The earth's crust is constantly changing, and plate material is always being created and destroyed. As a result, the continents and oceans cannot ride atop the plates without being changed.
There are ranges of seafloor mountains that circle the earth. These form the Global Mid-ocean Ridges. Magma is pushing up from Earth's interior all along these ridges, creating new crust. The same forces that pull the plates apart also allow magma from the earth's interior to come up along the ridges and create new crust. Areas where new crust is created and plates are forced apart are called spreading centres.
Although the movement of the plates away from spreading centres is very, very slow (about 2.5cm per year), it has been happening for millions of years. Thus, very small changes have become very large over time.
Two hundred million years ago the Atlantic Ocean was just a small bay between the continents of Africa, Europe, and the Americas. Now it is a huge ocean. The mid-Atlantic Ridge spreading centre that runs north-south beneath the Atlantic Ocean has been adding about 24km of new crust every million years, with the result that the Atlantic Ocean is now almost 5000km wide.
When scientists first discovered that new crust was being created continuously, they wondered if the earth were getting larger. Using mathematical models of the earth and sophisticated measuring tools they have confirmed that it is not. If this much new land has been created and the earth is no larger now than it was 200 million years ago, where is all the extra crust going? The answer is that it is being recycled.
The green lines are plate boundaries the pink lines are both plate boundaries AND spreading centres
This information was taken from the Scripps Institute of Oceanography web site. Please go there to learn more.