There are thousands of islands throughout all of the oceans and there are many more seamounts that grew upward from the seafloor but do not reach sea level. How do these features grow, and how long would it take for them to reach the sea surface so they become an island?
Volcanoes make nearly all ocean islands and seamounts, even though some may be capped with a coral reef. Seafloor volcanoes grow in spurts. When they are in an eruptive phase, they can easily grow about 300 meters (1,000 feet) in a few weeks or months, such as Nafanua Volcano on Vailulu’u seamount near Samoa in the Pacific Ocean.
That growth rate is good enough to form an island on shallow seafloor, but it is not enough to form one on average seafloor with a depth of about 3,600 meters (12,000 feet). At this depth, volcanoes would have to have many growth pulses to eventually breach the sea surface and that may take thousands to hundreds of thousands of years. In fact the chances of a seamount becoming an island is very small. Once islands form, they are very likely to eventually drown, because they subside with the underlying aging seafloor, and because they are worn down by ocean waves.
– Hubert Staudigel, geologist, Geosciences Research Division