Chapter (Thompson, 1991). Chapter 2. Convergent plate boundaries

1.                                                      Introduction

General statement:

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Plate tectonics is a logical hypothesis that recognizes the
extensive scale movements of Earth’s lithosphere. Where plate’s boundaries
combine, the movement of these plates decides that what type of boundary form
here, Example convergent plate boundary, divergent plate boundary, transform
plate boundary. Along these plate boundaries, Earthquakes, volcanic activity, mountain-building,
and Oceanic trench arrangement happen along these plate limits. The horizontal
relative development of the plates normally shifts from zero to 100 mm every

The Earth’s crust is divided into 12 plates which changes there
position in various directions with the passage of time. The motion of these
plates causes, collision (collide each other), Pull apart or slide past with
one another. With plate interaction different type of geologic structures or
tectonic feature are developed (Figure 1).

theories of plate tectonic are present which explain that at ocean ridges the
plates are generated and along Subduction zones the plate are consumed, and
areas where transform fault present the plates are slide past each other and
when two plates collision occur, then suturing of plates occur (Condie, 1997).



Types of plate boundaries:

Plate boundaries are classified into three types, on the
basis of motion relative to one another (Kious et al, 1996).

Convergent plate boundary: One
plate dives beneath another (Subduction) or two plates collide without either

Divergent plate boundary: New lithosphere forms as plates
pull apart.

Transform plate boundary: Plates grind past each other, no
change in Lithosphere.

three plate boundaries have different type of stresses so because of this
produce different structure. At divergent plate boundary extensional stresses
are generated which make normal faults and graben which is uncommon. At
transform plate boundary shear stresses produced which causes bending and
fractures in the rock, fault, folds and uplift of rock occur. At convergence
plate boundary compressive stresses are produced, because of which folds,
reverse faults and thrust fault (Thompson, 1991).




Chapter 2.                              Convergent
plate boundaries

Convergent plate boundaries:

At convergent plate boundaries collision of two plate occur
because of when two plates are moving towards each other. In these boundaries
the heavier (Denser) plate drive undergo the less heavy (lighter) plate. The
process in which one plate will go below the other is called Subduction, the
plate which is going below the other are reach into the mantle and here they
melts because temperature is very high in the mantle.

At convergence plate boundaries continuously new Lithosphere
is produced occur along oceanic ridges, but there is no changing occur in the
earth total volume. The volume of earth remains same as previous one (earth
volume remain constant). There is a cyclic process continue, the older oceanic
lithosphere will go into the mantle as same rate as the rate of production of
sea floor.

2.2 Subduction zones:

Subduction zones which is also known as Convergent
boundaries. Subduction zones are the place where one plate is undergo beneath
the other (subducted) and will reach to the mantle. Subduction of two plates
occur when there is density contrast between the two plates for example the oceanic
lithosphere is subduct below the continental plates because oceanic plates are
denser than continental plate. The continental lithosphere is less dense so
there is no subduction occur and the oceanic plate will have high density and
will subduct up to greater depth
(Zhao et al., 1992).


2.3 Deep-ocean

the lithosphere having oceanic origin descend down and will go into mantle a
surface like feature are produced called deep ocean trenches, these trenches
are in the form of long and deep depressions. Example Pero Chile trench which
is 4499 km in length and 7.9 km below sea level present along the west coast of
S. America. Other example of western Pacific are Mariana trench and Tonga
trench which are deeper than those which are present in the Eastern Pacific.

oceanic plate slabs are goes down into the mantle with small angle or at 90
degree. The angle downward movement is depend upon the density of the plate.
For example when the spreading zone is around the subduction zone along Peru Chile
trench, the subducting material are younger and are hot and floating outward.
Here small angle of descent are form. Because of this the area around the Peru
Chile trench have large no of earthquakes i.e., 2010 Chile earthquake which is
also included in top 10 largest recorded earthquake. Properties of all the
convergent plate boundaries are same and they have very complex structures.
Each of these feature is form from different material and different tectonic
environment (DeLong and Fox, 1977).

2.4 Types
of convergent plate boundaries:

two ocean plate, or one continental and ocean plate or either two continental
plate meet or collide convergent boundaries are generated On the basis of this
there are three type of convergent plate boundaries (Frederick, et al 2012).




