A volcanic island is an island, or mass of land isolated by water on all sides, formed by volcanic eruption instead of by continental plate shift. When a volcano erupts, it emits red-hot magma, which is termed lava as it exits the crust. This lava cools as it enters its new environment; in the ocean, the lava cools faster, making the sloping sides of the volcano more contained and more dense than the flowing lava on the sides of a land surface volcano. This lava cools and forms hard rock, which slowly grows in size as the volcano continues to erupt. Sometimes, the volcano can seal itself off by erupting too slowly; the lava solidifies into rock and the weak crustal area where the volcano erupts is plugged or strengthened.
In this case, the volcano continues to erupt, with the volcanic shaft increasing in height as lava builds up the volcanic cone. Eventually, the volcano breaches sea level, and starts to erupt into the air; the lava from this eruption continues to slide down into the ocean, cooling and creating a solid mass. Over time, the mass becomes larger and larger, and accrues different minerals from ocean activity. Some of this material erodes and gains biological content, becoming fertile soil; birds and sea creatures add to this biological content, and eventually the volcanic island is able to support plant and animal life. A good example of a volcanic island is Hawaii, which is composed of several volcanoes that merged their lava output; some of the volcanoes in Hawaii are still active, adding to their island landmass to this day.