floor created each year may play a significant role in the chemical 

 composition of sea water. 



Structure and Age of the Ocean Lithosphere 



As the oceanic lithosphere moves down the flank of a midocean 

 ridge, it evolves both petrologically and structurally. The first 

 suggestion of this evolution came from studies of the relationship 

 between sea-floor depth and age, which were sponsored by NSF and 

 ONR. The sea-floor depth showed a logarithmic increase in going 

 from the youngest crust at ridge crests to the oldest crust. This 

 increase of depth with age is apparently related to thermal 

 contraction of the lithosphere as it cools while moving away from a 

 spreading center. Because the logarithmic profile is independent of 

 the spreading rate, it provides another tool for determining sea-floor 

 age. 



The age-versus-depth relationship is only one clue to the puzzle of 

 how the lithosphere evolves. First-order models of heat flow and 

 gravity across a midocean ridge have been devised. Regional 

 anomaly maps show the departures of observed data from the 

 models. Analysis and interpretation of such maps, coupled with 

 additional field observations and geodynamic modeling, will 

 ultimately shed a great deal of light on the dynamics and properties 

 of the oceanic lithosphere and underlying asthenosphere. 



A clearer understanding of the detailed structure and petrology of 

 the lithosphere is one of the primary objectives of the crustal drilling 

 portions of the IPOD program. Detailed and well-controlled seismic 

 refraction data obtained by ocean-bottom seismometers will be 

 integrated with information from rock cores to be obtained during 

 the drilling program. Initial results from the ocean-bottom 

 seismometer refraction program suggest a complex velocity struc- 

 ture in the upper lithosphere. 



Near the crest of the East Pacific Ridge, a recent study has shown a 

 low-velocity zone, which may be evidence of the magma chamber 

 below the zone of accretion. Out to 150 kilometers (3 million years), 

 very little is seen in the way of discrete crustal layering. At about 250 

 kilometers (5 million years), however, numerous identifiable layers 

 are present in the refraction results. In the future it may prove 

 possible to trace these distinct velocity layers using multichannel 

 seismic reflection techniques. The USGS and NSF have recently 

 acquired a multichannel profile from the Mid-Atlantic Ridge to the 

 U.S. east coast. Analysis of data from this profile will attempt to 

 define and map crustal layering and will be used to determine the 

 locations of future IPOD drill holes. 



Although most of the oceanic lithosphere is produced by sea-floor 

 spreading at midocean ridges, significant volumes of material are 



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