By: Dr. Zaghlool El-Naggar:
“And by the mountains He (Allah) has stabilized it (the Earth)*, as a matter of convenience for you (mankind) and for your cattle*”. (Surat An-Nazi’at: 32,33).
In these two Qura’nic 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 creation of our planet and is still a very important factor in making that planet suitable for living.
The following question, however, arises: how can mountains function as means of fixation for the Earth?
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, out 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 fault (transcurrent or transform fault boundaries, sliding or gliding boundaries).
In this way, the lithospheric plates are constantly shifting their positions on the surface of 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 actin.
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 through 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 (test-figs.2,7), 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 don’t occur continuously, but in interrupted, sudden jerks, which release accumulated strain.
Moreover, it has to be mentioned that both the pattern and the speed of movement of lithospheric plates very 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 lave 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 side 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 measure, and 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 lithospheric plate moves away from its spreading center represents half the movement at the center 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 1 cm/year in the Arctic Ocean to about 18 cm/year in the Pacific Ocean, with the average being 4-5 cm/year. Apparently, the Pacific Ocean is now spreading almost ten times faster than the Atlantic (cf. 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 long 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 pattern 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 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, loc.cit.).
In as much as volcanoes abound at divergent boundaries under the seam 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 ling period 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 and hence 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 belt 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 result 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 considerable 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 extension of the mountains are commonly known as “mountains roots” and are several times their protrusion above the ground surface. The sea 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 (asthenophere) 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 thick (30-40) km) than it is beneath the oceans (5-8 km) and why the thickness of the continental plates (100-150 km) 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/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 corresponding (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 (test-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. These is enough geologic evidence of 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 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 steady decrease in the amount of heat arriving from the interior of the Earth 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 would shake and jerk, making life virtually impossible on the surface of our planet. This fact is stressed in ten Qur’anic verses as follows: [Surat Ar-Ra’d:3; Surat Al-Hijr:19; Surat An-Nahl:15; Surat Al-Anbya’:15; Surat Al-Anbya’:31; Surat An-Naml:61; Surat Luqman:10; Surat Fussilat:10; Surat Qaf:7; Surat Al-Mursalat:25-27; and Surat An-Nazi’at:32-33].
These verses also indicates 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. Via 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 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 out 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 do not only stabilize the lithospheric plates, but also the whole planet in its spinning around its own axis (nutation,recession, etc.).
The precedence of the Holy Qur’an with more than 14 centuries in describing these phenomena is a clear testimony of the fact that this Noble Book is the word of The Creator in its divine purity and the Prophet Mohammed (PBUH) is His final messenger.
In an authentic saying, this noble Prophet is quoted to have said that: “When Allah created the Earth 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. E.) when no other man was aware of such facts, which only started to unfold by the beginning of twentieth century, and was not finally formulated until towards its very end.
This article is from Science’s archive and we’ve originally published it on an earlier date.