Historical Persp & Evidences of Cont. Drift. Lec#5 28th Sep_2011

8/3/2019 Historical Persp & Evidences of Cont. Drift. Lec#5 28th Sep_2011

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GEOTECTONICSGEOTECTONICS

BS Geology(4BS Geology(4thth semester)semester)

BAHRIA UNIVERSITY ISLAMABADBAHRIA UNIVERSITY ISLAMABAD


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HISTORICAL PERSPECTIVE OFHISTORICAL PERSPECTIVE OF

PLATE TECTONICSPLATE TECTONICS


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Historical Perspective

The theory of plate tectonics has largelybeen developed since 1967 but the historyof ideas concerning the mobilist view of theearth extends back considerably longer

The theory of plate tectonics states thatthe Earths outermost layer is fragmentedinto a dozen of large & small plates that

are moving relative to one another on thetop of mobile asthenosphere.


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Before the advent of plate tectonics

,however, some people already believedthat the presentday continents were thefragmented pieces of preexisting largerlandmasses (supercontinents).

Plate tectonics is the relatively newscientific concept, introduced in the 19th

centuary. The theory has unified the study

of the earth by drawing together manybranches of earth sciences, frompaleontology to seismology.


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Plate tectonics theory has provided

explanation to the distribution ofearthquakes & volcanic active zone aroundthe globe.


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Abraham Ortelius(1527-1598)

He was the first to note the similarity& suggest an ancient separation in1756. He suggested that theAmericas were torn away fromEurope & Africa by earthquake &floods.


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Abraham OrteliusAbraham Ortelius(Abraham Ortels) (April(Abraham Ortels) (April14, 152714, 1527 June 28, 1598)June 28, 1598)was awas a FlemishFlemish cartographercartographerandand geographergeographer, generally, generallyrecognized as the creator ofrecognized as the creator of

the firstthe first modernmodern atlasatlas, the, theTheatrum Orbis TerrarumTheatrum Orbis Terrarum

(Theatre of the World). He(Theatre of the World). Heis also believed to be theis also believed to be the

first person to imagine thatfirst person to imagine thatthe continents were joinedthe continents were joinedtogether before drifting totogether before drifting totheir present positions.their present positions.


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Theatrum Orbis TerrarumTheatrum Orbis Terrarum


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Sir Francis Bacon:

In 1620, he commented on thesimilar form of the west coasts ofAfrica & South America. He alsonoted the similar configuration of theNew & Old world, both of which arebroad & extended towards the north,narrow & pointed towards south.


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Sir Francis Bacon


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Theodor Christoph Lilienthal:

Theodor Christoph, Professor ofTheology at Konigberg in Germany,

also note A fit of Atlantic Coasts ofSouth America & Africa & Suggestthat they might once have been side

by side


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Alexander Von Humbolt:

The German explorer Humbolt alsonoted the geometric & geologicalsimilarities of the opposing shores ofthe Atlantic but also suggested thatthe Atlantic ocean was formed bycatastrophic event.


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Catastrophism VsCatastrophism VsUniformitarianismUniformitarianismCatastrophismCatastrophism

A principle that statesA principle that statesthat catastrophicthat catastrophicevents have beenevents have beenimportant in Earthimportant in Earthhistory and modifyhistory and modifythe path of slowthe path of slowchangechange..

UniformitarianismUniformitarianism

The principle that states thatThe principle that states thatgeological change occurs overgeological change occurs over

long periods of time, bylong periods of time, bysequence of almostsequence of almostimperceptible events.imperceptible events.

In addition, processes andIn addition, processes andscientific laws operating todayscientific laws operating today

also operated in the past andalso operated in the past andthus past geologic events canthus past geologic events canbe explained by forcesbe explained by forcesobservable today.observable today.


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In the mid of 19th centuary concept ofcatastrophism is replaced by that ofuniformitarianism or actualism asproposed by the Scottish geologist JamesHutton in 1785.

The Principle of uniformity is commonlystated as:

The present is Key to the past


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James Hutton


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It means that slow processes going onat & beneath the Earths surface todayhave been going on throughoutgeological time & have shaped the

surface record. However the geologic forces &

processes are gradual as well as

catastrophic, acting on the Earth todayare the same as those that have actedin the geologic past.


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For the first time it was considered

that drift is taking place today & hastake place at least through the past100-200 Ma of Earth history.

In this way drift was considered toaccount for the geometric &geological similarities of the trailing

edges of the continents around theAtlantic & Indian oceans & theformation of the young fold mountain

systems at their leading edges.


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Continental drift


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Continental drift & AlfredWegners Theory

In 1912, the German meteorologist AlfredWegener (1880-1930), first proposed thetheory of continental drift , which statethat parts of the Earths crust slowly driftatop of a mobile asthenosphere.

Wegener detailed much of the older, pre-drift, geological data & maintained that

the continuity of older structures,formations & fossils faunas & floras acrosspresent continental shorelines was morereadily understood on predrift

reconstruction.


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Alfred LotharAlfred LotharWegenerWegener(1880(1880--1930),1930),the originatorthe originatorof the theory ofof the theory ofcontinentalcontinental


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Wegener hypothesized that all the

continents were assembled intosupersontinents 200 Ma ago, which henamed Pangaea, meaning All earth.

