The Earths interior (University of Chicago). The crust is the outermost layer of the Earth; itâs a thin shell of rock that measures just 30 km thick at the continents, and 5 km thick underneath the oceans. Just 1% of the Earthâs mass is made up of its crust.he Earthâs crust is the outermost layer of the Earthâs structure. In fact, the crust is what youâre standing on. But geologists have learned that if you dig down into the Earth, even just a few hundred meters, and things start to heat up.