Origin of Mountains

Origin of Mountains

By: Dr. / Zaghloul El-Naggar

Two
main hypotheses were put forward to explain the formation of mountains:
the vertical —tectonics hypothesis which claims the predominance of
vertical movements in the Earth’s crust, and the horizontal-tectonics
hypothesis, which states that the major land movements responsible for
the building of mountains are primarily horizontal in nature and are
directly connected with both plate tectonics and the drifting of
continents.

Both
hypotheses, however, recognize the close association of orogenesis with
geosynclines. As previously mentioned, geosynclines are very large,
elongated troughs, several thousands of kilometers long and several
hundred kilometers wide, that have been infilled with very thick
accumulations of both sediments and layered volcanics (more than 15,000 m
thick). Such infill becomes later squeezed and uplifted to form
mountains, with or without a crystalline core of igneous and metamorphic
rock.

The
vertical-tectonics hypothesis postulates that thermal expansion can
cause gravity faulting (or sagging) to produce geosynclines in the form
of half grabens or full grabens, while plate-tectonics assume that such
troughs are formed by the subduction of one lithospheric plate below
another as a result of a driving force in the underlying mantle such as
convection currents or thermal plumes.

The
central idea of plate tectonics is that the solid, outermost shell of
the Earth (the lithosphere) is riding over a weak, partially molten, low
velocity zone (the asthenosphere). Continents are looked upon as
raft-like inclusions embedded in the lithosphere, while only a thin
crust (5 km thick) tops the lithosphere in ocean basins. The thickest
continental crust, about 70 km, is reported to lie beneath the Alps.

The
lithosphere (about 100 km thick) is broken up into about 12, large,
rigid plates by rift systems. Each of these plates has been moving as a
distinct unit, diverging away or converging towards each other and
slipping past one another.

Along
divergent junctions, plates spread apart, being accompanied by
intensive volcanicity and earthquake activity. The resulting space
between the receding plates is filled by molten, mobile, basaltic
material that rises from below the lithosphere. This basaltic magma
solidifies in the cracks formed by the rift, producing new sea-floor
material that adds to the edges of the separating plates and hence, the
name “seafloor spreading” for the whole process which is continuously
repeated over and over again.

Origin of Mountains 54298_250

Most
basaltic magmas are believed to originate from the partial melting of
the rock peridotite, the major constituent of the upper mantle. Since
mantle rocks exist under high temperature and high pressure, melting
most often results from a reduction in the confining pressure, although
the influence of increasing temperature cannot be excluded. This can
result from the heat liberated during the decay of radioactive elements
that are thought to be concentrated in both the upper mantle and the
crust.

Along
convergent junctions, plates collide against each other, producing
volcanic island-arcs, deep-sea trenches, both shallow and deep
earthquakes and volcanic eruptions. In the framework of plate tectonics,
orogeny occurs primarily at the boundaries of colliding plates, where
marginal sedimentary deposits are crumpled and both intrusive and
extrusive magmatism (volcanism) are initiated. However, mountain belts
formed at such junctions differ with the different rates of spreading as
well as with the nature of the leading edges of the colliding plates
(continental or oceanic).

When
the abutting edges are ocean floor and continent, the heavy, oceanic
lithosphere descends beneath the lighter, continental one to subduct
into the underlying mantle. This downbuckling is marked by an offshore
trench, while the edge of the over-riding plate is crumpled and uplifted
to form a mountain chain parallel to the trench. Great earthquakes
occur adjacent to the inclined contact between the two plates, and
increasing in depth with the increase in the downward movement of the
descending plate, while oceanic sediments may be scraped off the
descending slab and incorporated into the adjacent mountains. Such zones
of convergence, where the lithosphere is consumed are called subduction
zones. Here, the lithospheric material is consumed in equal amount to
the production of new lithosphere along the zones of divergence. Rocks
caught up in a subduction zone are metamorphosed, but as the oceanic
plate descends into the hot mantle, parts of it may begin to melt, and
the generated magma may float upwardly, in the form of igneous
intrusions and/or volcanic eruptions. The production of magma in the
subduction zone may be a key element in the formation of granitic rocks,
of which continents are mainly composed.

