It is believed that the upper layer of the tectonic plates - the crust - was produced by substances which have flowed from the inside of our planet.

Oceanic crust consists mainly of basaltic rocks which are rich in magnesium, iron and some silicates, with lower density granitic rocks dominating continental (land) crusts, which are enriched in mostly lighter elements, including oxygen, aluminium, sodium, silicon, and potassium.

Due to the recycling nature of the tectonic plate movement, the oldest ocean crust is though to be about 200 millions years old, and the oldest continental crust is believed to be about four billion years old.