| 释义 |
▪ I. sima Geol.|ˈsaɪmə|
Also Sima.
[a. G. sima (E. Suess Das Antlitz der Erde (1909) III. ii. xxiv. 626), f. L. si-licium silicon + magnesium magnesium.]
The continuous basal layer of the earth's crust, composed of relatively heavy, basic rocks rich in silica and magnesia, that underlies the sialic continental masses and forms the crust under the oceans; the material of which it is composed.
The lower limit of the sima is generally taken to be the Mohorovičić discontinuity.
1909[see Nife]. 1925Glasgow Herald 29 Sept. 9 In continental regions it [sc. sial] both rises higher to form the land surface, and extends downwards into or displaces somewhat the sima, just as a ship floats on and displaces the water. 1944A. Holmes Princ. Physical Geol. iv. 41 Certain rocks of the kinds grouped together as sima (e.g. basalt) contain calcic plagioclase, but others are free from felspar. All of the sima rocks, however, are characterized by the abundance or predominance of heavy, greenish silicate minerals. 1950P. H. Kuenen Marine Geol. ii. 127 If America were plowing through the sima westwards..one would expect a raising of the sima in front of the continent. 1970L. Knopoff in Johnson & Smith Megatectonics of Continents & Oceans vi. 120 It seemed plausible to assert that the top of the mantle was a chemically homogeneous material. In many of the older geology texts, this material was simply called sima to describe a dense, basic rock from which the less dense, more acidic sialic crust could be derived by some process of differentiation.
Hence siˈmatic a., of or pertaining to the basal crust or the material of which it is made.
1942R. A. Daly Floor of Ocean ii. 59 The oceanic sectors of the earth are characteristically simatic... Also simatic is any vitreous basalt which may form ‘pockets’ in the crust. 1955[see sialic a.1]. 1971Proc. 2nd Symposium Upper Mantle Project (Council of Sci. & Industr. Res., New Delhi) p. xix, Resting on the simatic crust, the continental blocks move, as if riding on the conveyor belt.
▪ II. sima
obs. variant of cyma. |