Physics. -
About Mountain-formation on the Earth . By F. A.
VENING
MEINESZ.
(Communicated at the meeting of September 25, 1948.)
If we study the way in which mountains and mountain-ranges have
formed on the Earth's surface we shall find much th at is unknown or
conjectural. We can no doubt state that there are several different types
between which we can distinguish. In many cases we shall find that these
types merge into each other.
In the first place we can distinguish between mountain-formations of a
linear kind where one horizontal dimension is much smaller than the other
and those of which the horizontal dimensions in the two directions have
the same order of magnitude. Besides the isolated volcano the la'tter type
as far as the continents are concerned mainly exists of areas which for
some reason or another have risen to some elevation above sea-level and
which have since been dissected byerosion. The resulting topography
depends on the time during which this erosion has operated and the
conditions to which it was subject; it is hardly necessary to go into details
here as we know that geomorphologists have made extensive studies of
these processes.
Sub-oceanic topography of this kind has been less intensively investigated 1) and so a"few remarks may be made. It is c1ear that n6 erosion
can alter the submarine topography but sedimentation may perhaps do so,
although in many cases probably all features will be covered by approximately the same layer and so the topography must in its general lines
remain the same. It might be possible that steeper slopes may thus be made
somewhat more gradual but the writer has no evidence in this sense. He
had the opportunity to make soundings over several deep troughs and
several submarine mountains of probably volcanic origin and no sign could
be found of a softening of the slopes. As an instanee he may mention the
northern slope of the Romanche Trough (near the equator between Africa
and S. America) which is unusually steep; the grade is about 60°. This may,
however, be caused by its being a young formation and so we can not
conclude much for the topography in general. Leaving aside again the case
of isolated volcanoes 2) and considering now banks or plateaus of the nonlinear kind we may try to make out their way of originating. We have only
two series of data which can give us a hold on this problem, detailed
1) We shall not deal here with the topography of the continental shelves and the
sub marine canyons about which much research bas been made.
2) An important study of this subject is given by H. H. HESS in "Drowned ancient
islands of tbe Pacific Basin" , Am. J. O . Sc. 244, Nov. 1946, pp. 772-791.
930
soundings and gravity resuIts, and in general th is will still leave us a wide
field of conjecture. If we find a bank which in general shows local isostatic
compensation as e.g . two gravity profiles running up to the Bromley
Plateau (S. Atlantic, eas't of S. America at about 31 0 S) appear to indicate,
the most obvious supposition is th at it is caused by alocal thickening of
the sialic layer which seems a not unlikely assumption considering the fact
that the continents themselves are no doubt much larger features of this
same kind . But it might of course be also possible that some unknown
cause in the substratum has brought about a rising of the crust in th is
area as we no doubt find them in so many continental areas where epeiro~
genetic movements are not uncommon, especially near or over old tectonic
areas of former periods. The causes of such vertical movements, rising as
well as sinking, are still unknown; in some cases perhaps convection~
currents in the substratum might be supposed to be responsible for them
but this is no more than a hypothesis.
A third possibility is no doubt that the topography is caused by plateau
exItrusions but in that case a purely local isostatic compensation seems
hardly likely.
We shall not further enlarge here on the case of topography of the non ~
linear kind ; in this paper we shall especially consider the mountain~
formation on the earth's surface of the more or less linear type. We can
distinguish here between two cases with a third case in a more or less
intermediate position.
In the first place we have the well~known geosync1ine beIts which give
rise to folded mountain~ranges ; we shall presently come back to them more
in detail. They generally occur in the continents or in areas bordering on
them; the last occurs in the Antilles, in Ithe Southern Antilles and along
the eastern and south~eastern border of Asia and the eastern border of
Australia and New Zealand. AIthough geologists have of ten surmised
connections of these beIts Ithrough the oceans, as e.g. between the Antilles
and the Mediterranean and Alpine geosync1inal belt there is no proof that
these connections do exist. In the North Atlantic in particular no evidence
in this sense can be found and ,the topography as well as the gravity~field
show quite different features. In general these geosync1inal beIts are
characterized by strong folding and overthrusting of the surf ace layers
and follow slowly curving tracks; in the belt themselves the individual
tectonic axis are also curved as e.g. in the Mediterranean and Alpine belt.
