What is the evidence for plate tectonics?

The main lines of evidence are: the jigsaw fit of the continents (especially their continental shelves); identical fossils such as Mesosaurus and Glossopteris on continents now separated by oceans; the profile of the ocean floor, including mid-ocean ridges and trenches; the age of sea-floor rocks, which increases with distance from the ridge; and symmetrical magnetic reversal stripes in the sea floor either side of the ridge.

Why was Wegener’s continental drift rejected at first?

Wegener had good evidence that continents had moved — their fit and matching fossils — but he could not propose a believable mechanism, a force able to move a continent across solid ocean floor. Without a mechanism, his evidence was dismissed as coincidence, and the idea was not accepted until sea-floor spreading supplied the missing explanation decades later.

How did sea-floor spreading prove the continents move?

Harry Hess proposed that new oceanic crust forms at mid-ocean ridges and spreads outward, carrying the continents like passengers on a conveyor belt, rather than continents ploughing through the floor. Vine and Matthews then showed the sea floor carries symmetrical magnetic stripes, and Glomar Challenger drilling showed sea-floor rock gets older away from the ridge — both exactly as spreading predicts.

Who contributed to the theory of plate tectonics?

Alfred Wegener proposed continental drift; Arthur Holmes proposed mantle convection as a driving force; Harry Hess proposed sea-floor spreading; Fred Vine and Drummond Matthews linked magnetic reversals in the sea floor to spreading; and drilling by the Glomar Challenger confirmed the ages of the oceanic floor. Together their work built the theory of plate tectonics.

The evidence for plate tectonics

Wegener saw that the continents had moved and was laughed out of the room for fifty years — because he could not say how. The proof that finally settled it came not from the continents he pointed at, but from the floor of the sea he never studied. This powerbook walks that story, from the jigsaw fit to the magnetic stripes that closed the case.

short sections

~50 yr

from idea to proof

lines of evidence

The evidence for plate tectonicsSection 1 of 5

0 of 5 locked in

This book has 5 short sections. Read each one, answer the check, then use Next to move on — the bar above tracks how far you've come.

The idea almost nobody believed

In 1912 a German meteorologist, Alfred Wegener, pointed at a world map and said something that sounded absurd: the continents are not fixed. They were once joined in a single supercontinent he called Pangaea, and they have drifted apart since. He called the idea continental drift.

His clues were good. South America and Africa fit together like two pieces of a torn page, right down to the curve of their continental shelves. Identical fossils turn up on opposite sides of oceans no land animal could cross — Mesosaurus, a small freshwater reptile, in both South America and Africa; the fern Glossopteris across India, Australia, Antarctica and South America. Whole mountain belts and rock sequences line up across the gap, as if a single feature had been cut in two.

And almost everyone rejected it. The problem was not the clues but the missing engine: Wegener could not say what force could shove a continent across solid ocean floor. Without a believable mechanism, the fit and the fossils were dismissed as coincidence, and the idea sat on the shelf for a generation.

The continents fit, and the same fossils run straight across the join — as if a torn page had been matched up.

New words

continental drift

Wegener’s idea that the continents were once joined and have since moved apart.

Pangaea/pan-JEE-uh/

The single supercontinent the continents are proposed to have split from.

🔒 Lock it in

Wegener had strong clues. Why was continental drift rejected for decades anyway?

Answer the check to continue

Quick reference & sources

The interactive book above is the lesson. What follows is a plain recap you can revise from, plus the sources and how this maps to the syllabus.

The five lines of evidence

Plate tectonics rests on evidence that fits together: the jigsaw fit of the continents and their shelves; matching fossils (Mesosaurus, Glossopteris) on continents now separated by ocean; the profile of the ocean floor (mid-ocean ridges and trenches); the age of sea-floor rocks, increasing away from the ridge; and symmetrical magnetic-reversal stripes either side of the ridge.

Who built the theory

Alfred Wegener (1912) proposed continental drift but lacked a mechanism. Arthur Holmes proposed mantle convection as the missing force. Harry Hess (1962) proposed sea-floor spreading. Vine & Matthews (1963) linked the symmetrical magnetic stripes to spreading and reversals. Drilling by the Glomar Challenger (from 1968) confirmed sea-floor ages.

Why the sea floor settled it

Wegener’s fatal gap was the mechanism. Sea-floor spreading closed it: continents ride outward on crust created at the ridge, so nothing has to plough through solid floor. The magnetic stripes (a mirror image about the ridge) and the sea-floor ages (youngest at the ridge, none older than ~200 million years) were the predicted, hard-to-fake confirmation.

For HSC students

This powerbook covers the Evidence for the Theory of Plate Tectonics thread of Module 2 (Plate Tectonics), framed by the inquiry question what is the current evidence for the theory of plate tectonics, and how did the theory develop? Working through it, you should be able to:

analyse the evidence for plate tectonics: the continental jigsaw fit, matching fossils, the ocean-floor profile, the age of sea-floor rocks, and magnetic reversals;
evaluate the contributions of Wegener (continental drift), Holmes (mantle convection), Hess (sea-floor spreading), and Vine & Matthews (magnetic reversals);
explain the role of the Glomar Challenger in dating the oceanic floor;
account for why continental drift was rejected until a mechanism was found.

Explanations here are original and are a study aid, not a copy of the syllabus. See the course hub for the full set of powerbooks and your progress.

Sources

Wegener, A. (1915) Die Entstehung der Kontinente und Ozeane (The Origin of Continents and Oceans) — the continental drift hypothesis.
Hess, H. (1962) “History of Ocean Basins” — the sea-floor spreading model.
Vine, F. & Matthews, D. (1963), Nature — magnetic anomalies over ocean ridges linking spreading and reversals.
US Geological Survey, This Dynamic Earth — overview of the development of plate-tectonic theory and the Deep Sea Drilling Project / Glomar Challenger results.
NSW NESA Earth and Environmental Science Stage 6 Syllabus (2017) — used only to scope the Module 2 content; explanations above are original.

Keep going

Wegener had the answer in 1912 and the wrong half of the planet to prove it. The case was won on the sea floor, by people reading the magnetism of rock he never saw.