Sima is a short form for magnesium silicate, which primarily makes up the oceanic crust rocks. Hence, oceanic crust and sima are, sometimes, interchangeably used. Sima is the lower layer of Earth's crust and represents much younger rocks (as compared to the rocks comprising the continental crust). Typically, oceanic crust is composed of rocks which are no older than 250 million years. The most common form of sima is basalt.
Another term commonly used to describe the crustal rocks is sial, which is short form for silica and aluminum. Sial is the upper layer of Earth's crust and is much thicker (30-40 km thick, on average) as compared to sima or oceanic crust (6-10 km thick). Sial is comprised of much older rocks, with an average age of about 1500 million years.
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