Earth

Cave evolution

Generally, the evolution of karst systems is similar to that of the mountain massif in which they are found. The general tendency is a gradual deepening of the cave systems as a consequence of the deepening of the base level of the valleys. But this is not always the case: the base level can also rise, bringing about the flooding of galleries that previously were fossilized. This has taken place, for example, in all the caves of coastal areas where, in the course of the last 2 million years, continental glaciations determined fluctuations in the sea level. The formation of big continental glaciers, in fact, causes the entrapment of enormous quantities of water on lands above sea level: this implies that during each ice age the expansion of glaciers provoked the lowering of the average sea level, and hence the lowering of the base level by about 100-120 m. This led to the formation of continental caves at altitudes presently below sea level. During warm interglacial periods, on the contrary, waters freed by the ice melt caused the sea level to rise and brought about the flooding of the ‘terrestrial’ caves that had been formed previously. As a result of the last Ice Age, during the last 10,000 years the average level of the Mediterranean has risen by about 100-120 m, while 125,000 years ago, not long before the last Ice Age, it was 8 m higher than today (as can be deduced from ancient nips and remains of sea caves). Naturally, it is not always that simple: variations in the sea level can also be a consequence of other causes among which, for example, tectonic activity that can raise or sink lands, and isostasy raising areas formerly covered by the weight of thick ice sheets, as is happening in Scandinavia. These variations can amplify or thwart the eustatic variations of the sea level, producing different effects from one place to another. In general, however, the result is that the majority of the sea caves does not actually have a marine origin but is the result of the flooding of continental caves with sea water. Confirmation of this is given by the study of the morphologies that are typical of continental karst caves but not of marine ones. The finding of speleothemes, in particular, is the proof of this fact and is a very precious element in the reconstruction of the history of the caves and of climatic evolution. Speleothemes can in fact be dated and the study of their morphology and of the minerals they are made of at times allows surprisingly detailed reconstructions. By studying the stalagmites formed at a depth of 20-30 m in sea caves, in Southern Italy, it has been possible to observe, for example, consecutive deposits of minerals of a continental environment and those of marine organisms, at times even with the holes of stone-boring mussels, with a cyclical alternation showing the advance and retreat of the continental glaciers. Cave-divers are a precious allies for geologists! Fluctuations in the sea level, particularly the rise of the last 10,000 years, have created vast systems of flooded caves: the most beautiful examples are the cenotes in Yukatan, entrances to ancient systems of sub-aerial caves that have been flooded by the rise in the water table to a depth of just a few metres or the blue holes of the Bahamas or Belize, where a very old karst plain, pitted with karst systems, has been completely flooded by the rising sea level.