At dusk, Lac de Chéserys above Chamonix in the French Alps reflects the Mont Blanc massif. The moon is rising above the jagged peaks of Les Grandes Jorasses, while the first stars are already appearing in the quickly fading light.
About 45 minutes after sunset, the twilight sky takes on a distinctive deep blue color caused by ozone high in the atmosphere. The last rays of sunlight travel through the atmosphere at an altitude of about 16 miles (25 kilometers). Ozone absorbs much of the incoming light, while the shorter blue wavelengths remain more prominent, giving the sky its intense blue hue. The rising moon illuminates the mountains as the first stars emerge in the deepening silence.
The rocks that make up the panorama of the 15,781-foot (4,810-meter) Mont Blanc massif have a geological history dating back to the Carboniferous and Permian periods, about 300 million years ago. At that time, all major landmasses were united into the supercontinent Pangaea. The Variscan mountain range formed along the continental collision zones between Africa, Eurasia, North America, and South America. Over the following eons, this mountain range was almost completely eroded away. What remains today is its crystalline basement, composed largely of granite.
During the Mesozoic Era, the age of the dinosaurs, this granitic basement was covered by ocean sediments several miles thick. About 40 million years ago, the Alpine orogeny began to raise the region, with particularly intense phases of uplift around 22 million years ago and again between 12 and 5 million years ago. The granite basement of the former Variscan mountain range was thrust upward into a dome, while the overlying softer sediments were gradually stripped away. The removal of this enormous weight allowed the granite to expand and contributed to further uplift of the mountain mass.
Finally, the glaciers of successive ice ages carved and sculpted the uplifted rocks, creating the dramatic relief of today’s Mont Blanc massif.
