Crust
Earth’s relatively rigid outer shell, the lithosphere, is made up of the crust and uppermost mantle layer. Oceanic crust is thinner (5–10 km) but denser than continental crust (20–90 km; average 35 km). Currently, 1/3rd of the crust is continental, 2/3rds oceanic.
Crust composition differs. Oceanic crust is rich in iron and magnesium. Continental crust, derived from oceanic crust over eons, and fed by volcanoes, has more granite.
The continental crust contains the oldest rocks: up to 4 billion years. Nevertheless, continental crust changes constantly, due to erosion, sedimentation, volcanic activity, and tectonics.
Oceanic crust is constantly recycled by tectonic plate subduction and regenerated by magma plumes rising from the bottom of the mantle. No location in the oceanic crust is older than 200 million years.
