Morphology

GEOMORPHOLOGY

From the geomorphological point of view, the region has well-defined structural elements: i) sediments of marine origin in the limestone foundations of the Prenestini Mountains (with outcroppings that indicate a shallow sea shelf protected from the motion of the waves, similar to what is found today in the Bahamas); ii) structures produced more recently as a result of Laziale Volcano activity.

The entire district falls within a zone of particular tectonic structure with several fault systems located at the border between the Umbro-Marchigiana facies (batial facies) and the Abruzzese facies (neritic facies) where a succession of carbonate formations of 3,000- 4,000 m each are found: mainly dolomite at the base, limestone at the summit. The sedimentary substrate is made of Mesozoic units that, since the Middle Miocene, have been subject to intense phenomena, first compression and later extension.

Thereafter, approximately two million years ago, the area underwent a generalized subsidence and the ensuing depression was invaded by pliocene, calabrian and sicilian waters. During the millennia that followed, powerful blankets of clay filled a great portion of the existing valleys although, after being deposited, the materials remained subject to tectonic movements. The existence of fractures — lines of minor resistance — allowed the underlying magmatic mass to rise up to the surface.

In the Albana region, close contact between the carbonatic rock and the magma caused the limestone and dolomite to be assimilated within the magma, thus producing a leucitic composite. A basic, predominantly explosive mechanism ensued, its products reaching far and wide, even brushing up against the Prenestini Mountains, diverting the Tiber River to the west and forcing the coastline to move south-west. Between 700,000 and 500,000 years ago this produced a series of multiple eruption plumes.

While, in some cases, the volcanic activity wore itself out in a single episode, around the middle of this period numerous episodes, both explosive and efflusive, formed a volcanic layer that spread the products in concentric rings. Between 500,000 and 360,000 years ago, enormous lava and pyroclastic emissions built up the great Tuscolano-Artemisio structure, more than 2,000 m high. So much material was expelled that the internal walls of the volcanic conduits collapsed, forming a great cauldron approximately 10 Km in diameter.

The edge of cauldron can still be seen in the outer Tuscolano-Artemisio ring of hills which includes Monte Tuscolo (m 670) and Monte Artemisio (m 931), the remains of the original cone. After a period of dormancy, between 270,000 and 180,000 years ago, the entire cauldron awoke to another cycle of activity: the gas that had accumulated pushed the magmatic mass upward, opening a new conduit and building up a structure — the Faete — inside the previous one.

At the same time explosive phenomena in the Molara atrium, a depressed area between the old and new sections, created a series of debris cones the most evident of which is today’s Monte Fiore (723 m). Once again the material was emptied out and the Faete structure collapsed, leaving a caldera of 2 Km in diameter. The base, represented by the Fields of Hannibal, is rimmed by the internal ring of hills (the Faete), interrupted by the cones of the Iano Hills (m. 938) and Monte Cavo (949 m).

Likewise, the Fields of Hannibal themselves later gave rise to the small cone of Monte Vescovo (822 m). Over the last hundred thousand years, as incandescent matter came into contact with the water in the aquifer, intense fissuring of the rock ensued. This, in turn, caused a build up of pressurized steam and then powerful explosions that demolished the S-SW side of the external ring. This last hydromagmatic phase formed broad calderas, including those that, today, hold the Nemi and Albano lakes.

New data, such as Nemi's discovery of hydromagmatic expansion dating back as much as 7,000 years, reopens the question of the recent volcanic history of the Albani hills, deemed completed 67,000 years ago. Other manifestations have been reported by authors in classical times: Virgil (Aeneid, book VIII), Tito Livius (The History of Rome, book I), Ovid (The Festivals, book I) and Plutarch (Parallel Lives XII). As regards the late volcanic phenomena, various events are worth mention: sulphuric acid emission from the crater of Ariccia in 1754, the formation of the sulphur lake at La Faiola in 1806, the jets of steam emitted by the debris cone at Montecompatri in 1809, the sudden oscillations in the Albani lacustrine basins in 1829, as well as hydrothermal outcroppings and numerous springs of mineralized water found throughout the area.

Finally, various sectors of the volcanic complex are still affected by ongoing seismic activity and bradyseism which, over the last century, has lifted the region an average of several centimetres.