Plate Tectonics（terms explanation）

Internal Layers：inner core，outer core，mantle，crust。

inner core : the innermost  region of the earth. It is solid and consists primarily of iron with minor amounts of light elements that likely include nickel, sulfur, oxygen.

outer core : a region surrounding the inner core. It is liquid and consists primarily of iron with minor amounts of lighter elements that likely include nickel, sulfur, oxygen.

mantle : main bulk of Earth between the crust and the core; increasing pressure and temperature with depth divide the mantle into concentric layers.

crust : outer shell of the solid Earth; the lower limit is usually considered to be the Mohorovicic discontinuity.

Mohorovicic discontinuity : boudary between crust and mantle, marked by a rapid increase in seismic wave speed.