IN THE NEWS Geology under the Oceans Scientists have developed a new map (image 1 - download the images for more detail) of the Earth’s seafloor, much more accurate than the previous version produced nearly 20 years ago. Knowledge of the seafloor is generally poor because saltwater is opaque to standard techniques used to map mountains on land. Shipborne echo sounders can gather very high resolution information by bouncing sound off bottom structures, but less than 10% of the oceans have been properly surveyed in this way because of the time and effort it involves. An alternative, indirect method of mapping the oceans is the use of satellites fitted with radar altimeters. These satellites can infer the shape of the ocean floor from the shape of the water surface. Because water follows gravity, it is pulled into highs above the mass of tall seamounts and slumps into depressions over deep trenches. Most maps showing gross outlines of mountains on the seafloor have relied on this approach. The new map was developed by researchers affiliated with Californias Scripps Institution of Oceanography, courtesy of the European Space Agencys CryoSat-2 satellite. CryoSat-2 primarily captures polar ice data but also operates continuously over the oceans. Data also came from Jason-1, NASAs satellite. The team has developed and proved a powerful new tool for high-resolution exploration of regional seafloor structure and geophysical processes, says Don Rice, of the National Science Foundations Division of Ocean Sciences, which funded the research. In the previous radar dataset we could see everything taller than 2 000 m, and there were 5 000 seamounts, says the main author of the mapping report, David Sandwell. Thousands of previously uncharted sea mounts from 1.5km high have now been mapped. The number of seamounts increases exponentially with decreasing size. Knowing where the seamounts are is important for fisheries management and conservation, because it is around these topographic highs that wildlife tends to congregate. The new mapping will improve estimates of seafloor depth in the 80 % of the oceans that remains uncharted or where the sea floor is buried beneath thick sediment. Earthquakes (image 2) were also mapped by CryoSat, and the researchers discovered that seamounts and earthquakes are often linked. Many seamounts were once active volcanoes, and so are usually found near tectonically active plate boundaries, mid-ocean ridges and subduction zones. Combined with existing data and improved remote sensing instrumentation, the new mapping technique gives scientists improved tools to investigate ocean spreading centres (image 3) and remote ocean basins. Previously unseen features include newly exposed continental connections across South America and Africa and new evidence for seafloor spreading in the Gulf of Mexico (image 4). These ridges were active about 150 million years ago and are now buried by 1 500 m thick layers of sediment. The rough topography of the seafloor is also important in steering ocean currents and promoting mixing, and the enhanced knowledge now becoming available is critical to understanding how the oceans transport heat and influence climate. Look out for a forthcoming new edition of Googles ocean maps which will incorporate the latest data. In the meantime, check out the introductory video at google.co.uk/earth/explore/showcase/ocean.html#tab=ocean-seafloor , to look at seamounts and ocean ridges before heading off yourselves to explore the oceans on Google Earth. Image 5 shows the current Google Earth view of the South Sandwich Islands, a volcanic island arc. Sources: nsf.gov/news/news_summ.jsp?cntn_id=132771 bbc.co.uk/news/science-environment-2946544 Sandwell, D.T., Müller, R.D., Smith W.H.F., Garcia, E. and Francis, R. 2014 New global marine gravity model from CryoSat-2 and Jason-1 reveals buried tectonic structure. Science 346 (6205): 65–67. DOI: 10.1126/science.1258213 (3 October 2014). Images 1 The new gravity data gives us our clearest view yet of the shape of the ocean floor (Credit: Scripps Institution of Oceanography) 2 Gravity model of the North Atlantic; red dots are earthquakes (Credit: David Sandwell, SIO) 3 Seeing fracture zones tells scientists about the movement of the continents (Credit: David Sandwell, SIO) 4 Gulf of Mexico: The jagged outline of an extinct spreading ridge is discernible (Credit: David Sandwell, SIO) 5 The South Sandwich Islands – an island arc and trench (Google Earth: 57°00 S, 27°00 W)
Posted on: Sat, 04 Oct 2014 10:00:01 +0000
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