How are sea floor spreading and plate tectonics related?
New sea floor is produced at mid-ocean ridges and that ocean is growing — generally at a rate of some cm/yr. If, as in most of the Atlantic, the ocean is attached to the continent — to Europe and to Africa and to the Americas, the latter move apart from each other. As the Earth is not expanding, somewhere else there is ocean crust sinking into the deeper parts of the Earth. These oceans are not attached to the continents — they have broken off — and the ocean is shrinking and the continents are approaching each other — that is the case around much of the Pacific. If we go back about 200 million years, there was no Atlantic Ocean, the continents on either side of what is now the Atlantic were joined. You could walk from Washington D.C. to Cadiz without getting wet unless some large creature ate you. The Pacific was much larger and, in fact, all the continents were joined up. Then the Atlantic formed, the continents split, the sea spread, and the Pacific shrank. One day, maybe, the
