represent a change in the mechanism that generates the field, it most 

 likely represents a deterioration of the record of polarity changes as 

 the crust ages. Recent investigations suggest that two crustal layers 

 may be contributing to the production of anomalies. The upper layer 

 corresponds to extruded pillow basalt with a high initial magnetiza- 

 tion and a narrow transition width between normal- and reversed- 

 polarity rocks. The lower layer corresponds to intruded basalt with 

 lower magnetization and a wider transition zone. As the crust ages, 

 the upper layer loses its high magnetization because of alteration of 

 the basalt, possibly by hydrothermal circulation. Over old crust, 

 then, the surface anomalies are more representative of deeper 

 magnetic sources. Such a model would explain a general lack of short 

 polarity intervals in older cust. 



Destruction of Ocean Crust 



The theory of plate tectonics assumes the destruction of oceanic 

 lithsphere in deep trenches bordering continents and island arcs. In 

 these areas the oceanic lithosphere is thrust beneath the overriding 

 continent or arc along a steeply dipping zone, which is highlighted by 

 intense earthquake activity. Melting of rock along this zone and the 

 subsequent rise of magma produces the chains of volcanoes that are 

 commonly adjacent to trench regions. Seismology studies and 

 analyses of the erupted volcanic material above the earthquake zone 

 give the best picture of the ultimate destruction of oceanic 

 lithosphere, though these studies are not the domain of the marine 

 geologist or geophysicist. Marine studies generally concentrate on 

 the shallower aspects of the process that are observable with 

 conventional geological and geophysical techniques, such as 

 gravity, refraction, reflection, heat-flow studies, and analyses of 

 volcanic history recorded in marine sediments. 



The great Alaska earthquake of 1964 triggered USGS efforts to 

 determine the effects of earthquakes on the sea floor. Since then, 

 continuing studies have been performed along the Aleutian trench 

 off the coast of Alaska to examine the total geologic framework, to 

 identify possible sources of future earthquakes, and to assess 

 potential effects of the earthquakes on the sea floor and adjacent 

 land areas. The USGS marine investigations, combined with 

 offshore studies sponsored by NSF and ONR, have provided strong 

 support to the concept that surface sediments on the Pacific plate are 

 being scraped off and accreted to the Alaskan continental margin as 

 the Pacific sea floor is thrust beneath Alaska. 



Continuing USGS investigations of the continental margin off the 

 coasts of northern California, Oregon, and Washington are directed 

 to problems similar to those encountered along the Alaska coast. As 



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