In the 1950s and 1960s, ships mapped the ocean floor and found a mountain range 65,000 km long winding through every ocean: the mid-ocean ridges. Down the middle of each runs a crack where hot rock rises from the mantle, melts a little, and cools into new basalt. This is seafloor spreading, the missing piece of continental drift.
Key idea
New ocean floor forms at mid-ocean ridges and moves away on both sides, like two conveyor belts. So the rock gets older the farther it lies from the ridge, in the same pattern on both sides.
Why it works
Three findings fit together. Rock samples are youngest at the ridge and older farther away. The sediment on the sea floor is thin at the ridge and thicker farther away, because it has had longer to pile up. And stripes of rock magnetized in alternating directions (Earth's magnetic field flips now and then) lie in matching, mirror-image bands on the two sides of the ridge. Each new strip of rock records the field as it cools, then gets carried away.
Worked example: How old?
A ridge spreads at 2.5 cm a year on each side. How old is ocean floor 100 km from the ridge?
- Change 100 km to centimeters: $100 \times 100{,}000 = 10{,}000{,}000$ cm.
- Time = distance ÷ rate: $10{,}000{,}000 \div 2.5 = 4{,}000{,}000$ years.
Answer: About 4 million years
Worked example: Mirror image
Ocean floor 300 km east of a ridge is 12 million years old. How old is the floor 300 km west of it?
- Spreading pushes new rock out equally on both sides.
- So rock the same distance away on the other side has the same age.
Answer: About 12 million years
Common mistake
Expecting the ocean floor to hold Earth's oldest rock because the oceans seem so ancient. Almost all ocean floor is younger than about 200 million years, because old ocean floor sinks back into the mantle at trenches. Some continental rock is about 4 billion years old. Check: the oldest sea floor is always far from the ridges, near the edges of the oceans.