SECT. 2] 



ABYSSAL PLAINS 



355 



C. The Gravity Field and the Magnetic Field 



On the basis of the relatively irregular gravity field shown by a series of 

 closely spaced pendulum observations in the Bay of Biscay, Browne and 

 Cooper (1950) concluded that there must be large topographic features buried 

 beneath the Bay of Biscay Abyssal Plain. Subsequent studies of the Madeira 

 Abyssal Plain with a continuous-recording Graf sea gravimeter gave further 

 support to this conclusion (Worzel, 1959) (Fig. 31). In the region of the adjacent 

 continental rise, the free-air gravity curve is by and large a perfect reflection 

 of the topographic })rofi]e. However, on the abyssal plain the gravity field 

 sliows variations of the same amplitude as those observed over the adjacent 



4000 



6000 



Abyssal Hills 



Madeira Abyssal Plain 



Madeira Rise 



Fig. 31. Free-air gravity anomalies across the Madeira Abyssal Plain. (After Worzel, 

 1959.) Free-air gravity anomalies were observed with a Graf sea gravimeter from a 

 surface ship. Note that the amplitude of the anomalies over the abyssal plain is not 

 markedly different from that over the adjacent rises. This suggests that the basement 

 structure is uniformly complex along the entire profile, and that the smoother topo- 

 graphy of the abyssal plain is due to sediment blanketing the complex topography. 



continental rise and abyssal hills. This clearly shows that the topography on the 

 buried rock surface beneath the abyssal plain must be essentially similar to 

 that of the continental rise.i The greater number of variable factors involved 

 makes the interpretation of total magnetic intensity anomalies a much more 

 difficult task than the interpretation of gravity anomalies. Profiles of total 

 intensity of the Earth's magnetic field show the same irregularities of field over 

 the abyssal plains as over the adjacent abyssal hills (Fig. 32), 



D. Heat Flow 



Heat flow measurements in the abyssal floor made by Bullard (1954), Bullard 

 and Day (1961) and Von Herzen (1959) reveal values of approximately 1.2 [xcal/ 

 cm2/sec (see Chapter 11 of this volume). Values of heat flow on the mid-oceanic 

 ridges range to seven or eight times this value and heat flows measured in the 



1 Recent simultaneous observations of gravity and seismic -reflection profiles obtained 

 during a roiuid- the -world cruise have confii'med this correlation in several abyssal plains. 



