Geology of the Euganean Hills
Like islands in the middle of the Po Valley, originating from volcanic phenomena
The peculiar morphology of the Euganean Hills is definitely the main distinctive feature of this mountainous area, considered unique of its kind. Everyone, getting in touch for the first time with the precise and geometric outline of these heights, can’t help wondering why these hills are perfectly conical shaped, with isolated peaks alternating with gently rounded slopes. Their geological origin is also crucial to know both naturalistic features and historical developments in the area.
Walking along the sundry trails in the Park, one can meet very clear limestone rocks, which are sedimentary stones, the most ancient existing on the Euganean Hills: they were born from the deposition of debris and sediment, when still the sea was there, instead of the plain. The oldest sedimentary rock is called Rosso Ammonitico (Red Ammonitic facies): it rose between 150 and 135 million years ago, during the early Jurassic. This rock contains numerous ammonites fossils from which it takes its name; it can only be observed in a cleft of Mount Resino, in the area of Fontanafredda. As per the genesis of the next rock, it originated between 135 and 90 million years ago in the late Cretaceous, and is called Biancone (White Giant). It's a white limestone composed almost entirely of calcareous shells of microscopic organisms that were deposited on the bottom of the deep open sea. Afterwards we can find the Scaglia Rossa (Red Splinter), which dates back 90 to 55 million years ago, between the early Cretaceous and the late Eocene. It’s a clayey limestone which often contains fossils such as sea urchins and shark teeth (the sea temperature was about 25° C, like in the tropics). The last sedimentary rock appearing is the Euganean Marl, 55 to 33 million years ago, from the early Eocene to the late Oligocene. It's a very clayey rock, grayish-green color, which can be found primarily in the area of Teolo. Exactly during the creation phase of this latter rock, about 43 million years ago, a first eruptive cycle took place: from fractures in the marine crust, some magma flowed out and expanded on the sea bottom, originating dark basaltic (basic) rocks. This type of volcanic rocks can be observed in the vicinity of Teolo and Mount Gemola. At the end of the first eruptive cycle, the following period was an inactivity stage allowing the constitution process of the Euganean marls to continue. The second eruptive stage occurred between 33 and 30 million years ago, during the late Oligocene. It still was an underwater phase, but unlike the first eruptions, in this second cycle, the magma escaped was more acidic and viscous. The different kind of magmatic material probably derived from the fusion of the basaltic mantle and the base of the earth’s crust. During its ascent, the lava stationed in an underground basin not very deep (between 3000 and 3500 m) and a fractional crystallization favored the emergence of a lighter and more acid magma, while the more basic and heavy material remained on the bottom of the eruptive channel. In this phase, three types of rocks developed: • Rhyolite, a light colored stone, containing a quantity of silica > 65% (it can be seen on Mounts Solone, Cinto, Ventolone, etc.). • Trachyte, which is yellowish-grey and has a higher silica between 65 and 55%: it can be observed on Mount Lonzina, on Colle della Rocca (the Fortress Hill), on Mount Rusta, etc.. • Latite, which is very dark and whose silica content is between 55-50% (found, for instance, on Mount Sengiari, Mount Cecilia, etc..). The second eruptive cycle led to a leakage of low content silica basaltic seam, which was more fluid and penetrated between the other existing rocks. An example of this phenomenon can be found in the cave on Mount Brusà, visible from the road that leads to Treponti, where one seam is embedded in basalt rhyolite. At the end of this second volcanic activity stage, the highest peaks of the hills appeared emerging out of the sea surface, creating a sort of archipelago of small islands. From the late Oligocene until the end of the Pliocene, a gradual withdrawal of water intervene, producing, around the hills, a transition environment similar to that of a brackish lagoon. During the Quaternary, about 2 million years ago, the rivers brought _in the peri-Euganea area_ large amounts of material derived from the erosion of alpine mountain ranges, forming the Po Valley and ultimately leading to the emergence of the Euganean Hills. During the last few millennia, in addition to the erosive action by atmospheric phenomena and further to the gravitational force (mudslides and landslips), a strong human action (man has been living on the hills since the Paleolithic era), have contributed to the current appearance of the Euganean landscape. Those interested in learning more about the geology of the Euganean Hills can visit the striking geo-paleontological museum of Cava Bomba in Cinto Euganeo district, which is also endowed with an external location, in addition to the exhibition halls. There are also a number of geo-sites scattered over the hills: they are remarkable geological sites of scientific interest, accessible both from a tourist and educational point of views. Among them, the following geo-sites can be highlighted: • Mount Brusà in the range of Lonzina-Sengiari: here are three quarry faces that create a great visual impact, thanks to the impressive columnar cracks of rhyolitic rocks; • Mount Cinto, where there are two quarries: the lowest which is in sedimentary rock and the closest to the summit, just as spectacular, that is in rhyolite; • Sasso delle Grotte (Cave Stone) on the west side of Rocca Pendice, which reveals a vertical trachyte wall with several small cavities, due to evaporation of magmatic gas during the cooling stage; • Mount Cecilia, in the municipality of Baone, where we may observe the typical onion exfoliation or weathering of latite.