On a five- ft globe the vertical relief between the highest suiimiit on 

 land and the most profound deep of the ocean would approximate 0.09 inch. The 

 depth to the main floor of the Pacific Ocean would be barely discernible, at a 

 depth of about 0.02 inch below the surface of the geoid. The vertical relief 

 along any great circle will be included within the most perfect possible 

 circle having a five- ft diameter if the line has sufficient width to be 

 visible a few ft away. The ocean basins have thus about the same outward 

 convexity as the earth's surface. 



In the traditional terminology of the geologist, the lithosphere is in 

 part separated from the atmosphere by a hydrosphere which is essentially 

 similar in shape to a thin membrane which might be thought of as covering a 

 spherical balloon. The hydrosphere is interrupted in continuity by dry land, 

 but its oceanic part forms a system that covers somewhat more than 70 percent 

 of the globe. 



The boundaries between the hydrosphere and its marginal lands are 

 complex. For the most part they are the shorelines of the world's ocean 

 system, with their many complicated ramifications. While the upper surface of 

 the oceans is regarded as sea-level, the mean position of that level varies 

 considerably from place to place. The radial distance from the earth's center 

 to the mean sea- level lengthens near mountainous coasts, such as western South 

 America, when gravitational pull distorts the ocean surface upward. Sea-level 

 varies temporarily according to tidal forces, barometric pressure, and changes 

 in wind. 



Leaving aside the question of departures between geoid and water 

 surface, changes in level between land and sea also depend on other factors. 

 At a given place, mean sea- level can be lowered as a result of subsidence of 

 an area of ocean floor many thousands of miles away. The down- faulting of the 

 trough of an ocean deep produces some minor effect along all coasts of the 

 ocean system. The building of deltas or the deposition of terrigenous 

 sediments around continental and island shores has a basin- filling effect, and 

 hence tends to displace ocean surfaces toward slightly higher levels. During 

 earth history there have been many secular changes such as these which have 

 affected sea- levels enormously, the greatest being volumetric growth of the 

 hydrosphere itself. Our discussion, however is not directed toward these long 

 term changes of level. It will concentrate on problems of more immediate 

 interest and less theoretical nature. It will involve mainly the closing 

 chapter of earth history, the time in which we are now living, including the 

 recent past, with some mention of the immediate future. There have been 

 several notable changes in sea-level during the Quaternary. (Author). 



305 SAHAGIAN, D. 1988. "Ocean Temperature -Induced Change in Lithospheric 

 Thermal Structure: A Mechanism for Long-Term Eustatic Sea-Level Change," 

 Journal of Geology. Vol 96, pp 254-261. 



Eustatic sea- level change can be produced by a change can be produced by 

 a change in ocean bottom water temperature through an alteration of the upper 

 thermal boundary condition of oceanic lithosphere. The consequent evolution 

 of lithospheric thermal structure can cause thermal expansion or contraction 

 of the lithosphere. If continental lithosphere remains unaffected by the 

 change in ocean temperatures, "eustatic" sea- level will change as measured in 

 the continental frame of reference. A two-dimensional finite difference model 



140 



