TAILIEUCHUNG - The Self-Made Tapestry Phần 7

Giả sử các thông số gần gũi với những người tìm thấy bằng thực nghiệm cho quá trình chuyển đổi này, họ đã tìm thấy rằng đối lưu là một hỗn hợp của hai quá trình này. Chìm dòng điện trong lớp vỏ phía trên nhìn chung đã không xuyên qua ranh giới này ở 660 km. | Page 17 temperature . Assuming parameters close to those found experimentally for this transition they found that convection was a mixture of these two processes. Sinking currents in the upper mantle generally did not penetrate the boundary at 660 km but if two such currents were pushed together by one of the broad upwelling flows together they might achieve a threshold size that would enable them to punch through the boundary and flush into the lower mantle. A similar picture of both layered and wholemantle convection was painted by Paul Tackley of the California Institute of Technology and colleagues when they conducted the same kind of computer simulation but with a somewhat more realistic model in 1993. They found that again the flow pattern organized itself into hot rising plumes and cold sinking sheets. The plumes were able to punch their way from the base of the mantle straight through the 660-km boundary to the top but the cold sinking sheets the analogue of mantle slabs generally stopped at this boundary where the cold dense fluid accumulated in spreading puddles. When these cold pools became large enough they would suddenly flush through to the lower mantle in an avalanche creating a broad sinking column that than spread in a vast pool at the core-mantle boundary Plate 15 . What limited direct evidence we have of the nature of mantle convection seems to support this picture sometimes slabs of cool material borne downwards at subduction zones seem to be stopped or deflected at the 660-km boundary but others appear to pass right through. Little by little we are starting to piece together the mysteries of the Earth s bowels. Air water earth and fire There are ample examples of natural convection patterns to keep US diverted at the Earth s surface too. The canvas of the sky is streaked with their imprint. The towering piles of cumulus clouds are erected by convective updrafts as warm air locally heated by the Sun rises and bears water vapour with it Fig.

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