Impermeable stratum overlaying a reservoir which prevents the hydrocarbons contained in it from migrating to other rocks. (2) The interior of the Earth, like the other terrestrial planets, is chemically divided into layers. The layer above the core of the earth is the mantle. It begins about 10 km below the oceanic crust and about 30 km below the continental crust and makes up 70-80% of the Earth’s total volume and is about 2,900 km thick.