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.
A hike through tectonic history
Trekking through the Meratus Mountains is a look back in time, across a landscape that was once at the bottom of the sea. Today, the terrain is mountainous and covered in dense rainforest, but the rocks underfoot tell the story of their violent submarine origins and tectonic emplacement. The region is recognized as the Meratus Geopark, protecting this unusual geological heritage.
Hikers can find greenish, waxy-textured serpentinite, a rock formed when the mantle rock peridotite is altered by seawater. Outcrops of pillow basalts are also visible, their distinctive shapes preserved for over 100 million years. These rocks are shows the structure of oceanic crust, which is normally hidden beneath kilometers of water. The entire complex records of a long-vanished ocean and the powerful collision of tectonic plates that built the island of Borneo. The mountains separate the Barito Basin to the west from the Asem-Asem Basin to the east, a direct result of this ancient tectonic uplift.