Question

why is the upper crust called sial
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Answer 1

Answer:

In geology, the term sial refers to the composition of the upper layer of Earth's crust, namely rocks rich in aluminium silicate minerals. The name 'sial' was taken from the first two letters of silica and of alumina.

Explanation:

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Answer 2

In geology, sima (/ˈsaɪmə/) is an antiquated blended term for the lower layer of Earth's crust. ... The name 'sima' was taken from the first two letters of silica and of magnesia. Comparable is the name 'sial' which is the name for the upper layer of Earth's crust (continental crust).
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