Granitic
magmas are thought to be generated by the partial melting of water-rich
rocks, subjected to increased pressure and temperature. Therefore,
burial of wet, quartz-rich material to relatively shallow depths is
thought to be sufficient to trigger melting and generate a granitic
magma in a compressional environment characterized by rising pressures.
Most granitic magmas, however, loose their mobility before reaching the
surface and hence, produce large intrusive features such as batholiths.

Andesitic
magmas are intermediate in both composition and properties between the
basaltic and the granitic magmas. Consequently, both andesitic
intrusions and extrusions are not uncommon, but the latter are usually
more viscous and hence, less extensive than those produced by the more
fluid, basaltic magma. A single volcano can, therefore, extrude lavas
with a wide range of chemical compositions and hence of physical
properties.

Again,
when an oceanic plate with a continent at its leading edge collides
with another plate carrying a continent, convergence (accompanied by the
gradual consumption of the oceanic lithosphere by subduction) gradually
closes the oceanic basin in between, producing magmatic belts, folded
mountains and mElange deposits on the over-riding continental boundary.
This can continue until the two continents collide, when the plate
motions are halted, because the continental crust is too light for much
of its composition to be carried down to the mantle. Here, the
descending oceanic plate may break off, with the complete cessation of
subduction at the continent/continent suture, but this can start up
again, els1ewhere on the colliding plate. Such continent/continent
suture is marked by lofty mountainous chains, made up of highly folded
and thrust-faulted rocks, coincident with or adjacent to the magmatic
belt. Both giant thrusting and infrastructural nappes lead to
considerable crustal shortening and are accompanied by much thickening
of the continental crust. An excellent example of continent/continent
collision is the Himalayan chain, which began forming some 45 million
years ago. This magnificent mountainous chain, with the highest peaks on
the surface of the Earth, was created when a lithospheric plate
carrying India ran into the Eurasian plate in the Late Eocene time. This can explain how the very thick root underlying the Himalayas was formed.

The
plate tectonic cycle of the closing of an ocean basin by continued
subduction of an oceanic plate under a continental one until a
continent/continent collision takes place and an intra-continental
(collisional) mountain belt is formed, has been called the “Wilson
cycle,” after J.T. Wilson, who first suggested the idea that an ancient
ocean had closed to form the Appalachian Mountain Belt, and then
re-opened to form the present-day Atlantic Ocean. As partly mentioned by
Dewey and Bird (1970), any attempt to explain the development of
mountain belts must account for a large number of common features which
are shared by most of the fully developed younger mountain chains such
as:

1) Their overall long, linear or slightly arcuate aspect.

2)
Their location near the edges of present continents or near former
edges of old continents that are presently intra-continental.

3) The marine nature of the bulk of their sediments, and the intense deformation of such sediments.

4) Their frequent association with volcanic activity.

5) Some of their thick sedimentary sequences were deposited during very long intervals, in the complete absence of volcanicity.

6)
Short-lived, intense deformation and metamorphism, compared with the
lengthy time during which much of the sedimentary succession of mountain
belts was deposited.

7)
Their composition of distinctive zones of sedimentary, deformational,
and thermal patterns that are in general, parallel to the belt.

Their complex internal geometry, with extensive thrusting and mass
transport that juxtaposes very dissimilar rock sequences, so that
original relationships have been obscured or destroyed.

9) Their extreme stratal shortening features and, often, extensive crustal shortening features.

10) Their asymmetric deformational and metamorphic patterns.

11) Their marked sedimentary composition and thickness changes that are normal to the trend of the belt.

12)
The dominantly continental nature of the basement rocks beneath
mountain belts, despite the fact that certain zones in these belts have
basic and ultrabasic (ophiolite suite) rocks as basement and as upthrust
slivers.

13)
Presence of a thrust belt along the side of the mountainous chain
closest to the continent, usually with thrust sheets and exotic blocks
(or allochthons).

14)
Presence of melange belts (composed of mappable rock units of crumpled,
chaotic, contorted and otherwise deformed, heterogeneous mixtures of
rock materials, with abundant slumping structures and ophiolitic
complexes).

15) Presence of a complexly deformed metamorphic core, with severe metamorphism, magmatization and plutonic intrusions.

