168 



WORZEL AND HARRIROX 



[chap. 9 



probable. The type B solution would imply that the volcanic material came 

 from dee]) within the mantle. The type C solution would imply that either the 

 volcanic material which forms the seamount originated in the region of the 

 compensation or that the vulcanism originated deep within the mantle 

 depositing some material as the island and some as the root. 



FREE 



AIR ANOMALIES 



— Computed onomoiy 



• Observed onomoty USiS. Redfish ^ 195Z 



• Observed anomaly 



Harrison and others, 1957 



F IS 



20 



P = l.03 



5" = 2.84 



^' = 2.21"' 



P--3.27 



Vertical Exoggeralion = 10^ 



100 



■50 



-100 



20 



25 



400 



DEDUCED STRUCTURE SECTION (b) 

 (with assumed sedimentary layer) 



Fig. 26. Mendocino Escarpment. (After Talwani et al., 1959.) 



D. Mendocino Escarpment 



A mass distribution which accounts for the observed gravity anomaly over 

 the Mendocino Escarpment has been given by Talwani et al. (1959). Fig. 26 

 shows the crustal structure deduced assuming a sedimentary layer as shown 

 and assuming that the residual anomaly is all due to the fluctuations of the M- 

 discontinuity. As this section is improbable on geological and seismic grounds, a 

 solution of the type shown above for the Mid-Atlantic Ridge, i.e. a low density 

 mass within the crust, would seem more likely. 



E. Ocean Trenches 



The discovery by Veiling Meinesz of the belt of large negative-gravity 

 anomalies associated with the oceanic trenches is one of the greatest discoveries 

 of contemporary geology and geophysics. Since their discovery, many geo- 



