ORGIN OF EARTH
Two sets of theories about origin or earth:
a. Catastrophic
b. Evolutionary
the catastrophic theories have hardly any evidence
uniformitarianism supports evolutionary theories
Nebular theory – Laplace
Dust cloud Hypothesis – Weizsaker
Nebular Hypothesis suggests that the matter which forms the Sun and the
Planets originated as a disc shaped cloud of gas or nebula which eventually
contracted into discrete bodies.
STRUCTURE OF THE EARTH
Earth has a layered structure
• Core
• Mantle
• Crust
The Nature of Seismic Waves
1. The velocity of seismic waves depends on the density and elasticity of the
intervening material. Seismic waves travel most rapidly in rigid materials.
8
2. Within a given layer, the speed of seismic waves generally increases with
depth because pressure increases and squeezes the rock into a more compact
elastic material
3. Compressional waves (P waves) travel through solids as well as liquids,
because when compressed, these materials behave elastically. Shear waves
(S waves ) cannot travel through liquids because, unlike solids, liquids have
no shear strength.
4. In all materials P waves travel faster than S waves
5. When seismic waves pass from one material to another, Based upon the
seismological data, the earth has been divided into various layers.
CORE:
• 1/3rd of mass
• 1/6th of the earth’s volume
• Pressure: millions of times that of atmospheric pressure at the surface.
• Temperature 4000oC - 5000oC
• Relative Density at the center 13.5 relative density.
INNER CORE
• Solid
• Predominantly Fe; also Nickel
OUTER CORE
It is in liquid form.
1. Addition of lighter elements, which when mixed with iron, lower its melting
point.
2. In outer core, the pressure is comparatively lesser, to allow the hot iron to
melt.
• There is a gradual flow of molten iron in the outer core and is very important
for maintaining earth’s magnetism.
9
• The core behaves like a self sustaining dynamo
• The driving forces- Earth’s rotation and unequal distribution of heat in the
earth’s interiors
The existence of a liquid outer core when the inner core, which must be hotter, is
solid. Most probably in its formative stage the entire core was liquid. Further, this
liquid iron alloy was in a state of vigorous mixing . However, during the last 3.5
billion year’s the material of the core has been slowly segregating. As the core
cooled, a portion of the iron components gradually migrated downward while some
of the lighter components floated upward toward the outer edge of the core. The
sinking iron rich components, depleted of the lighter elements which act to depress
the melting point, began to solidity.
MANTLE
• 80% of the earth’s volume is contained within the mantle.
• Mantle is described as a solid rocky layer, and the most common rock is
peridotite
• Peridotite – ultra basic rock, consisting largely of olivine, hence its
predominantly dark green colour (olivine – silicate of magnesium mg2SiO4
to silicates of iron Fe2SiO4).
• The crust increases its temperature with depth, but this trend does not
continue downward into the mantle.
• This means mantle has an effective method to transmit heat outward i.e.,
some form of convection.
• Material in this zone exhibit plastic behaviour, i.e., when the material
encounters short lived stresses, such as seismic waves, the material behaves
like an elastic solid. However, in response to long term stresses, this same
rocky material will flow.
10
• So S waves can penetrate through mantle, yet, this layer is not able to store
elastic energy like a brittle solid and is thus incapable of generating
earthquakes
ASTHENOSPHERE
• Asthenosphere is located between 100 to 400 kms.
• P and S waves show a marked decrease in velocity one. The most probable
explanation for the observed slowing of seismic energy is that this zone
contains a small percentage of melt.
• But Asthenosphere is not continuous and is absent below the older shield
areas.
The asthenospher is the layer of Earth that lies at a depth 100 – 400 km beneath
Earth’s surface.
It was first named in 1914 by the British geologist J. Barrel, who divided
Earth’s overall structure into three major sections: the lithosphere, or outer layer of
rock like material; the asthenosphere; and the centrosphere, or central part of the
planet.
The asthenosphere gets its name from the Greek world for weak, asthenis,
because of the relatively fragile nature of the materials of which it is made. It lies
in the upper portion of Earth’s structure traditionally known as the mantle.
The material of which the asthenosphere is composed can be described as
plastic-like, with much less rigidity than the lithosphere above it. This property is
caused by the interaction of temperature and pressure on asthenospheric materials.
Any rock will melt if its temperature is raised to a high enough temperature.
However, the melting point of any rock is also a function of the pressure exerted
on the rock. In general, as the pressure is increased on a material, its melting point
increases.