16) Presence of magmatic belts of both plutonic, hypabyssal and volcanic igneous activity.

17) Presence of folds of several stages and with unified or divergent trends.

18) Presence of block faulting, especially at the peripheries of the mountainous chain.

19)
Presence of deep roots that are proportionately related to both the
mass and elevation of the mountainous range, and can be as deep as 5
times the mountain’s height, or even more.

These
features are clearly suggestive of geosynclinal deposition, or
deposition in mobile belts that are generally referred to as
orthogeosynclines and are typically produced by the subduction of an
oceanic plate below a continental one. Orthogeosynclines are usually
separated into eugeosynclines (characterized by intensive volcanicity)
and miogeosynclines (distinguished by being non-volcanic).

Eugeosynclinal
belts (with their basic lavas, radiolarian cherts and graywackes,
intermediate lava and fragmental volcanic rocks, as well as other
sedimentary, volcanic and plutonic rocks that are metamorphosed to
varying degrees) usually characterize the central cores of mountain
systems. However, these can be notably narrow and may even be absent in
some of the major mountains, probably due to severe tectonism in
recurring phases of orogenesis. Extrusive lavas and agglomerates that
fringe eugeosynclinal belts are identical to those currently being
deposited in modern island arcs. Thick sequences of shallow-water
sedimentary rocks without volcanic material (characteristic of
miogeosynclines) sometimes occur in a belt parallel and adjacent to the
eugeosynclinal belt. These usually occur on that side of the mountain
chain nearer to the old cores of continents (known as the continental
cratons), which are themselves believed to be old mountain roots.

Such
features of youthful mountains have strongly supported the contention
that the present-day, paired island arc/trench systems, with their
intensive seismicity and volcanicity, are quite probably mountain belts
in the process of formation.

Miyashiro (1967) observed that the mountainous islands of Japan
belonged to an old island arc/trench system that had been compressed
and subjected to metamorphism and uplift during the later pan of the
Mesozoic era. These mountains exhibit a pair of different metamorphic
belts parallel to the length of the islands and adjacent to one another.
On the Pacific side, the main outcrops are schists containing minerals
indicative of formation at relatively low temperature but high pressure
(e.g. glaucophane, aragonite, lawsonite), and without any evidence of
granitic basement. On the western side of the islands, the other belt
does have granites and metasediments with minerals indicative of
relatively high temperature and low pressure (e.g. sillimanite).

Such
paired metamorphic belts, also formed during a late Mesozoic orogeny,
were found elsewhere around the Pacific (e.g. in both New Zealand and California),
with the “glaucophane-schist’ (or “blue-schist”) belt always occurring
on the ocean side, and the high-temperature, metamorphic belt (the
“sillimanite-schist” belt) on the continental side.

The
“blue-schist” belt is interpreted to have formed under ocean trench
conditions, where the required low temperature and high pressure are
likely to be obtained. Similarly, the high temperature metamorphic belt
is debated to represent uplifted island arcs, where high heat flows must
have been obtained. This is especially true where a collisional suture
zone marked by blue schist ophiolite melanges is recorded.

Stemming
from this, Dewey and Bird (1970) suggested that mountain belts are a
consequence of plate evolution and that they develop by the deformation
and metamorphism of the sedimentary and volcanic assemblages of
Atlantic-type continental margins. These authors proposed two main types
of mountain building. The first “island arc/cordilleran type,": is for
the most part thermally driven and develops on leading plate edges above
a descending plate (i.e. above a subduction zone) and is marked, by
paired metamorphic belts, paired miogeosyncline (continental shelf)
eugeosyncline (region between continental shelf edge and trench)
relationship, and divergent thrusting. The second “collision type”
results from continent/island arc or continent/continent collision. It
is for the most part mechanically driven, lacks the paired metamorphic
zonation, its metamorphism is dominantly of the low-temperature type
(“blue schist” facies) and its thrusting is dominantly towards and onto
the consumed plate. This often involves the complete remobilization of
basement near the site of collision, and gravity slides further onto the site of the old continental shelf.

