1. Brief presentation of planet Earth and its dynamics

The Earth is a roughly spherical, predominantly solid planet. It is made up of four concentric layers of unequal thickness, delimited by geophysical discontinuities: the inner core, the outer core, the mantle and the crust. Only the outer core is liquid.

The Earth's crust, the Earth's thin superficial envelope, varies in nature and thickness depending on whether we're dealing with the oceans or the continents and their margins. Oceanic crust is dense, thin (a few km to around ten km) and relatively simple in nature and structure: from bottom to top, it is composed mainly of gabbro (+/– peridotite), basalt and sediment. The crust is constantly being formed by the addition of magma at oceanic ridges.

The continental crust, which is lighter and therefore essentially emergent, is thicker (30-40 km on average, up to 70 km under recent mountain...