Kinds of Mountains

Kinds of Mountains

By: Dr. / Zaghloul El-Naggar

As
mentioned above, individual mountains within a range, system chain, or
cordillera can be related to individual geological structures such as
folding, faulting, igneous activity, or to a combination of such events.
However, the development of a whole chain of mountains (orogenesis) has
to be interpreted in terms of much larger tectonic episodes
(megatectonics or global tectonics).

Again,
regardless of their mode of formation, the present shape of individual
mountains is also related to a large number of factors such as its age
and the stage it has reached in the mountain-building cycle, the
climatic conditions under which it has existed and the resistance of its
exposed rock types to erosion. Indeed, mountains are born, grow,
achieve youth, maturity and old age, then they are worn down and finally
disappear. The oldest known rocks on the Earth’s surface today are
believed to be the roots of some ancient mountains. These currently form
the relatively stable cratons or shield areas of the continents.

According
to their geometry, structure, rock composition, and/or age, four main
kinds of mountains have been recognized; these include: volcanic
mountains, folded mountains (or fold belts), fault-block (or
block-faulted) mountains and erosional (or upwarped) mountains. These
are, indeed, successive stages in the development of mountains, besides
being distinctive types. Volcanic mountains represent the initial stage
in the development of such gigantic landforms, while folded mountains
represent the peak of youthfulness and maturity and erosional (or
upwarped) mountains represent the old age. Fault-block Mountains
can be produced at any of these stages, but, nevertheless, have
traditionally been treated as a specific type of mountain. These four
types or stages in the development of mountains can briefly be described
as follows:

1) Volcanic Mountains: (such
as Kilimanjaro of East Africa, Paricutin of Mexico, Mauna Loa of
Hawaii, Vesuvius of Italy, Fujiyama of Japan, etc.): These are the
simplest known mountains and are usually in the form of isolated peaks,
constructed from accumulated lava flows, pyroclastic debris and other
extruded igneous rocks that might have piled up rapidly (in only a few
years), or might have grown slowly (over thousands or even millions of
years).

Such piling-up of eruptive material can take place around volcanic vents producing cinder cones (such as Vesuvius, near Naples)
or elsewhere, producing volcanic mountains. It can also flow out at the
surface and consolidate in the form of a broad, gently sloping flat
topped, volcanic dome, usually several tens or hundreds of square
kilometers in extent, being chiefly built of overlapping and
interfingering basaltic lava flows (volcanic shield). These can
gradually grow into volcanic mountains such as Mauna Loa in Hawaii
(which rises from a depth of 4270 m below sea level to a height of more
than 3960 m above sea level), Kilauea of the same island, and the Geat
basaltic accumulations of Iceland.

Volcanic
mountains seem to have their origins connected to deep faults that
extend below the Earth’s crust to the mantle which supplies their
building materials. In other words, volcanic mountains are directly
related to deep rifling in the Earth’s crust and hence are considered to
represent the earliest stage in the development of a mountainous chain.

In
terms of global tectonics, most of the volcanic types of mountains are
believed to be associated with movements near the boundaries of
lithospheric plate. These are created as a result of downward, subplate
disturbances (e.g. the Aleutian and the Cascade volcanoes) or as a
direct consequence to the pulling apart of lithospheric plates at
mid-oceanic rifts (e.g. both Kilimanjaro and the Kenya Mountains which are both directly related to the East Africa rift system).

Indeed,
active volcanoes are most abundant in narrow belts, particularly in the
island areas that rim the Pacific Ocean (where it is believed that the
Earth’s crust is currently being consumed by descending into the
mantle), as well as along mid-ocean ridges (where new oceanic crust has
been steadily produced since at least the time between 150 and 200
million years ago).

The
Aleutian Islands are peaks of volcanic mountains that stretch out for
3200 km along the circumference of a circle centered at 62~ 40’ N and
l78~ 20’ W Island arcs festoon the western borders of the Pacific Ocean,
with great oceanic deeps (trenches) on the outside curve of many of
them.

Similarly,
many geologists believe that mid-ocean ridges are true volcanic
mountain ranges. These attain heights of as much as 1800 m above the
ocean bottom and are covered—in places—by up to 2700 m of water.
Nevertheless, in the framework of plate tectonics, such ridges are
believed to have “antiroots” rather than roots, and hence their
inclusion among mountains can be strongly debated. Antiroots are
accumulations of higher-density material in the suboceanic crust that
compensates for the low density of oceanic water. These are injected
upwardly from the underlying upper mantle by either convection currents
or thermal plumes.

