T h e E n e r g y o f G r a v i t a t i o n a l D i f f e r e n t i a t i o n o f T h e E a r t h ' s m a n t l e (*) E . N . L Y U S T I K H Ricevuto il 7 d i c e m b r e I960 1 . - I N T R O D U C T I O N . TN general, the E a r t h ' s crust consists of sialic rocks, (i.e. acidic and basic igneous and metamorphic rocks). I t is suggested by many authors (Kropotkin 1953, Belousov 1954, Wilson 1957) t h a t the crust grows gradually on account of permanent income of sial from the E a r t h ' s mantle, which consists of simatic or ultrabasic rocks. (Sial is generated due to the differentiation of the mantle: it is lifted up by buoyancy force and emerges in geosynclines). The hypothesis may be proposed t h a t the floating up sial provides with energy all the tectonic processes. Thus the question arises whether the output of energy can be sufficient. We may compute all the values to within an order only, since the necessary amount of energy is hardly known even to such exactness. 2 . - E N E R G Y W H I C H CAN B E R E L E A S E D . We may assume a flat Earth for our computations. The volume of the crust is 1025 cm3 today (Poldervaart 1955). The beginning of the forming of the crust may be 3 . 5 x 1 0 ° years ago, as the radioactive ages of rocks have shown. This gives the mean output of emerging sial 3 x 1015 cm3/year. I t is believed t h a t differentiation goes in the upper 700 k m of the mantle only. Accepting this supposition we assume the mean depth, sial emerges from, to be as great as 350 km, i.e. 3.5 X10' cm. (*) P a p e r read a t t h e Ilelsinky Assembly of t h e I . U . G . G . I960. 1 7 0 E . N . L T U S T I K H The density difference between sima and sial is 0.5 g/cm3, or so. Gravity in the m a n t l e is 1000 gals to within 2-3 per cent. Taking these values we have. e = D(j IIv = 5 X1025 e r g / y e a r , where: e = mean o u t p u t of energy, D — density difference, (j = gravity, H = mean depth sial emerges from v = mean o u t p u t of sial The p a r t of the mantle undergoing differentiation loses light com- ponents and so grows heavier. A t last it m a y plunge down and sink to the b o t t o m of the mantle, provided the whole m a n t l e is chemically homogeneous. Fresh m a t t e r will t a k e its place, and differentiation m a y be resumed. F o r such " chemical convenction " we have to substitute 2.5 X 10s cm for H. So we get energy o u t p u t e — 4 X 102° erg/year, i. e. one order greater t h a n from the differentiation of t h e upper p a r t of t h e mantle. Not all t h e energy released b y the differentiation m a y be useful for tectonic movements. An allowance m u s t be made for the efficiency of the transmission mechanism. The transmission m a y be very effective if sial ascends as a continuous sheet along a planetary r u p t u r e (we sup- pose eveiy geosyncline is generated b y a great " planetary " inclined r u p t u r e cutting the upper p a r t of t h e mantle). On the contrary, if m a n y isolated balls of sial emerge up through the mantle, t h e efficiency m a y be much less. We estimate presumable the efficiency /? in t h e range 1 > fi > 0.01 , then t h e possible contribution of energy to tectonics T — fie have limits 1027 > j ' > 10 23 e r g / y e a r . 3 . - N E C E S S A R Y A M O U N T O F E N E R G Y . The only way to estimate the a m o u n t of energy necessary for tec- tonic processes is to s t a r t from t h e average annual energy of all t h e seismic waves on the E a r t h , since in is t h e only geotectonic energy mea- T H E E N E R G Y OF G R A V I T A T I O N A L D I F F E R E N T I A T I O N 1 7 1 surable. During 1890-1955 the mean power of seismic waves was 8.4 X1024 erg/year, as it follows from the d a t a of B. Gutenberg (1956). I t is doubtful whether this figure m a y be representative for t h e whole period 3 . 5 x 1 0 " years. To be cautious we estimate the seismic energy z in t h e range 10" > z > 1023 erg/year . Only a p a r t of energy released in foci t u r n s into wave energy; the another p a r t is immediately converted into heat. Taking this fact into considerations we assume for the whole o u t p u t of energy in foci Z the limits 1020 > Z > 1023 e r g / y e a r . The power of all tectonic processes T m a y be 10 to 1000 times as great as the power of seismic activity Z, so 1031 > T > 1024 e r g / y e a r . 4 . - C O N C L U S I O N . As you have seen above, the tectonic power needed and t h e tectonic power available m a y coincide in the range 1027 > T > 1024 erg/year. Thus t h e mechanism of differentiation can provide geotectonics with energy if the efficiency of this mechanism is high enough, and t h e required amount of energy is not much greater t h a n t h e present energy of seismic waves. SUMMARY The Earth's crust has been forming owing to the differentiation of the mantle, «sial» being lifted by the forces due to gravity. As a result, the mean energy supply for tectonics might be as great as 10-3-1027 erg /year. We estimate the energy of all tectonic processes at 102i-1031 erg /year. So it may be suggested that the mechanism of differentiation provide geotectonics with energy. 172 E . N . L Y U S T I K H RIASSUNTO La crosta terrestre si e andata formando a causa della differenziazione del mantello, dato che il « sial » viene sollevato dalle forze dovute alia gravita. Di conseguenza il contributo medio di energia per la tettonica potrebbe raggiungere i 10'a-10'" erg/anno. Noi calcoliamo Venergia di tutti i processi tettonici a lO^-lO31 erg/anno. Si pud cost ritenere che il meccanismo della differenziazione fornisea Venergia alia geotettonica. R E F E R E N C E BELOUSOV V . V . , (EEJioycoB B. B . ) Oaiomibie eonpocti eeomenmoimKn. « Tocreoji- Texn3flaT » , MocKBa ( 1 9 5 4 ) . G U T E N B E R G B . , Tr. AGTJ, 3 7 , n . 5 , ( 1 9 5 6 ) . K R O P O T K I N P . N., (KponoTKHH n . H . ) H3eecmuH AH CCCP, cepnj[ reojionm., 1 ( 1 9 5 3 ) . LYUSTIKH E . N . , (JIIOCTHX E. H . ) Pheecmun AH CCCP, cepHH rco())H3HH., 3 ( 1 9 6 0 ) . P O L D E R V A A R T A . , Qeol. Soc. Am., Spec. Paper 6 2 , ( 1 9 5 5 ) . W I L S O N J . T u z o , Nature, 1 7 9 , N o . 4 5 5 3 , ( 1 9 5 7 ) .