The lower layer of Earth's crust, composed mainly of dense rock like basalt, rich in silicon and magnesium.
A portmanteau formed from 'silicon' + 'magnesium.' The term was invented by geologists in the early 1900s to create a technical name distinguishing this dense layer from the lighter 'sial' above it.
The word 'sima' is so specialized that even many scientists don't use it anymore, having been replaced by more precise terms—it's a fossil word from early 20th-century geology that survives mainly in textbooks.
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