Somewhere down the middle of the Atlantic Ocean, running roughly from Iceland to near Antarctica, there's a seam in the Earth's crust that never stops opening. Magma rises through it, cools into new rock, and pushes the older rock on either side further apart — a few centimetres a year, which sounds negligible until you notice it's also roughly how fast a fingernail grows. The comparison isn't a cute simplification; it's the actual order of magnitude geologists use to explain why plate tectonics feels so slow from a human vantage point and adds up to so much over deep time.
A seam that manufactures its own ocean floor
This seam is a mid-ocean ridge, and it's the visible edge of two tectonic plates — enormous slabs of crust and upper mantle — being pulled apart. As they separate, pressure on the mantle beneath drops just enough for rock there to melt, and that magma rises to fill the gap, solidifying into brand-new seafloor. The Atlantic's ridge has been doing this for roughly 180 million years, ever since the supercontinent Pangaea began breaking apart, and every strip of ocean floor on either side of it is, in a real sense, a growth ring: older and colder the further you get from the ridge, younger and warmer the closer you get to it.
Nothing about this requires the ridge to push. The leading theory isn't that magma shoves the plates apart from underneath — it's that the plates are also being pulled by their own far edges, where older, colder, denser ocean floor sinks back into the mantle at a subduction zone and drags the rest of the plate down and along behind it. The spreading ridge is as much a symptom of that pull as a cause of it.
Growth somewhere means shrinkage somewhere else
The Earth's surface area isn't increasing, so new seafloor at a spreading ridge has to be balanced by old seafloor disappearing at a subduction zone somewhere else on the planet — the Pacific's edges are ringed with exactly these zones, which is most of why the Pacific Ocean is slowly shrinking while the Atlantic grows. Plate tectonics isn't a single expanding system; it's a closed one, constantly recycling crust between creation at ridges and destruction at trenches.
What we're still unsure about
"Ridge push" versus "slab pull" is a genuinely live question, not one this post can settle in a paragraph — most current models treat slab pull as the dominant force, but the relative contribution of the two, and how much mantle convection itself drives both rather than just responding to them, is still actively debated in geodynamics. Spreading rates also aren't uniform: some ridge segments open closer to twice the Atlantic's rate, others barely a third of it, and treating "a few centimetres a year" as one universal number for all mid-ocean ridges would be flattening real variation the data shows.
This sits inside Plate Tectonics & Geological Structures, one of seven topics in Physical Geography, one of five domains in Geography, one of seventeen subjects the app can quiz you on.