island arc; and an island
arc colliding with an island arc (Figure 4.11):
1
Continent-continent collisions create
interconti-
nental collision orogens
. A splendid example is the
Himalaya. The collision of India with Asia produced
an orogen running over 2,500 km.
Collision margins
Landforms of collision margins vary according to the
properties of the colliding plate boundaries. Four types of
2
Island arc-continent collisions occur where an island
arc moves towards a subduction zone adjacent to
