In today's video you can actually see the process that formed the Hawaiin Islands in action. The islands are the tops of massive volcanoes, most of whose bulks lie below the sea surface. Puu Oo on the eastern rift zone of the Kilauea volcano has been erupting continuously since January 3, 1983, making it the longest-lived rift-zone eruption of the last two centuries.
The eruption has added 544 acres (2.2 km²) of land to the island of Hawaii. Watch the video and you can clearly see how.
Watch a video of Puu Oo on Google Maps