Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
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 1 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. Fantini, Monesi, Piazzini - Elementi 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. di Scienze della Terra • Italo Bovolenta editore - 2013 2