Why is the seafloor striped?

That time, that place

Thirty years had passed since Wegener's death, and continental drift was still on the margins.

Then data accumulated from an unlikely direction. Through the Second World War and the Cold War, navies surveyed the ocean floor exhaustively in order to find submarines.

Why this question

The seafloor was nothing like expected. Not a flat plain of mud but a vast mountain range running north to south down the middle of the Atlantic. And the rock of the seafloor was far younger than continental rock — nowhere older than about two hundred million years.

If the oceans are as old as the Earth, why is the floor young?

What was found

Hess proposed an answer: the floor is not fixed but is created at the ridge, spreads sideways, and descends back into the Earth at the deep trenches near continental margins. It is continually replaced, so no old floor survives.

The decisive evidence came from magnetism.

The Earth's magnetic field reverses every few hundred thousand years. And when hot rock solidifies, the field direction of that moment is locked inside it.

So if new crust forms at the ridge and spreads outward, there should be stripes of normal and reversed magnetization, alternating and mirrored on either side of the ridge.

There were. The ocean floor had been keeping its own record.

Terms on this card

Sea-floor spreadingPlate tectonics