The outer core of the Earth begins about 1,800 miles below the Earth's surface and is between 1,370 and 1,430 miles thick. It is composed of liquid iron and nickel with some trace elements.
The outer core surrounds the solid inner core, which is about 750 miles thick and is itself surrounded by the mantle. The boundary between the outer core and the mantle is the Gutenberg discontinuity, and it is marked by the disappearance of shear waves from seismic activity and the loss of about 30 percent of velocity for other types of seismic waves. The outer core is believed to be the source of the Earth's magnetic field.