Orogeny, a term used by geologists, refers to periods of mountain-building within the Earth’s crust. About 100 such epochs have been identified worldwide, ranging from ancient continental terrains to some of the tallest, actively growing mountains.
Mountain-building, or orogenesis, is driven by the movement of the Earth’s lithospheric plates. When continental plates collide, the immense tectonic forces involved thrust or crumple the crust upwards, forming mountain ranges along the collisional boundaries. The deformation resulting from these forces extends deep beneath the surface mountains.
Many of the world’s highest and most pristine-looking mountain ranges are relatively young, their appearance yet unweathered by natural erosive forces. However, these younger mountains often have ancient foundations. The Pyrénées, forming much of the border between France, Spain, Portugal, and Andorra, serve as a prime example. These mountains, rising over 11,150 feet (3,400 meters) above sea level, were formed about 55 to 25 million years ago by the collision of the Iberian microcontinent with the European plate.
The roots of the Pyrénées, however, date back much further. The rocks comprising the range were initially formed around 500 million years ago during the Variscan orogeny. This ancient mountain-building event involved the collision of the massive continental plates Gondwana and Laurussia, which later fragmented to form modern continents. The tectonic forces of this collision metamorphosed, deformed, and melted continental and ocean crust materials, along with some upper-mantle rocks. Today, many of these ancient, heavily altered “basement” rocks are visible among the peaks of the modern-day Pyrénées, providing a window into the region’s deep geological past.
