DEEP EARTH PRC 





Evaluation of Present Knowledge 



In spite of recent advances, our 

 ideas and knowledge of the deep in- 

 terior remain largely qualitative. We 

 know roughly, but not exactly, what 

 the mantle consists of. We suspect 

 that phase changes occur at certain 

 depths, but we cannot pinpoint the 

 exact nature of these changes. We 

 can estimate roughly how much heat 

 is generated in the mantle and its 

 source (mostly radioactive disintegra- 

 tion), but cannot tell yet how the 

 sources of heat and temperature are 

 distributed. We do not have precise 

 information as to the mechanical and 

 flow properties of the mantle, and 

 haven't yet solved the mathematical 

 equations relevant to convection, even 

 though approximate solutions have 

 been found. We don't even know 

 whether the whole mantle, or only its 

 upper part, participates in the motion. 



The situation regarding the core is 

 similarly vague. We know that it con- 

 sists dominantly of iron, but cannot 

 determine what other elements are 

 present. We know that the outer two- 

 thirds of the core is liquid, and that 

 motion in this metallic liquid gener- 

 ates the earth's magnetic field, but the 

 details of the process are still obscure, 

 and the full set of equations that 

 govern the process has not yet been 

 solved. Important physical properties 

 of the core, such as its electrical con- 

 ductivity, are still uncertain by one 

 or more powers of ten. The source of 

 the energy that drives the terrestrial 

 dynamo is still obscure. Even though 

 we suspect that motions in the core 

 are the cause of observable effects at 



the surface (e.g., irregular changes in 

 the length of the day, small periodic 

 displacements of the earth with re- 

 spect to its rotation axis), we still can- 

 not assess these effects qualitatively. 

 We suspect interactions between the 

 core and mantle which ultimately af- 

 fect the crust, but cannot focus pre- 

 cisely on any of them. 



Goals and Requirements for 

 Scientific Activity 



To understand our total environ- 

 ment, to see how it came to be the 

 way it is, and how it is changing from 

 natural — as opposed to human — 

 causes, and to control it to our best 

 advantage (e.g., by curtailing earth- 

 quake damage or by muzzling dan- 

 gerous volcanoes with due regard to 

 their positive contributions to human 

 ecology), we need a better under- 

 standing of the constitution and be- 

 havior of the deeper parts of the 

 earth. To reach this understanding re- 

 quires a concerted and sustained ef- 

 fort in many directions, encompassing 

 a wide range of scientific disciplines, 

 from fluid mechanics to materials sci- 

 ence, from electromagnetic theory to 

 solid-state physics. 



Observational Networks — If we 

 can foretell the future from the recent 

 past, it is clear that a key to further 

 progress is the establishment and 

 maintenance of a first-rate global net- 

 work of observatories such as the 

 worldwide network of standard seis- 

 mographic stations established under 

 the VELA program of the Department 

 of Defense. This network has enabled 



seismologists to determine more pre- 

 cisely than ever before just where 

 earthquakes occur, and has brought 

 into sharp focus a remarkable corre- 

 lation between earthquakes and other 

 geological features that had only been 

 dimly perceived. This correlation is 

 fundamental to the notion of global 

 tectonics. Among other examples of 

 progress resulting from improvement 

 in instrumentation, one can mention 

 the determination of the depth in the 

 mantle at which some of the phase 

 changes occur and refinements in the 

 fine structure of the inner core. 



Deep Drilling — Much speculation 

 could be avoided, and much informa- 

 tion gained, from analysis (of the type 

 to which lunar samples are subjected) 

 of samples from the mantle obtained 

 by deep drilling. Drilling through the 

 sedimentary cover of the ocean floor 

 has already been most rewarding in 

 its confirmation of the relative youth 

 and rate of motion of the oceanic 

 crust. But more is needed, and deeper 

 penetration through the crust into the 

 mantle at several points will eventu- 

 ally become necessary. 



High-Pressure Experiments — 

 Finally, it would seem that a major 

 effort should be made to gain more 

 knowledge of the properties and be- 

 havior of materials subjected to pres- 

 sures of the order of those prevailing 

 in the deep interior (tens of millions 

 of pounds per square inch). Too much 

 is now left to guessing; solid-state 

 theory is presently inadequate and 

 would anyhow need experimental 

 confirmation. 



A Note on the Earth's Magnetic Field 



The earth's magnetic field was one 

 of the earliest subjects of scientific 

 inquiry. The field's obvious utility 

 in navigation, as well as the intrinsic 

 interest of the complex phenomena 



displayed, have led people to study it 

 ever since the sixteenth century. 



The study of the earth's field di- 

 vides into two parts: that of the main 



part of the field, which changes only 

 slowly (over hundreds of years), and 

 that of the rapid variations (periods of 

 seconds to a year). The latter are 

 caused by things that happen in the 



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