A continental collision
The ground here marks the geological seam where two worlds collided. This is the Yarlung-Tsangpo Suture Zone, the line of impact between the Indian and Eurasian tectonic plates. The collision, which began around 50 to 55 million years ago, closed an ancient sea called the Neo-Tethys Ocean. The forces involved were immense they crumpled the Earth's crust, forming the Tibetan Plateau and the Himalayan mountain range.
Before the collision, the Indian plate drifted northwards at a rapid pace for a tectonic plate—up to 20 centimeters per year. As it approached the Eurasian plate, the oceanic crust of the Neo-Tethys was forced underneath Asia in a process called subduction. The suture zone preserves the remnants of this long-vanished ocean. Geologists identify these remnants as ophiolites, which are sequences of rock corresponding to the structure of oceanic crust and the upper mantle. These dark, dense rocks formed on a deep ocean floor, are now found exposed to the thin air of the Tibetan Plateau.
The collision is not over. The Indian plate continues to push north-northeast at about 5 centimeters per year, while the Eurasian plate moves north at only 2 centimeters per year. This ongoing compression causes the Indian plate to crumple at a rate of four millimeters annually, continually driving the Himalayas and the entire Tibetan Plateau higher.
Reading the rocks
The rocks within the Yarlung-Tsangpo Suture Zone record tectonic violence. The ophiolite sequences here are often incomplete, but they contain mantle peridotites and overlying crustal rocks like pillow lavas—basalts that erupted and cooled rapidly on the seafloor. These ophiolites are considered relics of the Neo-Tethys ocean lithosphere. Geochronological studies suggest the ocean floor here formed around 120 to 130 million years ago.
Sitting on top of, and mixed within these oceanic rocks, are marine sedimentary layers. These layers contain the fossilized remains of organisms that lived in the Neo-Tethys Ocean. Fossils of ammonites, trilobites, and crinoids are found high in the Himalayas, showing that these rocks were once at the bottom of a sea. In some locations, scientists have even discovered the fossils of a 10-meter-long marine reptile called the Himalayasaurus, which lived around 210 million years ago. Finding these aquatic fossils thousands of meters above sea level confirms the massive scale of geological uplift that occurred. The continental crust in this region has been thickened to about 75 kilometers, roughly twice the global average.