1. Continents Hold Rock Billions of Years Old, but Almost No Seafloor Is That Old
USGS names the Acasta Gneisses in northwestern Canada, at about 4.03 billion years old, as the oldest rocks found on Earth so far. The U.S. Geological Survey (USGS) also says rocks more than 3.5 billion years old have been found on every continent [7].
The ocean floor tells a different story. By that report, most of it is younger than 200 million years, which is 5 percent of the age of the Acasta rocks. The oldest ocean crust found so far is under the Herodotus Basin, a deep area of the eastern Mediterranean Sea. It was reported in 2016 to be between 315 and 365 million years old [6]. If the oceans are so old, a young seafloor looks odd at first. Something must be erasing it.
2. New Seafloor Is Made Along a Mountain Range Hidden Under the Ocean
A connected system of mid-ocean ridges runs through the major ocean basins, a chain of underwater mountains nearly 65,000 km (40,390 miles) long. More than 90 percent of it lies underwater, in the deep ocean [3].
The ridge sits where two tectonic plates are moving apart. As they separate, molten rock rises to the seafloor and erupts as basalt, and that basalt hardens into new ocean floor [3]. The new floor is made all along the ridge, then moves away on both sides.
The speeds are slow but add up. NOAA reports that the Mid-Atlantic Ridge is spreading at 2 to 5 cm (0.8 to 2 in) a year, and the East Pacific Rise much faster, at 6 to 16 cm (3 to 6 in) a year [3]. Spreading rates describe how fast the two sides move apart. As a rough calculation of our own, at 3 cm (1.2 in) a year, the two sides end up about 3,000 km (1,900 miles) farther apart after 100 million years.
3. Old Seafloor Sinks Back Into the Earth at Deep Ocean Trenches
USGS says Hess's idea neatly explained why the Earth does not get bigger as the seafloor spreads: the ocean floor moves away from the ridges in a conveyor-belt-like motion, and oceanic lithosphere sinks back into the mantle at the ocean trenches [1].
The deepest known point in the ocean is in a trench. The Challenger Deep, at the southern end of the Mariana Trench in the western Pacific, is about 10,935 m (35,876 ft) deep, while the average ocean depth is 3,682 m (12,080 ft) [4]. Sinking plates also cause earthquakes: along each trench, earthquake zones dip into the Earth at roughly 40 to 60 degrees and reach several hundred kilometers down [1].
Putting the two ends together gives the answer to the title. Ocean floor starts at a ridge, travels for tens or hundreds of millions of years, and is then pulled down at a trench. Old seafloor is mostly gone, so very little of it is left to be found. Continental rock is not recycled in the same way, which is why old rock survives there. The sources used here do not explain exactly why. You could look for an answer in other sources.
4. Harry Hess Proposed the Conveyor-Belt Idea Before Anyone Could Prove It
The geologist Harry Hess circulated the idea in a manuscript in 1959 and published it in 1962 as "History of Ocean Basins" [2]. In his picture, magma oozes up along the mid-ocean ridges, forms new crust, and the crust moves away until it sinks into trenches. Hess also pointed out that the continents did not have to push through the ocean floor [2].
He had a clue from the seabed itself. By his reasoning, sediment on the ocean floor had piled up for about 300 million years at most, and the oldest fossils found there were no more than about 180 million years old. Yet many geologists of the time believed the oceans were at least 4 billion years old [2]. The seafloor looked far too young for that, which is the same puzzle this article started with.
5. Rock Ages and Magnetic Stripes Showed That the Seafloor Really Moves
A good idea has to be tested. Two kinds of evidence mattered most.
Rock ages. Along the ridge crest the rocks are very young, and they become steadily older farther away. In 1968 the drilling ship Glomar Challenger brought up samples from the seafloor. Their dated ages, USGS says, gave the clinching evidence for seafloor spreading [1].
Magnetic stripes. Surveys beginning in the 1950s found that seafloor rock forms alternating stripes of different magnetism, laid out in rows on both sides of the ridge. The youngest rock at the crest has the present-day (normal) magnetic polarity [1]. Frederick Vine and Drummond Matthews saw how this fit Hess's idea in a 1963 paper in Nature [8]. The Earth's magnetic field switches direction from time to time, and the mineral magnetite in cooling basalt lines up with the field and keeps that direction as a record. So as new rock forms at a ridge and slides away, each reversal is stored as a stripe, and the pattern is a mirror image on the two sides [5]. The Canadian geologist Lawrence Morley arrived at the same idea independently, which is why it is now called the Vine-Matthews-Morley hypothesis [5].
The ages and the stripes agreed with each other and with the conveyor belt picture. That agreement is why the idea moved from a bold guess to accepted science.
6. Why "Conveyor Belt" Is Only a Picture, and What Is Still Worth Asking
The conveyor belt is a helpful picture, but real seafloor is not a neat belt. Plates move at different speeds, and, as the Herodotus Basin shows, a few very old patches survive. The sources used here say only that this is an unusually old patch of ocean crust, so why it was not recycled is a question for further reading.
Scientists also keep refining the numbers. The 2016 report of crust 315 to 365 million years old was a study in Nature Geoscience that used magnetometer readings [6]. A future study could find an even older patch of ocean crust.
7. How to Find Ridges and Trenches on a Seafloor Map With an Adult
Look for a seafloor map in an atlas or an online ocean-floor map. Find the Mid-Atlantic Ridge, the long line of high seafloor running down the middle of the Atlantic. Then find the deep trenches along the edges of the Pacific Ocean, and use the map scale to estimate the straight-line distance from the ridge to a nearby trench. Spreading rates describe how fast the two sides move apart, so assume each side moves about half that fast. At a spreading rate of 3 cm (1.2 in) a year, each side moves about 1.5 cm a year. How many million years would that take? Compare your answer with the "mostly younger than 200 million years" figure; it is only a rough guess.
To read more, start with the USGS pages "This Dynamic Earth" and the NOAA Ocean Exploration page on the mid-ocean ridge, both listed below.