Download Geology of the deep oceans

SPAZIO CLIL
Geology of the deep oceans
A
topographic map of Earth’s surface reveals the
most important geologic features submerged
beneath the oceans: mid-ocean ridges, volcanic
tracks of hot spots, deep-sea trenches, island arcs,
and continental margins. How oceanographers
have obtained this fundamental information is a
fascinating story of scientific exploration.
Making a map of the deep ocean floor is no
easy task. Because sunlight can penetrate only
100 m or so below the sea surface, the deep ocean
is a very dark place. It is not possible to map the
ocean floor using visible light, nor can we use radio waves beamed from spacecraft, as we have
used to map the surface of cloud-shrouded Venus. Ironi­
cally, spacecraft photography has allowed us to map the surfaces of our planetary
neighbors with much higher resolution than we
have been able to map the deep ocean floor, even
to this day.
It is possible to view the seafloor directly from a
deep-diving submersible. Pioneered by the French
oceanographer Jacques-Yves Cousteau, these small
ships can observe and photograph at great depths
(see figure). With their mechanical arms, they can
break off pieces of rock, sample soft sediment,
and catch specimens of exotic deep-sea animals.
Newer robotic submersibles are guided by scien-
ALVIN
(manned
submersible)
ROV
(remotely
operated vehicle)
SeaBeam
(hull-mounted
swath-mapping sonar)
High-technology methods for exploring the deep seafloor. The
manned deep submersible Alvin and a remotely operated vehicle (ROV) are directed from a surface ship. SeaBeam, a hullmounted multibeam echo sounder, continuously maps seafloor
topography in a wide swath as a ship steams across the ocean
Fantini, Monesi, Piazzini - Elementi
tists on the mother ship above. But submersibles
are expensive to build and operate, and they cover
small areas at best.
For most work, today’s oceanographers use instrumentation to sense the seafloor topography
indirectly from a ship at the surface. A shipboard
echo sounder, developed in the early part of the
twentieth century, sends out pulses of sound
waves. When the sound waves are reflected back
from the ocean bottom, they are picked up by sensitive microphones in the water. Oceanographers
can compute the depth by measuring the interval
between the time the pulse leaves the ship and
the time it returns as a reflection. The result is an
automatically traced profile of the bottom topography. Powerful echo sounders are also used to
probe the stratigraphy of sedimentary layers beneath the ocean floor.
Many of today’s oceanographic vessels are outfitted with hull-mounted arrays of echo sounders
that can reconstruct a detailed image of seafloor
topography in a swath extending as much as 10
km on either side of the ship as it steams along.
These systems can map seafloor topography over
large regions with unprecedented resolution of
small-scale geologic features, such as undersea
volcanoes, canyons, and faults.
Joides Resolution
drilling ship
Permanent seafloor
observatory
surface. The drilling ship Joides Resolution uses bottom transponders to navigate a drill string into a reentry cone on the
seafloor. Permanent unmanned seafloor observatories monitor
processes in the subsurface and the overlying water column for
extended periods of time.
di Scienze della Terra • Italo Bovolenta editore - 2013
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SPAZIO CLIL
Other types of instruments can be towed behind a ship or lowered to the bottom to detect
such properties as the magnetism of the seafloor,
the shapes of undersea cliffs and mountains, and
heat coming from the crust. Underwater cameras
on sleds towed near the ocean bottom can photograph the details of the seafloor and the organisms
that inhabit the deep. Since 1968, the U.S. Deep
Sea Drilling Project and its successor, the international Ocean Drilling Program, have sunk hundreds of drill holes to depths of many hundreds
of meters below the seafloor. Cores obtained from
these drill holes have provided geologists with
sediment and rock samples for detailed physical and chemical studies). Plans are now afoot to
install a global network of unmanned deep-sea
observatories that will send back streams of data
about processes taking place on the deep seafloor
and in the overlying water column. Marine geologists today work in a beehive of high technology!
Despite all this fancy gear, there are still many
regions of the oceans that have not been surveyed
in detail by surface ships, and our knowledge of
the seafloor remains fragmentary. Recently, however, scientists have developed a tool that enables
a satellite to «see through» the ocean and chart
the topography of the seafloor on a global scale.
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The new method makes use of an altimeter
mounted on a satellite. The altimeter sends pulses of radar beams that are reflected back from
the ocean below, giving measurements of the
distance between the satellite and the sea surface with a precision of a few centimeters. The
height of the sea surface depends not only on
waves and ocean currents but also on changes in
gravity caused by the topography and composition of the underlying seafloor. The gravitational
attraction of a seamount, for example, can cause
water to «pile up» above it, producing a bulge in
the sea surface of as much as 2 m above average
sea level. Similarly, the diminished gravity over
a deep-sea trench is evident as a depression of
the sea surface of as much as 60 m below average sea level.
This method has allowed us to infer features
of the ocean floor from satellite data and display
them as if the seas were drained away. Marine
geologists have used this technique to map new
features of the seafloor not revealed by ship surveys, especially in the poorly surveyed southern
oceans. The satellite data have also revealed deeper structures below the oceanic crust, including
gravity anomalies associated with convection currents in the mantle.
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