A cross-section of the ocean floor
In the Himalayan foothills of Himachal Pradesh, you can place your hand on rocks that once formed the floor of an ancient ocean and the fiery mantle beneath it. The Mandi Ophiolite Complex is a piece of oceanic lithosphere—crust and upper mantle—that was scraped off a tectonic plate and thrust onto the Indian subcontinent. Geologists call this an ophiolite, and it represents a complete cross-section of the oceanic floor, from deep mantle rock to undersea lava flows.
The sequence of rocks records the creation at a mid-ocean ridge. The lowest, deepest layers are ultramafic rocks like peridotite and dunite, which are characteristic of the Earth's upper mantle. Above these lie gabbros, coarse-grained rocks that cooled slowly in magma chambers deep beneath the ocean floor. This is overlain by a "sheeted dike complex," a mass of vertical basalt dikes that acted as conduits for magma. At the very top are pillow basalts, distinctive bulbous lavas formed by eruptions directly into cold seawater. This entire stack of rock, once horizontal, was tilted and broken during its journey onto the continent.
From Tethys to the Himalayas
The story of the Mandi Ophiolite begins in the Neo-Tethys Ocean, a vast sea that separated the supercontinents of Gondwana and Laurasia during the Mesozoic Era. The oceanic crust that makes up the complex formed at a spreading ridge within this ocean. As the Indian plate drifted northward, the Neo-Tethys began to close, a process starting around 155 million years ago.
The final closure of the ocean occurred around 50 million years ago when the Indian plate collided with the Eurasian plate. This colossal, slow-motion impact gave rise to the Himalayan mountain range. During the collision, instead of being pushed down (subducted) into the mantle, a fragment of the dense oceanic plate was instead shoved upward onto the edge of the lighter continental crust. This process, known as obduction, is how the Mandi Ophiolite was emplaced into its current position. The rocks here are a record of a former ocean and the birth of the world's highest mountains.
The mantle rocks exposed here have undergone a transformation called serpentinization. This chemical reaction occurs when ultramafic rocks like peridotite, rich in olivine and pyroxene, are exposed to water at low temperatures. The original minerals are converted into a group of green, waxy, or slippery minerals called serpentine. This process significantly increases the rock's volume by 30-40% and reduces its density, which helps in its uplift to the surface. The name "ophiolite" itself comes from the Greek ophis, for snake, referring to the snakeskin-like texture of serpentinite.