It is distinguished from the underlying mantle rocks by its composition, lower density, and the lower velocity at which it conducts seismic energy. ... These two types of crust differ in their average age, composition, thickness and mode of origin. ... The Mantle makes up most of the earths mass and is composed of iron, magnesium, aluminum, silicon, and oxygen silicate compounds. ... The Earth is separated into layers based on mechanical properties in addition to composition. ... Ninety-seven percent of Earth's water is salty. ...

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• Grade Level: High School