ࡱ > O Q N '` R ? bjbj$$ 8F F F 2 8 $ : f b b " _ _ _ $ Z h ! x r _ _ r r p p p r p r p p C V ok [ 0 c , :" j " :" 4 n" $ T _ p l e _ _ _ X _ _ _ r r r r D Z Z H o w D o e s E a r t h s I n t e r i o r W o r k 0WtgQ萂YUOKL R I C H A R D A . K E R R tg_' y>r T h e p l a t e t e c t o n i c s r e v o l u t i o n w e n t o n l y s o d e e p . T r u e , i t m a d e w o n d e r f u l s e n s e o f m o s t o f t h e p l a n e t s g e o l o g y . B u t t h a t s s o m e t h i n g l i k e u n d e r s t a n d i n g t h e f a c e o f B i g B e n ; t h e r e m u s t b e a lot more inside to understand about how and why it all works. In the case of Earth, theres another 6300 kilometers of rock and iron beneath the tectonic plates whose churnings constitute the inner workings of a planetary heat engine. Tectonic plates jostling about the surface are like the hands sweeping across the clock face: informative in many ways but largely mute as to what drives them. 0 00WkgJXKR(W0WtuigQ|vu[vxuR N0Wt N'YRv0WFO1\I{TeNW'Y({gQ/fYUOK\OvSt N#j00WkhbN g6 3 0 0 lQ̑v\wT5&y Ne0WvN*eTWNxOSq_d Yck8^KL00WkhbvgJX]d1\}YP(uKbcNMRcSv'Y({hb N#jSN_0RvvuYvnj FOe/fuRREO[P{SN/fP0 0 0E a r t h s c i e n t i s t s i n h e r i t e d a r a t h e r s i m p l e p i c t u r e o f E a r t h s i n t e r i o r f r o m t h e i r p r e p late tectonics colleagues. Earth was like an onion. Seismic waves passing through the deep Earth suggested that beneath the broken skin of plates lies a 2800- kilometer layer of rocky mantle overlying 3470 kilometers of molten and at the centersolid iron. The mantle was further subdivided at a depth of 670 kilometers into upper and lower layers, with a hint of a layer a couple of hundred kilometers thick at the bottom of the lower mantle. 0 00Wtyx[[_0WkgJXx[[ሗ_0RN0WtgQi vW,g#jW00Wt1\P NFm%0WlN0Wtmd\vgQHQ/f2 8 0 0 lQ̑v0WT^\wcW g3 4 7 0 lQ̑v?o g_/f1uVKa5D}bv0W8h02 NekQ6 7 0 lQ̑U0}Rp N0WT^TN0WT^SY(WN0WT^^N g Nd\~^~vlQ̑S^vRd\0 0 0I n t h e p o s t r e v o l u t i o n e r a , t h e o n i o n m o d e l c o n t i n u e d t o l o o m l a r g e . T h e d o m i n a n t p i c t u r e o f E a r t h s i n n e r w o r k i n g s divided the planet at the 670-kilometer depth, forming with the core a three-layer machine. Above the 670, the mantle churned slowly like a very shallow pot of boiling water, delivering heat and rock at mid-ocean ridges to make new crust and cool the interior and accepting cold sinking slabs of old plate at deep-sea trenches. A plume of hot rock might rise from just above the 670 to form a volcanic hot spot like Hawaii. But no hot rock rose up through the 670 barrier, and no cold rock sank down through it. A l t e r n a t i v e l y , a r g u e d a s m a l l e r c o n t i n g e n t , t h e m a n t l e c h u r n e d f r o m b o t t o m t o t o p l i k e a d e e p s t o c k p o t , w i t h p l u m e s r i s i n g a l l t h e w a y f r o m t h e c o r e - m a n t l e b o u n d a r y . 0 0 0(W_i}TBfNm%!jW8od'Y0;NAmv0WtgQK\OW(W6 7 0 lQ̑vm^\0WtRb;Nv NPd\!k0(W6 7 0 lQ̑ Ne0WT^1\P NvmKq4l N#j}ba*eRW NecOqT\wf}wm N"uuev0Wk0Q{SgQTBf(WmwmwmnTcSW gJXvln0 N'YJXq\w(W6 7 0 lQ̑U NGS1\ gSb_b NPPYZ7Ykpq\,vqޞ0FO/fl gq\wSNN6 7 0 lQ̑vO\_N Ng gQ\wlnBf荊hLuP0SY0WT^_N0R N*eR1\P NPmonKqg N_8hT^Lubaba_ NGS0 0 0F o r t y y e a r s o f p r o b i n g i n n e r E a r t h w i t h e v e r m o r e s o p h i s t i c a t e d s e i s m i c i m a g i n g h a s b o o s t e d t h e v i e w o f t h e engines complexity without much calming the debate about how it works. Imaging now clearly shows that the 670 is no absolute barrier. Slabs penetrate the boundary, although with difficulty. Layered earth advocates have duly dropped their impenetrable boundary to 1000 kilometers or deeper. Or maybe theres a flexible, semi permeable boundary somewhere that limits mixing to only the most insistent slabs or plumes. 0 0VASt^O Nec,n0WtgQN_NUOBfPfpܖv0WbP:RoN1Y \er0|vR_j 0vܖwlFO/f{Sl gWXe NR0R^/f`K\Ov0q_PnZi0Whfl g@bU} \v6 7 0 lQ̑O\0\JXnLuBf֖6q gV0FON[If0Wd\ NSNuLuv[[^]}XR1 0 0 0 lQ̑bAVfm0bb_N1X[(W NPfH_'`vbJSn'`v0WeSP6RqTN gXVv\JXbqg0 0 0N o w s e i s m i c i m a g i n g i s a l s o o u t l i n i n g t w o g r e a t g l o b s o f m a n t l e r o ck standing beneath Africa and the Pacific like pistons. Researchers disagree whether they are hotter than average and rising under their own buoyancy, denser and sinking, or merely passively being carried upward by adjacent currents. Thin lenses of partially melted rock dot the mantle bottom, perhaps marking the bottom of plumes, or perhaps not. Geochemists reading the entrails of elements and isotopes in mantle-derived rocks find signs of five long-lived reservoirs that must have resisted mixing in the m a n t l e f o r b i l l i o n s o f y e a r s . B u t t h e y h a v e n t a c l u e w h e r e i n t h e d e p t h s o f t h e m a n t l e t h o s e r e s e r v o i r s m i g h t b e h i d i n g . 0 0vMR0WWPo:y giQ'Y0WT^Wrir[oυ(W^2mT*Ys^m^d\nboW;m^Xv҉r0vQq^s^GW