Download Continental Drift Theory of Alfred Wegener

Continental Drift Theory of Alfred Wegener
Continental Drift Theory of Alfred Wegener
Continental drift is the movement of the Earth's continents relative to each other by
appearing to drift across the ocean bed. The speculation that continents might have 'drifted'
was first put forward by Abraham Ortelius in 1596. The concept was independently and more
fully developed by Alfred Wegener in 1912, but his theory was rejected by because the lack
of a mechanism (though this was supplied later by Holmes) and others because of prior
theoretical commitments. The idea of continental drift has been subsumed by the theory of
plate tectonics, which explains how the continents move.
The hypothesis that the continents had once formed a single landmass before breaking apart
and drifting to their present locations was first presented by Alfred Wegener to the German
Geological Society on 6 January 1912. Although Wegener's theory was formed
independently and was more complete than those of his predecessors, Wegener later credited
a number of past authors with similar ideas: Franklin Coxworthy (between 1848 and 1890),
Roberto Mantovani (between 1889 and 1909), William Henry Pickering (1907) and Frank
Bursley Taylor (1908). Eduard Suess had proposed a supercontinent Gondwana in 1858 and
the Tethys Ocean in 1893, from a sunken land-bridge/ geosyncline theory point-of-view.
Wegener was the first to use the phrase "continental drift" (1912, 1915) (in German "die
Verschiebung der Kontinente" – translated into English in 1922) and formally publish the
hypothesis that the continents had somehow "drifted" apart. Although he presented much
evidence for continental drift, he was unable to provide a convincing explanation for the
physical processes which might have caused this drift, and suggests two possibilities;
i.
If the continents remained stationary at their places throughout geological history of
the earth, the climatic zones might have shifted thus a particular region might have
experienced varying climatic conditions from time to time.
ii.
If the climatic zone remaining stationary, the landmasses might have been displaced
and drifted.
He opted the 2nd alternative as he rejected the view of the permanency of the continents and
ocean basins. Thus, the main objective behind this theory was to explain the global climatic
changes which have taken place during the past of the earth.
He postulated that originally there existed one big landmass (Fig. 1.4) which he called
PANGAEA which was covered by a big ocean called PANTHALASSA. A sea called
TETHYS divided the PANGAEA into two huge landmasses, LAURESSIA (North part) and
Lecture Delivered By: Tariq Ahmad Andrabi
Continental Drift Theory of Alfred Wegener
GONGWANA (South part). The landmasses consisted of SAIL (lighter) crust, while the
ocean had a SIMATIC (heavier) base.
Fig. 1.4: Drift of the continents
According to Wegener, the drift started around 200 million years ago (Mesozoic Era), and the
continents began to break up and drift away from one another. The drift was in two
directions:
1. Equator wards due to gravity and buoyancy
2. Westwards due to tidal currents because of the earth’s motion. As per Wegener, the
drift is still continuing.
Evidences in support of the Theory
1. Jig Saw Fit: - South America and Africa seems to fit in the each other, especially, the
bulge of Brazil fits into the Gulf of Guinea. Greenland seems to be fit with Ellesmere
and Baffin Island. The east coast of the India Madagascar and Africa seems to have
been joined. North and South America on one side and Africa and Europe on the
other, fit along the mid –Atlantic ridge. The Caledonian and Hercynian of USA
seemto be one continuous series. Sierra de Tendillmountains (S. America) and Cape
Mountains (S. Africa) seem to exhibit a similar tendency.
Lecture Delivered By: Tariq Ahmad Andrabi
Continental Drift Theory of Alfred Wegener
2. Causes of Drift:-Gravity of the earth, buoyancy of the seas and the tidal currents
were given as the main factors causing the drift.
3. Interplay of the Sima and Sial:- the lighter sial was carried by heavier sima, thus
creating cordillera formation on western edges of North America. These formations
are folded and warped.
4. Shifting of Poles:-originally, the North Pole was positioned at 107 degree west. 3
degree N (SW of California) and has now shifted in a north-westerly direction. The
south pole on the other hand was situated where present day South Africa (Natal
coast) is, in the middle of Pangaea as shown in table 1.2.
Table 1.2: Position of North Pole in Geological history
S. No
Time (Million Years)
Geological Period
Position of North Pole
01
600
Pre-Cambrian
3N, 107W
02
350
Silurian
14N, 124W
03
285
Carboniferous
16N, 147W
04
70
Tertiary
51N,153W
Fig. 1.5: Fossil patterns across continents (Gondwanaland)
5. Presence of ice Sheets:-Evidences of such ice cover dated back to the carboniferous
period in the Falkland and other places in the southern hemisphere suggests that they
were closer once.
Lecture Delivered By: Tariq Ahmad Andrabi
Continental Drift Theory of Alfred Wegener
6. Botanical Evidences: Fig 1.5 shows the presence of glossopteris vegetation in
carboniferous rocks of India, Australia, South Africa, Falkland Island, Antarctica etc
can be explained on the basis of the fact that these parts were linked in the past.
7. Palaeomagnetism: The magnetic field of the earth has magnetised many iron-based
rocks in the past. The study of the old magnetic rock show different directions of
magnetism.
Criticism
1. Coastal areas are temporary features and are liable to change due to erosional work
and are not reliable as a basis for attempting this type of analysis of landforms. The
Niger delta, for instance, on west coast of Africa could not have been formed if Africa
and south America were one. For a delta to be formed there, the two continents must
be separated by 200 KM and it would take 50 million years for this process.
2. The various factors mentioned by Wegener, for these factors to be able to cause a drift
of such a magnitude, they will have to be millions of time stronger.
3. If sima was so rigid, it could not have drifted away and if it was light and mobile,
mountain building could not have occurred.
4. Poles may have shifted, not necessarily the continents.
5. Glossopteris vegetation is also found in the northern parts of Afghanistan, Iran and
Siberia.
6. Wegener ignored the time factor and took a two dimensional view of the subject,
trying to solve the mystery like JIG SAW FIT Puzzel. He failed to explain why this
drift began only in the Mesozoic era and not in the period of prior to it.
Lecture Delivered By: Tariq Ahmad Andrabi