D -
THE MOUNTAINS AS STABILIZERS FOR THE EARTH:
The Holy Qur’an reads:
(And by the mountains He (Allah) has stabilized it (the
Earth)*, as a matter of convenience for you (mankind) and
for your cattle)* (LXXIX:32, 33).
In these two Qur’anic verses it is explicitly stated that
the stabilization of the Earth by means of its mountains was
a specific stage in the long process of the creation of our
planet and still is a very important factor in making that
planet suitable for living. Now, the following question
arises: how can mountains function as means of fixation for
the Earth?
As mentioned above, the lithosphere (the outer rocky cover
of the Earth , which is 65- 70 km thick under the oceans and
100-150 km thick under the continents) is broken up by deep
rift systems into separate plates that vary greatly in both
dimensions and shape. Each of these rigid, outer rocky
covers of the Earth floats on the semi-molten, plastic, weak
zone of
the Earth’s mantle (the asthenosphere) and move freely away
from, towards or past adjacent plates. At the diverging
boundary of each plate, molten magma rises and solidifies to
form strips of new ocean floor, and at the opposite boundary
(the converging boundary) the plate dives (subducts)
underneath the adjacent plate to be gradually consumed in
the underlying asthenosphere, at exactly the same rate of
sea-floor spreading on the opposite boundary. An ideal,
rectangular, lithospheric plate would thus have one edge
growing at a mid-oceanic rift zone (diverging boundary), the
opposite edge being consumed into the asthenosphere, under
the over-riding plate (converging or subduction boundary)
and the other two edges sliding past the adjacent plates
along transform faults (transcurrent or transform fault
boundaries, sliding or gliding boundaries). In this way, the
litho spheric plates are constantly shifting their positions
on the surface of the Earth, despite their rigidity, and as
they are carrying continents with them, such continents are
also constantly drifting away or towards each other. As a
plate is forced under another plate and gets gradually
consumed by melting, magmatic activity is set into motion.
More viscous magmas are intruded, while lighter and more
fluid ones are extruded to form island arcs that eventually
grow into continents, are plastered to the margins of nearby
continents or are squeezed between two colliding continents.
Traces of what is believed to have been former island arcs
are now detected along the margins and in the interiors of
many of today’s continents.
The processes of both divergence and convergence of
lithospheric plates are not only confined to ocean basins,
but are also active within continents and along their
margins. This can be demonstrated, by both the Red Sea and
the Gulf of California troughs which are extensions of
oceanic rifts and are currently widening at the rate of
3cm/year in the former case and 6cm/year
in the latter. Again, the collision of the Indian Plate with
the Eurasian Plate (which is a valid example of
continent/continent collision after the consumption of the
oceanic plate which was separating them) has resulted in the
formation of the Himalayan Chain, with the highest peaks
currently found on the surface of the Earth.
Earthquakes are common at all plates boundaries (text but
are most abundant and most destructive along the collisional
ones. Throughout the length of the divergent plate boundary,
earthquakes are mostly shallow seated, but along the
subduction zones, these come from shallow, intermediate and
deep foci (down to a depth of 700 km), accompanying the
downward movement of the subducting plate below the
over-riding one. Seismic events also take place at the
plates transcurrent fault boundaries where it slides past
the adjacent plates along transform faults. Plate movements
along such fault planes do not occur continuously, but in
instituted, sudden jerks, which release accumulated strain.
Moreover, it has to be mentioned that both the pattern and
the speed of movement of lithospheric plates vary from one
case to another. Where the plates are rapidly diverging, the
extruding lava in the plane of divergence spreads out over a
wide expanse of the ocean bottom and heaps up to form a
deeply rifted, broad mid-oceanic ridge, with gradually
sloping sides (e.g. the East Pacific Rise). Contrary to
this, slow divergence of plates gives time for the erupting
lava flows to accumulate in much higher heaps, with steep
sides (e.g. the Mid-Atlantic Ridge). The rates of plate
movements away from their respective spreading axes (rift
zones) can be easily calculated by measuring the distances
of each pair of magnetic anomaly strips on both sides of the
axial plane of spreading. Such strips can be easily
identified and dated, the distance of each from its
spreading axial plane can be measured, hence the average
spreading rate can be calculated (test fig. 9).