The temperature of the materials that makeup the asthenosphere tends to be
just below their melting point. This gives them a plastic-like quality that can be
11
compared to glass. As the temperature of the material increases or as the pressure
exerted on the material increases, the material tends to deform and flow. If the
pressure on the material is sharply reduced, so will be its melting point, and the
material may begin to melt quickly. The fragile melting point pressure balance in
the asthenosphere is reflected in the estimate made by some geologists that up to
10% of the asthenospheric material may actually be molten. The rest is so close to
being molten that relatively modest changes in pressure or temperature may cause
further melting.
In addition to loss of pressure on the asthenosphere, another factor that can
bring about melting is an increase in temperature. The asthenosphere is heated by
contact with hot materials that make up the rest of the mantle beneath it.
In order for plate tectonic theory to seem sensible, some mechanism must be
available for permitting the flow of plate. That mechanism is the semi-fluid
character of the asthenosphere itself. Some observers have described the
asthenosphere as the lubricating oil that permits the movement of plates in the
lithosphere.
INNER OUTER MANTLE
• After about 400 kms, the velocity of seismic waves increases as a result of
phase change.
• A phase change occurs when the crystalline structure of a mineral changes in
response to change in temperature and pressure.
• The mineral olivine (Mg,Fe)SiO4 , which is one of the main constituents of
the rock peridotite, will collapse to a more compact high pressure mineral –
spinel.
• This structural change to a denser crystal form could explain the increased
seismic velocities observed.
12
INNER MANTLE
• Another boundary at a depth of around 700 kms because, mineral Spinel,
undergoes transformation to the mineral Perovskite (Mg,Fe)SiO4 ,
• Since Perovskite is the predominant mineral of lower mantle, it is the most
abundant mineral in the earth.
LITHOSPHERE
• Situated above the asthenosphere is the cool brittle layer about 100km thick
called the lithosphere
• Lithosphere included the entire crust as well as the uppermost mantle and is
defined as the layer of the earth cool enough to behave like a brittle solid.
• But it is not a single layer. A density discontinuity is there in the lithosphere.
It is broken/fractured along several lines. The different segments are known
as plates.
• So plates are essentially lithospheric plates are capable of sliding or, moving
over the plastic asthenosphere. Thus plates move from one point to another.
THE EARTH’S MAGNETIC FIELD
Anyone who has used a compass to find direction knows that the earth’s
magnetic field has a north pole and a south pole. In many respects our planet’s
magnetic field resembles that produced by a simple bar magnet. Invisible lines
of force pass through the earth and out into space while extending from one
pole to the other. A compass needle, itself a small magnet free to move about,
becomes aligned with these lines of force and points toward the magnetic poles.
It should be noted that the earth’s magnetic poles do not coincide exactly with
the geographic poles. The north magnetic pole is located in northeastern
Canada, near Hudson Bay, while the south magnetic pole is located near
Antarctica in the Indian Ocean south of Australia.
13
CONTINETAL DRIFT
Early in the 20th century Geologic thought was dominated by a belief in the
antiquity and Geographic permanence of oceans and continents, mountains were
thought to result from Earth’s cooling and contraction and were compared to the
wrinkles on a dried out piece of fruit, changes in sea level and occurrence of fossils
at depth were explained using the contraction model. Dramatic changes have taken
place over the past few decades.
Earth Scientists now realize the non-permanence of landmasses and ocean
basins, creation and continued destruction of crust. This profound reversal of
scientific opinion was termed as scientific revolution. An appreciable length of
time elapsed between the inception of the idea and its general acceptance. After
heated debates the idea of drifting continents was rejected, only to be resurrected
during the 1960’s.
HISTORICAL DEVELOPMENTS/LANDMARKS IN THE
REVERSAL OF THE SCIENTIFIC OPINION
Continental Drift:
An idea before its time
Although modern plate tectonics became an acceptable scientific theory
within only the past few decades, the concept of breaking up of an early super
continent into fragments that drifted apart is many decades old. Almost as soon as
good navigational charts became available to show the continental outlines,
persons of learning became intrigued with the close correspondence in outline
between the eastern coast line of South America and the Western coastline of
Africa.
As early as 1668 a Frenchman interpreted the matching coastlines as proof
that the two continents became separated during the biblical flood.
In 1858 Antoni-Snider-Pelligrini produced a map to show that the American
continents nested closely against Africa and Eurasia, and also suggested that the
14
reconstructed single continent explains the close similarity of fossil plant type in
coal bearing rocks in both Europe and N.America.
In 1910 two American Geologist, Frank B. Taylor and Howard B. Baker,
whose published articles presented evidence favouring the hypothesis that the New
World and Old World continents had drifted apart.