More
than 64,000 km of mid-ocean ridges have - so far-been mapped around
mid-ocean rift valleys. These have been pouring out fresh basaltic
material on both sides of such ruptures in the Earth’s crust, since the
early days of their initiation, to build-up new oceanic crusts. The
youngest oceanic crust will always be around deep rift valleys and will
steadily push older crusts away from it. The oldest existing oceanic
crust does not exceed the Mesozoic in age, and is currently being
consumed at the convergent edges of the plates with rates almost
equivalent to the rate of producing new oceanic crust.

Few
volcanic mountains are found on the continents such as the isolated
peaks of Ararat (5100 m), Etna (3300 m), Vesuvius (1300 m), Kilimanjaro
(5900 m), and Kenya
(5100 m). These are also associated with intra-cratonic, deep rift
systems that communicate with the upper layer of the Earth’s mantle.

2) Folded Mountains (or Fold Belts): These
represent the peak of the development of mountain belts, and hence are
represented by the great mountain systems of the world such as the
Andes, Carpathians, Urals, Alps, Juras, Himalayas,
etc. Such mountain systems normally comprise broad belts of varied rock
types and of structural patterns that involve folding, faulting,
over-thrusting and igneous activity. Faults are particularly numerous
along the borders of these highly folded belts. Some are normal faults,
but the majority is low-angle thrust faults that extend for hundreds of
kilometers, pushing gigantic masses of rock over one another for many
kilometers (over thrusting).

Field
observations clearly indicate that the development of folded mountains
was normally preceded by the formation of geosynclines. A geosyncline is
a large basin in the Earth’s crust, usually scores of kilometers wide
and hundreds of kilometers long, with sediments of marine origins that
do not usually exceed the 300 m depth that alternate with layered
volcanic accumulations in complexes of more than 15,000 m thick.
Consequently, geosynclines are believed to have been deeply rifled,
slowly and steadily subsiding basins to keep pace with the accumulation
of such thick sections of sediments and layered volcanic. The formation
of a geosyncline must then involve a slow, and continuous downwarping of
the Earth’s crust with the continuous deposition
of sediments, and a near access to molten basalts. Here, the theory of
plate tectonics can provide the clue to the formation of a geosycline.
Seismic evidence from many earthquakes confirms the motion of oceanic
plates away from mid-oceanic rifts towards and under other plates where
inter-oceanic island-arc-trench systems, or oceanic/continental trench
systems are formed and the lithosphere of the subducting (or
under-gliding) plate is gradually consumed into the mantle at a rate
equal to sea-floor spreading. Plate subduction can account for the
formation of oceanic trenches, and the partial melting of the descending
plate can explain both the availability of molten magma and the
formation of volcanic arcs. Such oceanic trenches are ideal sites for
the geosynclinal accumulation of sediments, and hence, geosynclines are
believed to have developed in such structurally mobile belts, where
subsidence is not only produced under the weight of accumulating
sediments, but is also maintained by the gradual sliding of one
lithospheric plate below another. When the oceanic plate between two
continental masses is completely subducted and consumed,
continent/continent collision takes place, forming folded mountains and
the highest peaks on earth.

The
sediments accumulated in a geosyncline eventually sink to levels where
they become surrounded by denser, more viscous rocks, and their own
buoyancy sets a limit to the depth to which they can sink under their
own weight. At that point, the whole system becomes isostatic, and the
thickness of the sediments cannot be increased just by load.

Both
folding and faulting occur continuously while sediments are
accumulating. Rocks at the surface are brittle and hence, they break
before they flow, but under deep burial, they become plastic and change
both their shape and volume by folding and/or slow flowing. When
sediments are buried deep enough, they melt. Expansion of such molten
rocks causes the whole overlying mass to rise, and their cooling will
produce basement rocks that often participate in the folding process.

