What is the crust and what is it mainly composed of?
The Earth's crust is the outermost layer of our planet. It is also the thinnest layer.
Over 90% of continental crust is less than 50km deep. You can see an image from the US Geological Society that shows the thickness of the crust across the planet here .
Oceanic crust is composed primarily of basaltic rock whereas continental crust is composed mainly of granite. Earth's crust is primarily made up of the elements iron, oxygen, silicon, and magnesium.