A cross-section of the Earth's crust
The Meratus Mountains of southeastern Borneo hold a geological secret: an immense slab of ancient oceanic crust and upper mantle, stranded hundreds of kilometers from any ocean. This is an ophiolite, a complete cross-section of the seafloor that has been thrust up onto a continent. The process, known as obduction, is a rare tectonic event where a slice of a subducting oceanic plate gets scraped off and pushed onto the overriding continental plate, rather than sinking into the mantle.
The rocks of the Meratus ophiolite complex began their life as the floor of the Meso-Tethys Ocean. Their formation dates back as far as 200 million years ago. The complex contains the classic sequence of rocks that define an ophiolite. At the lowest levels are ultramafic rocks like peridotite and dunite, which originate in the Earth's upper mantle. Above these lie intrusions of gabbro, followed by sheeted dikes, and finally, at the top of the sequence, pillow basalts. These bulbous, pillow-shaped rocks are a definitive sign of lava erupting and cooling rapidly underwater. The sequence is capped by deep-sea sediments like radiolarian cherts. The entire slab was pushed onto the edge of the Sundaland continental core during the Early Cretaceous, between 137 and 110 million years ago.