No evidence of such beIts has as yet been found in the central parts of the
Atlantic, the Indian Ocean or the Pacific.
In the second place we have linear topography along more or less
straight lines which keep their direction over greater distances than the
former group and where no large overthrusts are found in the surface
layers of the crust. As we shall see these beIts of topography lack many
of the properties of the geosync1inal beIts as e.g. the presence of deep
931
earthquakes. Usually a second direction is also prevailing in the topo~
graphy which makes an angle of 60°-90° with the first . The topography
makes the impression to be for a great part of volcanic origin and we
find especially great outcrops in areas where a belt in the first direction
crosses one in the second. We find this type of topography in the North
and South Atlantic and in the centra I Pacific east of the andesite line. It
is not certain whether it is also found in the Indian Ocean and perhaps
also in the continents .
In connection with the properties described it is obvious to attribute this
type of topography to a set of fault~planes in the crust along which shearing
has mainly taken place in a horizontal sense; as it is always the case with
shear it can occur in two directions enclosing an angle of 60°-90° .
For the geosynclinal beIts we shall adhere to the usual view for which
much evidence is present that in this case the blocks of the Earth's crust
on both sides of the belt are pressed together and that the crust in the belt
gives way and allows a shortening of the distance of the order of several
ten s of kilometers.
A third case of an intermediary character can be distinguished if in
geosynclinal beIts straight tectonic axis occur making a small angle with
the direction of the relative move ment of the two crustal blocks on both
sides of the axis . In that case the relative movement of these blocks is
mainly shearing along a fault~plane coinciding with the tectonic axis
while. if any. only a sm all component of the movement is present at right
angles to the fault~plane causing a slight overriding of one of the blocks
over the other which thus is somewhat pressed downwards . An example
of this may probably be found on the west~coasts of Sumatra and of Cali~
fornia and Mexico, where slight ocean~deeps are found at the foot of the
continental slope while fault~planes with strong relative movements are
known parallel to the coast. In the case of Sumatra this belt is forming
part of the great Indonesian arc which is curved over the greatest part of
its course. For this curved part the crust may be supposed to bulge down~
wards under the effect of the great compression in the way the strong
negative gravity anomalies over these beIts have led us to suppose. Where
the belt runs over Sumatra in a direction nearly coinciding with the relative
movement of the two crustal blocks it is straight and the gravity field no
longer points to a more or less symmetrical crustal down~bulge in the
tectonic belt but to an overriding of the Sumatra block over the ocean~
block. We thus see that the facts seem to indicate that a lso in the geosyn~
clinal beIts there are parts of the tectonic axis where the relative move ment
of the crustal blocks on both sides has the t haracter of a mainly horizontal
shearing movement along a fauIt~plane. These parts are not accompanied
by deep earthquakes. This case thus seems to approach the second one
although it forms part of a belt of the first~mentioned kind.
We shall now examine our cases somewhat more in detail. In the first
place we shall look at the geosyncline beIts. As a typical example of such
932
a belt in the present period we may mention the eastern part of the
Indonesian archipelago where we find the outer arc around the Banda Sea
characterized by folding and overthrusting of the surface layers. by
shaIlow~focus earthquakes obviously caused by the crust's deformation in
this belt ançl by strong negative anomalies which may probably be interpreted as the effect of the downward bulge of the crust in the denser substratum which is brought about by the crust's compression and which thus
brings lighter matter in the place of heavier masses.
Inside this arc we have the second one consisting of volcanoes. and under
or near this arc we find the intermediate shocks. The topography is more
or less locally compensated. lnside this arc we have the deep Banda bas in
and the deep earthquake foci.
This general distribution is typical for orogenic island arcs. We usually
find the shallow shocks in the zone of folding and the intermediate and
deep shocks on the inside of the curve and mostlyon the continental side.
the deep shocks at larger distances than the intermedia te on es and both at
di stances from the tectonic zone equal in order of magnitude to their
depths.
