Download Profile across the Atlantic

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