Another
essential difference between the two types of mountain belts is that
the cordilleran type has a dense, basic root, probably related to the
emplacement of basic intrusions beneath the high-temperature, volcanic,
metamorphic axis, while the root of collision mountain belts is sialic
and probably results from continental underthrusting and thickening.

Ophiolite
belts usually mark the presence of former zones of subduction between
two colliding plates, and are a significant feature of most mountain
belts. These are commonly associated with radiolarian cherts which are
believed to be of deep marine origin. Ophiolites are said to be
well-developed in cordilleran mountain belts and form extensive upthrust
regions behind the “blue schist” trench terrains, in the form of huge
thrust slices or slivers of peridotite, gabbro and basaltic pillow lava.
The composition and structure of the rocks strongly suggest oceanic
crust and mantle which have been forced upwardly into the overlying
rocks by the subducting plate. These also occur as smaller, detached
rafts in the melanges of trenches, representing blocks that might have
slid down the inner trench wall, slices of oceanic crust, of upper
mantle, or of both, and of seamounts torn off the descending plate.
Thick, intensely deformed oceanic sediments might also have been scraped
off the descending plate and plastered to the inner trench wall or
incorporated into the adjacent mountains. Subsequent uplifts expose the
so-called melange terrain of highly complicated nature, in which shear
surfaces replace bedding as the dominant structural feature.

In
collisional mountain belts, ophiolite blocks are extruded from the
trench during collision and lie in flysch-mElange suture zones that mark
the collision “join lines." The composition of ophiolite pillow basalts
may be a criterion for distinguishing between the crust of the main
oceans (tholeiite and spilite) and the alkalic crust of small ocean
basins, if the latter are produced by the separation of arcs from
continents. These authors concluded that: “Although the
cordilleran/island arc and collision mechanisms are probably the
fundamental ways by which mountain building occurs, mountain belts are
generally the result of complex combinations of these mechanisms.” They
referred to the evolution of the Appalachian orogen which involved
Ordovician cordilleran/island arc mechanisms, followed by Devonian
continental collision.

Dewey
and Bird also mentioned that the Alpine—Himalayan system has been
developing since the early Mesozoic times by multiple collision
resulting from the sweeping of microcontinents and island arcs across
the Tethyan—Indian Ocean. Similar inland mountain belts such as the
Urals, were also looked upon as complex combinations of cordilleran
belts, microcontinents, and volcanic arcs, of widely different ages,
that became juxtaposed by the closing up of a major ocean basin.

The
possibility of expanding and contracting transform offsets of consuming
plate margins was mentioned by these authors to raise the likelihood of
distinctive belts of volcanism, deformation and metamorphism coming to
an abrupt termination along the strike of a mountain belt.

From
the above discussion it becomes obvious that the two main types of
mountain building suggested by Dewey and Bird (1970) which are: the
“island arc/cordilleran type” and the “collision type" are no more than
successive stages in the mountain-building cycle as each
continent/continent collision must be preceded by closing the ocean
basin in-between. In other words, collisional mountains represent the
final stage in the development of these magnificent landforms, and must
be preceded by both the island arc and the cordilleran stages. This is
clearly demonstrated by the Himalayan orogeny, which is considered to be
the product of a combination of both the cordilleran and the
collisional types of mountain building. This author concluded that “The
present boundary between the Indian Plate and the Eurasian Plate is
delineated by the belt of ophiolites and colored melange rocks
separating the ‘Tethys’ Himalayas from the Karakoram and Tibetan Plateau
region of Central Asia…" and added: "…the Himalayan orogenic belt has
resulted through a combination of the two principal mountain-building
processes. The first phase of the Himalayan orogeny, taking place at the
junction between the continental margin of the Indian Plate and the
Tethyan oceanic crust, during the Upper Cretaceous to Eocene period,
could be considered as of the “cordilleran” type. Available geological
data appear to indicate that the subsequent phases in the Himalayan
orogeny, commencing probably from Late Eocene, were the result of the
collision between the Indian and the Eurasian Plate."

Athavale also reiterated that both Hamilton (1970) and Bird and Dewey (1970) had already evolved similar models for each of the Ural Mountains and the Appalachian chain, respectively.

الموضوع الأصلي : Origin of Mountains

المصدر : ملتقى الجزائريين والعرب