We find great differences in elevation in the tectonic zone. South of
Java e.g. it lies at depths of 1300 m to more than 2000 m while in Timor
it comes up to elevations above sea-Ievel of more than 2900 m. These last
elevations may perhaps be explained by a beginning of the readjustment
of the isostatic balance by some local disengaging of the crust from its
surroundings. The rising thus taking place must have an irregular character
depending on the amount of loosening in the belt. Probably the rising is
usually accompanied by a sinking of the neighbouring beIts which have
been pushed upwards by the central belt but which sink back as this belt
detaches itself.
A second possibility of explaining differences in elevation in the
tectonic belt may be found in a thinner surface layer being squeezed out
and folded when the downbulge of the main crust takes place. This may
weIl be dependent on the thickness of the sedimentary layer; as this layer
is no doubt less resistant than the deeper crustal layers. it can weIl be
understood that a great part of the layer is not engulfed in the downward
movement. This line of explanation has e.g. been advanced by HESS 3)
.lor making dear that the southern part of the tectonic belt in the Antilles
comes here and there above sea-leveI. as e.g. in Barbados and Tobago.
while the northern part which is indicated by the belt of negative anomalies
is found at great depth; Eor a great part it does not even show a ridge. In
the southern part the neighbouring South American .continent can account
Eor a thicker sedimental layer than in the north. UMBGROVE adopts the
same line of reasoning for explaining the absence of a tectonic second arc
3) HESS. H. H .• Recent advances in interpretation of gravity anomalies and island.arc
structure (Adv. Rep. Comm. 00 Cont. ao Oceanic Stro. 1939).
933
in the Marianas-Bonin arc where only the volcanie arc has been developed; he assumes a much thinner sialic layer here than nearer to the
Asiatie continent.
The inner arc is always volcanie and the volcanoes are often reaching
above sea-Ievel. lts volcanie character gives to this arc likewise an irregular
length-profile. It is possible that in this arc the surface layers also show
some folding as e.g. in the south of Java but this is always less intensive
than in the tectonie arc itself.
We have already mentioned that in the straight parts of these arcs as
e.g. in the Sumatra area, the relative move ment of the crustal blocks may
probably be assumed to be mainly shear in the sense of the tectonic axis
while only a small component at right angles to this axis is present; this
last component may be supposed to cause an overriding of the Sumatra
block over the Indian Ocean block and no longer a large down-bulge of
the crust; this view is in harmony with the smaller size of the negative
anomalies in the tectonie beIt here than in the other parts of the arc and
in the asymmetrie character of the anomaly-curve. This straight part is
not accompanied by deep shocks.
The deep earthquakes only occur in the tectonic beIts surrounding the
Pacific.
It is not yet quite certain whether in the present period also tectonie
beIts are present on the Earth's surface where early stages of the orogenetie
cycle occur as e.g . the first syncline stage when folding has not yet taken
place on a large sc ale or takes place for the first time. Later stages than
the one described above can no doubt be found as e.g. in the Alps where
the folding appears to be nearly finished and where the tectonic belt has
ris en to so great a height that only slight remnants of the negative
anomalies are left. Earthquakes are rare here although not yet quite absent
and volcanoes have become extinct.
As it is weIl known many geologists think that these great tectonic
deformations of the Earth's crust take place in cycles of some 50-70
million years which more or less periodically occur over the whole globe
and whieh are separated by periods of rest of some tOO-150 years. The
writer may refer here to UMBGROVE's book, "the Pulse of the Earth",
which deals especially with this subject, to KUENEN's study 4) and to
GRIGGS's 5) and his own papers ij) which try to account for this periodieity
by assuming great scale convection-currents between the crust and the
core. We shall presently come back to th is hypothesis.
4) KUENEN, PH. H ., Major geological c;ycles; Proc. Ned. Akad. v. Wetenseh.,
Amsterdam, oH (1941) .
