Core
The region beneath the mantle is called the core, and consists of two parts, a
liquid outer core that is around 2250 km thick and a solid inner core 1220 km
thick. The core is primarily made up of iron, with a small amount of nickel. The
liquid iron in the outer core is particularly important in that it is the
primary source of the Earth's magnetic field. Unlike a common magnet, though,
the north and south ends of our "global magnet" are not exactly situated at
Earth's poles. Instead, the magnetic north pole is actually situated in northern
Canada, and the magnetic south pole resides north of Antarctica and south of
Australia. Another interesting feature of the magnetic poles is that their
precise location moves over time. Every few million years, even the polarity of
the Earth's magnetic field reverses (called a geomagnetic reversal, where
magnetic north and south "switch"). While scientists still do not fully
understand why geomagnetic reversals occur, the presence of changing magnetic
orientations preserved in rocks containing iron was a fundamental clue in
unravelling the puzzle of Plate Tectonics. Almost all of our direct knowledge of
the Earth's interior is from the upper 10 km. Our knowledge of the remaining
6,300 km is based largely on indirect evidence from seismology, laboratory
studies of igneous and metamorphic rocks, computer models and meteorites.