Which ocean is still getting wider?

The Atlantic, by a couple of centimetres a year. New sea floor is being made along the Mid-Atlantic Ridge, a mountain chain running the length of the ocean, where magma rises between two plates pulling apart and freezes onto the trailing edges of both. The Americas and Europe are moving away from each other at about the rate your fingernails grow.

The proof of it is written in magnetism. The Earth's magnetic field reverses every few hundred thousand years, and each new strip of sea floor locks in the field direction as it cools. The result is a pattern of magnetic stripes running parallel to the ridge and mirrored perfectly on both sides - surveyed in the 1950s and 60s, and the single piece of evidence that turned continental drift from a fringe idea into plate tectonics.

The Pacific is doing the opposite, being consumed at its edges faster than new floor is made, which is why it is ringed with trenches, volcanoes and earthquakes. The Atlantic has quiet margins and the Pacific has the Ring of Fire, and it is the same process seen from both ends.