TART II— D\NAMICS Or THE SOLID EARTH 



cause deformation, flow, and vol- 

 canism. 



What observations, and what meth- 

 ods of study, do we have? 



Seismic Waves — Seismic (elastic) 

 waves are propagated between the 

 focus of an earthquake and receivers 

 (seismographs) appropriately located 

 on the earth's surface. The speed of 

 propagation depends on the physical 

 properties of the propagating mate- 

 rial; this knowledge of speed versus 

 depth within the earth provides a clue 

 as to variations of physical proper- 

 ties — hence, of composition — with 

 depth. A serious problem arises in 

 that physical properties are sensitive 

 to pressure, and pressures inside the 

 earth greatly exceed those that can 

 conveniently be created in the labora- 

 tory for the purpose of studying their 

 effects on physical properties. High 

 pressures, of the order of those exist- 

 ing in the core, can be created by 

 means of explosive shock waves, but 

 precise measurement of physical prop- 

 erties under shock conditions remains 

 exceedingly difficult and costly. 



Measurable Properties of the Eartli 

 as a Whole — Properties such as 

 earth's total mass, its moment of in- 

 ertia (best determined from observing 

 the motion of artificial satellites), and 

 the frequency of its free oscillations 

 (i.e., the "tone" at which the earth 

 vibrates, like a struck bell, when dis- 

 turbed by a sufficiently violent earth- 

 quake) provide constraints on density 

 distribution and physical properties in 

 the form of global averages. For in- 

 stance, the variation in physical prop- 

 erties with depth deduced from seis- 

 mic studies must average out to the 

 values deduced from these global con- 

 straints. 



Lava — The nature of molten mate- 

 rial (lava) that rises from the mantle 

 and spills out on the surface (from 

 volcanoes) provides information on 

 the chemical nature of the source. The 

 problem is not straightforward, how- 

 ever, for the chemical composition of 



the liquid that forms by partial melt- 

 ing of a system as complicated as 

 ordinary rock is not generally the 

 same as that of the parent rock; it 

 varies, moreover, as a function of 

 pressure and temperature. A great 

 deal of painstaking experimental work 

 at high pressure is required before the 

 chemistry of the earth's mantle will 

 be understood. 



The "Heat Flow" — The heat that 

 escapes across the surface of the 

 earth from the interior provides in- 

 formation on the distribution of heat 

 sources and temperature within the 

 earth. (This heat flow, incidentally, 

 amounts to some 30 million mega- 

 watts and is equivalent to the output 

 of about 30,000 large modern power 

 plants.) The manner in which this 

 heat is transferred within the earth is 

 not precisely known; it is generally 

 believed that transfer in the deep in- 

 terior is mostly by "convection": 

 mass motion of hot stuff rising while 

 an equivalent amount of cold stuff 

 is sinking elsewhere. Convection is 

 generally believed to provide a mech- 

 anism to move the crust. Earthquakes 

 may well be the expression of strains 

 set up by this motion, and the geo- 

 graphic distribution of earthquakes 

 may reflect the present pattern of con- 

 vective flow in the mantle. 



The chief problems are: (a) How 

 does the solid mantle flow? How do 

 we best describe its response to me- 

 chanical forces? (b) What flow pat- 

 tern do we expect in a body as com- 

 plicated and heterogeneous as the 

 earth? We are faced here with some 

 difficult mathematical problems in 

 fluid dynamics. It must also be re- 

 membered that, contrary to the com- 

 mon state of affairs in engineering 

 studies of fluid dynamics, where the 

 initial conditions are precisely stated 

 and controllable, conditions in the 

 earth regarding flow properties and 

 distribution of heat sources and tem- 

 perature are not known and must be 

 deduced from a comparison of theory 

 with geological or geophysical obser- 

 vations. 



Sea-Floor Spreading — Geological 

 studies regarding past history of the 

 earth provide information as to what 

 has happened. Most importantly, they 

 provide information as to the rate at 

 which the crust deforms or moves, the 

 sense of its motion, and its duration. 

 This is essential input to the solution 

 of the dynamical problems mentioned 

 in the previous paragraph. 



In this respect, the last decade has 

 seen what may well be the most im- 

 portant and far-reaching development 

 since the days of Hutton (1795). 

 What has now become known as 

 "sea-floor spreading" (or "plate tec- 

 tonics" or "global tectonics") is the 

 general proposition that new oceanic 

 crust is constantly generated from the 

 mantle along submarine ridges while 

 an equivalent amount of crust is re- 

 sorbed into the mantle at other places; 

 in between, the whole crust moves at 

 rates of a few inches per year. This 

 general pattern of motion provides an 

 important and much-needed clue to 

 the behavior of the mantle. 



Phase Cliauges — It is well known 

 in materials science that, at high pres- 

 sure or temperature, substances may 

 occur under forms with properties 

 quite different from those of the same 

 substance under normal conditions 

 ("phase changes"). A typical example 

 is that of common carbon that occurs 

 either as graphite (a soft material used 

 for lubrication) or diamond (the hard- 

 est known mineral). It is now clear 

 that phase changes do occur in the 

 mantle, the lower half of which has 

 properties quite different from those 

 of its upper half even though its gross 

 chemical composition may be roughly 

 the same. Again, a very large amount 

 of difficult experimentation on high 

 pressure is needed to ascertain the 

 form under which common minerals 

 could occur in the earth's deep in- 

 terior. It is not unlikely that such 

 studies could lead to the discovery 

 and synthesis of new materials of 

 engineering importance; for exam- 

 ple, very hard substances might be 

 produced. 