Oceanic-continental convergence

is the type of convergence in which one continental plate and one oceanic
collide with one another. In this convergence the plate having higher density
(oceanic plate) goes down and reach into the mantle and the lesser density
plate (continental plate) floating. When these plate slabs reaches into the
mantle here they started melting of the block of asthenosphere, these processes
occur at a depth of 90-100 km (Figure, 3.A).

this type of convergence partial melting of rock occur, which produced about 11
percent molten material and these molten material are then mixed with mantle
rock those which is not melted. When the oceanic plate goes downward, they
contain sediment which have high amount of water, these are derived from
oceanic crust, and the high pressure and temperature condition remove water
from the pore spaces. This process causes melting of rock which is called
partial melting. These melting rock are less dense than the surrounding rock
i.e. mantle rocks so these material rises upward in the form of volcanoes, but
all of these material not reaches to the surface and solidifies within the earth
crust which causes the increase in the crust volume.

of the above process is Andes Mountain, which is form from the subduction of
Nazca plate below the South American plate, these are form from subduction of
oceanic lithosphere and known as continental volcanic arcs. Other examples
include the mountains of California contain Mount Rainer, Mount Shasta, and
Mount of St. Helens which is reach to the Canada and here it include Mount
Garibaldi, Mount Silverthorne and others (Frederick, et al 2012)  (Figure 3.B).

Oceanic-Oceanic convergence:

two ocean plate are meet or collide each other, they form as oceanic-oceanic
convergence boundaries. Many feature which are produced along this type of
convergence are commonly present in continental plate boundaries. At this
boundary also subduction of one plate occur which reaches into the mantle and
triggers volcanic activity, as same process which is occur along
continental-oceanic convergence boundaries (FIGURE 4.A).

two ocean plate convergence setting, the volcanic activity generated from the
ocean floor. In this a chain of volcanic mountains are produced which is occur
in large amount and form as Islands. These are called as volcanic Island arc or
Island arc (FIGURE 4.B)

Mariana and Tonga Islands (young island arc). The Island arc produced 99-301 km
from deep ocean trench. The western pacific ocean contain most island arcs, the
Atlantic Ocean contain only two island arcs. Antilles Island are form because
of the subduction of the Atlantic beneath the Caribbean plate, present in
United States, British virgin island and also present in Martinique island
example of this is Mount Pelee which is erupted in 1902 and kill 28,000 people.

island arc which is form newly have simple structure and underlain by oceanic
crust, commonly less than 21 km thick. And the older island arc have complex
structures and are underlain by highly deformed crust which reaches to 36 km in
thickness. Examples Japan, Indonesia, and Philippines. These island arc are
generated form early form island arc system (Frederick, et al 2012).


Continental-continental convergence:

type of convergent boundary is form from the collision of two continental
plates. In this type of boundary no subduction occur, because of this boundary
mountain ranges are form. (FIGURE 5.A). When the collision of two continental
plate occur it will causes the formation of folds and deform the sediment and
sedimentary rock which is accumulate here along continental plate boundaries.
This collision causes the formation of high mountain ranges which is highly
deformed and contain sedimentary and metamorphic rocks (FIGURE 5.C).

include the collision of indian plate and the Eurasisan plate, starts before
51-60 million years ago, produced Himalayas, which is the very important and
have the world most highest peaks. In this collision the crustal shortening
occur and increase in thickness takesplace.

example include, Alps, Appalachians, and Urals which are also form as a result
of collision of two plates (Frederick, et al 2012) (Figure 5.C).

Chapter 3.                                                      Summary

This report is about the convergent plate boundaries, in
which explain the formation of convergent plate boundary, how subduction occur
and how oceanic trenches are formed then explain the different types of
convergent plate boundaries, Three
distinctive types of convergence are recognized: (1) the convergence of two
oceanic plates, (2) the convergence of a continental plate and an oceanic
plate, and (3) the convergence of two continental plates.

plate boundaries (continental-continental) is a place where two tectonic plates
meet each other. All tectonic plates are constantly being moved because of the
convection currents that occur in the mantle. Continental plates are creating
mountains when they pushed into each other. A few examples of this feature are
the Himalaya Mountain Range and the Appalachian Mountain Range.

subduction zone also develops where oceanic and continental plates converge.
The less-dense continental crust always resists subduction into the dense
mantle and over rides the oceanic plate. Consequently, the volcanic arc forms
on the continent, and compression may deform the continental margin into a
folded mountain belt.

simplest type of convergent plate boundary is ocean-ocean convergence which consists
of two oceanic plates. As the plates collide, one is thrust under the other
forming a subduction zone. The subducting plate descends into the mantle, where
it is heated, triggering the generation of magma. The magma, being less dense
than the surrounding rock, rises and erupts on the seafloor, ultimately
building an arc of volcanic islands. Andesite is the volcanic rock that
characteristically forms at such sites.