<span>Seamounts form by submarine volcanism. After repeated eruptions, the volcano builds upwards into shallower water. If a seamount eventually breaches the water's surface, it becomes an island. ... They may form in groups or clusters, or can be found aligned in submarine volcanic mountain chains known as oceanic ridges.</span>
To calculate the anomalies, from each monthly data point you subtract that month’s average. These monthly averages, called the “climatology”, are shown in the top row of Figure 2.
