GEOPHYSICS 



THE DIVISIONS OF GEOLOGIC TIME 



geophysics — The physics or nature of the earth. 

 It deals with the composition and physical 

 phenomena of the earth and its liquid and 

 gaseous envelopes ; it embraces the study of ter- 

 restrial magnetism, atmospheric electricity, and 

 gravity; and it includes seismology, volca- 

 nology, oceanography, meteorology, and related 

 sciences. (24) 



geosphere— The "solid" portion of the earth, in- 

 cluding water masses; the lithosphere plus 

 the hydrosphere. 



Above the geosphere lies the atmosphere 

 and at the interface between these two regions 

 is found almost all of the biosphere, or zone 

 of life. (5) 



geostrophic current — A current defined by as- 

 suming that an exact balance exists between the 

 horizontal pressure gradient and the coriolis 

 force. The usual manner of deriving geo- 

 strophic currents is to prepare a dynamic 

 topography chart which is based on observations 



of temperature and salinity at various depths 

 for a network or a line of oceanographic sta- 

 tions. The direction of the current is indicated 

 by the contours of dynamic topography and its 

 speed by the spacing of the contours. 

 geostrophic equation — An equation which repre- 

 sents a balance between the horizontal pressure 

 gradient and the coriolis force. The equation 

 used to compute the speed of the current, is : 



C{ Da-Di)n 

 L 

 where 



v=. 



G=- 



2a) sin 4, ■ 10= 

 Da — Di= difference in dynamic height anom- 

 alies at stations A and B in dy- 

 namic meters, 

 n= unit conversion factor, 

 Z= distance between stations (in kilo- 

 meters or nautical miles) , 

 V= speed in knots, 

 <o= angular velocity of the earth, 



</)= latitude. 



I 



geosyncline — A large generally linear subsident 

 trough in which many thousands of feet of sedi- 

 ments are accumulating or have accumulated. 

 Deep oceanic trenches paralleling island arcs 

 are considered to be developing geosynclines. 



(2) 



geothermal gradient — The change in tempera- 

 ture of the earth with depth, expressed either in 

 degrees per unit depth, or in units of depth per 

 degree. (35) 



giant clam — One of a genus {Tridacna,) of large 

 bivalves inhabiting coral reefs in the Indo-Pa- 

 cific region. The shells may attain weights of 

 several hundred pounds and lengths to four or 

 more feet. Although said to have caused fatal- 

 ities by trapping divers and waders, documenta- 

 tion of such incidents is lacking. 



giant floe — An obsolete term since adoption of 

 WMO code ; term now used is big ice floe. 



giant kelp — One of a genus {Macrocystis) of large 

 vinelike brown algae, which grow attached to 

 the sea bottom by a massive holdfast and reach 

 lengths to 150 feet. Members of this genus are 

 the largest algae in existence. 



giant squid — One of a genus (Architeuthis) , par- 

 ticularly ArchiteutMs ■princeps, of large cephal- 

 opods. These animals inhabit the middepths in 

 oceanic regions but may come to the surface at 

 night normally or by accident. The body of one 

 measured 10 feet long and the tentacles 42 feet 

 long. It is food for the sperm whale but often 

 may give battle. The fabulous sea monster, 

 kraken, as -originally drawn, resembles the giant 

 squid. See squid. 



gill — A platelike or filamentous outgrowth; res- 

 piratory organ of aquatic animals. 



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