units with interlayered fossil oozes. Magnetic properties of the 

 volcanic sequence are complex. The linear sea-floor spreading 

 anomalies observed over site 332 are not caused by a simple 

 sequence of uniformly magnetized lavas, but rather by the sum effect 

 of numerous lavas whose magnetic intensity and polarity vary 

 widely. This result, as well as the weak magnetization of the top few 

 hundred meters of basalt, strongly conflicts with standard inter- 

 pretations of marine magnetic-anomaly sources, which assume a 

 uniform magnetization strongest in the upper few hundred meters. 

 As the DSDP enters its International Phase of Ocean Drilling 

 (IPOD), significant effort will be placed on drilling deep holes into 

 the oceanic crust to clarify and expand knowledge gained at site 332. 



Other studies supported by NSF and ONR are examining the 

 history of particular ridge systems. Studies of the East Pacific Rise 

 have revealed a complicated spreading history during the last 25 

 million years, with large lateral shifts, or "jumps", in the location of 

 sea-floor genesis. These "jumps" have produced complicated 

 magnetic and bathymetric patterns in the eastern Pacific. 



The actual topography of ridge crests and its relation to the 

 spreading process have been under careful examination. Whereas 

 the slowly spreading Mid-Atlantic Ridge has a large central rift 

 valley with steep walls, the rapidly spreading East Pacific rise is 

 characterized by a central axial peak. Several theories have 

 attempted to account for these characteristics in terms of the width 

 of the accretion zone, the rate of basalt intrusion, the rate of 

 separation of the lithospheric plates, or the rate at which heat is 

 transferred away from the intrusion zone. Continuing research in 

 this area should provide valuable information on the tectonic 

 processes that accompany the creation of new sea floor. 



In addition to the USGS program of monitoring large earthquakes 

 from spreading centers, ocean bottom seismometers are now being 

 deployed to examine the fine-scale distribution of microearthquake 

 activity. Such studies will be important in determining the 

 distribution and extent of magma chambers beneath ridge crests. 



The axis of the oceanic ridge system is offset at many places, 

 departs from its characteristic midocean position in the eastern 

 Pacific, and intersects the continental margin at a few locations. 

 Many scientists now agree that the San Andreas Fault System of 

 Southern California represents a zone of offset that connects the 

 ridge axis between its intersection with land at the head of the Gulf 

 of California and the southern terminus of its northward continua- 

 tion to the west of northern California. Because it is the source of 

 frequent damaging earthquakes, the fault system, which lies both 

 onland and offshore, has been the object of intense investigation for 



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