A tectonic collision in plain sight
The island of Seram sits within one of the most tectonically complex areas on Earth. It is a part of the Banda Arc, a curve of islands formed by the collision of the northward-moving Australian continental plate with a series of microplates in the Banda Sea. North of the island, the seafloor buckles into the Seram Trough, which reaches depths of over 3,500 meters. This trough is not a subduction trench where one plate slides smoothly under another. Instead, it is a fold-and-thrust belt, a zone where the Australian continental crust is being compressed, buckled, and bulldozed upwards.
This intense compression is the direct cause of Seram's mountainous landscape. The ongoing collision forces the island to rise, lifting ancient seafloor and deep crustal rocks high above sea level. This process has created peaks like Mount Binaiya, which stands at 3,027 meters (9,931 feet). The entire island is an active orogen, or mountain range. The process began in the Late Miocene and Early Pliocene and continues today, marked by frequent seismic activity in the surrounding region.
Mountains made of mantle
The upward thrusting on Seram has created a geological cross-section through the Earth's crust and upper mantle. Rocks that were once deep beneath the ocean are now exposed on the island's surface. Geologists refer to these displaced sections of oceanic crust and upper mantle as ophiolites. Seram contains extensive tracts of these rocks, including large formations of peridotite, a dense rock that makes up most of the Earth's upper mantle.
On Seram, these deep-earth rocks are part of a formation known as the Kobipoto Complex. This complex contains peridotite and metamorphic rocks like granulites and migmatites. Analysis of the minerals in these rocks reveals the extreme conditions of their formation. Some of the granulites show they were created at temperatures approaching 900°C and pressures corresponding to depths of 25 to 30 kilometers. The island of Seram allows people to walk on rocks that were once part of the planet's deep interior.