Is Himalayan Mountain in Isostatic balance?
The word Isostasy
is derived from the german word ‘isostatios’ meaning to be ‘in equipoise’ and
was first proposed by the American geologist Dutton.
It simply means a mechanical stability between
upstanding parts and the low lying basins on a rotating earth.
The theory is developed out of the discrepancy
that arose during the geodetic surveying of the Indo-Gangetic plain for the
latitude determination under the supervision of Sir G.Everest,the then Surveyor
General of India
The problem gave rise to several explainations
to the problem proposed by – Sir G.Airy,Pratt,Hayford and Bowie and so on.
However,the problem that arises with the theory
of isostasy is that in all the concepts proposed ,all the aforementioned people
considered different landforms (like the plateau,plain,mountain etc.) as coloumns
while in reality it’s not so.Moreover the critics of this theory pointed that
there is no complete isostasy over the globe due to constant endogenetic
forces.Critics have also pointed out if isostatic adjustments don’t occur at
the local level,it may exist at the regional level.Isostasy in India is in
jeopardy.Gravity surveys show that natural or actual geoid in India did’nt
coincide with the plumb line deflected markedly southward from Indo-Gangetic
plain with no apparent topographic causal reason.The geoid in India is marked
by an upwarp (east-west) in the middle and one down the warp in the north and
another in the south.glennie has
pointed out that Indian region is marked by deficiency of mass.Uplands and
mountains are believed to be underlain by the light rocks while lowlands and
oceans are underlain by denser rocks.the high density zone of the ‘Hiden
Range’,whose axis roughly passes – Barmer,Udaipur,Bilaspur and
Sambalpur,then,should have been the lowest surface of India,if the previous sentence
is considered to be true.However,this is’nt the case,the zone is 300 – 600 meter
above sea level.
The low density ‘trough’ zone whose axis passes
through Cuddapah and Dharwar should have been a high mountain belt but it is
not.The Indo-Gangetic plain should have been a zone of high density but it is
not so either.Glennie believed
that the northern downwarp gave rise to Tethys sea and the Indo-Gangetic
depression.The Deccan Trough was the result of the ‘coast effect’,which states
that due to long continued erosion and deposition of lighter loose sediments on
the three oceanic sides of Deccan in the west zone has resulted in an area of
low density as a whole.Studies in the form of Hayford anomalies shows that
isostasy exists in the Himalayas.The order of the under compensation in outer
Himalayas is about 10% and that of over compensation over the Gangetic plains
by 40%.The process of erosion and deposition tends to maintain isostatic
equilibrium in the region as a whole.Large positive anomalies in the lesser
Himalayas and excessive thickness of the crust in the belt are believed to be
due to injection of the heavy sub-crustal material or thickening of the sima which is authenticated by the
discovery of basic material in the deep holes in the Ganga basin and Schwarderski’s density anomaly compensation from
satellite data.The array of the high peaks in the Himalaya with the intervening
deep gorges are result of the isostatic uplift following de-loading associated
with erosion of the vast amounts of sediments from the gorges.Other evidences
of isostatic uplift includes – frequent river terraces in the
Himalayas,earthquakes and growing esication of the Tibetan lakes.Measurements made
by the National Geographic Society (Washington) using G.P.S. on 5th March,2000
found that the height of Mt. Everest to be 8850m.Himalaya,thus,stands an
exception to the principles of isostasy.
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