Evidence for continental drift

Detail

Background: what Wegener claimed

• Continental drift: the idea that the continents are not fixed. They have slowly moved across the Earth's surface over millions of years.
• Alfred Wegener, a German meteorologist (a scientist who studies weather), put forward the theory in 1912, in two published articles [2].
• Pangaea (Greek for "all lands"): one huge supercontinent that held all of today's land. It began to break up about 225–200 million years ago [2].
• Alexander Du Toit later said Pangaea first split into two parts [2]:
• Laurasia in the north

• Gondwanaland in the south (South America, Africa, India, Madagascar, Antarctica, Australia)

• Others had noticed the fit of the coasts earlier. Abraham Ortelius (1596) suggested the continents had separated, and Antonio Snider-Pellegrini (1858) drew maps showing them fitted together [2].

• Wegener's evidence falls into five groups. NCERT lists them as: jigsaw fit, rocks of the same age, tillite, placer deposits and the spread of fossils.

1. Jigsaw fit of continents

• Jigsaw fit: the facing coasts of two continents match like pieces of a puzzle. The clearest case is the east coast of South America and the west coast of Africa, across the South Atlantic Ocean.
• Why the shoreline is not used:
• The present shoreline changes as sea level rises and falls. Erosion and deposits also change it.
• The real edge of a continent is the outer edge of the continental shelf (the shallow, drowned edge of a continent), which lies under the sea.

• So the fit was tested along an undersea depth line: a line on the sea floor joining points of equal depth.

• Bullard's computer fit:

• Sir Edward Bullard fitted the margins of the continents around the Atlantic by computer. The fit was almost perfect, with very little gap or overlap.
• The work was published by Bullard, Everett and Smith in 1965 in the Philosophical Transactions of the Royal Society. They matched the 500-fathom contour (about 915 m deep) [5][6]. (NCERT: done in 1964, at the 1,000-fathom line.)
• 1 fathom = 6 feet ≈ 1.83 m.
• Later work extended the same 500-fathom computer fit to most of the southern continents, and geological and geophysical data supported it [5].

2. Rocks of the same age across oceans

• Radiometric dating: a way to find the age of a rock. Radioactive elements such as uranium and potassium break down (decay) at a fixed, known rate. Measuring how much has decayed gives the rock's age.
• The match:
• A belt of very old rocks, about 2,000 million years old, runs along the coast of Brazil.
• The same kind of rock belt, of the same age, is found in western Africa.

• These belts look like one belt that was later torn apart by the Atlantic.

• When the ocean opened:

• The oldest marine deposits (layers laid down by the sea) on both coasts date only from the Jurassic period (about 201–145 million years ago).
• So there was no sea between South America and Africa before the Jurassic.
• This fits the date given for the break-up of Pangaea (about 200 million years ago).

3. Tillite: evidence of an old ice age

• Tillite: a sedimentary rock (rock made from layers of deposited material) formed when deposits left by a glacier harden into rock. It shows that ice sheets once covered the place.
• The Gondwana system of India:
• The Gondwana system is a thick series of sedimentary rocks in India. It is named after the Gond region of central India and is famous for its coal.
• Matching rock series are found on six southern landmasses.

• Thick tillite lies at its base. This shows a long period of glaciation (the land was covered by ice).

• Same rocks elsewhere: Africa, the Falkland Islands, Madagascar, Antarctica and Australia.

• Why this shows drift (cause → result):
• Today these lands lie in very different climates, some of them hot and near the tropics.
• One ice sheet could not cover all of them in their present, widely spread positions.
• So they must once have been joined, near the South Pole, and shared one ice age.

• This is clear evidence of palaeoclimate (the climate of the distant past) and of drift.

• Other climate clues [2]:

• Glacial deposits are found in today's dry Africa, for example in the Vaal River valley of South Africa.
• Fossils of tropical plants, now coal deposits, are found in Antarctica.

Clue	Where found today	What it shows
Tillite (glacier deposits)	India (Gondwana base), Africa, Falklands, Madagascar, Antarctica, Australia	The lands were once joined near the South Pole
Coal from tropical plants	Antarctica [2]	Antarctica was once in a warm belt

4. Placer deposits: gold from Brazil found in Ghana

• Placer deposit: a mineral such as gold that was worn out of its source rock, carried by rivers, and dropped in sand or gravel.
• The puzzle:
• The Ghana coast in West Africa has rich gold placers.
• There is no gold-bearing source rock nearby in Ghana.

• The gold-bearing veins (the source rock) are in the Brazil plateau of South America.

• The answer (cause → result):

• Africa and South America once lay side by side.
• Rivers carried gold from the Brazil plateau onto what is now the Ghana coast.
• The Atlantic then opened, and the gold deposit and its source ended up on two separate continents.

