CONTINENTAL STRUCTURES AND PROCESSES AND SEA-FLOOR SPREADING 



Figure 11-3— SIX SHIFTING PLATES OF THE EARTH 



EURASIA 



EURASIA 



This diagram shows the six major "plates" of the earth. The double lines indicate 

 zones where spreading or extension is taking place. The single lines indicate zones 

 where the plates are converging or compression is taking place. Earthquake 

 activity is found wherever the plates come in contact. 



into the deep basins. The basins are 

 merely former ridges. 



Sea-Floor Spreading 



The magnetic field of the earth re- 

 verses periodically, and a record of its 

 polarity is forever preserved in the 

 orientation of magnetic minerals in 

 volcanic rocks which cooled at any 

 particular time. This apparently unre- 

 lated fact gives us clues to the motion 

 of the plates. The new rocks at the 

 trailing edges of the plates record the 

 magnetic polarity like a tape recorder 

 and then, like a magnetic tape, they 

 move on and the next polarity change 

 is recorded. This occurs in each of the 

 plates moving away from their com- 

 mon boundary. As a result, the sea 



floor in the Atlantic, for example, is 

 a bilaterally symmetrical tape record- 

 ing of the whole history of the earth's 

 magnetic field since the basin first 

 formed as a result of Africa and South 

 America splitting apart. We usually 

 read a tape recording by moving the 

 tape, but the stereo records of the 

 oceans are read by moving a ship or 

 airplane with the proper instruments 

 over the sea floor. From work on 

 land and at sea, the changes in mag- 

 netic polarity have been dated. We 

 can thus convert the magnetic records 

 into age-of-rock records and prepare 

 a geological map of the sea floor. In 

 the Atlantic, to continue with the pre- 

 vious example, the youngest rocks are 

 in the middle; they grow progres- 

 sively older toward the continents. 



The remains of sea-surface micro- 

 organisms rain constantly onto the 

 sea floor to form layers of ooze and 

 clay. Where the crust is young, the 

 layers are thin; they thicken where 

 they have had more time to accumu- 

 late. The very youngest crust, which 

 has just cooled, is exposed as black, 

 glossy, fresh rock of the type seen 

 in lava flows in Iceland and Hawaii. 

 The outpouring of lava occurs at a 

 relatively constant rate but the plates 

 spread apart at different rates depend- 

 ing on the geometry. Consequently, 

 lava piles up into very long volcanic 

 ridges with a relief that varies with 

 the spreading rate. A slow spreading 

 rate produces mountainous ridges; 

 fast spreading produces low, gently 

 sloping, but very long hills. 



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