Why is the oceanic crust different than the mantle?

Layers that are less dense, such as the crust, float on layers that are denser, such as the mantle. ... Because continental crust is less dense than oceanic crust it floats higher on the mantle, just like a piece of Styrofoam floats higher on water than a piece of wood does.

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A Stevenson screen or instrument shelter is a shelter or an enclosure to meteorological instruments against precipitation and direct heat radiation from outside sources, while still allowing air to circulate freely around them