5) G~IQGS, DAVID, A theory of mountain-building, Am. J. o . Sc. 237, Sept. 1939,
pp. 611-650.
G) VENING MEINESZ, F. A ., Major tectonic phenomena and the hypothesis of
convection-currents in the Earth; J. o. the Geol. Soc. o. London, 103, 3, 1947, pp. 191-207.
VENING MEINESZ, F . A .. Gravity Expeditions at sea, Vol. IV. eh. 11. 1948, Waltman.
Delft.
934
It seems difficult to understand the curved track of the tectonic axis in
the orogenetic beIts and up to now no c1ear explanation has been given. It
seems strange that in this way the leng th of the deformed belt becomes
greater than is necessary and that thus the deformation energy should
appear to be larger than needed. We may perhaps come to some deeper
understanding by first considering the parts of the tectonic belt where the
main phenomenon is shear. In the Indonesian archipelago these are the
Sumatra and Philippine parts.
Examining the problems of mechanics involvedin these parts where the
shear occurs along a plane enclosing only a small angle with the direction
of the relative movement of the crustal blocks on both sides, it is difficult
to co me to another conclusion than that the fault-plane must already have
been present before the tectonic phenomenon took place: it is otherwise
hard to explain how a plane in th is direction could come into existence.
Accepting its presence beforehand it is c1ear that only little energy is
needed for causing shear in this sense and so we can understand that the
tectonic belt will as far as possible follow these old lines. It is thus perhaps
possible to account for the whole arc as of course the two lines on the si des
- in the East Indies the beits west of Sumatra and east of the Philippines
- have to be connected by a curved line for becoming a continuous belt.
The presence of old lines on the Earth's surface is more and more widely
admitted and as these lines are straight, it seems likely that they really
represent fault-planes through the crust. Since a long time geologists as
DAUBRÉE, SEDERHOLM, SONDER, a.<O. have written about such old lines
which they usually thought to have the directions SE-NW and SW-NE
and they have in general been supposed to be very old, having been
reopened from time to time when new forces occurred in the crust. The
writer himself has advanced a hypothesis for explaining their pres en ce 7):
he showed that a shift of the poles over some 70° along the meridian of
90° E could bring about a net of shear-planes in the cru st which would
weil agree with the existing directions.
In general we may state that the presence of a pre-existing net of faultplan es in the crust would make it much more easy to understand the
twisted and complicated courses which the tectonic axis have taken in the
orogenetic beits. Particularly marked instances of such complicated courses
may be found in the Mediterranean-Alpine beIt, in the West Indies and
in the East Indies.
A last point which has to be broached in connection with the orogenetic
beits on Earth is the question about the cause of these deformations of
the crust. It has of ten been supposed that this cause has to be looked for in
currents in the substratum which because of the very high viscosity must
exert great stress es on the lower boundary of the crust. Aplausible
7) VENING MEINESZ, F. A., Shear Patterns of the Earth's crust; Transactions Amer.
Geophys. Union, 28, I, 1947, pp. 1-61.
935
explanation of such currents could perhaps be found in the cooling of the
Earth which must bring about a cause of instability in the plastic layer
below the crust and if this layer is homogeneous over a sufficient thickness
this must give ri se to convection-currents. As most seismologists do not
accept a density discontinuity in the mantIe of the Earth between the
rigid cru st and the core, it seems at least possible that this entire layer
would be subject to such currents which thus would assume continental
dimensions in a horizontal sense. Smaller currents going less deep would
of course also be possible. As the writer has put forward eIsewhere 8) he
thinks that both types are present. The smaller type could perhaps account
for the deep earthquakes and for the sinking down of the deep basins in
the eastern part of the East Indian archipelago and in other is land-arc
areas while the larger type could explain the great tectonic phenomena
in the orogenetic geosyncline beIts of the Earth.
In both cases, however, the hypothesis gains in plausibility if we assume
a small streng th to be present in the convection-layer which has to be
overcome before the current can set in; the current is then supposed to
make only about a half-turn, bringing the layer of lower temperature down
and the layer of higher temperature on top and thus restoring the stability.
