rust is the thin solid layer covering the outside of the earth . the thickness of the crust differs from place to place. beneath the sedimentary rocks of the soild earth, the rocks are rich in sial ( silica + aluminium). sial is made up of lighter rock material and is on an average 35 km beneath the continents . beneath the sial layer lies sima, the rocks rich in silia and magnesium . sima is made up of heavier rock material and is on an average 4-7 km thick beneath the ocean .
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