294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1947 



Aside from vulcanism and its effect of producing local points of 

 instability, convection currents of lesser intensity than those produc- 

 ing island arcs may result in vertical movements of the suboceanic 

 crust at times. 



HYPOTHETICAL DEVELOPMENT OF THE HISTORY OF THE PACIFIC BASIN AND 



THE ORIGIN OF GUY0T8 



Most discussions of Pacific historical geology jam all the known 

 history into the late Tertiary, Pleistocene, and Recent ages. To be 

 sure the rocks visible on the surface of volcanic islands are mostly very 

 young, predominantly Recent plus some Pleistocene, and very rarely 

 rocks that can be demonstrated to be as old as Tertiary. Many writers 

 seem inclined to place Pacific atoll formation in the Pleistocene though 

 others extend it back into the Tertiary (Stearns, 1946). On the other 

 hand, the Pacific Basin is generally considered to be very old, probably 

 dating from early pre-Cambrian time (Kuenen, 1937). It seems 

 reasonable to suppose that volcanic activity in the Pacific Basin and 

 hence island formation has gone on sporadically since early pre-Cam- 

 brian. Vvliere then are the pre-Cambrian, Paleozoic, and Mesozoic 

 islands? In order to answer this it is necessary to digress along 

 several other channels. 



Any island formed in the Basin can be assumed to have begun as a 

 volcano or group of volcanoes. After vulcanism ceased and the island 

 had become stabilized, the following sequence of events would neces- 

 sarily take place. The island would be eroded to low relief, and after a 

 long period of time (providing growth of reef -forming organisms did 

 not interfere) the island would completely disappear as a result of 

 marine planation. Such must have been the fate of all pre-Cambrian 

 islands before reef -forming organisms existed. 



Kuenen (1937) and 1941) has concluded that there has been little 

 change of sea level since early pre-Cambrian time. He estimated that 

 the rate of sedimentation in the deep sea is approximately 1 cubic 

 centimeter in 10,000 years for red clay, since the end of the pre-Cam- 

 brian, and 1 cubic centimeter in 5,000 years for globigerina ooze. Since 

 most of the material deposited on the ocean floor has ultimately come 

 from the continents, isostatic adjustment of the load on the sea floor 

 and the loss of weight from the continents has resulted in the sinking 

 of the former and rise of the latter so that the relative sea level with 

 respect to the continents has not changed very much. One obviously 

 cannot put a layer of several thousand feet of sediments into the oceans 

 without causing the water to rise by an equivalent amount (less the 

 water included in pore space in the sediments ) . Thus, quite apart from 

 the discussion of isostatic adjustment mentioned above, every centi- 

 meter of sediment put into the ocean causes sea level to rise with respect 

 to an oceanic island by just a little less than^a centimeter (less by the 