Spreading rates at mid-oceanic ridges are usually given as
half-rates, while plate velocities at trenches are full
rates. This is simply because the rate at which one litho
spheric plate moves away from its spreading centre
represents half the movement at that centre as the full
spreading rate is the velocity differential between the two
diverging plates which were separated at the axial plane of
spreading (the mid- oceanic ridge rift or its axial plane of
rifting).
In studying the pattern of motion of plates and plate
boundaries, nothing is fixed, as all velocities are
relative. Spreading rates vary from about 1cm/year in the
Arctic
Ocean, to about 18cm/year in the Pacific Ocean, with the
average being 4-5cm/year.
Apparently, the Pacific Ocean is now spreading almost ten
times faster than the Atlantic (c.f Dott and Batten, 1988,
p. 167).
Rates of convergence between plates at oceanic trenches or
at mountain belts can be computed by vector addition of
known plate rotations (c.f Le Pichon,1968). These can be as
high as 9cm/year at oceanic trenches and 6cm/year along
mountain belts (Le Pichon, op. cit.). Rates of slip along
the transform fault boundaries of the lithospheric plates
can also be calculated, once the rates of plate rotation are
known.
Both the patterns of magnetic anomaly strips and the
sediment thicknesses on top of such strips suggest that the
spreading patterns and velocities of oceanic lithospheric
plates have been different in the past, and that the
volcanic activity along mid-oceanic ridges varies in both
space and time. Consequently such ridges appear, migrate and
disappear with time.
Spreading from the Mid-Atlantic rift zone began between 200
and 150 MYBP (Million
Years Before Present) from the north-western Indian Ocean rift zone
between 100 and 80 MYBP, while both Australia and Antarctica
did not separate until 65 MYBP (cf Dott and Batten, bc.
Cit).
In as much as volcanoes abound at divergent boundaries under
the sea, such eruptive features are also abundant on land.
Most of the current oceanic volcanoes have been active for a
period of 20-30 million years or even more (e.g. the Canary
Islands). During such long periods of activity, older
volcanoes were gradually carried away from the rift zone by
sea-floor spreading until they became out of reach of the
magma body that used to feed them hence these faded out
gradually and died. The floor of the present day Pacific
Ocean is spudded with a large number of submerged, non-
eruptive (dead) volcanic cones (guyots) that are believed to
have come into being by a similar process.
Continental orogenic belts are the result of plate boundary
interaction, which can take place between oceanic and
continental lithospheric plates and reaches its climax when
two continents come into collision, after consuming the
ocean floor that used to separate them. Such
continent/continent collision results in the scraping off of
all sediments and sedimentary rocks, as well as all volcanic
rocks that have accumulated on the ocean floor, squeezing
them between the two colliding continents, crumpling them
considerably in the form of mountains. This is immediately
followed by the cessation of movement for the two colliding
continental plates which become welded together, with
considerable crustal shortening (in the form of giant
thrusts and infrastructural nappes) and considerable crustal
thickening (in the form of the decoupling of the two
lithospheric
plates as well as their penetration by the deep downward
extensions of the mountain chains then formed). Such
downward extensions of the mountains are commonly known as
“mountain roots” and are several times their protrusion
above the ground surface. These deep roots stabilize the
continental masses (or plates), as plate motions are almost
completely halted by their formation, especially when the
mountain mass is finally entrapped within a continent as an
old craton.
Again, the notion of a plastic layer (asthenosphere)
directly below the outer rocky cover of the
Earth(lithosphere) makes it possible to understand why the
continents are elevated above the oceanic basins, why the
crust beneath them is much thicker (30-40 km) than it is
beneath the oceans (5-8 km) and why the thickness of the
continental plates (100-150km) is much greater than that of
the oceanic plates (65-70 km). This is simply because of the
fact that the less dense lithosphere (about 2.7 to 2.9 gm/cm
is believed to float on top of the denser, and more easily
deformed, plastic asthensophere (>3.5gm/cm3) in exactly the
same way an iceberg floats in the oceanic waters.
