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ROMAN TRAVERTINE DEPOSIT IN THE |
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ACQUE ALBULE BASIN |
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INTRODUCTION |
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Travertine deposits may be found around the world. In the Italian peninsula, travertine sediments come to light in Tuscany, Marche, Umbria, Campania, Sicily and in Lazio as well , in the surrounding area of the basin of Acque Albule. Certainly the most important travertine-bed for its continuity, richness of stratum, uniformity of colouring and aptitude for cultivation is that of Acque Albule near the city of Tivoli. This basin, about 85 metres deep in the centre, which thins out and then peters out in its extreme point has a rather oval frame: the major axle, direction ENE-WSH, is 9 km. long, the minor axle pointing to NW-SE is about 7 km. long. The basin area covers a surface of about 45 square km.; it borders , to the East with the Tiburtini mountains, to the North with the Cornicolani mountains, to the West with the Tevere basin and to the South it touches the pyrolisistic materials of volcanoes of Lazio. The stratigraphy below the travertine includes a pliocenec sandy stratum which in turn stands on limestone sediments of mesozoic nature. During a core boring, performed by Maxia in 1950, several vertebrate and invertebrate fossiles of continental origins (terrestrial) were found inside the travertine. |
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GENESIS |
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According to the petrographic division travertine belongs to sedimentary rocks of chemical nature that were formed by the precipitation of calcium carbonate CaCo3 coming from hot waters (juvenili) rich or supersaturated of calcium bicarbonate Ca (HCO3)2 soluble in water. During the Pliocene, about 2-3 million years ago, great part of the Italian peninsula was covered by the sea ,therefore also the basin area of Acque Albule was filled in with this ancient sea wich reached up to the slopes below the Tiburtini and Cornicolani mountains (Marcellina e Palombara). At the establishment of Marcellina many sea fossiles ,that testify the presence of the sea at that time, are still evident . In the large CONCH-shaped basin of Acque Albule this old sea, maybe due to a glaciation, stated to recede so much that it ebbed completely to its present site. Therefore in this age the basin concerned appears as a wide conch-shaped low-lying area where the sandy soils, saved by the pliocenec sea, emerge; these soils lean on limestones of mesozoic nature. From this moment on, hot thermomineral (23°) waters mixed with sulphurous gases, deriving from the latest volcano eruptions of Colli Albani, begin to surface again. In effect these are the consequence of the strengthening of magma of the ancient volcano. These juvenili waters, during their course in depth , cross enormous limestone masses and subjected to strong terms of pressure and temperature have melted the carbonate of calcium and have become greatly enriched. When these waters come to the surface, exploit wide rifts (faults ) of the soils below and they are pushed to the top by sulpurous gases wich accompany them ( foaming effect). Once on the top, owing to the effect of gravity, they start to flow down to the lowest parts of the basin. At this point the juvenili waters enriched with bicarbonate of calcium, undergo a lowering of temperature because they mix with cold surface waters. Furthermore they also pass through a lowering pressure due to their rising to the surface. Such lowering of temperature and pressure establishes the chemical- physical terms wich support the precipitation and therefore the further sedimentation of carbonate of calcium under the form of a sort of limestone mud. At that time the basin area of Acque Albule was extremely florid of vegetation that was caught up and scaled by the unceasing sedimentation of bicarbonate of calcium.By decomposing, these vegetables left then some holes which gave birth to the typical vacuolar structure of travertine. But the porous aspect of travertine is greatly due to solphorous gases that rose up to the surface in the shape of boiling bubbles when they rose beyond the stratum of limestone mud. In fact in the travertine structure, when it is cut parallel to the direction and sedimentation, the circular bubble frame that conferms the ancient outlet of gases is quite clear. If the cut is orthogonal to the direction and sedimentation the shape of the holes is rather longer in the direction of the stratification. The new rising of thermomineral waters to the surface wasn’t continuous and constant in time but it was often disrupted by cyclical alluvial floods due mainly to the river Aniene that, by opposing the capacity to rise, caused a momentary stoppage of sedimentation. At the end of the flood the thermomineral waters recovered their ability to rise and a new cycle of sedimentation started again. This is the reason why travertine appears in stratum or seams (beds) sometimes defined by a soft clay level (ash fault) certainly due to an alluvion occured in that period. The combination of all these sedimentary cycles has filled the ancient depressed area of the basin of the Acque Albule by assuming the present rather flat morphology. Often a bed of whitish travertine is characterized by one or more streakings a few centimetres thick and coloured in a wide range of hues that goes from hazelnut to dark grey. By means of a microscopic analysis to the streaking level, many fluvial fossils are evident. Obviously an alluvion caused by a temporary overflowing of the river has polluted for a period the normal sedimentation of carbonate of calcium with clay materials which have given birth to the forming of a small dark grey clay level (striping). Let’s analyse the two most important features of travertine: its homogeneity and its colouring. The first aspect (consistency) is both due to the pressure of the overhanging beds on the ones below, and due to a further precipitation of carbonate of calcium. The overall effect is represented by the formation of travertine beds. Moreover the high quantity of water ,the travertine is a permeable rock, seeps to the base of the stratum defined by the clay level and it also deposits more carbonate of calcium causing therefore the further occlusion of the pre-existing holes and giving birth to a very dense and strong travertine (process of crystallization) in that zone of the stratum. By means of a microscopic study of a sample of travertine placed at the base of a stratum, the presence of crystals of calcyte defined by a clear rhomboedrical border is noticeable; such crystals testify and conferm the process of crystallization of calcite. The second aspect (the colouring) :if carbonate of calcium had the chance to sediment without being disrupted by external agents, it would take on an extremely white hue, but rarely does this particular condition occur. As a matter of fact the process of sedimentation is altered by external agents, such as the momentary flood of a river, which modifies the features by causing changes in the colours of ravertine that vary from: straw-coloured to hazelnut and dark brown. The darker types of travertine possess higher technical characteristics than those of brighter varieties of travertine. In fact the darker ones have an abrasion coefficient , abrasion due to the stamping of pavings and floors, equal and sometimes lower than that of marbles and lower than that of some granites. According to a recent study (1994) about Higher Pleistocene ,in the basin of Acque Albule both the speed of sedimentation and the beginning of the sedimentary phase of travertine was calculated with accuracy .The radiometrical studies by means of the method 230Th\234U have demonstrated that the stage of the sedimentation started to develop 165.000 years ago. Furthermore through the same process it has been calculated that the average speed of sedimentation is 0,4-0,6 mm. a year, or 1.000 years were necessary to form 50 cm. of sediment. In order to further demonstrate the precision of this analysis of travertine ,visiting the establishment of the Terme Acque Albule would be very interesting, where a short course of the old Via Tiburtina, covered by about one metre of travertine, is still visible. If we think that the old Tiburtina street dates back to 2.000 years ago…
Homogeneity, strength and flexibility are the main features that make travertine a material suitable to many uses in the field of building. The proof, as constructive and ornating factor of ancient monumental buildings and modern structures, makes travertine to surmount with excellent results the difficult test of time. I’ll sum up by adding an explanatory quotation of Vitruvio (46-30 a.C.) a popular Roman architect who said: " The rocks of Tivoli and those of the same class are resistant to everything , to the weight ,to the damages of time,and, but they aren’t fireproof ". Francesco Poggi |
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