the crust is the outermost layer of earth. it is the layer on which we live. the crust is thicker under continents and thinner under oceans. it might be 70 km thick under mountains. the crust is only about 5 km thick under some oceans. the photo on the cover of this card shows the crust as the surfaces of africa and the arabian peninsula. they are separated by the red sea. the crust is much thinner under the red sea than under the continents