From a wild idea to a unifying theory
In 1912, German scientist Alfred Wegener proposed continental drift: the continents were once joined in a supercontinent he called Pangaea ("all lands") and have slowly moved apart. Most geologists rejected the idea because Wegener couldn’t explain how continents could move.
Fifty years later, new evidence from the ocean floor revived and transformed his idea into the theory of plate tectonics.
Wegener’s evidence
| Evidence | What it shows |
|---|---|
| Fit of the continents | South America and Africa fit like puzzle pieces, especially at the edges of their continental shelves |
| Matching fossils | Mesosaurus (a freshwater reptile) is found only in Brazil and South Africa; Glossopteris (a seed fern) is found across the southern continents and Antarctica |
| Matching rocks and mountains | The Appalachians match mountains in Scotland, Ireland and Scandinavia |
| Ancient climates | Glacial scratches in now-tropical Africa and India; coal (from swamps) in Antarctica |
Evidence from the ocean floor
Mapping the sea floor
After World War II, sonar mapping revealed a 65 000 km chain of underwater mountains, the mid-ocean ridge system, and deep trenches near some continents.
Sea-floor spreading
In the early 1960s, Harry Hess proposed sea-floor spreading: magma rises at mid-ocean ridges, forms new crust, and pushes older crust outward. At trenches, old crust sinks back into the mantle (subduction).
Paleomagnetism: the magnetic stripes
When lava cools, iron minerals line up with Earth’s magnetic field, recording its direction. Earth’s magnetic field reverses every few hundred thousand years on average.
In 1963, Fred Vine and Drummond Matthews (and independently Canadian geophysicist Lawrence Morley) showed that rocks on either side of the mid-ocean ridges form symmetrical stripes of normal and reversed paleomagnetism. This was strong evidence that new crust forms at the ridge and spreads both ways.
Age of the sea floor
- The youngest rocks are at the ridges; rocks get older moving away.
- The oldest ocean crust is only about 180–200 million years old, while continental rocks can be over 4 billion years old.
- Sediment is thinnest at the ridges and thicker farther away.
Earthquakes and volcanoes
Earthquake and volcano locations outline plate boundaries. Deep earthquakes along sloping zones beneath trenches trace subducting plates.
Modern measurements
GPS stations now measure plate motions directly: the Atlantic widens by about 2.5 cm per year.
Canadian contribution
Canadian geologist J. Tuzo Wilson (1908–1993) made key contributions in the 1960s: he identified transform faults, explained hot spots like Hawaii, and described how oceans open and close in cycles (the "Wilson cycle").
Geologic time
Earth is about 4.6 billion years old. The geologic time scale divides this history into units based on major changes in rocks and fossils.
| Eon/Era | Time (millions of years ago) | Key events |
|---|---|---|
| Hadean and Archean eons | 4600–2500 | Earth forms; oceans form; first single-celled life (by about 3500) |
| Proterozoic eon | 2500–539 | Oxygen builds up in the atmosphere; first multicellular life |
| Paleozoic era | 539–252 | "Cambrian explosion" of animal life; fish, land plants, amphibians, reptiles; ends with the largest mass extinction |
| Mesozoic era | 252–66 | "Age of reptiles": dinosaurs, first mammals and birds, flowering plants; Pangaea breaks up; ends with an asteroid impact |
| Cenozoic era | 66–present | "Age of mammals"; modern humans appear about 300 000 years ago |
If Earth’s history were squeezed into one calendar year, dinosaurs would appear in mid-December, and humans would show up about 20–30 minutes before midnight on December 31.
Dating rocks
Relative dating
Relative dating puts events in order without exact ages:
- Superposition: lower layers are older.
- Cross-cutting: a fault or intrusion is younger than the rock it cuts.
- Unconformities: gaps in the record where layers were eroded.
- Index fossils: an index fossil comes from a species that was widespread but lived only a short time, so it pins down a layer’s age. Trilobites and ammonites are classic examples.
Absolute dating
Radiometric dating measures the decay of radioactive isotopes at known half-lives.
| Isotope pair | Half-life | Useful for |
|---|---|---|
| Carbon-14 → nitrogen-14 | 5730 years | Organic remains up to about 50 000 years |
| Potassium-40 → argon-40 | 1.25 billion years | Volcanic rocks |
| Uranium-238 → lead-206 | 4.5 billion years | The oldest rocks and minerals |
The oldest known minerals on Earth are zircon crystals from Western Australia, about 4.4 billion years old. The Acasta Gneiss in the Northwest Territories is among the oldest known rocks, at about 4.0 billion years.
Common mistakes
- Thinking Wegener’s idea was accepted right away.
- Thinking ocean floor is as old as the continents.
- Using carbon-14 to date dinosaur bones. They’re far too old; other isotopes are used for the rocks around them.
Try it yourself
- Cut out continent shapes from a world map and try to assemble Pangaea.
- Explain how magnetic stripes support sea-floor spreading.
- A volcanic rock contains 25% of its original potassium-40. How old is it?
- In a rock outcrop, a basalt dike cuts through three sedimentary layers. List the order of events.
(Answers: 3. Two half-lives have passed, so it is about 2 × 1.25 = 2.5 billion years old. 4. The three layers were deposited from bottom to top, then the dike intruded through them.)
Quick recap
- Wegener proposed continental drift using fossils, rocks, climate clues and continent shapes.
- Sea-floor spreading and magnetic stripes explained how plates move.
- Plate tectonics unites these ideas.
- Earth is about 4.6 billion years old; the geologic time scale divides its history.
- Relative dating orders events; radiometric dating gives absolute ages.