3. Seafloor Spreading: The Proof

In the 1960s, geologist Harry Hess studied the ocean floor and proposed seafloor spreading. At long undersea mountain ranges called mid-ocean ridges, magma rises from the mantle, hardens into new oceanic crust, and pushes the older crust outward on both sides — like a conveyor belt slowly carrying the seafloor away from the ridge.

The proof came from magnetism. Earth's magnetic field flips direction every so often (Lesson on the core). As new crust hardens at the ridge, it records the field's direction at that moment. The result: the seafloor carries a pattern of magnetic stripes — and they are a perfect mirror image on both sides of the ridge. The only way to make that pattern is if new crust forms at the ridge and spreads symmetrically outward.

Two more clues clinched it:

• The seafloor is youngest at the ridge and gets older with distance from it.
• The oldest ocean floor is less than about 200 million years old — far younger than the continents (billions of years) — because old seafloor is constantly recycled back into the mantle.

Seafloor spreading was the mechanism Wegener lacked. Combined with continental drift, it became the modern theory of plate tectonics.

⚠️ Common misconception: Earth is not expanding. New crust made at ridges is balanced by old crust destroyed at subduction zones (Lesson 4) — the planet stays the same size.

💡 Tip: the symmetric magnetic stripes are the 'barcode' that proves the seafloor spreads.