Nevertheless, credit for a full-scale hypothesis of continental drift goes to a
German Scientist, Alfred Wegner, a Meteorologist and Geophysicist who worked
on several lines of geologic evidence to prove that the continents had once been
united.
He first presented his ideas in a lecture in 1912, the expanded version of
which was published in his book “ The Origin of continents & Oceans” in 1915.
His Major work on the subject appeared in 1922 and his work was translated
into English in 1924.
EVIDENCE OR ARGUMENTS TO SUPPORT WEGNER’S
HYPTHESIS
1.Fit of the Continents:
There is Striking/Remarkable similarity / parallelism between the opposing
coasts of the Atlantic and (the mid Atlantic Ridge*), India-Africa-Madagascar,
Greenland, Baffinland etc and several other coasts in the world. If they are drawn
together they make a rough zig saw fit.
However his use of present day shorelines to make a fit of the continents
was challenged immediately by earth scientists. The opponents correctly argued
that shorelines, which are dynamic and are being continuously modified by
erosional processes, upliftment & subsidence are a temporary phenomenon, the use
of which, to prove the existence of Pangea and its break-up would be untenable.
A much better approximation of the outer boundary of the continents is the
seaward margin of the continental shelf. In early 1960s Sir Edward Bullard and his
two associates, with the aid of computers produced a remarkable fit of continents
15
using the 900 meters isobaths. The fit of better than what even the supporters of the
continental drift theory suspected it would be. The continents overlap in a few
places, these are places where streams have deposited large quantities of
sediments, thus enlarging the continents.
2.Fossil evidences:
To add credibility to his argument for (the existence of ) Pangea, Wegner
used the already documented evidence regarding the existence of strikingly
identical fossils, particularly of Mesozoic life forms, on the widely separated
landmasses.
Fossil fern GLOSSOPTERIS was known to be widely dispersed in the
Southern continents of Africa, Australia, and S. America during the Mesozoic era.
Later the remains were also discovered in Antarcitica.
Fossils of Mesosaurus (reptiles-capable of swimming in shallow waters)
were found in eastern S. America and West Africa.
Distribution of modern day species with common ancestries-the Australian
Marsupials have a direct fossil link to the marsupilal Opposum found in the
Americas. For Wegner fossils provided undeniable proof that the landmasses were
joined together as the super continent Pangea.
Most Paleontologists were in agreement that some type of land connection
existed. Land Bridge hypothesis conjectured the existence of some land bridge
between Africa and S.America. We are now quite certain that land bridges of this
magnitude did sea level, but are nowhere to be found.
Wegner’s proposals were quickly countered using the logic of parallel
evolution-but in the process of evolution there are scores of random factors-how
could then the evolution be strikingly so similar? Land seeds and shells are
transported for thousands of kilometers across oceans. Floral similarities could be
explained by this mechanism.
16
3.Back type and structural similarities:
Anyone who has worked a picture puzzle knows that in addition to the
pieces fitting together, the picture must be continuous as well. The picture that
must match in the “Continental drift Puzzle” is represented by rock types and
mountain belts found on the continents. If continents were once together, the
adjacent rocks should match in age and type. Wegner made a good correlation
between rocks found in N, W Africa and Eastern Brazil.
Recent re-examination of this early evidence has supported Wegner’s claim.
In both regions, 550 million years-old rocks lie adjacent to rocks dated at more
than 2 billion years in such a manner that the line separating them is continuous
when the two continents are brought together.
Several mountainous belts which appear to terminate at one coastline only to
reappear again on a landmass across the ocean. For instance, the mountain belt that
includes the Appalachians trends northeastwards through the eastern U.S.A and
disappears off the coast of Newfoundland. Mountains of comparable age and
structure and found in Greenland and Northern Europe. When these landmasses are
reassembled, the mountain Chains form a nearly continuous belt.
Numerous other rock structures exist that appear to have formed at the same
time and were subsequently split apart.
In the words of Wegner “It is just as if we were to refit the torn pieces of a
newspaper by matching their edges and then check whether the lines of print run
smoothly across. If they do, there is nothing left to conclude except that the pieces
were in fact joined this way”.
4.Paleoclimatic evidences:
Since Wegner was a Climatologist by training, he was keenly interested in
obtaining pale climatic data in support of the continental drift. His efforts were
rewarded when he found evidence for apparently dramatic climatic changes.
17
Glacial deposits indicated that near the end of the Palezoic era(220-300
million year), ice sheets covered extensive areas of S. Hemisphere, layers of
Glacial till were found at the same stratigraphic position in southern Africa,
S.America, India and Australia. Much of the evidences of late Paleozoic glaciation
come from land areas which presently lie within 30o of equator in a subtropical –
tropical climate.
