produced as the result of intraplate magma sources. Long, linear 

 volcanic ridges in the Pacific Ocean are apparently produced as the 

 ocean plate drifts over one of these magma sources, or "hot spots." 

 The submarine ridge of which the Hawaiian Islands are the exposed 

 peaks is probably the best studied of these features. Under programs 

 sponsored in part by the USGS, scientists have defined an apparent 

 northwest-to-southeast decrease in the geologic age of volcanism. 

 Today the most active volcanoes of the Hawaiian Ridge are at the 

 southeast end on the Island of Hawaii. 



Initial hypotheses have attempted to explain the origin of these 

 "hot spots" as the result of either a rising plume of material from the 

 deep mantle or sinking and melting of portions of the shallow mantle. 

 If either of these explanations is correct, the "hot spots" may 

 represent fixed points within the Earth, over which the plates have 

 drifted for at least 100 million years. Thus, by analysis of the age and 

 orientation of volcanic ridges left on the plates by the "hot spots," it 

 may be possible to determine the "absolute" direction and rate of 

 plate movements. 



The "hot spots" hypothesis, however, has been called into serious 

 question as a result of drilling activities along the Line Islands, a 

 submarine ridge and island chain similar to Hawaii. On DSDP leg 33, 

 scientists found that volcanism apparently occurred simultaneously 

 along the 1,200 kilometers length of this ridge. A counter theory has 

 been proposed that suggests that at least some linear submarine 

 ridges are produced by fracturing of the lithosphere as it drifts over 

 the nonspherical Earth. Continuing research is attempting to 

 improve our understanding of these linear volcanic features in terms 

 of plate tectonics. 



Analyses of magnetic anomalies in oceanic areas near "hot spots" 

 reveal extensive regions of high amplitude. These are known to occur 

 southeast of Australia, around the Galapagos Islands, and on the 

 Juan de Fuca Ridge west of North America. Correlation with 

 chemical and magnetic analyses of dredged basalts shows that the 

 high amplitudes reflect iron and titanium, hence titanomagnetic 

 enrichment, which gives the crustal layer greater magnetization 

 intensity. Thus, "magnetic telechemistry" may be a viable tool in 

 assaying the bulk chemistry of the oceanic crust by the use of 

 magnetic-anomaly amplitudes. 



Other studies of "hot spots" showed that (1) volcano or major 

 seamount spacing is of the order of 70 kilometers and reflects 

 fracture spacing controlled by and roughly equal to the thickness of 

 the lithosphere; (2) volcano height is also controlled by the thickness 

 of the lithosphere; and (3) major "hot spots" on the Mid-Oceanic 

 Ridge — such as Iceland or the Azores — cause regional uplifts in the 

 spreading axis and associated regional gravity highs. As the plate 



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