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SPAZIO CLIL Profile across the Atlantic T o gain an appreciation of the geologic features that lie beneath the oceans, we will take a brief tour across the Atlantic and the Pacific, as if we were driving a deep-diving submarine along the ocean floor. The Atlantic profile shown in figure extends from North America to Gibraltar. Starting from the coast of New England, we descend from the shoreline to depths of 50 to 200 m and travel along the continental shelf. This broad, flat, sand and mud-covered platform is a slightly submerged part of the continent. After traveling about 50 to 100 km across the shelf, down a very gently inclined surface, we find ourselves at the edge of the shelf. There, we start down a steeper incline, the continental slope. This mud-covered slope descends at an angle of about 4°, a drop of 70 m over a horizontal distance of 1 km, which would appear as a noticeable gradient if it were on dry land. The continental slope is irregular and is marked by gullies and submarine canyons, deep valleys eroded into the slope and the shelf behind it. On the lower parts of the slope, at depths of about 2 000 to 3 000 m, the downward incline becomes gentler. Here it merges into a more gradual downward incline called the con tinental rise, an apron of muddy and sandy sediment extending into the main ocean basin. The continental rise is tens to hundreds of kilometers wide, and it grades imperceptibly into a wide, flat abyssal plain that covers large ar- A topographic profile across the Atlantic Ocean from New England (left) to Gibraltar (right). (Used by special permission - Studio H.C. Berann) NORTH AMERICA eas of the ocean floor at depths of about 4 000 to 6 000 m. These plains are broken by occasional submerged volcanoes, mostly extinct, called sea mounts. As we travel along the abyssal plain, we gradually climb into a province of low abyssal hills whose slopes are covered with fine sediment. Continuing up the hills, the sediment layer becomes thinner, and outcrops of basalt appear beneath it. As we rise along this steep, irregular topography to depths of about 3 000 m, we are climbing the flanks and then the mountains of the Mid-Atlantic Ridge. Abruptly, we come to the edge of a deep, narrow valley a few kilometers wide at the top of the ridge. This narrow cleft, marked by active volcanism, is the rift valley where two plates separate. As we cross the valley and climb the east side, we move from the North American Plate to the Eurasian Plate. Continuing eastward, we encounter topography similar to that on the west side of the ridge, only in reverse order, because the ocean floor is more or less symmetrical on either side of the ridge. On the path we have taken, this symmetry is disturbed by some large seamounts and the volcanic Azores Islands, which mark an active hot spot, perhaps caused by the heat from an upwelling mantle plume. Passing over the rough topography of the abyssal hills on the flank of the Mid-Atlantic Ridge, we descend to an abyssal plain, and then ascend up the continental rise, slope, and shelf off the coast of Europe. ATLANTIC OCEAN A Spain B AFRICA NORTH AMERICA Continental shelf Slope Rise North American Plate Rise A 0 500 1000 Fantini, Monesi, Piazzini - Elementi Abyssal plain 1500 GIBRALTAR Eurasian Plate Mid-Atlantic Ridge rift Atlantis valley Seamount Seamounts Shelf Azores Slope Rise B 2000 2500 3000 3500 di Scienze della Terra • Italo Bovolenta editore - 2013 4000 4500 5000 5500 km 1