. The Earth beneath the sea : History . 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. c
. The Earth beneath the sea : History . 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 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 [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.
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