Could the earth have gone through a period sufficiently cold to have
generated extensive continental glacier in what is presently a tropical region.
Wegner out rightly rejected this explanation because during the same period large
tropical swamps existed in the northern Hemisphere. These swamps with their lush
vegetation eventually became the major Coal Fields of the eastern United States,
Europe and Siberia. How could these two situations co-exist?
As Wegner proposed, a better explanation is provided if the landmasses are
fitted together as a super continent and moved nearer to the South Pole. This would
account for the conditions necessary to generate extensive expanses of glacial ice
over much of the Southern Hemisphere. At the same time this shift would place the
Northern landmasses nearer the tropics and account for their vast coal deposits.
In spite of compelling evidences as these – it took almost 50years for the
scientific community to accept the idea of continental drift and the logical
conclusions to which it led.
THE GREATE DEBATE
Wegner’s proposal did not attract much open criticism until 1924 when his
book was translated into English. From this time on, until his death in 1930, his
drift hypothesis encountered a great deal of hostile criticism.
To quote the American Geologist Chamberlin “Wegner’s hypothesis in
general is of the foot-loose type, in that it takes considerable liberty with our globe
and is less bound by restrictions or tied down by awkward, ugly facts than most of
its rival theories. Its appeal seems to lie in the fact that it plays a game in which
18
there are few restrictive rules and no sharply drawn code of conduct”. SCOTT
described in much fewer worlds, the theory as “Utter Damned Rot!”
Objections:
One of the main objections to Wegner’s hypothesis stemmed from his
inability to provide a driving mechanism for continental drift. Wegner proposed
two possible energy sources for drift.
1. The Tidal force/influence of the moon was presumed by Wegner to be
strong enough to give the continents a westward motion. However, the
prominent Physicist Harold Jeffrey’s quickly countered with the argument
that the tidal friction of the magnitude needed to displace the continents
would bring the earth’s rotation to a halt in a matter of few years.
2. Further Wegner proposed that the larger and sturdier continents broke
through the oceanic crust, much like icebreakers cut through ice.
However no evidence existed to suggest that the ocean floor was weak enough
to permit passage of the continents without themselves being appreciably
deformed in the process.
Despite criticisms from all corners he wrote the fourth and final edition of
his book in 1919, maintaining his basic hypothesis and adding supporting
evidence.
1930, Wegner made his 3rd and final trip to the Greenland ice sheet to test
his hypothesis by precisely establishing the locations of specific points and
measuring the changes over a period of years, he could demonstrate the
westward drift of Greenland with respect to Europe. In November, while
returning from Eismitte (an experimental station) Wegner perished. His
intriguing idea, however, did not die with him.
Although the hypothesis was correct in principle, it also contained many
incorrect details. The hypothesis was criticized largely on grounds of its
inability to suggest a satisfactory means of engineering continental movements.
19
It must be conceded that the cumulative evidence supporting the theory of
continental drift was massive and the theory was very attractive. If all the points
in the theory could be established and an adequate motive force discovered then
as Professor SHAND has said”.
“We would have to credit (prof.) Wegner with the greatest piece of
geological synthesis that has ever been accomplished”(1933).
Wegner himself in response to his critics said “Scientists still do not appear
to understand sufficiently that all earth Sciences must contribute evidence
towards unraveling the state of our planet in earlier times, and the truth of the
matter can only be reached by combining all this evidence”.
‘As one derides the past theories in the light of newly discovered Physical
facts, those theories which are deemed as modern, may be mocked at in the
future after the discovery of newer facts.’
Although most of Wegner’s contemporaries opposed his views, even to the
point of openly ridiculing him, a few considered his ideas plausible. Amongst
the noted supporter were.
Arthur Holmes (1928) contributed to the cause by proposing a plausible driving
mechanism for continental drift. Since the time he first proposed, he kept on
modifying his hypothesis and in his book Physical Geology (1966) he suggested
that convection currents operating within the Mantle were responsible for
propelling the continents across the globe. Although, even to this day geologists
are not in agreement on the exact nature of the driving mechanism, the concept
proposed by Holmes is still one of the most appealing.
Wegner also in 1929 suggested thermal convection currents. 1930 Death of
Wegner.
South Africa Geologist ALEXANDER DU TOIT (1937) published “Our
Wandering Continents” in which he eliminated some of Wegener’s errors and
added a great deal of new evidence in support of the drift idea.
20
Very little new light was shed on the continental drift hypothesis between
the time of Wegner’s death and the early 1950s.