5. Fossil evidence for continental drift

• Fossil evidence: the same land or freshwater species are found on continents that are now separated by oceans.
• Such animals and plants could not cross wide salt-water oceans. So the land masses must once have been joined.

Lemurs and "Lemuria"

• Lemurs (small tree-living primates) live in India, Madagascar and Africa.
• To explain this, some people imagined a lost land bridge across the Indian Ocean called "Lemuria". Drift explains the pattern without any lost land.

Mesosaurus

• Mesosaurus: a small reptile that lived in shallow brackish water (water a little salty, between fresh and sea water). It was only about 1 m long [3][4].
• It is found only in two places:
• the southern Cape province of South Africa

• the Irati formations of Brazil

• These places are now 4,800 km apart, across the Atlantic.

• A small freshwater or brackish-water reptile could not have swum across the Atlantic Ocean [3][4].

Other standard fossils

Fossil	Type	Period	Found in	Why it matters
Glossopteris	Seed fern (woody plant) with tongue-shaped leaves	Permian–Triassic, about 300–200 million years ago [7]	India, Africa, South America, Australia, Antarctica (all of Gondwana) [7]	One of the first fossil proofs of drift [7]. Its seeds were too heavy to be carried across oceans by wind. Its presence in now-polar areas also points to drift [2]
Mesosaurus	Freshwater/brackish reptile, about 1 m long [3]	Late Palaeozoic to early Mesozoic	South Africa, Brazil	Could not swim across the Atlantic [3]
Lystrosaurus	Land reptile; laid eggs on land; a poor swimmer [3][4]	Early Mesozoic (Triassic) [3]	Africa and Antarctica [3][4] (also India)	Shows that Africa, India and Antarctica were joined
Cynognathus	Land reptile (a cynodont, an early mammal-like reptile)	Triassic [8]	South America and Africa [8]	A land animal found on both sides of the South Atlantic

Why Wegener's evidence was not enough at first

• Wegener showed that the continents had moved, but not how. The forces he suggested were pole-fleeing force (from the Earth's rotation) and tidal force. Scientists found these far too weak.
• The proof of movement came later from sea-floor spreading (Hess, 1960s) and plate tectonics (1967). These gave a working mechanism, and the old evidence above then fitted into the new theory.

Prelims Hooks

• Bullard, Everett and Smith published the computer fit of the Atlantic continents in 1965, using the 500-fathom contour [5][6]. NCERT gives 1964 and the 1,000-fathom line. Either way, the fit was made along an undersea depth line, not the present shoreline.
• Rocks about 2,000 million years old on the Brazil coast match rocks in western Africa. The oldest marine deposits on both coasts are Jurassic.
• Tillite is a sedimentary rock formed from glacial deposits. It lies at the base of India's Gondwana system.
• Tillite matches: Africa, Falkland Islands, Madagascar, Antarctica, Australia (with India).
• Gold placers are on the Ghana coast, and their source veins are in the Brazil plateau.
• Mesosaurus is found in the Cape province (South Africa) and the Irati formations (Brazil), which are 4,800 km apart. It was a brackish/freshwater reptile, not a marine one.
• Lemurs live in India, Madagascar and Africa. The imagined land bridge is called "Lemuria".
• Glossopteris is a plant (seed fern) of the Permian–Triassic Gondwana [7]. Lystrosaurus, Cynognathus and Mesosaurus are reptiles. This is a common trap in "which of the following" questions.
• Wegener (1912) was a meteorologist, not a geologist [2]. Du Toit split Pangaea into Laurasia and Gondwanaland [2].

Mains Points

• Several kinds of evidence agree: the shape of the coasts, rock ages, ancient climate (tillite and coal), mineral deposits (Ghana gold) and fossils all point to the same past position of the southern continents. This makes the case for Gondwana strong, even though Wegener could not explain how continents move.
• India's place in Gondwana: India's Gondwana rocks hold glacial tillite at the base and coal-bearing layers with Glossopteris above them. This shows that India moved from a cold, near-polar position towards the tropics. It also explains why India's Gondwana coal lies in the Damodar, Mahanadi, Godavari and Son valleys (GS-I and GS-III, mineral distribution).
• Why the theory was first rejected: the evidence was good, but the mechanism (pole-fleeing and tidal forces) was too weak. The answer came only with sea-floor spreading and plate tectonics, which is a good example of how a scientific theory develops.
• Practical use: matching geology across old continental joins guides mineral exploration. Brazil and West Africa share gold belts, and India, Africa and Australia share Gondwana coal and gem-bearing rocks.