What is the thickness of the earth’s crust under the oceans; under the plains?

The thickness of the earth’s crust is distributed unevenly across our planet. This is primarily due to the number of layers of rocks. There are only two of them under the oceans – basalt and sedimentary. And under the plains and mountains there are three – sedimentary, granite and basalt. The thickness of the earth’s crust under the oceans is about five kilometers. Under the plains about forty, and under the highest mountain systems it reaches up to eighty kilometers.