Two sets of theories about origin or earth:
a. Catastrophic
b. Evolutionary
the catastrophic theories have hardly any evidence
uniformitarianism supports evolutionary theories
Nebular theory – Laplace
Dust cloud Hypothesis – Weizsaker
Nebular Hypothesis suggests that the matter which forms the Sun and the
Planets originated as a disc shaped cloud of gas or nebula which eventually
contracted into discrete bodies.
STRUCTURE OF THE EARTH
Earth has a layered structure
• Core
• Mantle
• Crust
The Nature of Seismic Waves
1. The velocity of seismic waves depends on the density and elasticity of the
intervening material. Seismic waves travel most rapidly in rigid materials.
8
2. Within a given layer, the speed of seismic waves generally increases with
depth because pressure increases and squeezes the rock into a more compact
elastic material
3. Compressional waves (P waves) travel through solids as well as liquids,
because when compressed, these materials behave elastically. Shear waves
(S waves ) cannot travel through liquids because, unlike solids, liquids have
no shear strength.
4. In all materials P waves travel faster than S waves
5. When seismic waves pass from one material to another, Based upon the
seismological data, the earth has been divided into various layers.
CORE:
• 1/3rd of mass
• 1/6th of the earth’s volume
• Pressure: millions of times that of atmospheric pressure at the surface.
• Temperature 4000oC - 5000oC
• Relative Density at the center 13.5 relative density.
INNER CORE
• Solid
• Predominantly Fe; also Nickel
OUTER CORE
It is in liquid form.
1. Addition of lighter elements, which when mixed with iron, lower its melting
point.
2. In outer core, the pressure is comparatively lesser, to allow the hot iron to
melt.
• There is a gradual flow of molten iron in the outer core and is very important
for maintaining earth’s magnetism.
9
• The core behaves like a self sustaining dynamo
• The driving forces- Earth’s rotation and unequal distribution of heat in the
earth’s interiors
The existence of a liquid outer core when the inner core, which must be hotter, is
solid. Most probably in its formative stage the entire core was liquid. Further, this
liquid iron alloy was in a state of vigorous mixing . However, during the last 3.5
billion year’s the material of the core has been slowly segregating. As the core
cooled, a portion of the iron components gradually migrated downward while some
of the lighter components floated upward toward the outer edge of the core. The
sinking iron rich components, depleted of the lighter elements which act to depress
the melting point, began to solidity.
MANTLE
• 80% of the earth’s volume is contained within the mantle.
• Mantle is described as a solid rocky layer, and the most common rock is
peridotite
• Peridotite – ultra basic rock, consisting largely of olivine, hence its
predominantly dark green colour (olivine – silicate of magnesium mg2SiO4
to silicates of iron Fe2SiO4).
• The crust increases its temperature with depth, but this trend does not
continue downward into the mantle.
• This means mantle has an effective method to transmit heat outward i.e.,
some form of convection.
• Material in this zone exhibit plastic behaviour, i.e., when the material
encounters short lived stresses, such as seismic waves, the material behaves
like an elastic solid. However, in response to long term stresses, this same
rocky material will flow.
10
• So S waves can penetrate through mantle, yet, this layer is not able to store
elastic energy like a brittle solid and is thus incapable of generating
earthquakes
ASTHENOSPHERE
• Asthenosphere is located between 100 to 400 kms.
• P and S waves show a marked decrease in velocity one. The most probable
explanation for the observed slowing of seismic energy is that this zone
contains a small percentage of melt.
• But Asthenosphere is not continuous and is absent below the older shield
areas.
The asthenospher is the layer of Earth that lies at a depth 100 – 400 km beneath
Earth’s surface.
It was first named in 1914 by the British geologist J. Barrel, who divided
Earth’s overall structure into three major sections: the lithosphere, or outer layer of
rock like material; the asthenosphere; and the centrosphere, or central part of the
planet.
The asthenosphere gets its name from the Greek world for weak, asthenis,
because of the relatively fragile nature of the materials of which it is made. It lies
in the upper portion of Earth’s structure traditionally known as the mantle.
The material of which the asthenosphere is composed can be described as
plastic-like, with much less rigidity than the lithosphere above it. This property is
caused by the interaction of temperature and pressure on asthenospheric materials.
Any rock will melt if its temperature is raised to a high enough temperature.
However, the melting point of any rock is also a function of the pressure exerted
on the rock. In general, as the pressure is increased on a material, its melting point
increases.
