An inverted tectonic sandwich
The Talaud Islands are at the center of one of the most complex tectonic settings on Earth: the Molucca Sea Collision Zone. This is where the massive Eurasian, Philippine Sea, and Australian plates converge. The main feature of this area is a unique process called divergent double subduction. Here, a small oceanic plate known as the Molucca Sea Plate is being forced downwards, or subducted, from two directions at once. To the west, it sinks beneath the Sangihe Arc, and to the east, it sinks beneath the Halmahera Arc. This action has bent the plate into an inverted U-shape.
Typically, a chain of volcanoes, called a volcanic arc, forms on the overriding plate, directly above the sinking slab. Sangihe and Halmahera arcs are classic examples of this process. The Talaud Islands, however, present a geological contradiction. They are an ancient volcanic arc located in a "rear-arc" position, on the part of the plate that should not have volcanism. The rocks here, including basalts and andesites, are of Neogene age (spanning from roughly 23 to 2.6 million years ago) and are interbedded with marine sedimentary layers like tuffaceous siltstone and marl. The presence of these volcanic rocks in this specific location challenges the standard model of subduction zone volcanism.