How were the Alps formed?
Limestone, marble, dolomite—it's all connected
The Alps were formed over many millions of years by the collision of the African and Adriatic plates with Europe. In the process, rocks from the Earth’s interior and sediments from the ancient Tethys Sea were folded, pushed over one another, uplifted, and transformed into a wide variety of rocks due to pressure and heat. Many of the rocks in the Alps were once formed on the floor of tropical seas. (GeoSphere Austria)
When we think of the Alps, we usually picture snow, ice, and rugged cliffs. Geologically speaking, however, we are looking at a gigantic mosaic from the deep sea. Large parts of today’s Alps consist of rocks whose source material was once deposited on the seafloor or in shallow tropical seas, including the calcareous skeletons of trillions of algae, mussels, corals, and other marine organisms. (GeoSphere Austria) As the African and Adriatic plates moved northward, they collided with Europe. The result? The seafloor of the ancient Tethys Sea was compressed, folded, and pushed kilometers into the sky. Over the course of millions of years, this process gave rise to the Alps.
If you look closely, you’ll quickly notice that the rocks in the Alps don’t all look the same. Nature has worked its magic here on a massive scale, using the ancient seafloor and the continental crust to create a fascinating puzzle—one that isn’t as straightforward as it might seem at first glance. There are many different types of individual rocks, as well as hybrid forms. Here are the most important ones:
Limestone is essentially a giant, fossilized prehistoric graveyard. Limestone often tells the story of an ancient sea: it contains the remains of organisms that once formed calcareous shells or skeletons.
When ancient marine limestone was buried deep underground by tectonic movements, it was transformed by heat and high pressure. It did not melt, but recrystallized into marble—such as the famous Laas marble.
It formed deep within the Earth's crust long before the Alps were formed, as magma slowly cooled and solidified. The granite of the Mont Blanc massif, for example, is about 300 million years old. It was not until much later, during the formation of the Alps, that the rock was uplifted and exposed.
Gneiss can form when rocks such as granite are metamorphosed under high pressure and high temperatures. During this process, the minerals rearrange themselves and form the characteristic banding. Gneiss is, so to speak, the “kneaded” version of its parent rock and often sparkles silvery in the sunlight.
This rock formed, among other things, through massive underwater debris flows that transported sand, mud, and other sediments into the ocean basins. Because flysch consists of relatively soft sedimentary rocks such as sandstone, marl, and clay shale, it weathers quickly. It gives parts of the Alpine foothills their gentle, green grassy mountains, but during heavy rains, it is also prone to spectacular landslides.
Many types of slate originated as fine-grained sediments that were metamorphosed under pressure and heat. The ingenious thing about this is that many of these rocks can be split into paper-thin slabs. People in certain regions of the Alps use this dark, glossy rock to cover the roofs of their mountain huts, making them storm-proof.
Anyone who has ever stood before the Three Peaks in South Tyrol knows: These mountains look completely different from the rest of the Alps—even though, geologically speaking, they are part of the Limestone Alps. No rounded summits or massive, gray walls, but rather vertical, pale towers rising from the green alpine meadows like the spikes of a petrified dragon. Why is that?
About 240 million years ago, the region that is now the Dolomites was a tropical underwater paradise. Coral and other calcium-forming organisms lived in the warm, shallow seas, and their remains were deposited on the seafloor. At the same time, the region was characterized by intense volcanic activity, which influenced the chemical composition of the seawater and contributed to the formation of magnesium-rich fluids. How the original limestone gave rise to the dolomite rock that gives the region its name today is a complex process: magnesium-rich fluids played an important role in what is known as dolomitization—the exact process remains the subject of scientific research to this day.
Dolomite rock is relatively durable, yet it is permeated by fissures and layering. Over time, combined with weathering, erosion, and the work of glaciers, this gave rise to the landscape typical of the Dolomites, characterized by towers, pinnacles, and vertical cliffs. Another distinctive feature of the Dolomites: They often glow at sunrise and sunset in the magical colors of the famous Enrosadira. When the sun is low in the morning or evening, its light travels a particularly long distance through the atmosphere. The bright dolomite walls take on spectacular shades of pink, red, and violet.
The next time you’re hiking in the Alps, take a close look at the rock beneath your feet. Now you can impress your fellow hikers with your impressive expert knowledge!
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