Trekking through an ancient sea in the Italian Dolomites

The main purpose of our August–September trip to Italy was to explore the much-lauded Dolomite Mountains, located in northeastern Italy (Trentino-Alto Adige province) just a few hours north of Venice.

We used a local company (Mont Trekking: https://monttrekking.com/) to get trail information and to make reservations in refugios where we stayed at the end of each trekking day. From their web site we choose the “Fassa Dolomites” 8-day trek described below. We spent another 9 days doing shorter hikes around the periphery of the mountains, mainly from the towns of Bolzano and Trento in the Adige Valley that is located just to the west of the area shown in the image below.

How did these beautiful white mountains form?

The name of the mountains is a big clue! The Dolomites are named after the French geologist Déodat de Dolomieu (1750–1801) who recognized an unusual carbonate rock that resembled limestone (CaCO₃) but was somewhat different. Carbonates are a type of sedimentary rock that are said to be “born” not made. That is because most limestone originates as shells of animals (e.g., clams and corals) or as precipitates of various types of algae. The minerals with the chemical formula CaCO₃ are calcite and aragonite. Both of these minerals effervesce when drops of weak hydrochloric acid (HCl) are applied. Dolomite is a carbonate mineral that does not effervesce so easily. This 1-minute-long YouTube video shows how to use weak HCl acid to distinguish calcite from dolomite: https://www.youtube.com/watch?v=DX6ZMPbA09U. It is now understood that in dolomite magnesium (Mg) atoms substitute for some of the calcium (Ca) atoms—its chemical formula is MgCa(CO₃)₂. For recognizing this different type of carbonate, Dolomieu’s name was given to the Dolomite mountains and to the dolomite mineral. What a legacy!

Organisms extract calcium, carbon and oxygen from sea water to create their hard parts of CaCO₃. There are many factors—e.g., temperature, sea level—that control the rate of carbonate productivity. Rates are highest in shallow, warm, tropical parts of the ocean referred to as “carbonate factories”. Today, these “factories” are only small areas like the Bahamas and Great Barrier reef in coastal Australia. In contrast, during the Triassic Period, large parts of the Tethys Ocean were in favorable locations for high carbonate productivity (see Triassic map above). These rocks have since gotten caught up in the Tethyan closure and are exposed on land throughout the Mediterranean region.

Now look at the Dolomite photographs above and imagine that these carbonate platforms were in ocean water like the platforms in the Bahamas today. All that’s needed is to drop sea level, or to lift the land up out of the sea, and the individual platforms are left high and dry! [Hint: it was uplift.]

Fossils in the Dolomites

Depending on the ocean’s chemistry, most carbonate-secreting organisms make their hard parts from calcite or aragonite (CaCO₃). So where did the Mg come from? We rarely see dolomite forming today, and although it could have been more common in ancient oceans, it’s believed that most of the dolomite formed as a secondary process (called diagenesis), when Mg-rich seawater flowed through the sediments and replaced the calcite or aragonite with dolomite. This process has been observed along the edges of carbonate platforms today. Some of the rocks in the Dolomites are still limestone, as the replacement process was not complete. Nevertheless, most of the rocks have been recrystallized.

Although recrystallization has obscured the fossil shapes, it has made the rocks more resistant to chemical weathering and erosion. Rain water is slightly acidic and more easily dissolves limestone than dolomite (see video link above that shows how to distinguish calcite and dolomite). As the mountains were uplifted, the surrounding rocks were removed and the more resistant dolomite peaks remain.

Refugios and views