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Environment
Tsunamis: The
Movers and Shakers
by VK
Joshi
We were taken
unawares when the tsunami struck the east coast of India 26th December
2004. It was hard to imagine for the people that a tsunami could inflict
so much of damage! Those who studied the earth science had read about
the term, though it was never taught or studied seriously by the
students in India, including the author. Since no tsunami had struck the
Indian coast in the recent memory, no one took it seriously.
The post tsunami scare has considerably changed the scenario. More and
more geo-scientists are coming up with the findings of paleo-tsunami
deposits or other evidences suggesting mega-tsunamis. The quantum of
rocks/sediments moved and the distances sometimes they have traveled
indicate the might of the past disasters when a tsunami acted as a mover
and shaker.
The tsunami that hit the Andhra Pradesh, Tamilnadu and Kerala coasts was
an aftermath of an earthquake of magnitude 9.3 that originated off the
western Sumatra coast. The United States Geological Survey (USGS),
ranked it as the fifth largest earthquake in the world in the past 40
years (Read 'When
Death Danced on Sea Coast').
We already know that how deadly an earthquake generated tsunami could
be! The last tsunami that hit the eastern coast of Indian peninsula took
a toll of 16000 lives. In view of the loss of lives and properties
further research on tsunami became imperative. Somewhat like an
earthquake a tsunami can not be predicted, but yes if warning systems
are in place at times it is possible to forewarn the far flung coastal
populations to move to safer areas. But this is possible only when the
tsunami is generated by an undersea earthquake or a consequential
landslide.
In case the
tsunami is generated by some other mechanism then? This is a question
that is bugging the experts these days. The two recent reports are
presented here to highlight the point.
N.C. Mehrotra a renowned paleobotanist, (presently Director Birbal Sahni
Institute of Palaeobotany, Lucknow) along with his coauthors reported
reworked spores and pollen of land plants from the drill cores recovered
from the depths of ocean in Bombay offshore. The rocks (Panna Formation)
that contained these pollens and spores were 65 to 33.7 million years
old, while the flora recovered was much older (Carboniferous Era). The
report (2001) raised curiosity, because during the Carboniferous Era
(354 to 298 million years ago) when the micro-continents of Asia were
still a part of Australia and Gondwanaland and as such the only
Carboniferous pollen and spores that are known from the sub-continent
are marine and from the Himalayas. The fossil spores and pollens
recovered from the drill cores did not show any affinity with the marine
flora.
The reworked fossils as such are of no use to the palaeontologists for
the purpose of establishing the age or palaeogeography of the era in
which they were deposited. But in conditions like the present one, they
gained significance as there was no nearby source from where they could
have landed there. Then from where they were transported and how? One
explanation could be that these pollens and spores were present in the
sediments which now lie buried under a huge pile of lava flows or Deccan
Volcanic rocks, which constitutes a major part of the Indian Peninsula.
The sediments with carboniferous flora were eroded, transported and
deposited in the area now covered under the Arabian Sea off shore from
Mumbai. But there is no evidence so far of the presence of Carboniferous
sediments under the Deccan Volcanic rocks. Or they might have come from
Carboniferous sediments of Africa-Arabia by waves, says Arun Kumar of
Centre For Petroleum and Minerals Research Institute, King Fahad
University of Petroleum and Minerals, Saudi Arabia, in his paper
(Current Science, June 2008).
Well sea waves normally do not transport materials for such large
distances. But yes in case of tsunamis the waves are known to move an
assortment of materials over long distances. Of course generation of
such a tsunami which could carry sediments over long distance also needs
a viable explanation. Here the story takes an interesting turn.
We all know
that meteors hit our planet regularly. Sometimes when their size gets
larger we call it a meteorite. But if the size of the meteorite becomes
several kilometers in diameter we term it a bolide. Fortunately there is
no record of a bolide hitting our earth since we came in to being. But
some 65 million years ago one such bolide supposedly around 40km
diameter hit the western continental shelf of India. A large crater
around 40 km diameter was formed, termed as the 'Shiva Crater'.
This catastrophic event generated mega-tsunamis that raced around the
globe and severally impacted the coastlines of western India, east
African and southern Arabian Peninsula. Some of the tsunamis traveled
northwest wards and impacted the Arabian continental shelf and moved on
land till they hit the eastern and western margins of the Arabian
shield.
Such gigantic waves can erode large tracts of existing rocks and that is
exactly what they did. Carboniferous strata of Arabia containing
terrestrial plant pollen and spore fossils were eroded and the flora was
deposited with the sediments of Panna Formation in the Mumbai offshore
basin.
Quoting two Japanese Tsunami experts K. Ida and Iwasaki, (1983) Arun
Kumar says in his paper that tsunamis in the open ocean are not high,
but on approaching the coast they gain height. With the decrease in the
depth of water their wavelength and velocity also decreases, but the
wave energy remains nearly constant. It is the level of wave energy that
determines the degree of damage on the coast or the coral reefs and also
the rate of erosion of sediments from the coast. Often the old marshes
and peat get covered under a thin layer of tsunami sand. Thus coastal
erosion, re-deposition, and deformation of coastal sediments are other
important impacts of tsunamis.
The mega tsunami that caused deposition of terrestrial pollen and spores
in Panna Formation occurred some 65 million years ago. That is during
the transition from Cretaceous to Tertiary Era of the earth's history.
One must understand the paleogeography of the Indian continent. Those
days India was a floating Island in the Indian Ocean, located east of
present day East Africa and approximately 1500 to 2000 km southeast of
the Arabian Peninsula.
The Shiva bolide impact created giant tsunamis that traveled in circular
waves in all directions. Amongst them the mega-tsunamis traveling
northwestward hit the coast of the Arabian Peninsula and the gigantic
waves moved further inland till they were reflected back by the rigid,
high crystalline rocks of the southern and eastern margins of the
Arabian Shield. Those reflected southeastwards came back to their place
of origin, i.e. Mumbai offshore region. The mega tsunami waves as they
hit the Arabian coast carried out intensive erosion and on their return
journey brought the Carboniferous sediments and associated pollen and
spore fossils and deposited them in the sediments of Panna Formation.
Well the transport of sediments with terrestrial pollen and spores took
place millions of years ago. The evidences to prove that it was due to a
tsunami are circumstantial. 'Nature' reported in October 2008
about another tsunami that hit the same region some 600 years ago. Teams
of researchers found sheets of sands that were rushed inland and dropped
by a tsunami in Thailand. Quoting Katrin Monecke a researcher from
Pittsburg the report says similar sand sheets were deposited by 2004
tsunami as well.
These findings have lots of meaning for the planners of the coastal
region. When a tsunami is the past can scour ad erode piles of sediments
as far as Arabia and dump it in the Indian Ocean in Mumbai basin, who
can stop another tsunami in future from repeating the feat! That is why
as per the coastal zone regulations there are restrictions on
constructing permanent or temporary dwellings close to the coast line.
As per that no dwelling should come up within a distance of 500 meters.
It is unfortunate that often these regulations are flouted. If a past
event is repeated then the results would be disastrous.
January 25,
2009
Image: 1. Tsunami wrecked
ships
Image: 2. Tsunami did not spare even concrete buildings at the coast.
(Images curtsey Geological Survey Of India)
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