The rsl has fluctuated throughout geologic time as the volume of ocean 

 water has fluctuated, the shape of the ocean basins has changed, and 

 continental masses have broken apart and reformed. Table 15 lists some of 

 the factors contributing to long-term (geologic time scale) factors that have 

 caused changes in rsl. Readers interested in details of this fascinating subject 

 are referred to Emery and Aubrey's (1991) excellent book. This volume and 

 Gorman (1991) contain extensive bibliographies of the subject. Detailed 

 analyses of United States tide curves are documented in Hicks, Debaugh, and 

 Hickman (1983). 



Table 1 5 



Long-Term Causes of Changes in Relative Sea Level 



Changes in ocean water volume 

 Waxing and waning of glaciers and Arctic ice cap 

 Juvenile water from volcanic eruptions 

 Thermal effects - volcanism, hot spots, aesthenospheric bumps 



Plate tectonics and seafloor spreading 

 Changes in spreading rates (affect the hypsometric curve - average depths of the ocean 



basins) 

 Changes in areas of ocean and land 

 Changes in direction of plate motions 

 Deep-ocean sedimentation 



Isostacy (isostatic adjustment of the earth's crust) 



Glacio-isostacy - waxing and waning of glaciers 



Hydro-isostacy - changing water load on crust below ocean basins 



Sediment-isostacy - depositional-erosional cycles cause varying sediment load 



Glacial surges and melting 

 Departures from the geoid (level surface of equal gravitational potential) 



Shifts in the hydrosphere, aesthenosphere, core-mantle interface 



Shifts in the rate of the earth's rotation, tilt of the spin axis, and precession of the equinox 



External gravitational changes 



Geological faulting 

 Vertical and horizontal movement of large pieces of the crust in response to fault motions 



Sediment compaction and subsidence 

 Reduction of volume of poorly packed sediments into more dense matrix - common in river 



deltas (Mississippi, Nile, Niger) 

 Draining of low-lying areas 



Compaction under earthquake-induced vibration 

 Compaction as a result of loss of interstitial fluids - oil drilling and withdrawal of 



groundwater for urban development 



(Adapted from Emery and Aubrey (1991)) 



Tide gage records may be analyzed for spatial interpolation and for 

 assessing temporal variations such as surges, tides, seasonal changes, and 

 long-term trends. Discrepancies between the predicted tide at one site and the 

 actual tide measured only a short distance away may be considerable. A 

 method for adjusting between predicted tides at a station and those at a nearby 

 study area using only limited field measurements is discussed by Glen (1979). 

 Other analysis methods are discussed in HQUSACE (1989) and the Shore 

 Protection Manual (1984). 



Chapter 5 Analysis and Interpretation of Coastal Data 



93 



