Volcanic Island Arc
Depth: 30 m · Sunlight Zone
Where one tectonic plate dives beneath another, rock melts deep underground and rises back up as magma. This builds a whole chain of volcanoes next to the ocean trench, called an island arc. The tallest volcanoes poke out of the water as islands, just like the Mariana Islands next to their trench.
For scientists
Once the descending plate has sunk to about 100 km, it squeezes water out of its rock; this lowers the melting point of the mantle above, so magma rises and builds the chain of volcanoes. The arc therefore always runs parallel to the trench, a few hundred kilometers inland. Such subduction volcanoes often erupt especially explosively.
Source: Encyclopaedia Britannica
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