SEMINAR
REVIEW
by Carol Tresa of Std. IX B
TOPIC: PLATE
TECTONICS
Date: 7th July 2012
Resource Person:
Mr.Unnikkrishnan Warrier
Scientist, CWRDM
WANDERING
CONTINENTS
On
July 07th, Mr. Unnikrishnan Warrier, a scientist from CWRDM,
conducted a class on Plate Tectonics. The class started with the Continental
Drift Hypothesis of 1970 by Dr. Alfred Wegenar who was a German Climatologist
and Geophysician. He wrote a book called “Origin of Continents and Oceans” in
1915 which by 1970 had evolved into a revolution in Geosciences called - ‘The
Theory of Plate Tectonics’.
A Theory has evidence while a hypothesis is
proved to only some extent. The class was about how a hypothesis became a Theory.
The Hypothesis stated that about 200 million years ago, a super continent called
PANGAEA, Latin for all Lands, started breaking and drifting to different positions.
The ocean that was there was Panthalassa which named after the Greek God of the
sea. The first sea that formed after the division was Tethys.
A
great debate took place then. Wegenar said that there was similarity between
the coastlines on opposite sides of the South Atlantic. But it was put down by
the fact that shorelines were modified by erosion. Another point was that
fossils of animals such as Mesosaurus, Lystrosaurus and Glossopteris were found
in places that had so many kilometers of sea in between the land that these
animals could not have travelled long distances. This was opposed by the fact that
there were lots of sea channels through which animals could have migrated.
Wegenar
believed that the centripetal force caused the continents to move away, but he
was not able to answer why they did not move in the same direction as they
should and also why the radius of the earth remained same even after a lot of
shifting. Thus, it remained a hypothesis.
Some
people continued to prove the hypothesis. They found that the Atlantic Ocean
had a ridge in the middle of the ocean floor where there was a lot of volcanic
activity. They found that the age of rock increased as they moved
towards the coastlines from the ridge. They found that the seafloor spreaded
out and named it sea floor spreading. But why it happened was not answered.
Interior of the earth consists of crust, mantle, outer core and inner core. The
density is different for all these layers. Continents have a composition – SIAL
(Silica and Aluminium), while oceans have SIMA (Silica and Magnesium). When
certain waves were passed through the interior of the earth, it was found that
the upper mantle is like a paste and also elastic.Thus, there came the
Lithosphere (crust and upper mantle) and Asthinosphere (lower mantle). It was
also found that the earth was made up of different plates that consisted of
both solid crust and ocean floor. Plates are divided as Major (Eurasian plate),
and Minor (Arabian plate) and Micro (Madagascar) based on their size. In a
major plate, there may be many minor and micro plates. Movement in a plate is
the activity along the plate borders like:
1. Convergent or
destructive boundary – the area where twp plates come close and collide, where
crust is destroyed and islands form. Eg:
Pacific Ocean where there are lots of
islands.
2. Divergent
or constructive boundary – the area where two plates move apart and mid-ocean
ridges form. This was misunderstood as ‘ocean floor spreading’. Eg:
- Atlantic Ocean where there are no
islands.
3. Transform
or conservative boundary – the area where two plates slide past in opposite
directions to form transform faults. Eg: -
San Andrea’s fault.
It was
also found that equal amount of earth’s crust is destroyed and constructed, so
the radius remains same. Plumes are the cyclic motions inside the earth that
carry the plates. This is how the Himalayas were formed. They are young
mountains. Indian plate was taken to the North North-East and collided with the
Eurasian plate. The Tethys Sea was elevated and its sediments formed the
Himalayas. Tethys was deeper in the North, so Himalayan Mountains are highest
in the northern side. This can be proved because fossils of sea animals and
limestone- which only forms in water- are found in the Himalayas. It is still
building higher by 1cm/year. The plateau was formed due to an outbreak of the
plume that carried the Indian plate. The Gangetic plains were formed due to
weathering of sediments on the mountains. The Tethys sea water went South
South-West to Pakistan. It is said that Tethys sea water flowed as the River
Saraswati, but it is having lot of controversies. We saw how the continents
have moved over different periods like Triassic period, Jurassic period and
Cretaceous period.
Earthquake
is a release of energy. The circum Pacific belt is prone to earthquakes.
Tsunami is the after effect of a sub-marine earthquake. It is called ‘Harbour
waves’ in Japan as it does not affect any ships that are sailing in mid-ocean.
It affects ships in the harbours. Tsunami waves have ‘crust’ and ‘truff’. When
a crust hits the coast first, there is huge increase in water level without
notice. When a truff comes first, the sea will withdraw and hit back, a lot
stronger.
The
factors to find out if two continents were together are geographical
continuity, ethological continuity, structural continuity and chronological
continuity. Earthquakes have compressional waves, i.e. primary waves that are a
very big threat to buildings, and secondary waves. Buildings are sometimes
placed on ‘concrete balls’. That is an engineering technique to prevent damages
due to earthquakes. The requirements for emergencies are:
* Co-operation
* Proper
civilian defense
* Mutual
understanding
* Infrastructural
development
* Practice
We must
also have general knowledge as on what to do in time of an earthquake. We must
be in an open space during an earthquake. We must also have everything like
cupboards and clocks attached to the walls. We also learnt that local time is
very important as its absence will cause less usage of man power.
