The diagram illustrates a natural process in which a volcanic island grows from beneath the sea, explodes, and then eventually sinks to become an underwater reef. The process is divided into nine stages.
In the first two stages, called the ‘preshield’ and ‘protoshield’ stages, a mountain gradually builds under the surface of the sea. In the third or ‘explosive’ phase, hydro-explosions occur which deposit a cone of ash. A large number of cinder cones then pile up to form a ‘shield’ on top of the volcano.
This shield gradually erodes and subsides to form reefs below the surface of the sea. However, there may be ongoing volcanic activity including lava flows. By the time of the ‘coral atoll’ stage, the original form of the volcano has disappeared, and the whole structure sinks further under the sea during the ‘guyot’ stage.
In summary, huge volcanoes rise and fall under the sea in a natural process known as the volcanic island growth cycle.