DROWNED ANCIENT ISLANDS — HESS 



293 



In this sense they are unstable. The expansion during magma genera- 

 tion, injexjtion of magma into the crust below the volcano, crystalliza- 

 tion of magma and contraction, extrusion of magma from a central 

 vent and isostatic adjustments to the load, out-fiow of weak oceanic 

 clays from beneath the volcanic load, etc., all tend to result in vertical 

 movements of the volcanic island. Such islands may have terraces 



Figure 12. — Fathometer recorder trace of guyot near latitude 21° N., longitude 

 173° E. Ship's speed, 13.5 knots; course 059° true. 



extending to hundreds of feet above sea level and at the same time 

 have drowned shore lines and exhibit a series of submerged terraces 

 as well. Once this vulcanism dies, the island will probably become 

 stable. Of the hundreds of atolls and banks with their volcanic 

 pedestals beneath them, one can find very few in the Pacific Basin 

 which have had their coral reefs uplifted by as much as 150 feet.* 



« Vening Meinesz (1941) reexamines gravity <!ata for oceanic islands. Though large, 

 local, positive, isostatic anomalies are found on sucli islands, the regional anomalies show 

 that such small islands are regionally and not locally compensated and thus closely approach 

 isostatic equilibrium. This Indicates a geologically rapid adjustment to the disturbance 

 of equilibrium brought about by vulcanism. 