In as much as mountains have very deep roots, all other
elevated regions such as plateaus and continents must have
coltesponding (although much shallower) roots, extending
downward into the asthenosphere. In other words, the entire
lithosphere is floating above the plastic or semi-plastic
asthenosphere, and its elevated structures are held steadily
by their downwardly plunging roots (text-fig. 10).
Lithospheric plates move about along the surface of the
Earth in response to the way in which heat flows arrive at
the base of the lithosphere (text-fig. 11), aided by both
the rotation and the wobbling of the Earth around its own
axis. There is enough geologic evidence to support the fact
that both
processes have been much more active in the distant geologic
past, slowing gradually with time. Consequently, it is
believed that plate movement have operated much more rapidly
in the early stages of the creation of the Earth and have
been steadily slowing down with the steady building up of
mountains and the accretion of continents. This slowing down
of plate movements may also have been aided by a steady
slowing down in the speed of the Earth’s rotation around its
own axis (due to the operating influence of tides which is
attributed to the gravitational pull of both the sun and the
moon). This steady slowing down of plate movement could also
have been aided by a steady decrease in the amount of heat
arriving from the interior of the Barth towards its surface
as a result of the continued consumption of the source of
such heat flows which is believed to be the decay of
radioactive elements.
The above mentioned discussion clearly indicates that one of
the basic functions of the mountains on land is its role in
stabilizing continental masses lest these might shake and
jerk, making life virtually impossible on the surface of our
planet.
This fact is stressed in the following ten Qur’anic verses:
[XXT:19; X\71:l5; XXI:l5; XXI:31; XX\7II:61; XXX1 XLI:lO;
L:7; LXXVII:25-27; and LXXIX:32-33].
These verses also indicate that the outer rocky cover of the
earth has been spreading out and accreting since the early
phases of creation of the earth, through intensive volcanic
activity. Through such activity both the atmosphere and the
hydrosphere of the earth have been outgassed, its
lithosphere has been built and rifted into separate plates,
its lithospheric plates have been set in constant movement
and mountain building (orogenesis) has been progressing to
halt such movement and stabilize the
lithospheric plates as well as the whole planet. The
stabilization of lithospheric plates by mountains is
effected by their sinking deeply into the zone of weakness
of the Earth (the asthenosphere) as wooden pegs sink into
the ground to stabilize the corners of a tent. Such process
of stabilization cannot take place without the presence of a
viscous, plastic material under the outer rocky cover of the
Earth, into which the mountains’ “roots” can float. In as
much as the ship casts its anchor into the anchorage of a
port to avoid the dangers of rolling and swaying by winds
and waves, the Glorious Qur’an uses the term “Rawasi”
(moorings or firm anchors) to describe mountains. Such firm
anchors not only stabilize the litho spheric plates, but
also the whole planet in its spinning around its own axis
(nutation, recession, etc).
The Quranic foretelling of these facts more than 14
centuries ago is a clear testimony of the fact that this
Noble Book is the word of the Creator in its Divine purity
and that Prophet Mohammad (PBUH) is His fmal messenger. In
an authentic saying, this noble Prophet is quoted to have
said that: “When Allah created the Barth it started to shake
and jerk, then Allah stabilized it by the mountains”. This
unlettered Prophet lived at a time (between 570 and 632 C.
B.) when no other man was aware of such facts, which only
started to unfold by the beginning of the twentieth century,
and was not finally formulated until towards the very end of
this century.
by Dr. Z.R.M. EL-NAGGAR.
The Seventh International Conference on Scientific Signs in
Quran & Sunnah
* In each
of these paired numbers, the first (or the Roman Number)
indicates the number of the Qur’anic chapter (or Surah),
while the second (or the Arabic Number) indicates the number
of the Qur’anic verse or verses (Ayah or Ayat) in the Surah
(chapter).
* MYBP = Million Years Before Present |