GRIGGS [J) has already pointed this out for the larger type of convectioncurrent in 1939 in his important paper on mountain-formation in which he
explains that thus the large periods of rest can be accounted for between
the periods of great tectonic activity as mentioned before while the writer
has shown the same to be the case for the great time-lag of some 15.000.000
years between the last folding-period in the outer Banda arc and the sinking
down of the Banda basin.
A further asset of the hypothesis that large scale convection-currents are
responsible for the great tectonic phenomena can be found in the possibility thus to explain the regressions of the oceans during the first part of
the period of tectonic activity and the long period of transgression during
the period of rest; the difference in temperature in the rising and sinking
columns during the time that the current is going can account for these
surface movements.
It appears to the writer that the number of phenomena which can thus
be understood is sufficient for at least considering the supposition of
convection-currents as a serious working hypothesis.
Much less can be said about the second kind of topography on Earth,
the straight-line type of mountain-formation of which we find. examples
in the central parts of the Pacific and in the North and South Atlantic,
possibly likewise in the Indian Ocean, in Africa and Europe and perhaps
S) VENING MEINESZ, F. A., Major tectonic phenomena and the hypothesis of
convectioncurrents in the Earth, J. o. the Geol. Soc. of London, 103, 3, 1947, pp. 191-207.
0) GRIGGS, DAVID, A theory of mountain-building. Am. J. o. Sc. 237, Sept. 1939.
pp. 611-650.
936
also in other continents. We do not know much about the period in which
this topography came into being nor whether the history of these mountain~
formations shows the same more or less periodic character as the geosyncline
beIts. The important discovery by HESS of many flattened sub marine cones
in the Pacific of which the upper flat surface is found at variabie depths
up to more than 2000 m 10) may perhaps shed light on this difficult pro~
blem. It led him to the interpretation that each cone has been a volcano
subjected to marine planation and sinking down below sea~level because
of the slow sedimentation in the ocean and the resulting sinking of the
ocean~floor with the volcanoes resting on it. If th is ingenious hypo thesis
can be accepted - and the writer thinks that there can hardly be any
doubt about it - we might come to an es ti mate of the age of the volcanoes
and if the observed material increases sufficiently this might enable us to
get an idea wh ether th ere have been periods of great volcanic activity
separated by intervals of small action.
Another question which at least for the ocean-areas can not yet be
answered is whether part of the topography along these straight lines is
non~volcanic. It makes the impression that the general features of the
Mid~Atlantic Ridge in the North Atlantic have local isostatic compensation
while most of the submarine volcanoes are regionally compensated but the
gravity material leading to these conclusions is yet insufficient for making
sure about them. If they prove to be true it appears probable that part of
the Mid~Atlantic Ridge in its generallines has been brought about by other
phenomena than volcanism. We might perhaps suppose that the shear
along the fault-planes in the ernst not only causes volcanoes but also a
pressing up of matter along the plane.
Such a mechanism has already been supposed for the west-coasts of
Sumatra and California and Mexico and we might no doubt surmise it
also for other straight-line parts of the mountain-systems in the continents
as we may perhaps find in the three ranges around Bohemia, the Thüringer
Wald, great parts of the Apennines and the Dalmation mountains and
other ranges which may possibly be reckoned to this type of mountainformation. Besides their straight courses we may perhaps find a reason
for doing so in the fact that they all seem to belong to the system of two
directions of topography usually admitted to be present in the European
continent. They are certainly in agreement with the before-mentioned net
of shear-planes derived by the writer from the hypo thesis of a shift of
the poles.
Among the few data we seem to have about the volcanic topography
in the oceans which we have attributed to shear along a system of straight
fault-planes we found a certain predominating of volcanic activity in the
places where faults in the two prevailing directions cross each other. We
10)
H . H . HESS, Drowned ancient islands of the Pacific basin, Am.
1946, pp. 772-791.
J.
o. Sc., 244, Nov.