The temperature of the materials that makeup the asthenosphere tends to be
just below their melting point. This gives them a plastic-like quality that can be
11
compared to glass. As the temperature of the material increases or as the pressure
exerted on the material increases, the material tends to deform and flow. If the
pressure on the material is sharply reduced, so will be its melting point, and the
material may begin to melt quickly. The fragile melting point pressure balance in
the asthenosphere is reflected in the estimate made by some geologists that up to
10% of the asthenospheric material may actually be molten. The rest is so close to
being molten that relatively modest changes in pressure or temperature may cause
further melting.
In addition to loss of pressure on the asthenosphere, another factor that can
bring about melting is an increase in temperature. The asthenosphere is heated by
contact with hot materials that make up the rest of the mantle beneath it.
In order for plate tectonic theory to seem sensible, some mechanism must be
available for permitting the flow of plate. That mechanism is the semi-fluid
character of the asthenosphere itself. Some observers have described the
asthenosphere as the lubricating oil that permits the movement of plates in the
lithosphere.
INNER OUTER MANTLE
• After about 400 kms, the velocity of seismic waves increases as a result of
phase change.
• A phase change occurs when the crystalline structure of a mineral changes in
response to change in temperature and pressure.
• The mineral olivine (Mg,Fe)SiO4 , which is one of the main constituents of
the rock peridotite, will collapse to a more compact high pressure mineral –
spinel.
• This structural change to a denser crystal form could explain the increased
seismic velocities observed.
12
INNER MANTLE
• Another boundary at a depth of around 700 kms because, mineral Spinel,
undergoes transformation to the mineral Perovskite (Mg,Fe)SiO4 ,
• Since Perovskite is the predominant mineral of lower mantle, it is the most
abundant mineral in the earth.
LITHOSPHERE
• Situated above the asthenosphere is the cool brittle layer about 100km thick
called the lithosphere
• Lithosphere included the entire crust as well as the uppermost mantle and is
defined as the layer of the earth cool enough to behave like a brittle solid.
• But it is not a single layer. A density discontinuity is there in the lithosphere.
It is broken/fractured along several lines. The different segments are known
as plates.
• So plates are essentially lithospheric plates are capable of sliding or, moving
over the plastic asthenosphere. Thus plates move from one point to another.
THE EARTH’S MAGNETIC FIELD
Anyone who has used a compass to find direction knows that the earth’s
magnetic field has a north pole and a south pole. In many respects our planet’s
magnetic field resembles that produced by a simple bar magnet. Invisible lines
of force pass through the earth and out into space while extending from one
pole to the other. A compass needle, itself a small magnet free to move about,
becomes aligned with these lines of force and points toward the magnetic poles.
It should be noted that the earth’s magnetic poles do not coincide exactly with
the geographic poles. The north magnetic pole is located in northeastern
Canada, near Hudson Bay, while the south magnetic pole is located near
Antarctica in the Indian Ocean south of Australia.
13
CONTINETAL DRIFT
Early in the 20th century Geologic thought was dominated by a belief in the
antiquity and Geographic permanence of oceans and continents, mountains were
thought to result from Earth’s cooling and contraction and were compared to the
wrinkles on a dried out piece of fruit, changes in sea level and occurrence of fossils
at depth were explained using the contraction model. Dramatic changes have taken
place over the past few decades.
Earth Scientists now realize the non-permanence of landmasses and ocean
basins, creation and continued destruction of crust. This profound reversal of
scientific opinion was termed as scientific revolution. An appreciable length of
time elapsed between the inception of the idea and its general acceptance. After
heated debates the idea of drifting continents was rejected, only to be resurrected
during the 1960’s.
HISTORICAL DEVELOPMENTS/LANDMARKS IN THE
REVERSAL OF THE SCIENTIFIC OPINION
Continental Drift:
An idea before its time
Although modern plate tectonics became an acceptable scientific theory
within only the past few decades, the concept of breaking up of an early super
continent into fragments that drifted apart is many decades old. Almost as soon as
good navigational charts became available to show the continental outlines,
persons of learning became intrigued with the close correspondence in outline
between the eastern coast line of South America and the Western coastline of
Africa.
As early as 1668 a Frenchman interpreted the matching coastlines as proof
that the two continents became separated during the biblical flood.
In 1858 Antoni-Snider-Pelligrini produced a map to show that the American
continents nested closely against Africa and Eurasia, and also suggested that the
14
reconstructed single continent explains the close similarity of fossil plant type in
coal bearing rocks in both Europe and N.America.
In 1910 two American Geologist, Frank B. Taylor and Howard B. Baker,
whose published articles presented evidence favouring the hypothesis that the New
World and Old World continents had drifted apart.
