Priority of the EDU-GEO Project, sponsored by the FIST (see Cantelli & Venturini, this Vol.), is the production of six excursions as paradigm for next ones. They are intended to be used by last-class students of High School and their teachers to utilize the territory as a Natural Laboratory for geological s.l. observations and exercises. First realized among the six has been the middle But Valley excursion, in the Carnic Alps core (NE Italy). The geological importance of the area lies in its Holocene evolution. It can be summarised in three steps. 1) About 10.000 years ago (14C age) a seismic shock caused a massive rockslide; it gave origin to a large main scarp and a thick chaotic deposit in the valley bottom. 2) The blocking of the River But water flow formed a temporary lake more than 6 km2 wide and 100 m deep at least; the value corresponds to the estimated thickness of mud, sand and gravel that piled up in the lake during almost 5.000 years. 3) The collapse of the rockslide dam, occurred about 5.000 years ago (14C age), caused the emptying of the lake and deep terraced surfaces in the lacustrine deposits; at the same time the renewal of erosion in the main scarp of the slide gave origin to a localized alluvial fan in the valley bottom. The excursion area, that is about 10 km2 wide, is a sort of geological puzzle with some evident morphological and subordinate sedimentological features. They are respectively a) the well preserved scarp of the landslide; b) scattered outcrops of lacustrine mud and sand; c) well visible terraced surfaces distributed over the ancient lake area; d) a large alluvial fan in the same place where the rockslide mass stored up. The puzzle can be solved only by reconstructing the well-constructed succession of causes and effects, in which the previous effects often becomes the next causes. The excursion area is perfect to persuade students to find by themselves, under the teachers’ guide, the morphological features, as the sparse cover allows everywhere good observations. The starting point of the middle But Valley Holocene evolution is the large scarp of the landslide (50x106 m3 of rockslide), still clearly visible from long-distance. The first stop of the excursion is naturally devoted to this large morphological feature. Afterwards the useful educational strategy is to reconstruct together with the students the immediate effect produced by the rockslide. The effect (dam in the valley bottom) immediately changed into a cause, as it might have been responsible of the water stream blocking. As consequence the water storage could have generated a lake. From now on, the educational strategy useful in the middle But Valley is to change the students into detectives. “What we must expect if it was really so? If the lake formed how much deep it was? May somebody say anything about the position of the shoreline? Are there any field evidences to solve these questions? What kind of data might we have to look for in the area to have exhaustive answers?” The teachers must only introduce the right questions and guide the discussion. The students, better if split up into study teams, will become able to reconstruct by themselves the causes and effects sequence, learning the methodology of the scientific research. Moreover, they will feel protagonists in their travel back in the past of the territory.

Birth and death of a gorgeous holocenic lake (Carnic Alps, NE Italy)

VENTURINI, CORRADO
2007

Abstract

Priority of the EDU-GEO Project, sponsored by the FIST (see Cantelli & Venturini, this Vol.), is the production of six excursions as paradigm for next ones. They are intended to be used by last-class students of High School and their teachers to utilize the territory as a Natural Laboratory for geological s.l. observations and exercises. First realized among the six has been the middle But Valley excursion, in the Carnic Alps core (NE Italy). The geological importance of the area lies in its Holocene evolution. It can be summarised in three steps. 1) About 10.000 years ago (14C age) a seismic shock caused a massive rockslide; it gave origin to a large main scarp and a thick chaotic deposit in the valley bottom. 2) The blocking of the River But water flow formed a temporary lake more than 6 km2 wide and 100 m deep at least; the value corresponds to the estimated thickness of mud, sand and gravel that piled up in the lake during almost 5.000 years. 3) The collapse of the rockslide dam, occurred about 5.000 years ago (14C age), caused the emptying of the lake and deep terraced surfaces in the lacustrine deposits; at the same time the renewal of erosion in the main scarp of the slide gave origin to a localized alluvial fan in the valley bottom. The excursion area, that is about 10 km2 wide, is a sort of geological puzzle with some evident morphological and subordinate sedimentological features. They are respectively a) the well preserved scarp of the landslide; b) scattered outcrops of lacustrine mud and sand; c) well visible terraced surfaces distributed over the ancient lake area; d) a large alluvial fan in the same place where the rockslide mass stored up. The puzzle can be solved only by reconstructing the well-constructed succession of causes and effects, in which the previous effects often becomes the next causes. The excursion area is perfect to persuade students to find by themselves, under the teachers’ guide, the morphological features, as the sparse cover allows everywhere good observations. The starting point of the middle But Valley Holocene evolution is the large scarp of the landslide (50x106 m3 of rockslide), still clearly visible from long-distance. The first stop of the excursion is naturally devoted to this large morphological feature. Afterwards the useful educational strategy is to reconstruct together with the students the immediate effect produced by the rockslide. The effect (dam in the valley bottom) immediately changed into a cause, as it might have been responsible of the water stream blocking. As consequence the water storage could have generated a lake. From now on, the educational strategy useful in the middle But Valley is to change the students into detectives. “What we must expect if it was really so? If the lake formed how much deep it was? May somebody say anything about the position of the shoreline? Are there any field evidences to solve these questions? What kind of data might we have to look for in the area to have exhaustive answers?” The teachers must only introduce the right questions and guide the discussion. The students, better if split up into study teams, will become able to reconstruct by themselves the causes and effects sequence, learning the methodology of the scientific research. Moreover, they will feel protagonists in their travel back in the past of the territory.
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C. Venturini
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Utilizza questo identificativo per citare o creare un link a questo documento: http://hdl.handle.net/11585/51061
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