Continental drift theory
Look at a globe: the east coast of South America and the west coast of Africa look like two jigsaw pieces. In 1596 a Dutch map-maker, Abraham Ortelius, first noticed this.
In 1912 Alfred Wegener, a German meteorologist, said:
- About 200 million years ago all continents were joined in one super-continent, Pangaea ("all earth").
- It was surrounded by one mega-ocean, Panthalassa ("all water").
- Pangaea first split into a northern part, Laurasia, and a southern part, Gondwanaland, which later broke into today's continents.
Forces Wegener suggested
(1) Pole-fleeing force from the Earth's rotation and (2) tidal force from the Sun and Moon. Scientists found these far too weak to move continents, so the theory was not accepted for a long time.
Evidence in support of drift
- Matching coastlines (jigsaw fit): in 1964 Bullard used a computer to fit the edges at about 1,000 m depth; the fit is very close.
- Rocks of the same age: very old rock belts (about 2 billion years old) on the coast of Brazil match those of western Africa.
- Tillite: rock made from glacier deposits. Similar tillite in India (Gondwana beds) matches that of Africa, Madagascar, Antarctica, Australia and South America, showing they were once together in a cold region.
- Placer deposits: gold-rich sands on the Ghana coast have no source rock nearby; the gold-bearing veins are in Brazil, showing the two were joined.
- Fossils: fossils of the small freshwater reptile Mesosaurus are found only in southern Africa and Brazil. It could not have swum across an ocean. Lemur-like fossils in India, Madagascar and Africa also suggest a joined land.
The ocean floor
After the Second World War, ships used echo sounding (sending sound down and timing the echo) to map the sea bed. The floor has these parts:
| Part | What it is |
|---|---|
| Continental margin (shelf + slope) | The shallow edge of the continent under the sea and the slope down to the deep ocean |
| Abyssal plain | Wide, very flat deep plains, 3–6 km deep, covered with fine sediment |
| Mid-oceanic ridge | A mountain chain running through the oceans, with a rift valley on top; very active volcanoes |
| Trench | Long, narrow, very deep valleys near some continent edges; many earthquakes |
Earthquakes and volcanoes form narrow belts along ridges, trenches and young fold mountains, for example the Ring of Fire around the Pacific.
Surprises from the ocean floor
- Rocks near the ridge are youngest; they get older away from it, on both sides like a mirror.
- Sediment on the floor is thin; the oldest ocean rock is only about 200 million years old, far younger than the oldest continental rock (about 3.2 billion years).
- Stripes of rock record the Earth's magnetism flipping, and the stripes match on both sides of a ridge.
Sea-floor spreading
In 1961 Harry Hess explained these findings with the idea of sea-floor spreading:
- Hot magma rises along the mid-ocean ridge and cools into new ocean crust.
- The new crust pushes the older crust away on both sides.
- Far away, at trenches, the old crust bends and sinks back into the mantle (subduction).
- So the Earth does not grow bigger: crust made at ridges equals crust destroyed at trenches.
Convection currents in the mantle, first suggested by Arthur Holmes in the 1930s, provide the engine: hot material rises, spreads and sinks, like soup in a pot.
Try it at home: heat water with a few tea leaves in a clear pan. Watch the leaves rise in the middle, spread to the sides and sink at the edges. That is a convection current.
Plate tectonics and plate boundaries
In 1967 McKenzie, Parker and Morgan joined these ideas into the theory of plate tectonics. A tectonic plate is a huge, rigid slab of lithosphere (crust + upper mantle) that moves as one unit on the soft asthenosphere. Plates can carry continents, ocean floor, or both.
There are 7 major plates: Antarctic, North American, South American, Pacific, India-Australia-New Zealand, Africa, and Eurasia. Minor plates include Cocos, Nazca, Arabian, Philippine, Caroline and Fuji.
| Boundary | Plates | Result | Example |
|---|---|---|---|
| Divergent (constructive) | Move apart | New crust forms; ridges and rift valleys | Mid-Atlantic Ridge |
| Convergent (destructive) | Move together | One plate sinks (subduction) making trenches and volcanoes, or two continents crush up into fold mountains | Himalayas; Pacific trenches |
| Transform (conservative) | Slide past each other | No crust made or destroyed; strong earthquakes | Faults cutting across ridges |
Plates move at different speeds: the Arctic Ridge about 2.5 cm a year (slow); the East Pacific Rise over 15 cm a year (fast).
Movement of the Indian plate
The Indian plate includes the Indian peninsula and Australia. Its northern edge is the convergent boundary along the Himalayas; its eastern edge runs through the Rakinyoma hills of Myanmar towards the Java trench; in the west it meets along the Kirthar mountains; and in the south it is bordered by spreading ridges in the Indian Ocean.
| Time ago (about) | What happened |
|---|---|
| 225–200 million years | India was a large island near Australia, part of Gondwanaland, with the Tethys Sea to the north |
| ~200 million years | India broke away and began moving north |
| 60–65 million years | Huge lava floods formed the Deccan Traps as India passed over a hot spot |
| 40–50 million years | India collided with Asia; the Tethys closed and the Himalayas began to rise |
| Today | The plate still moves north (about 5 cm/year); the Himalayas keep rising |
That is why earthquakes are common along the Himalayas.
Key formulas and definitions
- Continental drift: Wegener, 1912; Pangaea (land) + Panthalassa (ocean); Laurasia (north) + Gondwanaland (south)
- Evidence: jigsaw fit, same-age rocks, tillite, placer gold, fossils (Mesosaurus)
- Ocean floor: continental margin → abyssal plain → mid-oceanic ridge; trenches near some margins
- Sea-floor spreading: Hess, 1961; new crust at ridges, destroyed at trenches (subduction)
- Plate tectonics: 1967; plates = rigid lithosphere moving on asthenosphere
- Divergent = apart (constructive); Convergent = together (destructive); Transform = slide past (conservative)
- 7 major plates; Indian plate moves north ≈ 5 cm/year; collided with Asia ≈ 40–50 million years ago
Worked examples
1. Why is Mesosaurus strong evidence for continental drift?
It was a small freshwater reptile that could not cross an ocean, yet its fossils are found only in southern Africa and Brazil. So those lands must once have been joined.
2. Rocks at a ridge are 0 years old; 500 km away they are 10 million years old. Find the spreading speed on one side in cm/year.
500 km = 50,000,000 cm. Speed = 50,000,000 ÷ 10,000,000 = 5 cm per year.
3. Why was Wegener's theory rejected at first?
His forces (pole-fleeing and tidal) were far too weak to move continents, so he could not explain how drift happened.
4. Classify each boundary: (a) Mid-Atlantic Ridge, (b) Himalayas, (c) a fault where two plates slide sideways.
(a) Divergent, (b) convergent (continent–continent), (c) transform.
5. How does sea-floor spreading explain why ocean rocks are young?
Ocean floor is constantly made at ridges and destroyed at trenches, so no ocean floor survives very long; the oldest is only about 200 million years.
6. The Indian plate moves 5 cm a year. How far does it move in 1 million years (in km)?
5 cm × 1,000,000 = 5,000,000 cm = 50 km.
Common mistakes
- Saying only continents move. Whole plates move, including the ocean floor around continents.
- Thinking the Earth grows as new crust forms at ridges. Old crust is destroyed at trenches, so the size stays the same.
- Mixing up boundaries: divergent = apart, convergent = together, transform = sliding past.
- Believing Wegener proved his forces. His idea of drift was right, but his forces were wrong; convection and spreading explained the motion later.