Nevertheless, credit for a full-scale hypothesis of continental drift goes to a
German Scientist, Alfred Wegner, a Meteorologist and Geophysicist who worked
on several lines of geologic evidence to prove that the continents had once been
united.
He first presented his ideas in a lecture in 1912, the expanded version of
which was published in his book “ The Origin of continents & Oceans” in 1915.
His Major work on the subject appeared in 1922 and his work was translated
into English in 1924.
EVIDENCE OR ARGUMENTS TO SUPPORT WEGNER’S
HYPTHESIS
1.Fit of the Continents:
There is Striking/Remarkable similarity / parallelism between the opposing
coasts of the Atlantic and (the mid Atlantic Ridge*), India-Africa-Madagascar,
Greenland, Baffinland etc and several other coasts in the world. If they are drawn
together they make a rough zig saw fit.
However his use of present day shorelines to make a fit of the continents
was challenged immediately by earth scientists. The opponents correctly argued
that shorelines, which are dynamic and are being continuously modified by
erosional processes, upliftment & subsidence are a temporary phenomenon, the use
of which, to prove the existence of Pangea and its break-up would be untenable.
A much better approximation of the outer boundary of the continents is the
seaward margin of the continental shelf. In early 1960s Sir Edward Bullard and his
two associates, with the aid of computers produced a remarkable fit of continents
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using the 900 meters isobaths. The fit of better than what even the supporters of the
continental drift theory suspected it would be. The continents overlap in a few
places, these are places where streams have deposited large quantities of
sediments, thus enlarging the continents.
2.Fossil evidences:
To add credibility to his argument for (the existence of ) Pangea, Wegner
used the already documented evidence regarding the existence of strikingly
identical fossils, particularly of Mesozoic life forms, on the widely separated
landmasses.
Fossil fern GLOSSOPTERIS was known to be widely dispersed in the
Southern continents of Africa, Australia, and S. America during the Mesozoic era.
Later the remains were also discovered in Antarcitica.
Fossils of Mesosaurus (reptiles-capable of swimming in shallow waters)
were found in eastern S. America and West Africa.
Distribution of modern day species with common ancestries-the Australian
Marsupials have a direct fossil link to the marsupilal Opposum found in the
Americas. For Wegner fossils provided undeniable proof that the landmasses were
joined together as the super continent Pangea.
Most Paleontologists were in agreement that some type of land connection
existed. Land Bridge hypothesis conjectured the existence of some land bridge
between Africa and S.America. We are now quite certain that land bridges of this
magnitude did sea level, but are nowhere to be found.
Wegner’s proposals were quickly countered using the logic of parallel
evolution-but in the process of evolution there are scores of random factors-how
could then the evolution be strikingly so similar? Land seeds and shells are
transported for thousands of kilometers across oceans. Floral similarities could be
explained by this mechanism.
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3.Back type and structural similarities:
Anyone who has worked a picture puzzle knows that in addition to the
pieces fitting together, the picture must be continuous as well. The picture that
must match in the “Continental drift Puzzle” is represented by rock types and
mountain belts found on the continents. If continents were once together, the
adjacent rocks should match in age and type. Wegner made a good correlation
between rocks found in N, W Africa and Eastern Brazil.
Recent re-examination of this early evidence has supported Wegner’s claim.
In both regions, 550 million years-old rocks lie adjacent to rocks dated at more
than 2 billion years in such a manner that the line separating them is continuous
when the two continents are brought together.
Several mountainous belts which appear to terminate at one coastline only to
reappear again on a landmass across the ocean. For instance, the mountain belt that
includes the Appalachians trends northeastwards through the eastern U.S.A and
disappears off the coast of Newfoundland. Mountains of comparable age and
structure and found in Greenland and Northern Europe. When these landmasses are
reassembled, the mountain Chains form a nearly continuous belt.
Numerous other rock structures exist that appear to have formed at the same
time and were subsequently split apart.
In the words of Wegner “It is just as if we were to refit the torn pieces of a
newspaper by matching their edges and then check whether the lines of print run
smoothly across. If they do, there is nothing left to conclude except that the pieces
were in fact joined this way”.
4.Paleoclimatic evidences:
Since Wegner was a Climatologist by training, he was keenly interested in
obtaining pale climatic data in support of the continental drift. His efforts were
rewarded when he found evidence for apparently dramatic climatic changes.
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Glacial deposits indicated that near the end of the Palezoic era(220-300
million year), ice sheets covered extensive areas of S. Hemisphere, layers of
Glacial till were found at the same stratigraphic position in southern Africa,
S.America, India and Australia. Much of the evidences of late Paleozoic glaciation
come from land areas which presently lie within 30o of equator in a subtropical –
tropical climate.