Near
the edges of the geosyncline, the rocks are squeezed upward and outward
along great thrust faults, while in the central area they are pushed
upward to form an inter-montane plateau. Evidence of preconsolidation
folding supports the contention that the mountain-building forces were
active during sedimentation. Indeed differential downwarping could have
produced folding while deposition was in progress, but at this stage,
the dominant forces were probably mainly vertical. Thrust faulting along
the margins of the geosyncline could have been initiated by a bordering
zone of differential subsidence, but as active horizontal and
tangential compressive
stresses are usually late in the geosyncline’s history (as a result of
the collision of plates) they may be the main cause of overthrusting.
Such stresses finally elevate the already deformed strata to mountainous
heights. Modem examples of geosynclinal zones growing slowly into
mountain ranges are thought to exist today between the Pacific border of
Asia and arcs of volcanic islands off the continental coast.

From
the above mentioned discussion, it is obvious that the major mountain
systems have evolved as a result of the movement of lithospheric plates.
At the boundary of two such plates, one plate can move down relative to
the other, a geosyncline develops and island arcs are built by the
piling up of eruptive volcanic material initiated by subduction. Later,
the geosynclinal infilling of sedimentary and volcanic rocks rises to
form a mountainous chain. As it rises, folds and faults develop either
through squeezing (the horizontal-tectonics hypothesis) or through
gravity sliding of material away from the rising welt (the
vertical—tectonics hypothesis) or by both. Mountain ranges could also
result from the collision of two continents being rafted along on their
lithosphere conveyor belts (e.g. the Alps and the Himalayas).
In both cases, folded mountain ranges were not formed by the
deformation of only one geosyncline, but rather by the deformation of
many.

Present-day
mountain ranges were definitely much higher. These were worn down over
time and were left as erosional remnants of the original, sharply folded
and faulted uplifts. Isostatic rebounds for the whole mountain range
would also intervene to compensate for erosion and keep the isostatic
adjustments. This can go on until the mountain roots are exposed to the
surface, attain the thickness of the surrounding lithosphere and the
mountain chain is almost completely leveled.

3) Fault-Block Mountains (Block-Faulted Mountains) Such
mountains are formed by uplifts of the Earth’s crust along steep
dipping or almost vertical faults. Differential tilting of blocks of the
Earth’s crust along areas of separation such as rifts can produce
fault-black (or block-faulted) mountains. These occur in many parts of
the world, frequently adjacent to incipient oceans (such as the Red Sea)
or at the periphery of fold belts. Subsequent to folding and low-angle
thrust faulting in such belts, a period of steep block faulting produces
fault-block Mountains at the periphery of the folded mountain range.

Fault-block
mountains are large, uplifted sections of the Earth’s crust that are
bounded by faults in the form of alternating horsts and grabens (e.g.
the Great Basin and Range province of Oregon, the Sierra Nevada of
California, the mountain ranges that border both the Red Sea rift and
the rift valleys of East Africa, etc.). Their rocks may be totally
crystalline, igneous and metamorphic complexes or may carry a thin or a
thick sedimentary cover. The sedimentary cover, being originally
deposited in a geosyncline, can sometimes be folded during an earlier
cycle of deformation, before the region was broken up into blocks and
uplifted by successive movements along the different planes of faults
over millions of years to attain mountainous heights. In the North
American Cordillera (along the western border of North America), the
fault-block mountains began to be elevated about the same time as their
neighboring folded mountains and plateaus, indicating that regional
deformative forces were acting.

Many
geologists believe that block-faulting is due to either stretching or
relaxation in the later phases of a geosyncline/mountain-building cycle.
But, according to the theory of plate-tectonics, large scale rifting
may be due to intraplate ruptures, followed by the pulling apart of the
ruptured lithospheric plate to diverge away from each other as two new
separate plates (such as the splitting of the Arabian/Nubian plate). Fault-block Mountains
can also be produced at a later stage in the development of subdued
folded mountains, when faulting can provide the necessary elevation of
the mountainous range.

4) Upwarped (or Erosional) Mountains: These
are the erosional remnants of previously existing mountain ranges and
owe their present heights and appearances to broad upwarpings of the
Earth’s crust as a result of isostatic adjustment (e.g. the Ozarks,
Adirondacks, Appalachians, Rockies, Black Hills,
the Highlands of Labrador, etc.). When the old mountain chains were
worn down by erosion and reduced to subdued topographies, isostatic
re-adjustment brought them to their present-day elevations. Such subdued
elevations represent the final stage in the history of a mountainous
chain, before they are almost completely leveled and added to a
previously existing craton.