The Great Tectonic Bend
In eastern Indonesia, the Indo-Australian plate is driving northwards, colliding with the Eurasian plate at a rate of about 7 centimeters per year. This collision has created a geologic structure of immense scale and unusual shape: the Banda Arc. This feature is a 1,000-kilometer-long, horseshoe-shaped chain of islands that displays an almost perfect 180-degree curve. The radius of this tectonic curvature is approximately 200 kilometers.
The process driving this is subduction, where the dense oceanic crust of the Indo-Australian plate dives beneath the Eurasian plate and sinks into the Earth's mantle. Using seismic tomography—a method akin to a CT scan for the planet—scientists can map this descending slab of rock. These images reveal a continuous "spoon-shaped" slab of crust that is twisted and folded deep within the mantle, in some places reaching depths of 650 kilometers. This process began around 15 million years ago, shaping the complex geology of the region. The arc itself consists of two parallel chains: a volcanic inner arc, home to active volcanoes, and a non-volcanic outer arc made of uplifted continental and oceanic rocks.
A Rolled and Torn Crust
The extreme curvature of the Banda Arc is the result of a process called "slab rollback." As the subducting plate sinks, it pulls the edge of the overriding Eurasian plate along with it. This backward-pulling motion stretched and thinned the crust, opening up the deep basin that is now the Banda Sea. The formation of the North Banda Basin occurred between 12.5 and 7.2 million years ago, followed by the South Banda Basin between 6.5 and 3.5 million years ago.
This immense stretching created one of the world's most dramatic underwater features: the Weber Deep. This is not a typical oceanic trench but a forearc basin—a depression in front of the volcanic arc. It plunges to a maximum depth of 7,351 meters (24,117 feet), making it the deepest such basin on Earth. Its formation is linked to a massive fault system called the Banda Detachment, a giant rip in the seafloor exposed over 60,000 square kilometers. This detachment fault allowed a huge piece of the crust to be pulled apart by about 120 kilometers, creating the abyss. The region remains highly active, with the fault system having produced a magnitude 9.2 megathrust earthquake in 1629 that generated a 15-meter tsunami.