Why do volcanic islands sometimes appear suddenly?

Volcanic islands can sometimes appear suddenly. This has happened several times. For example, in November 1963, an eight km high column of steam, ash, and volcanic fumes was ejected from the ocean bed when lava started building up in the middle Atlantic sea floor.

Within two days, this ejected material, which fell back into the sea, formed an island. In a week’s time, the island was 100 km long, and growing. This island is today known as Surtsey Island.

In November 2013, explosive volcanic activity was observed about 1000 km south of Tokyo in Japan, and huge lava outflows were seen rising up to the surface of the ocean. Within a month, a new island rose, 25 metres above sea level. In January 2015, a new island was formed in the South Pacific after the eruption of the Hunga Tonga volcano.