937
need hardly say that this appears to be in good harmony with our supposition of two sets of fault-planes along which shear occurs. IE at the
.same time move ment takes place in both directions. the points of crossing
must in particular show a lockering of the Earth's crust and so th is must
be favorable for the rising of magma.
Before leaving our subject we may examine one problem more which
presents itself. We have already drawn attention to the fact that in the
central parts of the oceans we seem mainly to find the straight-lined type
of topography; this appears at least to be true for the Pacific and the
Atlantic and there is not much reason for a contrary opinion in the Indian
Ocean although the scarcity of soundings there does not yet allow a good
conclusion in this case. The question ar is es why no geosynclines seem to
occur in those areas.
An answer to th is question may perhaps be looked for in two directions.
In the first place the crust under the oceans has another constitution than
under the continents; it has no doubt a thinner granite layer and perhaps
this is even absent over large areas while the sedimentary layer must likewise be much thinner and of a different type. It might be possible that
this different constitution would lead to other physical constants and to a
different behaviour with regard to the stresses working in the crust. The
deviation in thermal conditions causecL by the low temperature near the
ocean floor and probably also by a smaller content of radio-active matter
in the crust mayalso contribute to the bringing about of another reaction
of the crust to the stresses. This might have the effect of making the crust
more resistant against down-buckling which process must involve a good
deal of plastic deformation in the cru st and of gliding of one layer on
another. The result might. therefore. be that the cru st gives way to shear
before the stresses have reached the values needed for down-buckling.
In the second place we might try to find an explanation by assuming
that the conditions below the oceanic crust prevent convection-currents to
come into being and this could account for the absence of sufficient
stresses in the crust for causing down-buckling. We have al ready mentioned that probably the subcrustal matter has a certain strength which
must be overcome before a convection-current can set in and this involves
the need of sufficient temperature differences in a horizontal sense for
bringing about differences of pressure large enough for starting the movement. Now it could weIl be understood that such differences of tempera tu re
in a horizontal sense are present on the border of the continents where the
oceanic and continental temperature distributions occur side by side. or
even perhaps in the middle of the continents where the constitution of the
crust and . therefore. of the thermal and radio-active properties may change
sufficiently from place to place for causing temperature differences. but
that under the oceanic crust the conditions are too regular for such effects.
This could thus explain the absence of convection-currents below the
oceans and. as a consequence of this. the absence of all tectonic activity
in the crust.
938
The above sketch of the great features of mountain-formation on the
Earth's surface is only a first tentative attempt to arrive at some deeper
understanding of these great problems. Much more research will no doubt
be necessary before a weIl-founded treatment can be given.
Summary .
Af ter a short discussion of the non-linear topography on Earth a survey
is given of the linear mountain-formations among which three groups are
distinguished. firstly the geosyncline beIts characterized by folding and
overthrusting. by shallow as weIl as deep earthquakes and by a curved
track of the axis. in the second place the straight-lined type mostly
accompanied by strong volcanism where two directions predominate over
large areas of the crust probably representing fauIt-planes through the
whole crust along which shear occurs and wh ere deep shocks are absent,
and thirdly the intermediate case th at in geosyncline beIts straight fauItplanes are found along which mainly shear takes place. possibly coupled
with some overriding of one block over the other (e.g . Sumatra. California.
Mexico. etc.).
In the centra I part of the Pacific. in the Atla ntic and possibly also in
the Indian Ocean it is probable that only the second type occurs. A
possible cause might be the absence of convection-currents or the different
constitution of the crust in those areas.
A more detailed discussion is given of the three types of mountainformation as weIl as of the way they can have co me into being. For the
compression in the crust which leads to the phenomena in geosyncline
beIts. convection-currents are supposed to be responsible. but the writer
thinks that the course of the beIts can not be understood without admitting
pre-existing fauIt-planes. The presence of a net of such fault-planes over
the whole Earth might also explain the second type of mountain-formation.
The net is probably very old but the faults must sin ce continually have
been rejuvenated. The writer thinks that his hypothesis about the net
having originally been brought about by a shift of the poles. gives a
satisfactory explanation.