Definition of sial in US English:

sial

noun

Geology
dated
  • The material of the upper or continental part of the earth's crust, characterized as being of relatively low density and rich in silica and alumina.

    Contrasted with sima
    • ‘A mnemonic term derived from for silica, for alumina, and for magnesium, applied as a compositional term to a layer within the Earth that occupies a position intermediate between sial and sima.’
    • ‘The rocks found in the crust of the continents is called sial.’

Origin

1920s: from the initial letters of silica and alumina.

Pronunciation

sial

/ˈsaɪæl//ˈsīal/