A volcanic island arc is a curved chain of volcanic islands and undersea volcanoes that forms beside an ocean trench, where one oceanic plate sinks beneath another. The arc runs roughly parallel to the trench.
Key takeaways
- Island arcs form when two oceanic plates converge, and the volcanoes pile up until one breaks the surface.
- Arcs are generally curved and run close to the trench beside them.
- Many arc volcanoes never reach the surface: in our grid, Kavachi has a summit 140 m down and Ahyi 179 m.
- A volcanic island arc is not the same as a hotspot chain, which forms in the middle of a plate.
How a volcanic island arc forms
When two oceanic plates converge, one is pushed beneath the other along a subduction zone. Magma forms as the sinking plate and the plate above it partly melt, and it rises through the upper plate [1]. Over millions of years, lava and volcanic debris pile up on the seabed until a submarine volcano breaks the surface and becomes an island [1].
Because the melting happens in a line above the sinking plate, the volcanoes also form a line. They are strung out in chains called island arcs, and the arcs are generally curved and run close to the ocean trench beside them [1]. The Mariana Islands and the Aleutian Islands are the examples USGS gives [1].
An arc is mostly under water. Each island is the top of a volcano that is much larger beneath the surface, and between the islands there are many more volcanoes whose summits never reach the air. These submarine volcanoes are a large part of the arc, and they are the reason that a map of the seabed shows an arc as a long, curved ridge with peaks along it rather than a scatter of dots.
The glossary pages on ocean trenches and seamounts cover the other two parts of the picture.
An example from our data
The Tonga arc lies beside the Tonga Trench, which reads 10,755 m in the grid. On the arc, Hunga Tonga-Hunga Ha'apai has a summit that the grid places only 5 m below the surface, 58 km from Tonga. The Tonga-Kermadec Ridge runs along the same line.
Other arc volcanoes sit just below the waves. Kavachi, near the Solomon Islands, has a summit 140 m deep in the grid, 25 km from land. Ahyi, in the Mariana arc, is 179 m below the surface and 15 km from the Northern Mariana Islands, and Myojin-sho is 125 m deep and 50 km from Japan.
Our list has 420 named seamounts in all. The map does not mark which are arc volcanoes, so use the feature pages to see the depth and the nearest coast. The homepage shows the deepest trenches that sit beside arcs.
The thing people get wrong
People often assume every chain of volcanic islands forms the same way. Hawaii, for example, is not an island arc. Hotspots are mostly found in the interiors of plates, not at plate boundaries, and the islands they build form straight chains that get older along one direction [2]. Arcs form at convergent boundaries and curve around the trench.
A second mistake is to think the arc and the trench are the same feature. The trench is the deep line where the plate bends down. The arc is the volcanic chain a short distance behind it. Together they form a pair, with the trench on the outer side.
Island arc vs hotspot chain
Frequently asked questions
Which islands are volcanic island arcs?
USGS gives the Mariana Islands and the Aleutian Islands as examples. Each lies beside a deep trench, the Mariana Trench and the Aleutian Trench.
Why is an island arc curved?
Arcs closely follow the curve of the trench beside them, which marks where the sinking plate bends down. USGS describes arcs as generally curved and parallel to the trench.
Sources
- Understanding plate motions, USGS (This Dynamic Earth).
- Hotspots, USGS (This Dynamic Earth).
- Every figure from our own map is worked out from the datasets on the data sources page (checked against GEBCO_2026 grid).