This is called the crust: oceanic crust is around 5 km (3 miles) to 10 km (6 miles ) thick and the continental crust is 20 to 30 miles thick, (still, i think that the word you're looking for is "crust" or "oceanic crust" to be sure.
Although we don’t fully understand why it is easier for younger children to learn new languages than for older children and adults, learning a language does appear to have what psychologists call a __SENSITIVE PERIOD__, an interval when people are receptive to learning and can acquire new knowledge more easily.