Pangaea was a supercontinent consistingof all of Earths land masses.

It existed from Permian to Jurassic periods. It began breaking up during the Jurassic

period, forming two supercontinents


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Pangaea(all earth)

Laurasia(from Laurentia,a region of Canada

& Asia )

Gondwanaland(literally land of Gonds, after

an ancient tribe of northern India)


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Laurasia:

It is the northern portion of

supercontinents & consists ofNorth America, Greenland,Europe & Asia.


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Gondwanaland

It is the southern portion of

Supercontinent & includeSouth America, Antarctica,Africa, Madagascar, India &

Australia.


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These two supercontinents wasseparated by palaeo-Thethysocean (after the Greek goddess

of the sea).The ocean surrounding themwas Panthalassa (literally all

ocean).


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Alfred SupercontinentsAlfred Supercontinents


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Position of Continents inPosition of Continents in

different periodsdifferent periods


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Wegener proposed hisContinental drift hypothesis inbook Die Entstehung der

kontinente und ozeane (Theorigin of continents & oceans)


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Drawbacks of Alfred Wegeners

Theory Lack of plausible driving force was

missing.

Wegener suggested that continents driftoccurred in response to the centripitalforce experienced by the continents

because of the Earths rotation but it is nottrue


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Wegeners theory vindication

Although Wegener s continental theorywas rejected during his life time due tolack of reasonable mechanism forcontinental drift.

Later on, the British geologist ArthurHolmes championed the theory ofcontinental drift & proposed that

continents were moved by convectioncurrent powered by the heat ofradioactive decay.


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However, after his death, new evidencefrom ocean floor exploration and otherstudies rekindled interest in Wagener'stheory, ultimately leading to the

development of the theory of platetectonics


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Evidences of Continental driftEvidences of Continental drift


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Wegener devoted his life to

doggedly pursuing additionalevidence to defend his theory.

The evidence used by Wegener,du Toit, and others to support thehypothesis of continental drift are

as follow;


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1)GEOMETRICAL1)GEOMETRICAL FIT OF CONTINENTSFIT OF CONTINENTS

Wegener, like some before him, was impressedby the close resemblance between the coastlinesof continents on opposite sides of the Atlantic

Ocean, particularly between South America andAfrica. He argued that continents are fit togetherlike pieces of a giant jigsaw puzzle.

He cited these similarities as partialevidence that the continents were at onetime joined together as a supercontinentthat subsequently split apart.


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As his critics pointed out, though, theconfiguration of coastlines result from erosional

and depositional processes and therefore iscontinually being modified. So even if thecontinents had separated during the MesozoicEra, as Wegener proposed, it is not likely that thecoastlines would fit exactly.

A more realistic approach is to fit thecontinents together along the continental

slope where erosion would be minimal.


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In 1965 Sir Edward Bullard, an Englishgeophysicist, and two associates showedthat the best fit between the continentsoccurs along the continental slope at adepth of about 2,000m.

Since then, other reconstructions using thelatest ocean basin data have confirmed theclose fit between continents when they are

reassembled to form Pangaea.


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2)Similarity of Rock Sequences andMountain Ranges

If the continents were at one time joined,then the rocks and mountain ranges of thesame age in adjoining locations on theopposite continents should closelymatch. Such is the case for the Gondwanacontinents. Marine, non-marine, and glacial

rock sequences of Pennsylvanian to Jurassicage are almost identical for all five Gondwanacontinents, strongly indicating that they werejoined together at one time.


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The continuity of Paleozoic fold beltsacross the assembled Laurasia &Gondwanaland also support thehypothesis of continental drift.

The continuation of Appalachian foldbelt of North America with theCaledonian fold belt of northernEurope was noted.

Similarly the sedimentary depositsassociated with these fold belts also

supported theory.


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The grain size & chemical compositionof the sediments can be used todetermine the nature & direction of thesource.

The source of the sediments inCaledonian fold belt of northern Europelies to the west in location nowoccupied by Atlantic ocean, indicatethat in past both continents were sideby side.


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3) Age provinces3) Age provinces

The correlation of the patterns ofages across the southern Atlantic is

also noted. He observed similar rockunits up to Jurassic.


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4) Igneous provinces4) Igneous provinces

The igneous rocks of similar age canbe traced b/w various continents.

This consists of both extrusive &intrusive e.g. the Mesozoic doloritesspread through Africa, Antarctica,

America.


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4) Glacial Evidence

During the Late Paleozoic Era, massiveglaciers covered large continental areas of

the Southern Hemisphere. Evidence forthis glaciation includes layers of till(sediments deposited by glaciers) andstriations (scratch marks) in the bedrockbeneath the till.


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Fossils and sedimentary rocks of the sameage from the Northern Hemisphere,

however, give no indication ofglaciation. Fossil plants found in coalsindicate that the Northern Hemisphere had

a tropical climate during the time that theSouthern Hemisphere was glaciated.


