How are the Pyrenees geologically different from the Alps?

The Pyrenees and the Alps differ significantly in their geological origins and developmental history. While the Alps are characterized by a more recent and vigorous mountain-building process, the Pyrenees represent the geologic renewal of an older mountain chain.1

The core of the Pyrenees is formed from the Variscan (or Hercynian) orogeny, a mountain-building event that occurred between 370 million and 290 million years ago. Although this ancient base was later submerged, the modern range was primarily uplifted due to the collision between the Iberian and European tectonic plates, a process that began in the late Cretaceous Period and peaked during the Eocene and Oligocene epochs.1

In contrast, the Alps emerged from the Alpine orogeny, which began approximately 65 million years ago as the Mesozoic Era concluded. This process involved the folding of deep sedimentary layers that had accumulated in the Tethys Sea against crystalline bedrock. While both ranges were shaped by tectonic convergence, the Pyrenees were specifically built by the rotation and collision of the Iberian Peninsula with southern France.23