Could the earth have gone through a period sufficiently cold to have
generated extensive continental glacier in what is presently a tropical region.
Wegner out rightly rejected this explanation because during the same period large
tropical swamps existed in the northern Hemisphere. These swamps with their lush
vegetation eventually became the major Coal Fields of the eastern United States,
Europe and Siberia. How could these two situations co-exist?
As Wegner proposed, a better explanation is provided if the landmasses are
fitted together as a super continent and moved nearer to the South Pole. This would
account for the conditions necessary to generate extensive expanses of glacial ice
over much of the Southern Hemisphere. At the same time this shift would place the
Northern landmasses nearer the tropics and account for their vast coal deposits.
In spite of compelling evidences as these – it took almost 50years for the
scientific community to accept the idea of continental drift and the logical
conclusions to which it led.
THE GREATE DEBATE
Wegner’s proposal did not attract much open criticism until 1924 when his
book was translated into English. From this time on, until his death in 1930, his
drift hypothesis encountered a great deal of hostile criticism.
To quote the American Geologist Chamberlin “Wegner’s hypothesis in
general is of the foot-loose type, in that it takes considerable liberty with our globe
and is less bound by restrictions or tied down by awkward, ugly facts than most of
its rival theories. Its appeal seems to lie in the fact that it plays a game in which
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there are few restrictive rules and no sharply drawn code of conduct”. SCOTT
described in much fewer worlds, the theory as “Utter Damned Rot!”
Objections:
One of the main objections to Wegner’s hypothesis stemmed from his
inability to provide a driving mechanism for continental drift. Wegner proposed
two possible energy sources for drift.
1. The Tidal force/influence of the moon was presumed by Wegner to be
strong enough to give the continents a westward motion. However, the
prominent Physicist Harold Jeffrey’s quickly countered with the argument
that the tidal friction of the magnitude needed to displace the continents
would bring the earth’s rotation to a halt in a matter of few years.
2. Further Wegner proposed that the larger and sturdier continents broke
through the oceanic crust, much like icebreakers cut through ice.
However no evidence existed to suggest that the ocean floor was weak enough
to permit passage of the continents without themselves being appreciably
deformed in the process.
Despite criticisms from all corners he wrote the fourth and final edition of
his book in 1919, maintaining his basic hypothesis and adding supporting
evidence.
1930, Wegner made his 3rd and final trip to the Greenland ice sheet to test
his hypothesis by precisely establishing the locations of specific points and
measuring the changes over a period of years, he could demonstrate the
westward drift of Greenland with respect to Europe. In November, while
returning from Eismitte (an experimental station) Wegner perished. His
intriguing idea, however, did not die with him.
Although the hypothesis was correct in principle, it also contained many
incorrect details. The hypothesis was criticized largely on grounds of its
inability to suggest a satisfactory means of engineering continental movements.
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It must be conceded that the cumulative evidence supporting the theory of
continental drift was massive and the theory was very attractive. If all the points
in the theory could be established and an adequate motive force discovered then
as Professor SHAND has said”.
“We would have to credit (prof.) Wegner with the greatest piece of
geological synthesis that has ever been accomplished”(1933).
Wegner himself in response to his critics said “Scientists still do not appear
to understand sufficiently that all earth Sciences must contribute evidence
towards unraveling the state of our planet in earlier times, and the truth of the
matter can only be reached by combining all this evidence”.
‘As one derides the past theories in the light of newly discovered Physical
facts, those theories which are deemed as modern, may be mocked at in the
future after the discovery of newer facts.’
Although most of Wegner’s contemporaries opposed his views, even to the
point of openly ridiculing him, a few considered his ideas plausible. Amongst
the noted supporter were.
Arthur Holmes (1928) contributed to the cause by proposing a plausible driving
mechanism for continental drift. Since the time he first proposed, he kept on
modifying his hypothesis and in his book Physical Geology (1966) he suggested
that convection currents operating within the Mantle were responsible for
propelling the continents across the globe. Although, even to this day geologists
are not in agreement on the exact nature of the driving mechanism, the concept
proposed by Holmes is still one of the most appealing.
Wegner also in 1929 suggested thermal convection currents. 1930 Death of
Wegner.
South Africa Geologist ALEXANDER DU TOIT (1937) published “Our
Wandering Continents” in which he eliminated some of Wegener’s errors and
added a great deal of new evidence in support of the drift idea.
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Very little new light was shed on the continental drift hypothesis between
the time of Wegner’s death and the early 1950s.
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