An Oceanic Collision in Action
The Sangihe volcanic arc owes its existence to a tectonic arrangement found nowhere else on Earth. Here, the Molucca Sea Plate is trapped in a vise, subducting simultaneously westward beneath the Sangihe Arc and eastward beneath the Halmahera Arc. This process, known as divergent double subduction, is the final stage of an arc-arc collision that has been underway for millions of years. The two volcanic arcs, Sangihe and Halmahera, are steadily closing in on each other, with the oceanic crust of the Molucca Sea Plate sinking beneath both.
This geological drama unfolds as three major plates (the Eurasian, Pacific, and Australian) converge in the region. The Sangihe Arc sits on the edge of the Eurasian Plate, while the Halmahera Arc is part of the Philippine Sea Plate. Seismic imaging shows the U-shaped slab of the Molucca Sea Plate descending to depths of over 600 kilometers under the Sangihe Arc and around 250 kilometers under the Halmahera Arc. The convergence rate on the Sangihe side is approximately 4 centimeters per year. This intense geological activity makes the region one of high seismicity. As the collision progresses, the two arcs are now only about 250 kilometers apart and will eventually fuse.
The Volcanic Front
The double subduction directly feeds a chain of active, dangerous volcanoes that form the Sangihe Islands. This chain includes volcanoes such as Karangetang, Ruang, and Gunung Awu. The magma produced is predominantly andesitic and basaltic andesite, typical of subduction zones. Geochemical analysis of the lavas shows that fluids from the subducting slab, including melted sediments, are incorporated into the mantle, influencing the composition of the magma that erupts.
Karangetang, also known as Api Siau ("Fire of Siau"), is one of Indonesia's most active volcanoes, with more than 40 eruptions recorded since 1675. It frequently produces explosive activity, lava dome growth, and dangerous pyroclastic flows. Gunung Awu, at the northern end of the main island, is known for its large, violent eruptions, which have produced pyroclastic flows and lahars, causing thousands of fatalities over the last few centuries. Ruang is another stratovolcano in the arc, with a long history of major explosive eruptions, one of which triggered a destructive tsunami in 1871. The unique tectonic pressure contributes to the volatile and explosive nature of these eruptions.