Earth mainly consists of four layers namely crust, mantle, outer core and inner core:
Crust: This is about 8 km thick inside and is highly explored. The continental crust is composed mostly of granite. The oceanic crust consists of a volcanic lava rock called basalt.
Mantle: It is the largest layer of the Earth which is 1800 miles thick. It is composed mostly of silicate rocks rich in magnesium and iron.
Outer core: It is located beneath the mantle. The outer core is composed of melted metals like nickel and iron.
Inner core: It is composed of an iron-nickel alloy and some light elements.
Layer that lies immediately after the mantle is the outer core.