Tre Cime di Lavaredo
The 3 peaks of Lavaredo (also called Dre Zinnen/3 chimneys), seen here in the mists, are some of the most famous peaks in the Alps. They are located in Northwest Italy, in the province of South Tyrol. The highest of them, Cima Grande (big peak) in the center reaches 2999 meters above sea level.
As they’re some of the key peaks of the Dolomites, it is no surprise that the rocks making up these towers are in fact dolomite, a carbonate mineral.
Their story starts during the Triassic Period, just after the peak of mountain building associated with the Assembly of Pangaea. At the time, Northwestern Europe was colliding with North America, but the area that is today southern Europe sat as a passive margin, far away from any plate boundary. A great sea sat between southern Europe and the nearest continent, the far off landmass of Gondwanaland, containing Africa, India, Australia, Antarctica, and South America.
Along the edge of this passive margin, a carbonate platform began to grow. Carbonate minerals form in tropical areas, where the seas are warm and where the pH is high – that helps the minerals nucleate and grow. Carbonates can grow in all sorts of formats; some form from corals, some form the shells of bivalves and other organisms, some will form the shells of tiny little planktonic organisms. Today in an area like the Bahamas, the best example of a modern carbonate platform, all of these mechanisms are active and carbonate minerals are regularly forming from the ocean.
A passive margin is not just a place where there are no mountains being built. Most passive margins gradually sink downwards into the ocean; as the crust ages it cools and the density increases, causing the crust to slowly drag downwards. Sometimes, old faults involved in the opening of the ocean can be involved as well. This sinking or subsidence creates new space for sediments to be deposited.
Sometimes, in a carbonate platform, the rate of carbonate production balances the rate of subsidence, and the carbonate just starts pouring out, building up thick sequences of sediment over many hundreds of thousands of years. This type of balance led to the formation of the Dolomia Principale, the “Principle dolomite” that makes up these mountains.
The Dolomia Principale is up to 2 kilometers thick in places, and was deposited over an area several hundred kilometers wide. It represents a gigantic deposition of carbonate rocks on the edge of the Tethys Seaway, over 200 million years ago. It can be found across many of the mountain ranges of Northern Italy, folded and faulted by what happened next.
About 100 million years ago, Pangaea had fully broken up, and the great seaway between Eurasia and the broken pieces of Gondwanaland was closing. As Africa impinged upon Europe, the sediments that had once been deposited on the margins of the Tethys found themselves thrust up above the surface.
Carbonate rocks like these weather into a pattern called a karst landscape. Rainwater dissolves carbonate rocks, so as it rains water flows into vertical cracks and the rocks begin to dissolve, forming caves that grow bigger and eventually collapse into sinkholes, leaving tall towers in-between the cracks where water once flowed.
-JBB
Image credit: https://flic.kr/p/2jbMVQZ
Reference:
https://onlinelibrary.wiley.com/doi/abs/10.1111/sed.12416
https://whc.unesco.org/en/list/1237/
