Faulting in tiie eastern Pacific is shown by 

 tiiis diagram. Turquoise Unas marl^ tiie 

 major fracture zones; darl<. siiading siiows 

 areas of ridges and trougiis; heavy 

 turquoise lines indicate trenches. 



away by waves and the top is eventually cut off. During this time 

 corals grow over the truncated part. Meanwhile, the great mass of 

 the entire cone presses into the oceanic crust, gradually the crust 

 yields, and the cone begins to sink. Provided the guyot is sinking 

 slowly, the coral reefs can grow upward fast enough to remain near 

 the surface, and so great thicknesses of coral limestone can be built 

 up. But if the guyot sinks too rapidly, or if the coral becomes less 

 active, then the whole column sinks below the surface until it finds 

 its equilibrium level, floating, as it were, in the mantle rock far below. 



Most of the seamounts cause some degree of local variation in 

 the intensity of the Earth's magnetic field. This happens because 

 the magnetic property of the volcanic rock forming them is different 

 from that of the sediments the seamounts displace. But there are 

 some seamounts near the continental slopes without magnetic 

 anomalies and without the exposed lava flows that are found on 

 the magnetic seamounts. These have certain properties rather similar 

 to continental rocks, so it may be that they are pieces of continent 

 that have broken off and separated from the shelf edge. 



In recent years American scientists have uncovered a striking 

 pattern of features in the Pacific Ocean. Extending westward for 

 thousands of miles from the edge of the continent are four or more 

 enormous fracture zones — represented as escarpments and chains 

 of submarine volcanoes. Now if the two sides of a fracture had 

 moved only up and down, we would expect to find the pattern of 

 magnetic anomalies across the fracture to be more or less contin- 

 uous. But detailed magnetic surveys have shown that the pattern 

 on one side of the fracture has been shifted horizontally relative to 

 the other side. The first fracture studied in this way showed that a 

 shift of eighty miles had occurred across the fault Hne ; but the most 

 recent work on the Mendocino Escarpment shows that the northern 

 side has shifted 750 miles to the west, relative to the southern side. 



