As a result of radioactive heating over millions of years, Earth's molten core is probably fairly hot today, around 11,000°F. By comparison, lead melts at around 800°F. Most of Earth's 2,100-mile-thick core is liquid, but the center of the core is mostly solid iron. The liquid outer portion, about 95% of the core, is constantly in motion. The interaction between the solid inner core and the fluid outer core creates a hydromagnetic dynamo that generates the magnetic field around Earth. The magnetic field protects Earth from harmful cosmic radiation and makes navigation by compass possible. Scientists have observed a 10% decline in the dipolar magnetic field over the last 150 years. There is a point off the coast of Brazil where the field is 30% weaker, causing glitches in satellites and spacecraft flying through it. This weakening of the magnetic field has scientists speculating that it may eventually collapse in about 1,500–2,000 years and lead to a reversal of the South and North Poles. Polar reversals are a periodic occurrence.
Within the last decade, scientists have made some important discoveries about Earth's solid-iron core. In 1996, geophysicists discovered that the core rotates slightly faster than the rest of the planet and gains a quarter-turn every century. X-ray images of the inside of Earth show that the core is not a perfect sphere—there are vast mountains 6 to 7 mi high and deep valleys. These features are in an inverse, or upside-down, relationship to similar features on Earth's surface.
Outside the core is Earth's mantle, 1,800 mi thick and extending nearly to the surface. The mantle is composed of heavy silicate rock, similar to that brought up by volcanic eruptions. It is somewhere between liquid and solid, slightly yielding, and therefore contributing to an active, moving Earth. Most of Earth's radioactive material is in the thin crust that covers the mantle, but some is in the mantle and continues to give off heat. The crust's thickness ranges from 5 to 25 mi.