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All of the Gondwana continents except

Antarctica are currently located near theequator in subtropical to tropicalclimates. Mapping of glacial striations inbedrock in Australia, India, and South

America indicates that the glaciersmoved from the areas of the present-day oceans onto land. This would be

highly unlikely because large continentalglaciers flow outward from their centralarea of accumulation toward the sea.


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If the continents did not move duringthe past, one would have to explainhow glaciers moved from the oceansonto land and how large-scalecontinental glaciers formed near theequator.

But if the continents are reassembledas a single landmass with South Africalocated at the south pole, the directionof movement of Late Paleozoiccontinental glaciers makes sense.


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Furthermore, this geographicarrangement places the northerncontinents nearer the tropics, which isconsistent with the fossil andclimatological evidence from Laurasia.


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5) Fossil Evidences5) Fossil Evidences

Some of the most compelling evidence forcontinental drift comes from the fossil

record.


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a)Glossopteris flora

Glossopteris, a tree-like plant from thePermian through the Triassic Period. It

had tongue-shaped leaves and was about12 ft (3.7 m) tall. It was the dominantplant ofGondwana.


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Eduard Suess was an Austrian geologistwho first realized that there had once been

a land bridge between South America,Africa, India, Australia, and Antarctica. Henamed this large land mass Gondwanaland

(named after a district in India where thefossil plant Glossopteris was found).

The present-day climates of South America,

Africa, India, Australia, and Antarcticarange from tropical to polar and are muchtoo diverse to support the type of plantsthat compose the Glossopteris flora.


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Distribution of GlosspterisDistribution of Glosspteris


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b)Mesosaurus

It is freshwater reptile whose fossils arefound in Permian-aged rocks in certainregions of Brazil and South Africa andnowhere else in the world.

Because the physiology of freshwater andmarine animals is completely different, it ishard to imagine how a freshwater reptile

could have swum across the Atlantic Oceanand found a freshwater environment nearlyidentical to its former habitat.


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Mesosaurus


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Moreover, if Mesosaurus could have swumacross the ocean, its fossil remains shouldbe widely dispersed. It is more logical toassume that Mesosaurus lived in lakes in

what are now adjacent areas of SouthAmerica and Africa, but were then unitedinto a single continent.


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c) Lystrosaurus and Cynognathus

Both are land-dwelling reptiles that livedduring the Triassic Period; their fossils are

found only on the present-day continentalfragments of Gondwana.

Because they are both land animals, theycertainly could not have swum across the

oceans currently separating the Gondwanacontinents. Therefore, the continentsmust once have been connected.


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d) Polar dinosaurs

Recently, paleontologists (specialists instudies of fossils) have carefully studied somewell-preserved dinosaur remains unearthed atDinosaur Cove, at the southeastern tip of

mainland Australia. Dinosaurs found in most other parts of the

world are believed to have lived in temperate

or tropical regions, but these Australianspecies, popularly called "polar" dinosaurs,seemed well adapted to cooler temperatureconditions.


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They apparently had keen night vision andwere warm-blooded, enabling them to

forage for food during long winter nights, atfreezing or sub-freezing temperatures.

The last of the dinosaurs became extinctduring a period of sharp global coolingtoward the end of the Cretaceous period(about 65 million years ago).

The discovery of the polar dinosaurs clearly

suggests that they survived the volcanicwinter that apparently killed other dinosaurspecies.


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Paleontologists do not have theanswers. Regardless, this recentlyacquired paleontologic evidenceconvincingly demonstrates thatAustralia has drifted north toward the

equator during the past 100 millionyears.

At the time when the Australian polar

dinosaurs thrived, their habitat wasmuch farther south, well within theAntarctic Circle.


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ApproximatelyApproximately100 million years100 million years

ago, the Dinosaurago, the DinosaurCove area (smallCove area (smallred outlinedred outlinedboxes) at theboxes) at the

southern end ofsouthern end ofAustralia was wellAustralia was wellwithin thewithin theAntarctic Circle,Antarctic Circle,

more than 40more than 40degrees closer todegrees closer tothe South Polethe South Polethan it is today.than it is today.


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Palaeoclimatology

& continental drift


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Palaeoclimatology

It is the study of ancient climates. It may be used to demonstrate that

continents have drifted at least in a north-south direction.


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Factor controlling climate

Climate

Latitude Wind direction& ocean currents

Elevation &topographic barriers


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1) Carbonates & reef deposits

Condition: restricted to warm water Latitude range: within 30 of the equator

at the present day. Temperature range: 25-30 C


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2) Evaporites

Conditions: formed under hot aridconditions in regions where evaporations

exceeds sea water influx/precipitation. Latitude: arid subtropical high pressure

zones b/w about 10 & 40


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3) Red-beds

Condition: hot climate Latitude: < 30


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Coal & oil

Condition: warm humid climate isnecessary for accumulation & degradation

of organic matter. Latitude: < 30.


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5) Glacial deposits

Latitude: within 30 of pole

The result of applying thesepalaeoclimatic techniques strongly indicatethat continents have changed their

latitudinal position throughout geologicaltime.


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Historical Persp & Evidences of Cont. Drift. Lec#5 28th Sep_2011

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